AIR CONDITIONING SYSTEM QUESTION AND ANSWER
What are the objectives of air conditioning on ships ?
To extract excess heat
To raise air temperature when required
To add moisture as required
To reduce moisture content as required
To maintain sufficient oxygen and air flow
To remove dust
How control of temperature is attained ?
Comfortable temperature range is about 22 °C and relative humidity (RH) about 60% (usually 40 ~ 70%)
All zone temperature
Controlled by compressor suction pressure, via solenoid valve as step controlling.
Thermostat, placed at some accommodation space actuates the Master Solenoid Valve of the plant, which will stop the Compressor, when preset temperature is reached
Capacity unloader of compressor units, does last step controlling, as required
Particular zone temperature
Controlled by flap valve fitted in each zone loop
Local cabin temperature can be adjusted by volume control at delivery point of air duct controller.
Write a note on Ozone depletion ?
Ozone is gas; between 15 & 30 kilometer above the surface of earth.
This layer controls our climate & protects us from radiation.
The release of industrial waste and other process are now increasing the ozone breakdown to disturb the natural balance.
Pollutant (e.g. CFC) gas from ground level travel to atmosphere.
In the upper atmosphere Ultra Violet Ray breaks off Chlorine atom from the CFC molecule.
The free chlorine atom attack the one ozone molecule. Breaking it form to chloromonoxide. The remainder of the ozone molecule formed into regular oxygen molecule.
A free oxygen atom now steels the oxygen from chloromonoxide molecule form an oxygen molecule. Chlorine atom is again free, attack and break up an ozone molecule. In this way single chlorine atom destroy 1 million of ozone molecule.
Ozone depletion leads to:
More ultra violet radiation
Global warming
Increasing skin cancer
Immune system affected
Reduced forest production and crops
Sea ecology disturbed.
More green house gas
Warmer & more humid climate
More desert
Less forest
Higher sea level
Sea ecology system destroy
Refrigerant affected to Ozone layer: R11 (CClF ), R12 (CCl2F2), R22 ( CHClF2)
Refrigerant not affected to Ozone layer: NH3, R134A
What are the leakage tests for various refrigerants ?
CO2 – Soap and water solution
NH3 – Wet litmus paper ( Red to Green ); Sulphur candles, which gives off white dense smokes when contact with ammonia.
Freon – Soap and water solution / Halide torch /Electronic leak detector (buzzer sound)
How to indicate the flame by halide torch ?
Content is methylated spirits type.
Butane can also be used.
When the leak is detected the flame will change from blue to green depending on the concentration of gas.
What is refrigeration ?
It is a process in which the temperature of a space or its contents is reduced to below that of their surrounding.
What is air conditioning ?
It is the control of temperature and humidity in a space together with the circulation, filtering and refreshing of the air.
What is ventilation ?
It is the circulation and refreshing of the air in the space without necessarily a change of temperature.
What is comfort zone ?
It is between the 40 to 70 % relative humidity at the temperature of 20 to 29 °C dry bulb temperature and air motion is 100mm per sec.
What are the advantages of secondary refrigerant ?
Low initial cost
Low maintenance cost
Suitable for large refrigeration plant
Easily produced on board by mixing CaCl2 and distilled water.
Easily store as a salt on board
What is defrosting ?
A method of removal of frost, built-up on evaporator coils. Defrosting should be done before snow thickness exceeds ¼”.
What are the reasons for defrosting ?
Affecting heat transfer properties
Affecting air flow and circulation
Liquid back to compressor
What are the defrosting methods ?
By stopping the system
By washing with warm water
By means of electric heater coil fitted at the evaporator
By hot gas defrosting method
How to defrost brine system ?
Hot brine thawing
Best and fastest method, used powerful brine heater with separate thawing system. Watertight trays under the pipes, collect the dripping water.
Hot air from atmosphere
It is important that isolating doors in air trunks are perfectly tight, so as to prevent hot air going into cargo spaces.
By shutting off brine
Allow the snows to be melted by the heat of the air in circulation .Very slow operation and tends to throw back great deal of moisture into cargo space.
What is the purpose of ventilation for cargo hold ?
To remove surplus heat and humidity
To prevent the condensing of moisture on cargo or hull
To remove gases produced in ripening process of some fruits and vegetables cargos.
What is short cycling ?
It is a condition of a compressor unit repeatedly running for a few second and then cutting out. This is the result of L.P controller.
Why high pressure cut out is fitted ?
It is fitted to shut down the compressor in the event of high pressure. After remedy the fault, it must reset manually.
Why fitted master solenoid valve for large plant ?
If compressor stops due to a fault, the master solenoid valve will close to prevent flooding by liquid refrigerant and possible compressor damage.
Explain about one method of refrigerant charging ?
Normally charging is made through the liquid charging valve at the high pressure side.
Firstly, weighting the gas bottle.
Connect the gas bottle and charging valve with the connection pipe.
Before tightening the cap on charging pipe, open bottle valve to remove air in the pipe.
Then tighten the cap and open bottle valve fully, charging valve is still closed.
Change compressor to manual running and start it.
Close the condenser outlet valve.
Pumping down the entire charge to the condenser.
Open the charging valve slowly when suction pressure just above zero.
Control the valve opening slowly that no frost formed on the compressor suction pipe.
Check the level in the condenser sight glass.
Close the charging valve and pumping down the entire charge until suction pressure just above zero.
Stop the compressor and close the discharge valve.
Cooling water kept running for some hour.
Then air is purged out through purging valve on condenser until the refrigerant gas appear at the valve.
Calculate the amount of refrigerant (charging) and enter the engine log book.
Why super heat is required at evaporator outlet ?
To prevent the liquid refrigerant entering into the compressor.
If no super heat, hammering may happen and the valve will suffer damage and breakage.
How to fill fridge plant compressor oil ?
Change the compressor to manual running.
Pumping down the entire charge to condenser.
Connect the L.O hand pump to L. O filling valve after air is purged out.
When compressor suction pressure just above zero, open the oil filling valve, inject the L.O into crank case.
Then stop the compressor and close compressor discharge valve. Then cool down the refrigerant.
Then purged out the air through the purging valve until refrigerant appears at purging valve.
What are the types of expansion valve ?
Thermostatic control
Electronic control
Manual control
How to prevent liquid flow back to compressor ?
Liquid shock valve (on cylinder head)
Thermostatic Expansion Valve
Master solenoid valve (when the plant is standstill, especially in Large plant)
Defrosting
Bursting disc (on cylinder head, between inlet and discharge manifold)
Why back pressure valve is fitted ?
It is fitted at the outlet of vegetable room to prevent under cooling of cargo.
In reheat system, the air is pre heated at he central unit; its temperature is being automatically controlled. The air terminals are equipped with electric or hot water heating elements. These raise the temperature of the air to meet the demands of the room thermostats which are individually set. The diagram below shows a reheat system for air conditioning.
Filter
Cooler
Pre heater
Pre insulated air pipe
Sound attenuating air terminal containing electric re-heater and overheat thermostat
Automatic steam valve
Steam trap
Multi step cooling thermostat
Compressor
Automatic Capacity Control valves
Condenser
Thermostatic expansion valve
Sea water pump
Fan starter
Compressor starter
Sea water pump starter
Heater Contractor
Room type thermostat
In summer, the air temperature is controlled by a multi step thermostat in the re-circulating air stream, which governs the automatic capacity control of the refrigerating plant. In the case of electric reheat, fire protection is provided by overheat thermostats which shut down he heaters in the event of air starvation, while a fan failure automatically cuts off the power supply.
In Double Duct system, two separate ducts are run from the central unit to each of the air terminals, as shown in figure below. In winter two warm air streams, of different temperatures, are carried to the air terminals, for individual mixing. The temperatures of both air streams are automatically controlled. In summer the air temperature leaving the cooler is controlled by a multi-step thermostat in the re-circulating air stream, which governs the automatic capacity control of refrigerating plant, as with the zone control. Steam is supplied to one of the heaters, so that two air streams are available at the air terminals for individual mixing.
Filter
Cooler
Low duty heater
High duty heater
Pre-insulated pipes delivering air
Sound attenuating air terminal, with volume and temperature control
Automatic steam valve for tempered air stream
Automatic steam valve for warm air stream
Steam traps
Multi-step cooling thermostat
Compressor
Automatic Capacity Control valves
Condenser
Thermostatic expansion valve
Sea water pump
Fan starter
Compressor starter
Sea water pump starter
Air conditioning systems may be divided into two main classes, the central unit type in which the air is distributed to a group of spaces through ducting, and the self contained type, installed in the space it is to serve.
The central unit type is most widely used, in one or other of a number of alternative systems, characterized by the means provided o meet the varying requirements of each of the spaces being conditioned. The systems in general use are as follows:
Zone Control System
Double Duct System
Reheat System
Zone Control System
This is the most popular because of its simplicity. The accommodation is divided into zones, having different heating requirements. Separate air heaters for each zone are provided at the central unit as shown in the figure below.
Filter
Cooler
One, Two, or Three zone heaters as required
Pre-insulated pipes delivering air to zones
Sound attenuating air terminal, with volume control
Automatic steam valves, one per zone heater
Steam trap, one per zone heater
Multi step cooling thermostat
Compressor
Automatic Capacity Control valves
Condenser
Thermostatic expansion valve
Sea water pump
Fan starter
Compressor starter
Sea water pump starter
The regulation of air temperature by individual air quantity control in this system can give rise to difficulties unless special arrangements are made. For instance, a concerted move to reduce the air volume in a number of cabins would cause increased air pressure in the ducts, with a consequent increase in the air flow and possibly in noise level at the other outlets. This can be avoided but the economic factors usually put a limit on this. Some degree of control is possible through maintaining a constant pressure at the central unit, but since most of the variation in pressure drop takes place at the ducts, the effect is very limited. A pressure sensing device some way along each branch duct, controlling a valve at the entry to the branch, strikes a reasonable mean, and is fairly widely applied.
The temperature of the air leaving the heater varies in accordance with the outside temperature prevailing. This can be effectively performed by a self actuating regulator controlled by two thermostat sensors, one in the air leaving the heater, the other outside. Air quantity control in each room served gives individual refinement. In summer, air temperature is controlled by a multi step thermostat in the re-circulating air stream, which governs the automatic capacity control of the refrigerating plant.
Provsion refrigeration system on board the ship
The
· Provision refrigeration system has three rooms meat,vegetable and handling room as shown in the fig and in some ships more than three rooms also being maintained.
· The temperature range maintained in rooms areas follows: meat room -12 to -16 veg and handling room +4 to +8
· The system runs automatically, when the desired minimum temperature reaches the flow of refrigerant is stopped to the room and when the temperature reaches higher range the refrigerant is allowed to flow.
· When there is no flow of refrigerant the compressor is made to stop and restart when the refrigerant stars flowing to the compressor
· The system consists of following components one set for normal operation and second set is kept for standby in case of any malfunction/maintenance to the first system, the main components are
Ø 1.compressor 2.condenser 3. Receiver 4. Oil separator 5. Filter drier. 6. Liquid line sight glass.
Ø 1. Low pressure cut in cut out switch 2. High pressure cut out switch 3. Differential oil pressure switch.used for automatic and safe operation of the system.
· Each room will have the following components to maintain the desired temperature range.
Ø 1.Evaporator and fan 2.Thermostatic expansion valve 3. Solenoid valve. 4.Master solenoid valve 4. Thermostatic switch 5. back pressure valve
DESCRIPTION OF COMPONENTS
1.Compressor
Generally of the single stage, reciprocating type. Larger systems have multiple cylinders with an un-loader system using the suction pressure as its signal.Refrigerant is compressed in the compressor to a pressure dependent upon the temperature of the cooling water to the condenser.
2.Condenser:
Generally a water cooled tube cooler.A safety valve and vent are fitted. The purpose of the vent is to bleed off non-condensable such as air which can enter the system when the suction pressure is allowed to fall below atmospheric or can be contained within the top up gas. The coolant flow to the condenser is sometimes temperature regulated to prevent too low a temperature in the condenser which can affect plant efficiency due to the reduction in pressure.
Below the condenser, or sometimes as a separate unit, is the reservoir.For system not fitted with a reservoir, a sight glass is sometimes incorporated on the side of the condenser to read the level of liquid refrigerant. Care should be given to ensuring that the liquid level is not too high as this reduces the surface area of the cooling pipes available for condensing the liquid and can lead to increased discharge pressures.
3.Filter drier:
Can be either a compacted solid cartridge or bags of dessicant. The main purpose of this unit is to remove the moisture from the refrigerant.
4.Sight glass;
Sight glass is often of the Bulls eye form. This allows the operator to ensure that it is only liquid, and not a liquid/gas mix going to the expansion valves.On some designs a water indicator is incorporated, this is a coloured ring in contact with the liquid, when water is detected it changes colour, typically from pink to blue.
5.Oilseperator
The purpose of the oil separator, situated on the compressor discharge line, is to return oil entrained in the gas, back to the compressor sump.
Low pressure cut out switch:
Low pressure cut out switch: is used to protect against too low a suction pressure and
set to stop the compressor at a pressure corresponding to a saturation temperature 5°C below the lowest evaporating temperature.It is also used as a control of stopping and starting the compressor to maintain the desired pressure and hence the temperature.
High pressure safety cut out
High pressure safety cut out is used to protect against high discharge pressure which will overload the compressor and may damage components. Set to stop the compressor motor at a pressure of about 90% of the maximum working pressure of the system. Some controls restart the compressor automaticallyon drop in pressure others have a manual reset mechanism.
Oil pressure safety cut out.
Oil pressure safety cut out is used to protect against too low oil pressure in forced lubrication system.It is a differential control using two bellows, one side represents low side pressure and the other responds to the oil pressure.The oil pressure must always be greater than the low side pressure for the oil to flow. If the oil pressure falls below a minimum value the control stops the compressor after a certain time has lapsed.
Thermostatic switch:
Thermostatic switch is used to control the temp in a refrigerated space by cycling the compressor on and off or by opening and closing a solenoid valve in the liquid line.
Back pressure valve
Back pressure valve is placed at the outlet of the evaporator which is fitted to rooms with + ve temperature
The function of Back pressure valve is :
· To control the back pressure
· To prevent the liquid entering into the compressor
· To drop the pressure of refrigerant flowing through a common line leading to the compressor
solenoid valve:
The solenoid valve is a electro magnetic valve which provides automatic opening and closing of liquid and gas lines. A solenoid valve is an electrically activated valve that can either be open or closed
Thethermostat and room solenoid valve forms the main temperature control of the cold rooms.
The Thermostat is set to the desired temperature and given a 3 to 4 degree differential to prevent short cycling. When the temperature in the room reaches the pre-set high level the thermostat switch makes and the room solenoid is energised allowing gas to the refrigerant liquid to the expansion valve.
Master solenoid valve
· Master solenoid valve is placed after the receiver is connected to the motor control which in turn is connected to HP cut out switch and LP cut in /cut out switch
· Master solenoid valve is operated by the motor control which allows refrigerant to flow only when the motor circuit is in ‘ON’ condition
· The motor control will stop the motor if any malfunction of the components takes place in the system
Thermostatic Expansion valve:
The purpose of this valve is to 1. Efficiently drop the pressure of the refrigerant 2. Control the quantity of refrigerant flow to the evaporator and maintains 3 to 5 degree of super heat temperature to the gas leaving the evaporator and entering the compressor.
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1. Describe with a line diagram its operation showing various pressures and
temperature of a Vapour Compression Cycle refrigeration system.
The compressor sucks the vapour from the evaporator; the temperature and
pressure of the vapour are raised by compression.
The compressed vapour flow through the oil separator gas discharge line in to the
condenser, where it is cooled with sea water as cooling medium.
The vapour in the condenser first gives up its super heat and then cooled from the
discharge temperature to the saturation temp corresponding to condensing pressure
then gives up its latent heat as it condenses back to liquid.The liquid then flows from
the bottom of the receiver to complete the cycle.
When the liquid temp is below the condensing temp it is said to be sub cooled.
The liquid refrigerant stored at high pressure in the receiver. It flows through the
liquid line to the flow control valve (Thermostatic expansion valve)which regulates
the rate of flow to the evaporator to suit the evaporation and also the pressure of the
liquid is reduced to the evaporating pressure so that the temperature of the refrigerant
entering the evaporator is below that required in the refrigerated space.
The liquid vapour refrigerant mixture then flows through the evaporator, extracts
heat from the refrigerated space and changes to a dry saturated vapour at approx the
same temp and pressure at which it left the flow control valve.
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2. Describe with a neat sketch a drier unit as fitted to a refrigeration system and why
is it required?
Filter driers:
The function of filter driers is to remove moisture from all refrigerant systems, with
the exception of those using ammonia (R717), which has a high tolerance for
moisture. Foreign particles, sediment, and the products of oil breakdown are also
filtered out. The filter driers are fitted in the liquid refrigerant line from the condenser
to prevent moisture and other contaminants entering and blocking the expansion valve.
1. Gauge connection. 2. Cover. 3. Joint. 4. Withdrawing handle. 5. Spring.
6. Distance piece. 7. Division plate. 8. Felt washers. 9. Gauze plates.
10. Casing 11. Inlet connection. 12. Oval flange. 13. Charging connection.
14. Center bolt 15. Drying agent.16. Retaining sleeve. 17. Outlet connection.
Refrigerant drier.
Filter driers contain a desiccant, which is a moisture absorbing substance which will
eventually become saturated with moisture and have to be replaced. Two widely used
desiccants are silica gel and molecular sieves. Where rechargeable units are fitted, the
desiccant can be removed and the drier refitted with a fresh charge. The commonest
drying agents used are silica gel and activated alumina, both of which can be
reactivated by heating to 140 deg.C for a number of hours.
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3.Sketch a thermostatic expansion control valve as fitted in a refrigeration system,
label the parts and state the location in the refrigeration system.
THERMOSTATIC EXPANSION VALVE:-
The expansion valve is fitted to the inlet of the evaporator after the drier unit. One
manually operated valves are fitted to either side of expansion valve and bypass line
provided used during pressure testing.
4. Describe with an aid of a sketch high pressure cut out fitted in refrigeration
system.
HP CUTOUT:-
Used to protect against high discharge pressure which will overload the
compressor and may damage components.
Set to stop the compressor motor at a pressure of about 90% of the maximum
working pressure of the system.
The bellows connected by small bore pipe between compressor and condenser.Excess
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pressure expands the bellow and moves the switch arm to activate electrically to trip the
compressor motor. The high pressure thus operated has to be manually reset after
rectifying the fault.
5a) Explain the term “Superheat” with reference to the thermostatic
Expansion valve and the term “under-cool” with reference to condenser.
Superheat
Superheat is referred to as the condition of gas at outlet of the evaporator and is
controlled by the thermostatic expansion valve.
The degree of superheat is set by the spring force at Factory. The sensing bulb senses
the gas temp at evaporator out let and controls the Qty of the refrigerant to maintain
the at degree of superheat.
· To ensure no liquid passes through to the compressor, the expansion valve is set
so that the gas at outlet from the evaporator has 2 to 3 degrees of superheat.
Undercool
The hot and high pressure gas is fed from the discharge side of the compressor to
a
“Condenser” .
The vapour in the condenser first gives up its super heat and then gives up its
latent heat from the discharge temperature to the saturation temp corresponding to
condensing pressure. The liquid then flows from the bottom of the receiver to
complete the cycle.
When the liquid temp is below the condensing temp it is said to be sub cooled.
If the liquid temperature is in sub cooled temp flash of gas Qty will he reduced
and refrigerating effect will be increased
5b) Explain the term entropy and enthalpy with respect to the refrigeration system.
Enthalpy
Enthalpy is the total amount of heat in one Lb. of a substance. It's units are therefore
BTU/Lb. The metric counter part is kJ/Kg. (kilo joules/kilogram)
Entropy
Entropy measures the energy dispersion in a system divided by temperature. This ratio
represents the tendency of energy to spread out, to diffuse, to become less concentrated in
one physical location or one energetic state. Entropy is measured in BTU per Lb. per degree
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change for a substance.
6. Explain the functions of the following with reference to the refrigeration system:-
a)Solenoid valve. b) High-pressure safety cut out. c) Oil pressure safety cut out.
Solenoid valves.
The solenoid valve is a servo-controlled electromagnetic valve, which provides
automatic opening and closing of liquid lines. It is fixed between the drier and
expansion valve and controlled by a thermostat.
When the coil energised, the pilot orifice is opened connecting the inlet and outlet due
to the diaphragm moving into open position (vice versa when the coil is de-energised).
The malfunction of this valve can occur due to burnt-out coil, a damaged diaphragm,
or blockage by dirt.
High pressure safety cut out: used to protect against high discharge pressure which
will overload the compressor and may damage components.
The control is set to stop the compressor motor at a pressure of about 90% of the
maximum working pressure of the system.
Oil pressure safety cut out : Used to protect against too low oil pressure in forced
lubrication system .It is a differential control using two bellows, one side represents low
side gas pressure and the other responds to the oil pressure. The oil pressure must
always be greater than the low side gas pressure for the oil to flow. If the oil pressure
falls below a minimum value the cut out controlol stops the compressor. The manual
reset of this safety cut out is required to put back the compressor to normal mode.
7.Describe the procedure of charging gas to a large refrigeration system after complete
overhaul of the compressor, assuming the condenser is empty.
The system have been opened to atmosphere during repair should be flushed through to
remove solid particles and evacuated to remove moisture or non condensable gases.
This is carried out on new system installations or when a plant has been discharged of
refrigerant prior to repair.
It involves the use of oxygen-free nitrogen (OFN) which is a high pressure gas.
This is used to obtain a higher pressure than that of the refrigerant in normal ambient
temperatures.
When an installation is completed and a pressure test is to be carried out.It is most
important to ensure that the compressor is isolated, irrespective of design, before
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pressurizing the system. This will also prevent rupture of the crankshaft seals in
compressors.
All pressure controls must be disconnected or by-passed.
The expansion valve is not capable of withstanding the test pressure must be removed
or by-passed.
The cylinder must be fitted with an approved regulator to control the test pressure.
When the system is pressurized, that pressure should be recorded leaving for few hours.
It could take a considerable time for a drop in the nitrogen pressure to become evident,
if a system has a small leak.
Drying by evacuation:
1.Connect a vacuum pump to system using a short length of large bore pipe and open
all valves in the system.
2.Evacuate the system to a pressure of 6mm hg.
3.Close the line between the system and vacuum pump. ( the pressure in the system
may not rise more than 2mm hg with in 5 mins. A rise in pressure indicates presence
of water and / or leak, where water is present, the system will be colder than
surrounding.)
4. Check for water and / or leaks carry out any repair and repeat evacuation procedure
until the pressure rise is less than 2mmHg.
When this is achieved the system is free of moisture and non condensable gases and
ready for refrigerant charging.
1.Low pressure side should be charged ONLY WITH GAS.
2.High pressure side should be charged ONLY WITH LIQUID REFRIGERANT.
CHARGING LIQUID BETWEEN CONDENSER OUTLET AND DRIER:-
1. Connect the Freon gas cylinder to the gas charging valve with a special flexible
hose(purge the air from the hose).
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2. Open the cylinder valve (to liquid side) and gas charging valve and run the
compressor.
3. The low pressure switch cuts in and the compressor starts on LP cut in.
4. Charge the liquid until the condenser level is normal and close Freon gas bottle
valve and gas charging valve.
5.The pumping down operation starts and compressor stops due to LP cut off.
6. Release the charging hose of pressure and remove from place.
Procedure for charging refrigerant in low pressure line:
CHARGING GAS THROUGH COMPRESSOR SUCTION VALVE:-
1. Connect the Freon gas cylinder to the gas charging valve with a special flexible
hose (purge the air from the hose).
2. Open the cylinder valve (to gas side) and gas charging valve and run the
compressor.
3. The low pressure switch cuts in and the compressor starts on LP cut in.
4. Charge the gas until the condenser liquid level is normal and close Freon gas bottle
valve and gas charging valve.
5.The pumping down operation starts and compressor stops due to LP cut off.
6. Release the charging hose of pressure and remove from place.
8.Explain with a neat sketch the drain system arranged in a cold room for draining
wash water and defrosted water.
Cold room
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Cold room Drain pipe overboard shipside
The drain pipe is designed with double bend as shown in the sketch above. The
defrosted drain water stagnates to form a water seal at the initial stage. The water
collected in excess due to defrosting, washing cold room would flow overboard and
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maintaining always some water in the bend. The purpose of this water is to act as a
“seal” avoiding outside air into the cold room effecting the temperature.
9.How will you know if the gas is leaking in the closed system of a refrigerated
compressor using F-12, Justify your answer.
The system under normal running condition is under pressure and the system can be
Leak tested by using simple soap solution test as a simple test. However leaks are
tested by using the electronic leak detector.
The detector contains an internal pump that draws air into a probe. If the gas is
present in the sample, the electrode sensing element generates a current and an
output signal is obtained.
In modern refrigerated cargo installations leak detector sensors are fitted in the holds
and machinery spaces activate audible and visual alarms located in the control room
in the event of refrigerant leakage.
10. How will you identify if the gas is leaking in a closed system of a refrigerated
compressor using F12 and how will you rectify it?
The easiest method of checking the leakage is by using soap solution.
After deducting the leak, if the pipe or valve connections are leaking, it should be
rectified by using proper gaskets/packing and tightening.
If the any hole identified on the pipe, then pumping down of gas to the condenser and
brazing of the holed portion to be carried out by isolating the system.
The repaired pipe is to be cooled and then re-start the system. After re-starting the
system the air in the system must be purged out from the condenser.
Leak detection equipments :-
Leak tested by using soap solution test as a simple test. However leaks are tested by
using the electronic leak detector. The detector contains an internal pump that draws
air into a probe. If the gas is present in the sample, the electrode sensing element
generates a current and an output signal is obtained.
11. Describe the procedure of charging gas to a large refrigeration system,
when condenser gas level is lower than normal.
TOPPING UP OF REFRIGERANT :-
1. Connect the Freon gas cylinder and gas charging valve with a special flexible hosepurge
the air from the hose. (If liquid is to be charged connect after the condenser and
if gas has to be charged connect to the suction side of the compressor.)
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2. Open the cylinder valve and gas charging valve and run the compressor.
3. Allow the liquid / gas to flow in to the system.
Check the liquid level in the condenser has reached to the normal level, then close
freon gas bottle valve and gas charging valve.
4.Release the charging hose of pressure and remove from place.
Precautions:
1. liquid refrigerant must never be charged directly into the compressor suction.
2. Properly tested charging hoses are used, and that non return valves are fitted when
charging into the high pressure side of the system.
12.Write short notes on maintenance of the following refrigeration system
quipments:- a)Condensers. b) Oil separators c)Filters and driers.
a)Maintenance of Shell and Tube Condenser:
In general the following work should be done at regular intervals.
1. To prevent fouling of water tubes with scale or marine growth, reducing the heat
transfer capacity of the condenser, the tubes should be cleaned by ‘tube brush’.
Alternatively, a ready mixed inhibitive scouring acid can be used, with subsequent
neutralization. Such agents must be suitable for use with the tube materials, and
applied in strict accordance with the chemical manufacturer’s instructions and
environmental protection.
2. The end cover should be inspected for wastage, caused by erosion and corrosion
especially the division plate. Zinc anodes to be renewed.
3. The tube ends on both sides of the end plate to be leak tested using soap solution
or any leak detector.
b) Oil separator:- Gas from the system to be pumped down to the condenser and the
circuit breaker of the compressor switched off with a sign board “men at work”. Close
the compressor and discharge valve. Release the pressure by slackening the drain plug
of the oil separator.
Open the float valve and take out the float out of the chamber carefully. Clean internal
thoroughly with a clean rag and blow dry air such that the partition mesh is cleaned.
Overhaul the oil return needle valve and fit back the float as original with new gaskets
and seal rings. After tightening all the fittings, open the compressor discharge valve
one turn and close back to pressurize the oil separator for leak test. Once leak test is
satisfactory, the suction and discharge valves of the compressor are fully opened and
the system set back to normal.
c)FILTERS AND DRIER:-
The filters are located one on the suction side of the compressor, one on the outlet
from the evaporator coil and one in each expansion valve. These filters are removed
Kngreddy Refrigeration and Air Conditioning System kngreddy
from location by shutting the inlet and outlet valves of the equipment. The filters are
cleaned using electro cleaner and later flushed with low pressure air. The filters are
fitted back to its original location by purging the air by opening the valves.
13.Explain the causes and rectification of the following defects, when observed
during the running of the refrigeration compressor:-
a) Too high delivery pressure.
b) Too high suction pressure.
c) Compressor starting and stopping too often on low-pressure switch.
Too high a delivery pressure. 1. Air or non-condensable gas
in the system.
2. The cooling water is too
warm or an insufficient
quantity is passing through the
condenser.
3. Mud or scales block the
condenser tubes.
4. Too much liquid R12 or
R22 in the receiver and some
liquid in the condenser.
1. Purge the foul gas from the
condenser.
2. Inspect and clean the water
valve and water filter, and
make sure that the water valve
is opened wide enough.
3. Clean the condenser tubes.
4. Drain off the excess R12 or
R22 in an empty cylinder.
Too high a suction pressure. 1. Too much liquid refrigerant
is being fed through the
expansion valve.
2. Leaky suction valves.
1. Adjust the expansion valves
and check the sensing bulb.
2. Remove the cylinder covers,
check all the valves, repair or
renew and test.
Compressor starting and
stopping too often (on low
pressure switch).
1.The filters for liquid
refrigerants, suction strainers
or filters for expansion valves
blocked/chocked.
2. The delivery valves of the
compressor leaking.
3. leaky solenoid Valve .
1. Clean all filters.
2. Inspect and clean the valves
or renew them if necessary.
3. Renew the solenoid valve
14. Explain the causes and rectification of the following defects, observed
during the running of the refrigeration compressor:-
a) Too low suction pressure.
b) Too low delivery pressure.
Kngreddy Refrigeration and Air Conditioning System kngreddy
c) Compressor stopping too often on high-pressure switch.
Too low delivery pressure. 1. Too much cooling
water to the condenser.
2. Cooling water is too cold.
3.Liquid R12 or R22 is
coming back from the
evaporator.
4. Leaky delivery valve.
1. Regulate the water supply.
2. Reduce the water supply.
3. Adjust the expansion valve
and check that the expansion
valve bulb in contact with
evaporator pipe and fastened
correctly.
4. Remove the cylinder cover,
inspect the valve plates and
piston rings; renew them if
necessary.
Too low a suction pressure. 1. Blockage in liquid pipe,
expansion valve or suction
filters.
2. Too little refrigerant.
3.Too much oil circulating in
the system.
4. Incorrect adjustment of the
expansion valves.
1. Evacuate; remove; inspect
and clean filters.
2. Charge with more R12 or
R22.
3. Inspect to see whether oil
has accumulated anywhere in
the system.
4. Set the expansion valves to
feed more refrigerant.
Compressor stopping too
often (on high pressure
switch).
1. Insufficient water flowing
through the condenser
(blocked condenser tubes).
2. High pressure switch set
incorrectly.
3. The system is over charged
with refrigerant.
1. Find out from cooling water
system the reason for
insufficient flow. Clean
condenser tubes.
2. Adjust the high pressure
switch to switch off at correct
pressure.
3. Reduce the liquid
refrigerant from the system.
15.State how each of the following faults identified and rectified for a vapor compression
refrigeration machine.
a)Air in the system.
b)Moisture in the system.
c)Undercharge.
d)Overcharged.
a)Air in the System
Indication:
Kngreddy Refrigeration and Air Conditioning System kngreddy
· This may cause the refrigeration compressor to overheat, with a high discharge
pressure and normal condensing temperature.
· There are possibilities of small air bubbles in the liquid sight glass of the
condenser.
· Condensing pressure of the refrigerant in the condenser may be high.
· If there is excessive air, it may reduce the cooling capacity of the system making
the compressor to run for the extended period of time.
· It may cause the gauge pointer of the condenser to jump indefinitely.
Causes:
· During charging, air may enter in to the system.
· If Freon-12 is used, air may leak in to the suction line because of the working
pressure of the refrigerant is less than the atmospheric pressure.
Action:
· Air in the system can be removed by collecting the system gas in the condenser,
leaving the condenser cooling water on and venting out the air from the top of the
condenser because air will not be condensed in the condenser but remains on top of
the condenser above the liquid refrigerant.
· Connect the collecting cylinder to the purging line of the condenser, open the
valve, and collect air in the cylinder.
· After purging the air from the system don’t forget to shut the purging valve.
· Check the level of the refrigerant in the system. If required, charge the system
with fresh refrigerant.
· Restart the compressor with all safety precautions.
b)Moisture in the System
This normally comes with the ingress of air in the system. Moisture may freeze at the
expansion valve, giving some of the indication of under charging. It will contribute to
the corrosion in the system. It may cause lubrication problems and breakdown of the
lubricating oil in the refrigerant compressor.
c)Undercharging of Refrigeration System
Indication:
· Compressor is running hot and performance of the compressor falls off due to
high
superheat temperature at the suction side of compressor.
· Suction and discharge pressure of the compressor is low.
· Large vapor bubbles in the liquid sight glass.
· Low pressure gauge readings in the condenser.
· Ammeter reading for the compressor motor is lower than normal.
· Rise in room temperature which is to be cooled.
· Compressor is running for extended period of time.
Causes:
· Leakage of refrigerant at the shaft seal, flange couplings, valve gland etc.
· Expansion valve inlet filter may be blocked.
Kngreddy Refrigeration and Air Conditioning System kngreddy
· Partial blockage of refrigerant at the filter or drier may cause undercharging.
Action:
· Identify and rectify the leakage of refrigerant from the system.
· Clean the filter and drier.
· Charge the system if required with fresh refrigerant.
d)Overcharge of Refrigeration System
Indication:
· The liquid level in the condenser is too high (high condenser pressure gauge
reading). This will reduce the available condensing level, with corresponding increase
in the saturation temperature and pressure.
· High pressure switch of the refrigerant compressor activates and stops the
compressor.
· There is possibility of excessive liquid refrigerant getting to the evaporating,
giving icing at the compressor suction, and a pressure drop across the expansion valve.
The cold room temperature may rise, if the evaporator is flooded.
Causes:
· It may be due to the reason that excessive refrigerant has been charged in the
system.
· Air in the system may also cause over charging indication.
· It may also be due to the formation of ice on the regulator.
Action:
· Remove the refrigerant from the system. This is done by connecting a cold
cylinder to the liquid line charging valve, starting the compressor, and then operating
the charging valve.
· Purge the air from the system and maintain effective cooling.
· Remove ice from the expansion valve by using any of the defrosting methods.
16.Draw a block diagram of an automatic accommodation air conditioning unit and
label the parts and indicate the directions of airflow.
Typical system
Kngreddy Refrigeration and Air Conditioning System kngreddy
17.Draw a block diagram of an automatic accommodation air conditioning unit and
explain the reason for installing the dehumidification and humidification unit.
Air conditioning is the control of humidity, temperature, cleanliness and air
motion. Winter conditioning relates to increasing temperature and humidity whilst
summer conditioning relates to decreasing temperature and increasing humidity.
In summer when air temperature is lowered the humidity is increased to the level
more than the comfortable level. Hence in Air conditioning system air temperature is
lowered much lower than the comfortable level about 10 deg C and then reheated to
comfortable temp 17 deg c and humidity in the delivery duct.
And when air is heated in winter the air becomes dry, and the humidity is
controlled by spraying and mixing wet Steam in the air path as shown in the diagram
Kngreddy Refrigeration and Air Conditioning System kngreddy
18.What are the readings taken and recorded of a domestic refrigeration system during
watch keeping on board a ship?
Watch keeping duties.
The following are the readings taken for a refrigeration system and recorded in the
Engine log book every four hours of watch keeping.
Item Check Procedure
1. Compressor……………………Check Suction, discharge, oil pressure, compressor oil level.
2. Compressors, fans, pumps…….Check no failure alarms in alarm condition.
3. Domestic refrigerant…………..Check temperatures.
Chambers.
4. Condenser and piping………….Check water pressure gauge that water flow satisfactory.
Check refrigerant liquid level. Examine for leaks.
5. Sight glass in refrigerant
liquid line………………….…Check that full flow of liquid.
6. Refrigeration blackboard……..Check any special instructions from Chief Engineer
The completion of the refrigerator Logbook should be the last task of a watch keeper
before going off duty, to ensure that he is leaving all temperatures in order.
19.Explain the following with reference to the refrigeration compressor:-
a) Sketch a mechanical shaft seal, method of lubrication and name its parts.
b) How is gas leakage deducted through shaft seal and what is its effect on
environment?
c) What factors can cause the shaft seal failure in service?
Kngreddy Refrigeration and Air Conditioning System kngreddy
ANS:- a)
The carbon seal acts as a fixed ring and its face is in contact with a soft bronze metal ring. A
metal ring firmly rotates along the shaft compressing a synthetic rubber ring against the
collar to form gas tight. The spring forces the fixed carbon ring against the stationary bronze
metal ring and prevents gas leakage. The synthetic rubber bellow rides on the shaft with
sufficient pressure to prevent leakage of gas along the shaft. This complete assembly is
forced with a strong spring to keep them in place during normal running. The complete shaft
seal is compressed with a seal mounting plate and bolted to the compressor housing with
proper gasket to prevent gas leakage.
Lubricating oil is fed to this packing, usually under pressure, to keep the shaft and packing
lubricated. The shaft normally moves forward and backward called “end clearance” of crank
shaft. The starting torque of the compressor is so high, which moves the shaft away from its
original position. This torque is balanced by the seal spring force to keep in equilibrium
position.
Ans b)The gas leak can be deducted by carrying out one of the following methods:-
Electronic leak detectors, Leak detector torch, Sulphur candles, Litmus paper and soap
solution.
Ans c) Moisture cause to form acids by reaction with the freon refrigerants. This acid
attacks the copper in the lines and deposits in other parts of the system. This can become
Kngreddy Refrigeration and Air Conditioning System kngreddy
particularly troublesome when it is deposited on the compressor mechanical seal faces
leading to damage and leakage. The air enters the system from the system valve glands
during pumping down operation or charging gas. This oxidizes the lubricant oil and
chemically reacts with the additives forming sludge in the presence of moisture affecting
the viscosity.
The belt is to be checked for tension and alignment of motor to compressor at regular
interval. If the belt tension is high or misalignment, it can cause the shaft seal breakage
due to high torque load transmitting to the shaft seal leading to failure.
If in-sufficient or contaminated lubricating oil supplied to shaft seal, the carbon seal
could be worn out due to abrasive wear damaging rotating seal face and causing loss of
refrigerant.
20. a) Draw a neat sketch of an oil separator fitted to an refrigeration compressor and name
its parts.
b) Why is oil separator required and how does it work?
Ans a)
ANS b)
Oil Seperator:-
The discharge from compressor carry some oil along with the compressed gas and this
must be removed.
The following are the reasons for fitting the oil separator:-
a) To prevent it entering and fouling the internal surfaces of the evaporator and condenser.
b) To separate oil from gas and return back to the crankcase, causing shortage of oil in the
crankcase.
Oil separators are placed in line between the compressor and condenser, and consist of a
pressure container with internal baffles and screens. The oil gas mixture enters the
Kngreddy Refrigeration and Air Conditioning System kngreddy
separator, where it is made to change direction. The heavier oil droplets tend to fall on the
mesh and slides down to the bottom and return back to the crankcase through the float
valve. The oil return may be float controlled or electric solenoid controlled needle valve
with a small bore capillary tube. With all of these methods a shut off valve is fitted
between separator and compressor to allow for maintenance.
21. a) Explain the properties of a refrigeration system lubricating oil.
b) How does the performance effect if lubricating oil of a refrigeration compressor
Deteriorate in service?
ANS a) Refrigeration oils should possess the following properties :
· Good chemical stability. There should be little or no chemical reaction with the
refrigerant or materials normally found in the system.
· Good thermal stability. They should not form hard carbon deposits at hot spots in
the compressor (such as valves or discharge ports).
· Low viscosity. This is the ability of oil to maintain good lubrication properties at
high temperatures and good fluidity at low temperatures, i.c. to provide a good
lubricating film at all times.
· Low wax content. Particularly important in the case of CFC and HCFC plants,
operating at low evaporating temperatures, as separation of wax particles from the
· refrigerant-oil mixture may cause problems by blocking expansion and regulating
valves.
· Low pour point. Ability of the oil to remain in a fluid state at the plant’s lowest
evaporating temperature. The pour point is particularly interesting in relation to
oils used in ammonia (R717) plants, as oils with a low pour point are easier to
drain from the plant’s low pressure side.
· Moisture free. Any moisture added with oil may cause corrosion, and in the case
of CFC and HCFC refrigerants would form as ice in a choked expansion or
regulating valve.
· When adding oil to a compressor, or doing an oil change, it is therefore important
that only the specified in the manufacturer’s operating manual is used. The oil
must be clean and have no moisture content. Oil should always be stored in
tightly sealed containers, in a warm place, to ensure it does not absorb moisture
from the atmosphere. It is important that the procedures given in the compressoroperating
manual for changing and topping-up the oil are strictly followed.
ANS b) Moisture cause two main problems. First, it can freeze to ice in the evaporator
and cause blockage. Second, it can form acids by reaction with the freon refrigerants.
This acid attacks the copper in the lines and deposits in other parts of the system. This
can become particularly troublesome when it is deposited on the compressor mechanical
seal faces leading to damage and leakage. Fine particles which could possible block the
expansion valve are removed. The system is installed with a drier to absorb any moisture
and also filter fine particles of debris in the gas using silica gel as an agent. If moisture
found on the higher side even after renewing silica gel a few times, then the gas has to be
renewed. The air enters the system from the system valve glands during pumping down
operation, charging gas and during standby mode. This oxidizes the lubricant oil and
chemically reacts with the additives forming sludge in the presence of moisture affecting
Kngreddy Refrigeration and Air Conditioning System kngreddy
the viscosity. The refrigerant is continuously filtered using gas filters at compressor
suction, expansion valve inlet and at the evaporator outlet. The contaminated gas will
foul the evaporator bringing the complete system to a halt.
22. Oxidation decomposes lubricating oil, explain briefly how does air
enters into the closed system of refrigeration compressor?
The air enters the system from the system valves glands during pumping down operation,
gas charging and during standby mode. This oxidizes the lubricant oil and chemically
reacts with the additives forming sludge in the presence of moisture affecting the
viscosity.
· In the refrigerant systems some oil always carried over from the compressor into the
condenser by the refrigerant gas, from where it is carried into the evaporator.
· The presence of oil in the circulating refrigerant reduces the heat transfer capacity
being greatest in the evaporator, since oil becomes more viscous and tends to congeal
at low temperature.
· To prevent oil related problems, the operation of the oil separator and the oil rectifier
(if fitted) should be checked regularly to ensure oil is being returned to the compressor
crankcase. The amount of oil added to the compressor crankcase should be strictly
monitored, a decreasing amount indicates that the oil is being trapped in the
evaporator or suction line.
· Oil and gas filters to be regularly cleaned and fresh oil to renew at regular intervals, to
avoid problems during normal running.
23. What action will you take if the moisture could not be removed from the
gas by renewing ‘silica gel’, due to contamination with excess moisture?
The following to be carried out in order to renew contaminated refrigerant in the
condenser:-
1. The pumping down of the gas to be carried out to collect the complete system gas to
the condenser and later close the condenser outlet valve.
2. Switch of the circuit breaker.
3. The empty freon gas cylinders must be cooled in a fish room to bring the temperature
approx. -18 deg.C.
4. The cooled cylinders are connected to the charging valve between condenser and drier
unit.
Kngreddy Refrigeration and Air Conditioning System kngreddy
5. Open the empty cylinder valve and condenser outlet valve, the liquefied gas from the
condenser is transferred to the bottle due to the differential pressure.
6. Renew bottles to collect all of the gas as a single bottle would not be enough.
7.The reason for more gas bottles is that temperature of bottles rise during collection
process equalizing the pressure of the condenser and bottle.
8. Renew silica gel on completion of removing entire contaminated refrigerant.
9. Charge fresh Freon gas to the system to normal condenser level.
10. It may be required to renew silica gel couple of times even after renewing the
refrigerant, due to left over moisture adhering to the walls of condenser.
24.What is humidity and how is it controlled on board ship?
Air conditioning is the control of humidity, temperature, cleanliness and air
motion. Winter conditioning relates to increasing temperature and humidity whilst
summer conditioning relates to decreasing temperature and increasing humidity.
In summer when air temperature is lowered the humidity is increased to the level
more than the comfortable level. Hence in Air conditioning system air temperature is
lowered much lower than the comfortable level about 10 deg C and then reheated to
comfortable temp 17 deg c and humidity either in the delivery duct or individual
re.heater fitted inside the cabin at delivery point.
And when air is heated in winter the air becomes dry, and the humidity is
controlled by spraying and mixing wet Steam in the air path as shown in the diagram
25.What are the basic consideration taken into consideration in calculating
the capacity of air condition plant required for installation on a ship?
The basic principals of air conditioning
Air conditioning is the control of humidity, temperature, cleanliness and air motion.
All air conditioner units must have the five basic components to work:
1. The compressor (To maintain required flow of gas and maintain the necessary
superheat at the evaporator outlet).
Kngreddy Refrigeration and Air Conditioning System kngreddy
2. The condenser ( the cooling of refrigerant and maintain its temperature balance
between inlet and outlet).
3. The expansion device (to regulate the correct quantity flow of refrigerant to
evaporator and maintain superheat).
4. The evaporator (for keeping the required refrigerant flow for optimum enthalpy).
5. The capacity of the fan (to enable correct quantity of air and in addition change of
air in rooms to maintain optimum comfort level).
AIR CONDITION
The coefficient of performance calculated to determine the size of the evaporator and
the expansion valve. The air conditioner evaporator size is important.
The following are taken into calculation in addition to the above:-sensible heat, latent
heat and sub-cool. Humidifier is fitted to the system to regulate the moisture level of
the air outlet to the rooms. Change of air rate (cubic meters/hr) in each cabin to
measure to find the capacity of fan.
26.What is capacity control / un-loader in a refrigeration system and how is
the automatic operation controlled?
The Operation of refrigeration compressor is to balance the compressor capacity
against the refrigerant requirement.
1. Variable speed drives run induction motors at variable speeds down controlling the
capacity of a single compressor.
2. Vertical single-acting compressors use bypass system for capacity reduction. These
are operated either by hand or automatically by a solenoid valve.
3. Most modern multi-cylinder compressors control capacity with cylinder un-loaders.
When a reduction in capacity is required, the pumping action of one or more cylinders
is interrupted. This is accomplished by lifting the suction valve off its seat and holding
it open. With the suction valve open, no compression is possible. Only some of the
cylinders are capable of being unloaded, so there is always some pumping action
taking place. A lever that lifts or drops the valve is operated by a small control piston.
Some manufacturers use oil pressure, others use discharge gas pressure on this control
piston.
Kngreddy Refrigeration and Air Conditioning System kngreddy
A change in suction pressure operates a valve in the oil or gas line to the control
piston. Design is such that the cylinders that can be unloaded remain unloaded when
the compressor is idle. Thus, the compressor can be started at reduced load. As the
compressor comes up to speed, a change in oil or gas pressure activates the control
pistons to allow the suction valves to operate normally. All the capacity-reduction
methods reduce the power required to turn the compressor.
27.What are the factors taken into consideration for optimum comfort level in air
conditioning system?
The term comfort is not something that can be measured precisely. Attempts have
been made to convert this term into measurable parameters Whether a person feels
comfortable or not in a room depends upon many factors:-
Room design (daylight, colours, height, plant, wall temperatures).
Noise pollution (machinery, people, surroundings, air conditioning).
Room atmosphere (quality, hygiene, odour, air speed, temperature, humidity).
Air condition has an influence on the areas highlighted in bold print.
Depending upon the intensity of body activities and air pollution, 20-30 cubic meters
per hour fresh hour is necessary for the removal of carbon-dioxide and moisture
28. What are the dangers of gas to the crew and the environment, with respect to
International regulation?
The gas can cause cold burns if come in contact with the skin. The gas emits a
pungent smell causing breathing problems if the concentration is high. It can cause
blindness if comes in contact with eyes. The Freon gas has to be handled carefully in
order to avoid the danger of accidents.
· The release of CFC refrigerant in to the atmosphere is harmful to the environment.
· The CFCs released broken down to release chlorine atoms which destroy ozone, the
stratospheric gas which acts as a filter of ultra violet light from the sun.
· The increased UV light on earth as a result of ozone depletion will cause skin cancer,
interfere with immune system and harm aquatic system and crops.
· CFCs along with other “green house gases” inhibit the release of heat radiated from
the earth, contributing to global warming. If the global temp increase the mean sea
level will rise.
Kngreddy Refrigeration and Air Conditioning System kngreddy
· To protect the global environment, an international agreement, Montreal protocol
signed in 1987 controls the use and production of CFC refrigerant and other ozone
depleting substances throughout most of the world.
· It currently mandates that the production of the refrigerant R11, R12 and R502 is to be
phased out by1997.
· CFCs are characterized under the Montreal protocol according to the extent to which
they damage the ozone layer.
29.Draw a neat sketch of an oil separator and describe its functions.
0il Separators:
Some oil is always carried over with the compressed gas and must be removed. This
is:
a) To prevent it entering and fouling the internal surfaces of the evaporator and other
heat exchangers.
b) To ensure its return to the crankcase or reservoir, preventing failure through the
shortage of oil.
Oil separators are placed in the line between the compressor and condenser, and
consist of a vessel fitted with internal baffles and screens. The separation of oil is
mechanical, the slowing down and change of direction of the gas/oil stream throwing
out the oil. The oil separated from the gas collects in the bottom of the separator and is
returned to the crankcase or receiver through an automatic regulating needle valve of
float type.
Kngreddy Refrigeration and Air Conditioning System kngreddy
30.Explain the functions with a neat sketch the following with reference to the
refrigeration system:-
A) Thermostatic expansion valve. B) Compressor shaft seal.
Ans A) The expansion valve consists of a gun metal housing with both ends threads are
provided for inlet and outlet connections. A long capillary tube filled with same liquid
refrigerant with a bulb is connected to the top of the valve housing. A fine metallic
bellow fixed to the push pins at one end slides through the bores of the valve housing and
attached to a valve disc. This valve disc is fixed on a hollow metal chamber acts as a
retainer for the spring The spring tension is adjusted using a screw in order to adjust the
superheat. A balancing line from evaporator outlet is connected below the bellow. An
orifice plate is fixed in the centre of the housing for the purpose of gas expansion.
A)Thermostatic expansion valve:
Kngreddy Refrigeration and Air Conditioning System kngreddy
Expansion valve working procedure:-
Kngreddy Refrigeration and Air Conditioning System kngreddy
Three conditions present themselves in the operation of this valve: first, the balanced forces;
second, an increase in superheat; third, a decrease in superheat. The remote bulb and the power
element make up a closed system (power assembly), and the remote bulb and power element are
charged with the same refrigerant as that in the system. The remote bulb and Power element
pressure which corresponds to the saturation pressure of the refrigerant gas temperature leaving
the evaporator, moves the valve pin in the opening direction.
Opposed to this opening force on the underneath side of the diaphragm and acting in the closing
direction are two forces: (1) the force exerted by the evaporator pressure and (2) that exerted by
the superheat spring. In the first condition, the valve will assume a stable control position when
these three Forces are in balance (that is, when P1 = P2 + P3). In the next step, the temperature
of the refrigerant gas at the evaporator outlet (remote bulb location) increases above the
saturation temperature corresponding to the evaporator pressure as it becomes superheated. The
pressure thus generated in the remote bulb, due to this higher temperature, Increases above the
combined pressures of the evaporator pressure and the superheat spring (P1 greater than P2 + P3)
And causes the valve pin to move in an opening direction. Conversely, as the temperature of the
refrigerant gas leaving the evaporator decreases, the pressure in the remote bulb and Power
assembly also decreases and the combine evaporator and Spring pressure cause the valve pin to
move in a closing Direction (P1 less than P2 + P3).
COMPRESSOR SHAFT SEAL
The compressor shaft seal consists of a carbon seal acting as a fixed ring and its face is in
contact with a rotating soft bronze metal ring. A metal ring firmly rotates along the shaft
compressing a synthetic rubber ring against the shaft collar to form gas tight. The spring is
fixed with force the carbon ring against the metal ring and to prevent gas leak.
The synthetic rubber bellow rides on the shaft with sufficient pressure to prevent leakage of
gas along the shaft line. The complete shaft seal is compressed with a seal mounting plate
and bolted to the compressor housing with proper gasket to make gas tight.
Lubricating oil is fed to this shaft seal, usually under pressure to keep the shaft and packing
lubricated. The crank shaft normally has some “end clearance”. The starting torque of the
compressor can affect the shaft seal if the assembly is in correct or alignment problem
arises.
Shaft Seal Sketch.
Kngreddy Refrigeration and Air Conditioning System kngreddy
31.Describe the danger of “Contamination” with reference to sump oil in the
refrigerated compressor and its effect on the system?
· In the refrigerant systems some oil always carried over from the compressor into the
condenser by the refrigerant gas, from where it is carried into the evaporator.
· The presence of oil in the circulating refrigerant reduces the heat transfer capacity
being greatest in the evaporator, since oil becomes more viscous and tends to congeal
at low temperature.
· To prevent oil related problems, the operation of the oil separator and the oil rectifier
(if fitted) should be checked regularly to ensure oil is being returned to the compressor
lubricating system. The amount of oil added to the lubricating system should be
strictly monitored and excessive amount indicates that the oil is being trapped in the
evaporator or suction line.
Kngreddy Refrigeration and Air Conditioning System kngreddy
· Ammonia (R717) is not oil miscible, so in ammonia systems the oil carried over
compressor separates out and, as it is heavier than the liquid refrigerant, accumulates
at the bottom of the condenser and evaporator. These vessels are usually fitted with
drain points and should be kept drained of oil.
32.How is Freon gas effect the “ozone” and effect the climate?
a) Release of CFC refrigerants into the atmosphere is harmful to the environment.
CFCs released into the atmosphere are broken down by photolysis to release
chlorine atoms, which catalytically destroy ozone, the stratospheric gas that acts
as a filter of ultra violet light from the sun.
b) CFCS, along with other ‘greenhouse gases’, inhibit the release of heat radiated
from the earth, thereby contributing to global warming, leading to rapid rise in sea
level.
c) The increased UV light on earth as a result of ozone depletion will cause skin cancer,
interfere with immune system and harm aquatic system and crops.
UV light
ClF3 F3+Cl
Cl + Ozone(O3) ClO +O2
ClO + O Cl+O2
33. How does a port state control inspection carried out for a refrigeration
system?
The following are inspected by port state control:-
1) Official Engine log book readings of the refrigerant compressor readings entered
by all watch keepers every four hours.
2) All cold room temperature maintained during the entire voyage.
3) Any breakdown or maintenance carried out on the refrigeration system and
documentation of the same.
4) Internal inspection of all cold rooms and the arrangement made in storing food
items.
Kngreddy Refrigeration and Air Conditioning System kngreddy
5) Inspection of meat product temperature.
6) Any other defects related to hygiene, including expiry date of food products.
7) A visual inspection of the refrigeration system and its condition of its
components.
34. How is the reciprocating compressor lubricated?
Oil is supplied to the bearings and crankshaft seal by means of a gear pump
Drive taken from the crankshaft. The oil is filtered through the suction strainer
mounted inside the crankcase. A pressure gauge is provided for confirming the
lubricating pressure and sight glass are fitted for checking the crankcase oil level.
A protection device is fitted for oil failure is provided by a differential oil pressure
switch. Compressor oil loss result due to oil being carried into the system by the
refrigerant.
Oil pressure is about 2 bar above crankcase pressure. Cylinder walls are
splash lubricated. Oil and refrigerant are miscible (i.e. if they form a mutual
solution) but oil is separated and returned to the crankcase using an oil separator.
35.What are the uses of psychometric charts and when do you use them?
· A psychometric chart shows vertical lines representing dry bulb temperatures, and
dotted lines relative humidity. To obtain the relative humidity from a pair of wet
dry bulb temperatures is marked on the chart, and the line of relative humidity
passing through this point is the required relative humidity.
· The indicated area in this figure defines the so-called “comfort zone”, i.e. the
range of air conditions (in still air) in which most people feel comfortable when
sedentary. By “most people” is meant a very high proportion of people – tests in
controlled environment chambers have shown that people generally agree well
with one another as to what is comfortable.
Kngreddy Refrigeration and Air Conditioning System kngreddy
Psychrometric chart.
36.What are the major components of a reciprocating compressor and how does it work?
1. Crankshaft of spheroidal graphite cast iron.
2. The alloy pistons and cast iron liners.
3. Piston rings of cast iron.
4. Forged Connecting rods.
5. Cast iron Liners.
6. Crankcase comprise as a single piece cast iron.
7. Main bearings and crankshaft bearings.
8. Compressor Safety devices such as Oil pressure switch, High Pressure switch and
Low Pressure switch.
Kngreddy Refrigeration and Air Conditioning System kngreddy
Compressor working process
The pressure raised instantly from low pressure to high pressure by the compressor. This
process is carried out by stroke of the compressor. Both the suction and discharge valves
are held closed by a combination of spring pressure and vapor pressure. On the down
stroke, with both these valves closed, the pressure in the cylinder is lowered as the volume
increases. When the pressure in the cylinder becomes less than the pressure below the
suction valves, the differential pressure opens the valves. The vapor from evaporator flows
through the suction line into this constantly increasing volume.
At the bottom of the stroke (BDC) there is no pressure difference, and the springs close
the suction valve, trapping the cylinder full of vapor. On the upward stroke this vapor is
compressed from its low pressure to a pressure greater than the discharge pressure on top
of the discharge valve. This high pressure forces the discharge valve open. At this point,
compression stops and compressed vapor flow out through discharge valve. When the
piston reached top of the stroke (TDC), there is no differential pressure to hold the valve
open, so the spring closes it.
A small amount of high-pressure vapor is still trapped in the space between the piston top
and the valve plate. This space is referred to as the clearance volume. The vapor in the
clearance volume is called clearance vapor and must be re-expanded to a pressure slightly
lower than the suction pressure before the suction valve opens again.
37.Why is expansion valve used in the refrigeration or air conditioning system?
The purpose of this valve is to efficiently drop the pressure of the refrigerant.
· The thermostatic expansion valve is designed to provide an ample supply of liquid
refrigerant to dry expansion evaporators, ensuring high heat transfer under varying load
conditions, without allowing liquid to pass into the suction line and enter the compressor.
· The liquid refrigerant stored at high pressure in the receiver flows through the
Thermostatic expansion valve, which regulates the rate of flow to the evaporator to suit the
evaporation.
· The pressure of the liquid is reduced to the evaporating pressure so that the temperature
of the refrigerant entering the evaporator is below that required in the refrigerated space.
· The liquid vapour refrigerant mixture flows through the evaporator extracting heat from
the refrigerated space and changing to a dry saturated vapour at the same temperature and
pressure it left the thermostatic expansion valve.
38.What is called “coefficient of performance” in a refrigeration or air condition system?
The conventional way to express the effectiveness of refrigeration is Coefficient of
Performance (COP). This is the ratio of heat extraction to energy input. Both are expressed
Kngreddy Refrigeration and Air Conditioning System kngreddy
in the same units, normally kilowatt (kW). For a system in which the mass flow rate is
constant, COP becomes equal to the ratio of ratio of enthalpy change in the evaporator and
enthalpy increase during compression. i.e. dh1/dh2. On the pH chart, enthalpy is shown as
enthalpy per per unit mass, or specific enthalpy. The ratio dh1/dh2 is the rate of transfer of
energy in the evaporator divided by the rate of transfer of energy in the compressor.
39.Enumerate the standard maintenance carried out in a refrigeration or air condition
system?
The following standard maintenance are carried out:-
· Filters-Should be washed in 50ppm solution on a regular basis
· Plenum insulation-Insulation to be examined at re-fit and damaged areas resealed.
· Drier units are removed and renewed silica gel (content of moisture in the system).
· System gas side filters are removed and cleaned at regular intervals for free flow of
gas in the system.
· Condenser to be cleaned sea water side tubes get choked with the mug present in the
sea water affecting the discharge pressure and raising the gas temperature.
· Oil level and gas level to be maintained at all times and oil to be regular intervals to
avoid choking of evaporator coils.
· To carry out regular leakage check of gas in the system using leak detectors.
40.What are the properties of air for an effective air conditioning system?
Air conditioning is a mechanical process for controlling the humidity, temperature,
cleanliness, and circulation of air in buildings and rooms. Indoor air is conditioned and
regulated to maintain the temperature-humidity ratio that is most comfortable and
healthful. In the process, dust, soot, and pollen are filtered out.
41.What are the basic principles of air conditioning?
Air conditioning is the control of humidity, temperature, cleanliness and air motion.
Winter conditioning relates to increasing temperature and humidity, whilst summer
conditioning relates to decreasing temperature and increasing humidity. Air
conditioning implies to the total automatic control of internal environment primarily for
the comfort of humans. The term is used to control the temperature and moisture at the
outlet of central air conditioning unit to the rooms.
The external air volume flow per cubic metre per hour is calculated according to:-
· Number of people.
· Level of air pollution.
· Extract heat from air.
Kngreddy Refrigeration and Air Conditioning System kngreddy
Humidity is controlled by installing humidifier to supplement the required moisture to
the flow of air to the rooms. The percentage rating of water vapour per cubic meter of
air is important for human comforts.
42.What is called central air conditioning and how is it carried out?
· Central air conditioning is one in which large air condition refrigeration
machines are installed and their output are distributed about the ship by a variety of
means.
· The outside and re-circulated air (50% or above) is mixed in the inlet section of
the unit fitted with dampers. It is then filtered and passes through pre-heater and
humidifiers in winter (cold) condition, or through a gas cooler in summer (warm)
conditions. The fan distributes the conditioned air via a low velocity duct to cabin.
· One common large fan is used to circulate the conditioned air with one common
Freon coil and steam heater with a humidifier to all the cabins.
· All of the machines are arranged and fitted in one common room including
condenser and compressor.
43.What are the safety features incorporated in a vapor compression cycle.
The following are the safety features fitted in a vapor compression system.
· High-pressure safety cutout .
· Low-pressure safety cutout
· Oil pressure safety cutout
· Pressure relief devices on condenser
· Refrigeration compressor abnormal alarm and stop .
· Refrigeration compressor motor overload stop .
44.What is the term “defrosting” and how does it work?
Electric defrosting of a fish room:-
The defrosting timer switch operates and shuts the solenoid valve of the fish
room.
Automatic pumping down takes place and the compressor stops on LP cut off.
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The evaporator fan stops and electric heater starts at the same time for about
15min.
The electric heater switches off automatically and cools of the heater.(Hot air is
not cycled into the cold chamber).
The evaporator fan starts after 10 min.
The defrost moisture is drained out through a non-return drain pipe from the cold
chamber.
The defrost cycle is thus completed with the solenoid valve of fish room opens
and compressor start back after defrost cycle.
45.What are the different types of Freon gas used for refrigeration system?
· Refrigerants RI 2, R22, R502 and R717 (ammonia), are the substances in general
use.
· The suitable alternative refrigerant for R12 and R502 currently limited to
R22,ammonia(R717) and R134a.R22 will continue to be the first choice for all new
marine installations.
· R134a has been developed as an alternative for R12, But it has a drawback in
being un suitable for use with mineral oil, and is expensive.
46.What is the use of the compressor in the vapour compression system?
The following are the reason for using the compressor in the vapour compression
system:-
It removes the vapor from the evaporator and introduces vapor to the high-pressure
side of the systems.
It maintains the low-side pressure at which the refrigerant evaporates, and the highside
pressure at which it condenses.
It supplies the pressure differences necessary to keep the system refrigerant flowing
through the system.
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47.How many gas filters are fitted in a refrigeration system and state the location of these
filters fitted in the system with three cold rooms?
b) How does the expansion valve work and control the superheat in the Evaporator?
ANS a)
The gas filters are fitted to filter the Solid particles which is formed by corrosion of
ferrous parts when exposed to oxygen and the non ferrous particles such as copper comes
out when lubricating oil mixes with moisture. There is one filter on the compressor
suction side, one on the return line to the compressor and one filter in each thermostatic
expansion valve inlet. Filter drier fitted on the liquid line before the solenoid Valve.
ANS b) Thermostatic expansion valve:
Kngreddy Refrigeration and Air Conditioning System kngreddy
The expansion valve consists of a gun metal housing with both ends threads are
provided for inlet and outlet connections. A long capillary tube filled with same liquid
refrigerant with a bulb is connected to the top of the valve housing. A fine metallic
bellow fixed to the push pins at one end slides through the bores of the valve housing
and attached to a valve disc. This valve disc is fixed on a hollow metal chamber acts as
a retainer for the spring The spring tension is adjusted using a screw in order to adjust
the superheat. A balancing line from evaporator outlet is connected below the bellow.
An orifice plate is fixed in the centre of the housing for the purpose of gas expansion.
Expansion valve working procedure:-
Three conditions present themselves in the operation of this valve: first, the balanced
forces; second, an increase in superheat; third, a decrease in superheat. The remote
bulb and the power element make up a closed system (power assembly), and the remote
bulb and power element are charged with the same refrigerant as that in the system.
The remote bulb and Power element pressure which corresponds to the saturation
pressure of the refrigerant gas temperature leaving the evaporator, moves the valve pin
in the opening direction.
Opposed to this opening force on the underneath side of the diaphragm and acting in
the closing direction are two forces: (1) the force exerted by the evaporator pressure
and (2) that exerted by the superheat spring. In the first condition, the valve will
assume a stable control position when these three Forces are in balance (that is, when
P1 = P2 + P3). In the next step, the temperature of the refrigerant gas at the evaporator
outlet (remote bulb location) increases above the saturation temperature corresponding
to the evaporator pressure as it becomes superheated. The pressure thus generated in
the remote bulb, due to this higher temperature, Increases above the combined
pressures of the evaporator pressure and the superheat spring (P1 greater than P2 + P3)
And causes the valve pin to move in an opening direction. Conversely, as the
temperature of the refrigerant gas leaving the evaporator decreases, the pressure in the
remote bulb and Power assembly also decreases and the combine evaporator and Spring
pressure cause the valve pin to move in a closing Direction (P1 less than P2 + P3).
48.What is the term “carry over” and what are the dangers of it in a refrigerated
system?
Kngreddy Refrigeration and Air Conditioning System kngreddy
The vapour in the condenser gives up its super heat and then cooled from the
discharge temperature to the saturation temp corresponding to condensing pressure
then gives up its latent heat as it condenses back to liquid. This process will not take
place as the quantity of sea water flow decreases due to fouled condenser.
The discharge temperature and pressure of gas from the compressor will be higher
than normal.
This high discharge carries more quantity of oil to the oil separator and condenser.
The oil “carry over” process to the condenser cannot be retrieved and would enter
into the evaporator.
The oil in the evaporator affects heat transfer and does not allow normal flow of
gas.
The desired room temperature cannot be maintained.
The gas flow is restricted and blocked due to oil in the evaporator and complete
system stops normal functioning.
49.What are the min and max temperature of all the cold rooms on board ship?
The following are the temperatures of the cold rooms on board ship:-
Vegetable room +4 to +8
Meat room -12 to -15
Fish room -15 to -18
50.How do you identify the condenser is dirty and what action will you take?
The condenser seawater side normally gets fouled. The heat transfer decreases
over a period of time.
The discharge of gas to the condenser would not be cooled to form liquid thus
affecting the performance of the evaporator.
The discharge temperature and pressure of gas from the compressor will be
higher.
The high pressure switch prevents the compressor from normal running.
The following are carried out to clean the condenser:-
o The Sea water valves to the condenser isolated and both side end covers are
opened.
o The tubes of the condenser are to be cleaned manually using tube brush.
o Wash internal of tubes with pressurized water.
o Zinc anodes to be renewed.
o Check division plate of end cover and repair if corroded to avoid short cycling of
cooling water.
o Tube ends on both sides of the end plates to leak test.
o Upon completion end covers are fitted with new gaskets.
51.Explain the term “Liquid knock” in a refrigeration system.
Kngreddy Refrigeration and Air Conditioning System kngreddy
The liquid refrigerant stored at high pressure in the receiver flows through the
Thermostatic expansion valve. It automatically adjusts the rate of flow to the
evaporator to suit the evaporation.
The pressure of the liquid is reduced to the evaporating pressure so that the
temperature of the refrigerant entering the evaporator is below that required in the
refrigerated space.
The liquid vapour refrigerant mixture then flows through the evaporator, extracts
heat from the refrigerated space and changes to a dry saturated vapour at approx the
same temp and pressure at which it left the flow control valve.
The compressor sucks the vapour from the evaporator; the temperature and
pressure of the vapour are raised by compression.
If more quantity of refrigerant flows than required by a faulty expansion valve,
the Freon sucked by the compressor will be liquid and ends up damaging the
complete compressor. This liquid entering the compressor is called “liquid knock”.
52.What are the two ways of charging gas to the refrigerated compressor?
Charging of refrigerant in the refrigerated system can be done on low pressure or high
pressure side.
1.Low pressure side should be charged ONLY WITH GAS.
2.High pressure side should be charged ONLY WITH LIQUID REFRIGERANT.
CHARGING LIQUID BETWEEN CONDENSER OUTLET AND DRIER:-
1. Connect the Freon gas cylinder to the gas charging valve with a special flexible
hose(purge the air from the hose).
2. Open the cylinder valve (to liquid side) and gas charging valve and run the
compressor.
3. The low pressure switch starts the compressor with LP cut in.
4. Charge the liquid until the condenser level is normal and close Freon gas bottle
valve and gas charging valve.
5. The pumping down operation starts and compressor stops due to LP cut off.
6. Release the charging hose pressure and remove from place.
Procedure for charging refrigerant at low pressure line:
CHARGING GAS THROUGH COMPRESSOR SUCTION VALVE:-
1. Connect the Freon gas cylinder to the gas charging valve with a special flexible
hose (purge the air from the hose).
Kngreddy Refrigeration and Air Conditioning System kngreddy
2. Open the cylinder valve (to gas side) and gas charging valve and run the
compressor.
3. The low pressure switch starts the compressor with LP cut in.
4. Charge the gas until the condenser liquid level is normal and close Freon gas bottle
valve and gas charging valve.
5. The pumping down operation starts and compressor stops due to LP cut off.
6. Release the charging hose pressure and remove from place.
53. What safety precaution will you take when working with a refrigerated compressor
during troubleshooting or repairs?
The following are carried out before Overhauling of compressor:-
· Pump down the entire gas to the condenser.
· Ensure that the entire system valves are shut and isolated.
· Isolate the compressor from its electric motor and switch off the circuit breaker.
· Place a safety board stating "MEN AT WORK." at the circuit breaker.
· Release the pressure on the compressor by slackening the oil filling plug with
great care due to high internal pressure.
· Correct tools to be used with proper torque while opening and tightening holding
bolts.
· Wear safety goggles and gloves.
54.How is the air cleaned in the air conditioned unit and how re-circulation controlled?
Kngreddy Refrigeration and Air Conditioning System kngreddy
The air filter material used is felt, it is soft and filters all dirt from the atmosphere. The
air conditioning system fan sucks the atmospheric air through this filter before
cooling. This filter is periodically removed and cleaned. The filter (felt) is fixed on to
the metal frame on the air intake side.
The re-circulation of air is normally carried out by allowing the same cold air from the
accommodation back to the inlet side of the air conditioning system . Small quantity of
fresh air is mixed to keep re-circulated fresh. The flaps located in the re-circulation
and fresh air trunking are operated to meet the comfort level of the crew.
55 . Describe the refrigerated ship refrigeration system and how does the cargo
temperature maintained.
The perishable cargo transporting long distance is always a complicated problem. The
cargo temperature has to be maintained through the voyage. A common agent of
decomposing of food stuff is heat and this can be removed by refrigeration.
Each cargo holds are well insulated with layers of insulation material. The floor is
double skinned to allow for even circulation of the cooling air. The air passages are
made to allow the cold air upwards through the cargo. The perishable cargo like fruits,
vegetables, fish and meat are carried in these cargo hold at temperatures -30 to +12
deg C. The most sensitive cargo is fresh bananas.
A reefer ship design carry large amount of pipelines with refrigerant, which lead to
each cargo hold. This refrigerant is the secondary cooling element and is brine. Each
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brine line feeds a bank of cooling coils which cools the forced air flow generated by
many number of cooling fans.
Reefer ship requires the following:-
1. Effective control systems to meet precision temperature requirement.
2. Higher capacity generators to meet the power needs.
3. Special monitoring equipment for safe monitoring of equipment and cargo.
4. Effective ventilation control system, high level of humidity needs to be maintained.
5. Extra hold bilge pumping requirements.
6. After loading, due to rapid drop in temperatures, condensation leads to large water
accumulation and requires to be controlled.
7.Modern reefer vessels also incorporate inert gas generators and systems thus
cooling and atmosphere control are both applied to the cargo.
The ripening of fresh fruits may cause lack of oxygen and produce unwanted gases.
The modern trend is to use inert gas generators called controlled atmosphere
transportation in the reefer world.
Controlled atmospheres are essentially those which deviate from the normal air
composition of 21% oxygen, 78% nitrogen and 300ppm of carbon dioxide. Other
gases are also present but negligible. The most common inert gas used is Nitrogen.
Special generators are employed to extract the atmospheric nitrogen (reducing oxygen
content). The resulting air mixture is then pumped into the cargo holds, purging the
existing mass of air. The atmosphere in the hold is controlled to preset levels
depending upon the cargo. Modern reefer ships can be monitored temperatures using
satellites and internet from shore to sea.
Kngreddy Refrigeration and Air Conditioning System kngreddy
Undercharging of Refrigeration System
Under charging means either the system is charged with less gas or gas leaked out from the system. The system runs with less amount of gas in circulation.
1. Indication:
1. Compressor is running hot and performance of the compressor falls off due to high superheat temperature at the suction side of compressor.
2. Suction and discharge pressure of the compressor is low.
3. Large vapor bubbles in the liquid sight glass.
4. Low gauge readings in the condenser.
5. Ammeter reading for the compressor motor is lower than normal.
6. Rise in room temperature which is to be cooled.
7. Compressor is running for extended period of time.
Causes:
Leakage of refrigerant at the 1. Shaft seal 2. Flange couplings and joints 3. Valve glands etc.
Leakage of gas through the condenser tubes due to corrosion.
Low suction pressure of the compressor may be due to 1. Blockage at expansion valve orifice
2. Partial blockage of refrigerant at the filter or drier.
3.Blockage of refrigerant at the evaporator coil due to moisture/oil and mixture of both.
For the reasons 1&2 the level in the condenser and receiver will be high as there is less gas flow.and for the reason 3the level in the condenser receiver will be normal .
Action:
1. Identify and rectify the leakage of refrigerant from the system.
2. Clean the filter and drier.
3. Charge the system with fresh refrigerant if required.
Over - charge of Refrigeration System
Indication:
1. The liquid level in the condenser is too high (high condenser gauge reading). This will reduce the available condensing surface, with corresponding increase in the saturation temperature and pressure.
2. High pressure switch of the refrigerant compressor activates and stops the compressor.
3. There is possibility of excessive liquid refrigerant getting to the evaporating, giving icing at the compressor suction, and a pressure drop across the expansion valve.
4. The cold room temperature may rise, if the evaporator is flooded.
Causes:
1. It may be due to the reason that excessive refrigerant has been charged in the system.
2. High delivery pressure may also be due to air/incondensable gases present in the system.
3. It may also be due to the formation of ice on the regulator.
Action:
1. Remove the refrigerant from the system. This is done by connecting a cylinder to the liquid line charging valve, starting the compressor, and then operating the charging valve.
2. Purge the air from the system and maintain effective cooling. While purging there is always some gas will pass along with air hence the regulation says the purging has to be collected in recycling cylinders and same has to be recorded.
3. Remove ice from the regulator by using any of the defrosting methods.
Moisture in the System
1. This normally comes with the ingress of air in the system. Moisture may freeze at the expansion valve, giving some of the indication of under charging.
2. It will contribute to the corrosion in the system.
3. It may cause lubrication problems and breakdown of the lubricating oil in the refrigerant compressor.
Air in the System
Indication:
1. This may cause the refrigeration compressor to overheat, with a high discharge pressure and normal condensing temperature.
2. There are possibilities of small air bubbles in the liquid sight glass of the condenser.
3. Condensing pressure of the refrigerant in the condenser may be high.
4. If there is excessive air, it may reduce the cooling capacity of the system, making the compressor to run for the extended period of time.
5. It may cause the gauge pointer of the condenser to jump indefinitely.
Causes:
1. During charging, air may enter in to the system.
2. If Freon-12 is used air may leaks in to the suction line because the working pressure of the Freon-12 refrigerant is less than the atmospheric pressure.
Action:
1. Air in the system can be removed by collecting the system gas in the condenser, leaving the condenser cooling water on and venting out the air from the top of the condenser because air will not be condensed in the condenser but remains on top of the condenser above the liquid refrigerant.
2. Connect the collecting cylinder to the purging line of the condenser, open the valve, and collect air in the cylinder.
3. After purging the air from the system don’t forget to shut the purging valve.
4. Check the level of the refrigerant in the system. If required, charge the system with fresh refrigerant.
5. Restart the compressor with all safety precautions.
Oil in the Refrigeration System.
Indication:
1. Temperature is not dropping in the cold rooms as normal, due to fact that oil act as insulation in the evaporator.
2. It may cause excessive frost on the suction line.
3. Refrigerant compressor runs for the extended period of time.
4. Lubricating oil level in the compressor will drop.
Causes:
1. This may happen if the oil separator is not working properly.
2. Oil my carry over from the compressor and may not come back to the compressor due to blockage in the system.
3. Defective piston rings or worn out liner of the compressor may cause the oil to carry over along with the refrigerant.
4. Compressor may take high capacity current during starting.
Action:
1. Check the oil separator for proper functioning.
2. Check the drier for proper cleaning and if its require cleaning clean it
3. Evaporator coil should be drained to remove any trace of oil.
4. If there is oil in the cooling coils, increase the condenser and evaporator temperature differentials and remove excess frost on the suction pipe.
Flooding of Refrigerant in the System
1. This is seen as liquid getting back to the suction of the refrigerant compressor.
2. It may be due to a faulty or incorrectly adjusted expansion valve and also due to solenoid valve leakage.
3. It may also result from overcharging of the refrigeration system. Flooding may lead to an iced up evaporator.
Moisture, dirt and air.
Preventing and eliminating moisture, dirt and air from entering a Freon-12 system during its installation or while being serviced is highly important and cannot be too often stressed. Any moisture in the system, regardless of how little, will not be absorbed by the Freon-12. Moisture will tend to develop intercrystalline embrittlement of brass or copper bellows, such as used in the high pressure control switch; and leads to corrosion on compressor valves, pistons, shaft, etc. Therefore, it is imperative that all care possible be exercised to eliminate moisture in the system.
Freon-12 has the ability of loosening and removing any scale and all factory installation dirt or scale that may be in the system. Eventually this dirt is deposited in the compressor suction strainer, and the liquid strainers ahead of each expansion valve. This is a physical property of the refrigerant, as it does not react chemically with these materials. Care should be taken to clean, dehydrate and seal all openings on the equipment during handling and installation.
Air should never be pumped into a Freon-12 system as a means of testing for leaks, or permitted to be drawn accidentally into the system. Never open any connections in the system unless the pressure within the system is above zero pounds. The atmosphere always contains moisture. Therefore, any air permitted to enter the system will carry with it a certain amount of moisture, endangering the correct performance of the installation as explained above. Furthermore, air permitted to enter the system after the installation has been in operation will lodge in the condenser, causing high condensing pressure and reducing the operating efficiency of the compressor.
TESTING FOR LEAKS
Bibliography
There are no sources in the current document.
Due to the fact that Freon-12 is stable and inert, the usual visual method called the "smoke" test cannot be used to determine leaks. Neither can leaks be determined by the sense of smell, because Freon-12 is practically odorless. The use of chemicals or odorants such as ammonia or peppermint, charged into the system with the Freon-12, is not recommended and should not be attempted. Serious trouble, particularly copper bellows failure, is certain to result from the use of ammonia as a leak detector.
Use of oil. Oil should never be used as a means of testing for Freon-12 leaks. This medium is unreliable because of the capacity of the oil for absorbing Freon-12. Should a small leak exist where oil has been applied, the Freon-12 will be absorbed by the oil and show no indications by bubbles until after the oil is saturated with Freon-12. Furthermore, should an attempt be made to test with a Halide Torch a leaky joint previously tested with oil, will result in a false indication because of the Freon-12 being released from the oil.
Use of soap suds. A Halide Torch will detect very small Freon-12 leaks. However, the Halide Torch is so sensitive that, if there are any very bad leaks, the atmosphere around the apparatus will become so contaminated with Freon-12 as to make it impossible to locate the source of the leak with the aid of the torch. This condition will prevail especially if the apparatus is located in a small or poorly ventilated room. Under such conditions, the Halide Torch is of little value for discovering the exact location of the leak, and soap suds must be used.
To prepare soap suds for testing, use a soap and water solution of about the consistency of liquid hand soap, which will lather freely, or work up a lather on the brush by rubbing the wetted brush on a cake of soap. A few drops of glycerine added to the solution will cause the lather to remain wet longer. When applying the soap suds, paint the soap lather on the joint all the way around, and examine the joint thoroughly for bubbles. When the joint is located so that a part of it is not visible, use a pocket mirror. It will sometimes take a full minute or more for bubbles to appear at a small leak. Questionable spots should be covered with lather and examined again.
Use of the Halide Torch.
Small Freon-12 leaks are detected by a special designed torch known as a "Halide Torch." Atmosphere suspected of containing Freon-12 gas is drawn through an exploring hose into the burner by an injector action. The air sample passes over a copper reactor plate in the burner chamber, which is heated to incandescence by the flame. If there is even a minute trace of Freon-12 present, the torch flame will turn from its normal blue or neutral color to a characteristic green color as it comes in contact with the reactor plate. The shade of green will depend upon the relative amount of Freon-12 present, being paler for small concen¬trations, and darker for heavier concentrations. Excessive quantities of Freon-12 will color the flame a vivid purple, and may even extinguish it by crowding out the supply of oxygen in the air.
A number of Halide Torches are available, most of which use acetylene gas or alcohol as a fuel. There are several precautions which must be observed in using the leak detector to obtain good results, namely:
1. Be sure the reactor plate is in place.
2. Adjust the flame low enough so that it does not extend beyond the top of the burner chimney.
3. A small flame is much more sensitive than a large flame.
4. If difficulty is experienced in lighting the torch with the small gas flow necessary, block the end of the exploring hose until the flame ignites, then gradually open.
5. If the flame persists in burning a white or yellow color, the exploring tube is partially blocked with dirt and should be cleaned out.
6. Try the torch in an atmosphere in which there is known to be a small amount of Freon-12, to finally make sure that it is working properly.
7. Check to see that air is being drawn into the exploring tube, by holding the end of the tube to the ear from time to time.
8. Hold the exploring tube close to the joint being tested, to prevent dilution of the sample by stray air currents.
9. Move the end of the exploring hose slowly, completely around each joint.
10. There is a definite time lag between the instant the air enters the exploring hose, until it hits the reactor plate. Leak testing cannot be hurried.
11. If a green tinge to the flame is noted at any time, repeat the test in the same vicinity until the source of the Freon-12 is determined.
12. Use soap bubbles if necessary to find the exact point at which a leak is occurring, after one has been indicated by the Halide Torch.
13. Do not use the torch in an atmosphere known to be heavy with Freon-12, as this will tend to foul it.
14. Always follow a definite order in leak testing, so that no joints will be missed.
When using the Halide Torch for detecting leaks, the following practice should be kept in mind:
Even the smallest leak is not to be considered negligible.
However insignificant the leak may seem, it will eventually empty the system of its charge to the point of faulty operation. Because Freon-12 is practically odorless, the first indication is the loss of refrigerating effect.
The extra time spent in testing all threaded, flared, soldered and valve cap gasket joints made in the field, as well as the factory fabricated connections, will be justified.
The system should never be recharged until all leaks are discovered and definitely repaired.
Upon locating one leak, do not assume that it alone is responsible for the difficulty; thoroughly retest the complete installation.
Pressure testing, purging and dehydration.
The system should be tested at a pressure of 10 bar, for the purpose of locating leaks. (All parts have been previously shop tested to confirm with standard specifications. The use of chemical dehydration when installing a new system is very risky, and distinctly not recommended.
The following is a recommended procedure for testing for leaks, purging, and dehydration:
Complete all piping connections, and open all liquid, discharge, and suction shut-off valves.
Connect a Freon-12 service cylinder to the charging connection in the liquid line, elevate the rear end of the Freon-12 drum so as to be sure that the outlet is below the liquid level, and slightly open the valve on the Freon-12 service cylinder, introducing enough refrigerant to raise the pressure in the system to 1bar to2 bar. Then close the valve and disconnect the refrigerant drum. Test the system for leaks at this pressure with a Halide Torch.
Connect a drum of dry nitrogen to the valve at the charging connection, and introduce enough nitrogen gas to raise the pressure in the system to the required test pressure.
Test the entire system for leaks with a Halide Torch. The amount of Freon-12 refrigerant in the system will act as an indicator.
When it is found that the system is perfectly tight, release the pressure on the system by opening the "purge" valve.
Dehydration. Using an approved vacuum pump (preferred).
Connect the suction side of the vacuum pump to the liquid charging valve, allowing the pump to discharge to the atmosphere.
Open all valves on the system to be evacuated.
Run the vacuum pump until the lowest vacuum it will produce is obtained, then stop the pump, and close the liquid charging valve.
Alternate Method, using the compressor.
1. Block the suction pressure cut-out switch in its running position.
2. Open the flared joint at the tubing connection between the main discharge stop valve on the compressor and the high pressure cut-out switch.
3. Open all valves on the system to be evacuated.
4. Run the compressor until the lowest vacuum it will produce is obtained.
5. Stop the compressor, and immediately close the flared joint on the high pressure cut-out switch connection.
The time required for this operation will vary with the capacity of the compressor and the amount of surface to be pumped out, but in general a few hours will suffice.
If it is impossible to obtain a 28-inch vacuum, it will probably be due to one of the following reasons, namely:
a) Presence of excess moisture in the system;
b) Presence of absorbed refrigerant in the oil in the crankcase; or
c) Leakage of air into the system.
If there is a leak in the system it should be stopped. After the desired vacuum has been obtained, allow it to remain over night.
If the next morning the system has not lost more than 2 or 3 inches of vacuum, it may be considered reasonably tight.
Connect a Freon-12 service cylinder to the charging connection, and introduce enough Freon-12 into the system to produce a pressure of 20 to 25 pounds, then test the joints again with the Halide Torch. If no leaks have been discovered, the system is then ready to charge with refrigerant.
To charge Freon-12 proceed as follows:
1. Observe and practice the caution under Handling Freon-12.
2. If the Freon-12 charge has been lost, pump a vacuum on the entire system.
3. Mount the Freon-12 drum on a portable platform scale.
4. Connect the Freon-12 drum to the Freon-12 charging connection.
5. Slightly open the valve on the Freon-12 service cylinder and test charging connections with soap suds for leaks.
6. Open the charging valve and the Freon-12 cylinder valve, and charge in sufficient Freon-12 to create 60 pounds pressure.
a. On a new system or one in which there may be leaks, it is advisable to check all connections with the Halide Torch before adding any more Freon-12.
7. Close the liquid valve at the outlet of the receiver, run the compressor, and charge sufficient Freon-12 into the system. Be sure that the compressor suction and discharge valves are open, also the valve between the condenser and the receiver.
Note. When the system has two compressors, condensers, and receivers, close both liquid valves at the outlet of the receivers, and run both compressors with suction and discharge stop valves open.
The liquid charging valve must be closed sufficiently to provide for a pressure at the charging connection lower than the pressure in the Freon-12 drum, so that the Freon-12 will flow from the drum into the line.
By observing the change in weight of the drum, the amount of charge added can be obtained.
Refrigerant charging connections.
Fig. Recommended apparatus for charging Freon-12.
Figure shows the connections for charging a system with Freon-12.
The charging valve, packless, angle or globe type, should be located in the liquid line between the receiver and the dehydrator.
The Freon-12 drum is preferably mounted on scales to enable the oper¬ator to determine the amount of refrigerant charged into the system.
The charging connection consists of a flexible section and a 6-inch length of f-inch O.D. copper tubing soldered to each end. The flexible section is made up of a seamless bronze bellows tube, reinforced and protected on the outside by a heavy bronze wire braid. The ends of the bellows tube and bronze wire braid are fitted with copper ferrules, to which are soldered the 6-inch lengths of copper tubing. The outer ends of these tubes are flared, and fitted with standard 3\4-inch SAE flare nuts.
A 3\4-inch male SAE flare to f-inch female pipe threaded adapter is furnished for connecting to a standard Freon-12 drum valve.
Removing Freon-12.
If a system has been overcharged with Freon-12 or if the charge is to be transferred from the system, proceed as follows:
Start the compressor and pump down the evaporator pressure to zero pounds pressure, with the liquid valve out of the receiver closed.
Close the discharge stop valve, and all liquid valves at the cooling coils.
Connect an empty Freon-12 drum to the liquid charging valve. Be sure the drum is large enough to avoid the hazard of overfilling.
Before connecting the drum to the Freon-12 system, set it in an ice water bath to thoroughly cool the drum.
Open the liquid charging valve, and the Freon-12 drum valve; the cooled Freon-12 drum will drain Freon-12 from the system until the pressure in the drum equalizes with the pressure in the system.
To remove the remaining Freon-12 from the system, it will be necessary to use a second empty cold drum. The colder the drum, the less Freon-12 will remain in the system.
Caution. After disconnecting the drum from the system, weigh it to be certain that it has not been overcharged. The net and gross weight is stamped on the drum, and includes the cast iron protecting cap.
Maintenance required for refrigeration and air conditioning plant
A large proportion of refrigerating and air-conditioning equipment is now fully automatic in operation, but that does not absolve the user from the responsibility of understanding how it operates and being able to observe this operation. If this is not done, or if there is no other way of monitoring performance, the plant may run abnormally for some time before a fault is noticed, by which time considerable damage may have occurred. The initial requirement is for the equipment to be fitted with sufficient pressure gauges and other monitoring devices to indicate the conditions under which it is working. It is helpful to mark these with the normal working limits when commissioning the plant.
Persons operating the plant should understand the meaning of any indicator or warning lights fitted to the control panels. It is important that the operator should be aware of the temperature gradients to be expected with the system so as to be able to compare actual working conditions.
A running log should be kept, to monitor working conditions.
Standby plant is often fitted, and it is part of the operation discipline to change over machines to ensure that they get even wear and keep all sets in running order. All staff should be aware of the method of bringing standby plant into use in an emergency. Where refrigerant valves need to be opened or closed as part of plant operation, this should be carried out only by competent and responsible staff.
If an open compressor is shut down for any length of time, it should be pumped down and valves are closed to prevent possible loss of gas. It is expected a senior engineer officer to take a close interest in the operation of the system, and not delegate all the responsibility to trainee engineers. This implies a good knowledge of the purpose, working and characteristics of the system and its control.
User maintenance
Where the user undertakes the day-to-day running of the plant, including most cases where the equipment is fully automatic in ordinary operation, basic maintenance will be assumed as part of this responsibility.
The maintenance includes operation where not automatic, cleaning of filters and strainers, attention to oil and lubricant levels, belt tensioning, general cleaning, running standby equipment, and verification of control operation. The correct and efficient operation of any plant requires full flow at all times through the heat exchangers. Air and water filters
need to be kept clean. Finned coils, especially outdoor condenser coils, if fitted must be cleaned frequently. The water side of heat exchanger coils should be cleaned of any accumulations of scale or algae as soon as a change in working conditions shows that they are getting dirty.
Accumulated dirt on air filters will increase the resistance and lead to reduced air flow. This is by far the most frequent cause of malfunction of air-conditioning equipment. It is the responsibility of the engineer officers to clean or replace air filters and knows how to carry this out and when. Spare filters should be available so that the change of clean for dirty can be made in the one operation and the dirty filters taken away in closed bags for cleaning or disposal, to prevent release of dirt in the conditioned area. It is an advantage if the person changing the filters has a hand vacuum cleaner to pick up dirt which may become dislodged, and to clean the filter frames.
Changing of large filters will need to be left until the plant can be shut down for the time required to carry out the work. Under no circumstances should fans run without filters in place, or dirt will be deposited in inaccessible parts of the plant. The provision of a manometer across the filter to indicate the pressure drop will give a positive indication of the need to clean or replace. Such resistances can be estimated from the filter manufacturer’s data and should be recorded at the time of commissioning and also marked at the filter.
Water strainers are of the cleanable type, either a single-mesh basket, which must be removed after isolating the water flow, or a duplex construction which permits the cleaning of one while the other is working. Indication of a dirty strainer will be an increase in pump pressure, and it is essential to have a pressure gauge on the pump discharge.
Strainers in closed water systems will need cleaning soon after first starting up the circuit, but little attention once the pipe dirt is flushed out.
A clear set of operating instructions should be posted in the main Plant room, enabling any authorized person to start, run and shut down the system in a correct, safe and efficient manner. All staff who may be required to operate the plant need to be instructed and have some practice. It is usual to mark the grade of lubricant on each item which might need periodic attention. Most equipment is designed to run for long periods without addition of lubricant and the dangers of adding too much should be noted. The user will not normally add oil to a refrigeration system, apart from an industrial R.717 plant which will have a routine for the draining of parts of the circuit and replenishing the compressor sump.
Drive-belt tensioning and the replacement of broken or worn belts is a normal maintenance procedure, but may be missed if equipment is out of sight. A routine check will find these out.
The general cleanliness of plant is an indication of the care and interest taken by the maintenance staff, and is an encouragement to others working on it. There is no reason or excuse for accumulations of dirt and refuse on or around any system. Standby plant needs to be run frequently, both to ensure that it is in working order, and also to keep items such as shaft seals oiled and run-in, and thus gas-tight. The location and function of any changeover valves which must be operated in conjunction with standby plant should be clearly marked.
Many malfunctions, and some dangerous situations, arise from incorrect setting of control and safety instruments. It is assumed that these are all set and the correct settings recorded at the time of commissioning, but such settings may afterwards be tampered with by uninformed or unauthorized persons. The correct adjustment of any instruments normally set by the user remains his or her responsibility as a matter of routine operation. It is good practice to arrange that instruments are locked, sealed, or otherwise guarded from tampering – even to the extent of putting dummy controls in a conspicuous place. The function of safety controls should be checked at least once a year. Water treatment and corrosion inhibition systems require periodic attention, and full instructions should be left on site by the supplier or installer of the apparatus, whether or not they will be responsible for later attention. Where water is evaporated from a circuit, such as a cooling tower, evaporative condenser, or humidifier, it must be remembered that there is no way of avoiding a steady bleed-off or frequent flushing, to restrict the concentration of dissolved solids. Much trouble arises from the efforts of well-wishing but misguided persons who stop the flow of bleed-off to ‘save water’. Many systems are shut down for periods of the year, either for process closure or if not required in winter. The advice of the supplier should be sought as to the correct procedure. In the case of refrigerant circuits, it is advisable to pump down into the receiver or condenser to minimize leakage losses
.
Leaks of Refrigerant
In the case of the halocarbons, a regular leak test should be part of the general maintenance schedule. Under no circumstances should refrigerant be added to a leaking circuit without first making a repair. Where gas is detected at the shaft gland of an open compressor which is not turning, the compressor should be run for a short time to re-lubricate the gland. The leak may then cease. Staff should be forbidden to smoke while leak testing or repairing. Many engineers are ignorant of the danger to their health if smoking in the presence of traces of the halocarbons.
Moisture in halocarbon circuits will be indicated by the colour trace on the sight glass, where this is fitted. Immediate action is required, especially with a hermetic or semi-hermetic compressor, before damage is caused. The drier should be changed and the sight glass watched for reversal of the colour to ‘dry’. Bad cases of contamination may need a second change of drier. If the liquid line leaving the drier or strainer (if separate) is colder than the inlet, there is a severe pressure drop within, indicating dirt. A new drier, cleaning of the strainer, will cure this.
Heat exchanger surfaces need to be kept clean. Aqueous circuits (evaporator or condenser) can be cleaned with a chemical such as sulphuric acid, brushed or subjected to high-pressure water jets. In each case, all traces of dirt and chemical need to be removed from the circuit before it is put back to work. In cases of doubt, the manufacturer’s advice should be sought. A layer of scale 2 mm thick on a condenser tube can cause a power increase of 16%, and the need to clean a condenser can usually be deduced from the condensing pressure. Persons using high-pressure water jets should wear face masks to avoid inhaling aerosol droplets. The checking and readjustment as necessary of all safety controls is an essential part of periodic maintenance – possibly annually.
Unsafe conditions can be set up by throttling valves, stopping pumps, or removing the load. In each case the relevant safety control should function at the pre-set conditions. Safety checks on specialized items such as fire dampers may be required from time to time by local authorities, and these checks, together with the expert advice available from the testing officers, should be welcomed as proof of the inherent safety of the installation.
It is essential that all major maintenance work and findings are recorded in the plant running log as a guide to the reliability of components, the need for cleaning, and other indicators to future.
work.
Refrigerant compressors, air compressors and some other items of mechanical equipment might be subjected to a periodic part strip-down inspection and overhaul, as may be recommended by the manufacturer and indicated by running time meters or estimated
running hours from the plant log sheet. Such planned maintenance entails, as its name suggests, some planning. Manuals, diagrams and drawings should be obtained beforehand, and sources of possible spare parts located. Special tools or instruments may be required. The manufacturer should be able and willing to advice and guide in this work. Failed, worn and other replaced parts should be retained for later examination and a post-mortem held in cases of doubt; records should be kept. In the case of a shut-down of a process plant, the major components should be tackled in rotation, lest a serious fault or shortage of spares prevents the process re-starting at the end of the closure.
Planned or preventative maintenance is not necessarily the best for all installations. If the service is not essential, and spares are known to be available within a reasonable time, nothing is done beyond obvious routine servicing work. Then, when a breakdown occurs, it is repaired. This approach, although not widely accepted, is an option which should be considered.
Fault-finding
System faults fall into two general classes: the sudden catastrophe of a mechanical breakdown and the slow fall-off of performance which can be detected as a malfunction in its early stages but will also lead to a breakdown if not rectified. Identification of the first will be obvious. To track down the cause of a malfunction will be more complicated.
Fault-tracing is seen as a multistep process of detection, ending in normal operation again and a record of the incident to inform.
1. Detection, i.e. detection of abnormal operation
2. Knowledge of the system to track down the cause
3. Observation of exact operating conditions
4. Identification of the fault
5. Decision: what to do? How? When? Can it be left?
6. Action to rectify the fault.
7. Test: is it now normal?
8. Record note in log, for future information.
A lot of help in fault-tracing may be had from charts for specific pieces of apparatus, prepared by the manufacturer.
Detailed examination of a sophisticated item may be beyond the skills of the plant operators and require the assistance of a specialist, such as an electronics engineer. Where such complications are part of the system, it is an advantage to know beforehand where such specialist help can be reached. It should be accepted that fault analysis can be a slow process and that it usually defies prior estimates of the time it will take, regardless of the pressures of persons who are affected by the interruption of the service. In any case, hasty decisions and random efforts to get the plant working again are to be avoided , since more damage may result.
Training courses are available in analytical methods of fault-tracing. Computers are also in use which monitor a number of parameters and draw attention to any observed abnormality. The control/ monitoring device may then make a judgment as to the cause, or this may rely on the interpretation of the operator. Considerable use is now made of logic control/monitoring devices which can oversee the operation of a large number of installations from a central computer/observation terminal.
Except in the case of a planned overhaul, spare parts will only be wanted in an emergency, and then in a hurry. Most manufacturers can guarantee supplies from their own stocks and will dispatch quickly, providing they are given enough information to correctly identify the parts required.
There are two predominant problems. First, there is the necessity to keep the plant in operation, which may vary from a non-essential service such as the comfort cooling of an infrequently used room, to the precise temperature control of an expensive or potentially dangerous process. The second problem is the time taken in transit, which might be as long as a year in parts of the world subject to excessive docks and custom delays.
The scale of spares to be held under these varying conditions must be judged by the user, taking into account the problems which might be met and seeking advice from the supplier. For remote installations, the supplier may be asked to recommend ‘Spares for one year’s operation.
The detection of abnormal operation can only occur if normal operation is monitored. Since refrigeration is a thermal cycle, the obvious readings to be taken will be temperature and the related refrigerant pressure. The skilled operator will have a working knowledge of the pressures and temperatures to be expected, but will not be able to make an accurate assessment of the actual conditions without plant measurements for comparison.
The ship’s trial report will show readings taken at that time, but only at one set of running conditions. It is therefore essential on a plant of any size to maintain some kind of running record, so that performance can be monitored with a view to detecting inefficiency and incipient troubles. The degree of complexity of this running log must be a matter of judgment, and a small amount of useful information is to be preferred to a mass of data which would be confusing.
The following would seem to be basic:
1. Compressor suction and discharge pressures and corresponding temperatures.
2. Oil pressure gauge. It would be helpful to add a column so that true oil pressure can be entered (i.e. oil – suction).
3. The load temperature (room, water, brine, etc.).
4. Load flow rate or pump pressure.
5. Ambient temperature, dry bulb and wet bulb if possible.
6. Condenser water flow rate or pump pressure.
7. Any motor currents where ammeters are fitted.
OPERATION AND TROUBLE DIAGNOSIS
Starting the compressor. To start the compressor in conjunction with its condenser and its respective receiver and refrigerator coils, proceed to:
1. Start the condensing water pump, open all water valves to the condenser,
2. and be sure that the proper amount of water is circulating through the condenser.
3. Test for flow of condenser water by slightly opening the drain valve in the bottom of the condenser-heads.
4. At short intervals, open the air vent on the condenser heads, and release all air until the condenser tubes are full of water, which will be indicated by water squirting out of the vent valves.
5. Close the vent valves.
6. Open discharge valve (A), Fig. 12, on the compressor, and suction valve (B).
7. Start the compressor at the push button station on the controller.
8. Open main liquid line stop valve (E).
9. Open the proper liquid and return valves on the evaporator.
Stopping the compressor.
Close main liquid valve (E).
Run the compressor until it cuts out on the low pressure switch.
Stop the compressor by means of the controller. (Push the "Stop" button.)
Close main suction valve (B), main discharge valve (A).
OPERATION DIFFICULTIES
Faulty operation of any system is indicated by very definite symptoms. These symptoms may be caused by one or more incorrect conditions which should be eliminated in a step by step process of correction. The following chart of symptoms, their causes and correctives, will assist in correcting faulty operation.
TROUBLE DIAGNOSIS CHART
Symptom or difficulty
High head pressure.
1. Condition may be due to Air or non-condensable gas in system.
2. Inlet water to the condenser is warm.
3. Insufficient water flowing through condenser.
4. Condenser clogged or limed.
5. Too much liquid in receiver, condenser tubes submerged in liquid refrigerant.
Correction
1. Purge air from condenser.
2. Increase quantity by adjusting water regulating valve.
3. Readjust water regulating valve.
4. Clean condenser water tubes.
5. Draw off liquid into service drum.
Low head pressure.
1. Condition may be due to Too much water flowing through condenser.
2. Water too cold, un-throttled. Liquid refrigerant flooding back, from the evaporator.
3. Leaky discharge valve.
1. Correction
2. Regulate water valve. Reduce quantity of water. Change expansion valve adjustment, examine fastening of thermal bulb. Remove head, examine valve diaphragms. Replace any found defective.
High suction pressure.
1. Overfeeding of expansion valve.
2. Leaky suction valve.
Correction
1. Regulate expansion valve, check bulb attachment.
2. Remove head, examine valve discs, replace if worn.
Low suction pressure.
1. Condition may be due to restricted liquid line and expansion valve or suction screens. Insufficient gas in system.
2. Too much oil circulating in system.
3. Improper adjustment of expansion valves.
Compressor short cycles (on high pressure cut¬out).
1. Condition may be due to
2. Insufficient water flowing through condenser.
3. clogged condenser.
4. High pressure cutout incorrectly set, low pressure adjusted incorrectly.
5. System overcharged with refrigerant.
Correction
1. Determine if water has been turned off.
2. Adjust water regulating valves.
3. Check for scaled or fouled condenser.
4. Check setting of high pressure cutout.
5. High pressure cutout may be tripping due to insufficient condenser capacity because condenser tubes are submerged.
Compressor runs continuously.
1. Shortage of refrigerant.
2. Discharge badly.
3. Valve leaks
Correction
1. Test refrigerant; if short of liquid add amount necessary.
2. Test for leaks. If leaking, remove head of compressor, and repair or replace valves.
Compressor short cycles (on low pressure cut¬out).
1. Coils in refrigerators clogged with frost.
2. Liquid, suction or expansion valve screens clogged.
3. Discharge valve leaks slightly.
4. Thermal bulb on expansion valve has lost charge.
Correction
1. Defrost coils.
2. Pump down and clean screens.
3. If leaking, remove cylinder head, examine, replace if necessary.
4. Detach thermal bulb from suction line and hold in palm of one hand, with the other hand gripping the suction line; if flooding through is observed, bulb has not lost its charge. If no flooding through is noticed, replace expansion valve.
Compressor noisy.
1. Vibration because not bolted to foundation rigidly.
2. Too much oil in circulation, causing hydraulic knock.
3. Slugging due to flooding back of refrigerant.
4. Wear of parts such as piston pins, bearings, etc.
Correction
1. Bolt down rigidly.
2. Check oil level.
3. Expansion valve open too wide, close. Thermal bulb incorrectly placed or loose, check.
4. Determine location of cause. Repair or replace compressor.
Water valve chatters.
1. Water pressure too high.
Correction
1. Reduce water pressure by throttling water stop valve.
Water runs continuously.
1. Water valve open too wide.
2. Dirt under seat of water valve.
3. Valve mechanism stuck.
Correction
1. Readjust valve to give correct head pressure, corresponding to water inlet temperature and condensing temperature.
2. Remove valve from lines, disassemble, examine and replace defective parts, clean and reassemble. If valve then does not function properly, replace.
3. Remove and disassemble. Clean valve seats, valve pins, etc.
Compressor will not start.
1. Overload tripped, fuses blown.
2. Switch out.
3. No charge of liquid in system.
4. operated by low pressure control.
5. Solenoid valves closed.
Correction
1. Reset overload, replace fuses and examine for cause of condition.
2. Throw in switch.
3. With no liquid in system there is insufficient pressure to throw in low pressure control. Recharge system with liquid; stop leaks.
4. Examine holding coil; if burned out or defective, replace.
Head gasket leaks.
1. Head bolts stretched, washers crushed.
Correction
1. Examine gaskets, replace if necessary. Tighten head bolts. Replace washers.
Cylinders and crankcase sweating.
1. Too much oil in circulation.
2. Too much refrigerant in circulation
Correction
1. Examine for conditions of refrigerant and oil charge. Correct anything found wrong.
Common faults & Symptoms in refrigeration systems:-
(1) Excessive high pressure at compressor discharge.
Air in the system. Air being non-condensable, will collect in the condenser vapor
Space and raising the delivery press. By Dalton's Law of partial press the delivery
pressure will be sum of that due to air and that of refrigerant.
(a) Overcharge of refrigerant.
(b) Insufficient cooling water to condenser.
(c) Temp, of cooling water too high.
(d) Fouling of condenser.
(e) Condenser cooling water valve inoperative.
(2) Heavy Icing up of crankcase -
(a)Liq. Refrigerant in suction line.
(b) Exp. v/v bulb wrongly placed.
(c) Exp. v/v gives too small superheat.
(d) Compr. Capacity too high during starting i.e. it sucks liquid from the evaporator.
(e) Oil is foaming in the crankcase.
(3) Excessive vacuum in Compr. Suction -
(a) Refrigerant charge too small.
(b) Restriction of refrigerant supply, expansion valve clogged.
(c)Expansion valve gives too large superheat.
(d) Wrongly adjusted capacity regulator.
(e)Filters in suction line clogged.
(f) Solenoid valve in suction line closed.
(g) Too much loading of the plant.
(4) Short Cycling-
(a) Low pressure control difference too small.
(b) Wrongly adjusted capacity regulator.
(c) Refrigerant too small.
(d) Expansion valve bulb wrongly placed.
Requirements of efficient operation of refrigeration plant:-
(1) Evaporating temp. & press should be as high as possible, such that all heat transfer surfaces are kept clean, so that temp, difference across them is at its minimum. It is essential that evaporator surface are supplied with liq. Refrigerant at the correct temp and in correct condition.
(2) Condensing temperature should be as low as possible to keep the compressor deliver) pressure to a minimum. This means correct flow of water in the condenser and to keep air out of the system as being non-condensable.- Air will raise the pressure by Dalton's law of partial pressure. Rule of thumb- A reduction in suction saturated pressure of 1°C would reduce the compressor capacity by approx. 4% and similarly an increase in condensing saturation pressure of 1°C would reduce the capacity by a further 0.9%.
(3)Suction & Delivery valves must not leak. Correct operation of unloading device should be ensured. Valve leakage means re expansion of gas from delivery to suction
Pressure. And hence reduced pumping capacity.
(4)Clearance volume should be minimum otherwise volumetric efficiency will suffer.
(5)Temp, at inlet and outlet of each side of heat exchanger must be correct to ensure correct superheat (at evaporator) and sub-cooling temps.(at condenser)
(6)Refrigeration Capacity is directly proportional to the weight of refrigerant evaporated in the evaporator i.e. directly related to the weight of vapour pumped by the compressor. This is the sum of flash gas and liquid, so to keep the weight higher, flash gas should be minimum.
(7)Weight of refrigerant vapour pumped by a compressor depends directly on the temperature and hence pressure difference between the evaporator and the condenser, and this must be kept to a minimum.
(8)Density of gas at suction is higher at higher inlet temperature, so volumetric efficiency decreases by lowering the evaporation temperature.
OPERATION AND TROUBLE DIAGNOSIS
(Water Cooled Condensing Units)
Starting the compressor. To start the compressor in conjunction with its condenser and its respective receiver and refrigerator coils, proceed to:
Start the condensing water pump, open all water valves to the condenser,
and be sure that the proper amount of water is circulating through the condenser.
Test for flow of condenser water by slightly opening the drain valve in the bottom of the condenser-heads.
At short intervals, open the air vent on the condenser heads, and release all air until the condenser tubes are full of water, which will be indicated by water squirting out of the vent valves.
Close the vent valves.
Open discharge valve (A), Fig. 12, on the compressor, and suction valve (B).
Start the compressor at the push button station on the controller.
Open main liquid line stop valve (E).
Open the proper liquid and return valves on the evaporator.
Stopping the compressor.
Close main liquid valve (E).
Run the compressor until it cuts out on the low pressure switch.
Stop the compressor by means of the controller. (Push the "Stop" button.)
Close main suction valve (B), main discharge valve (A).
OPERATION DIFFICULTIES
Faulty operation of any system is indicated by very definite symptoms. These symptoms may be caused by one or more incorrect conditions which should be eliminated in a step by step process of correction. The following chart of symp-toms, their causes and correctives, will assist in correcting faulty operation.
TROUBLE DIAGNOSIS CHART
Symptom or difficulty
Symptom or difficulty
Compressor short cycles (on high pressure cut¬out).
Condition may be due to
Insufficient water flowing through condenser; clogged condenser.
High pressure cutout in¬correctly set, low pres¬sure adjusted incor¬rectly.
System overcharged with refrigerant.
Correction
Determine if water has been turned off.
Adjust water regu¬lating valves.
Check for scaled or fouled condenser.
Check setting of high pressure cutout.
High pressure cutout may be tripping due to insufficient condenser capacity because condenser tubes are submerged.
Symptom or difficulty
Compressor runs continuously.
Condition may be due to
Shortage of refrigerant.
Discharge badly.
Valve leaks
Correction
Test refrigerant; if short of liquid add amount necessary. Test for leaks.
If leaking, remove head of com¬pressor, and repair or replace valves.
Symptom or difficulty
Compressor short cycles (on low pressure cut¬out).
Condition may be due to
Coils in refrigerators clogged with frost.
Liquid, suction or expan¬sion valve screens clogged.
Discharge valve leaks ' slightly.
Thermal bulb on expan¬sion valve has lost charge.
Correction
Defrost coils.
Pump down and clean screens.
If leaking, remove cylinder head; examine, replace if necessary.
Detach thermal bulb from suc¬tion line and hold in palm of one hand, with the other hand gripping the suction line; if flooding through is observed, bulb has not lost its charge. If no flooding through is noticed, replace expansion valve.
Symptom or difficulty
Compressor noisy.
Condition may be due to
Vibration because not bolted to foundation rigidly.
Too much oil in circula¬tion, causing hydraulic knock.
Slugging due to flooding back of refrigerant.
Wear of parts such as pis¬ton pins, bearings, etc.
Correction
Bolt down rigidly.
Check oil level.
Expansion valve open too wide, close. Thermal bulb incor¬rectly placed or loose, check.
Determine location of cause. Repair or replace compressor.
Symptom or difficulty
Water valve chat¬ters.
Condition may be due to
Water pressure too high.
Correction
Reduce water pressure by throt¬tling water stop valve.
Symptom or difficulty
Water runs con¬tinuously.
Condition may be due to
Water valve open too wide.
Dirt under seat of water valve.
Valve mechanism stuck.
Correction
Readjust valve to give correct head pressure, corresponding to water inlet temperature and condensing temperature.
Remove valve from lines, dis¬assemble, examine and replace defective parts, clean and re¬assemble. If valve then does not function properly, replace.
Remove and disassemble. Clean valve seats, valve pins, etc.
Symptom or difficulty
Compressor will not start.
Condition may he due to
Overload tripped, fuses blown.
Switch out.
No charge of liquid in sys¬tem operated by low pressure control.
Solenoid valves closed.
Correction
Reset overload, replace fuses and examine for cause of condition.
Throw in switch.
With no liquid in system there is insufficient pressure to throw in low pressure control. Re¬charge system with liquid; stop leaks.
Examine holding coil; if burned out or defective, replace.
Symptom or difficulty
Head gasket leaks.
Condition may he due to
Head bolts stretched, washers crushed.
Correction
Examine gaskets, replace if neces¬sary. Tighten head bolts. Replace washers.
Symptom or difficulty
Cylinders and crankcase sweating.
Condition may he due to
Too much oil in circulation.
Too much refrigerant in circulation
Correction
Examine for conditions of refrigerant and oil charge. Correct anything found wrong.
GENERAL INFORMATION AND PRECAUTIONS
Whenever it, is necessary to open a fully charged system for investigation or repairs, the final evacuation should be to a pressure slightly above atmospheric pressure. If the final pressure should reach a point lower than zero, sufficient refrigerant should be admitted into the evacuated part to raise the pressure to between one and two pounds gage. Connections may then be broken, and the necessary investigation or repairs made.
If more than a few minutes must elapse after breaking the connections, the open ends of the system should be plugged.
When connecting the part to the system again, make one joint first, and blow out the part under investigation with gas from the system, then quickly finish making up the other joint.
Refrigerant or oil charging lines, although of small size and short length, should be purged with refrigerant gas immediately before actual charging is started.
Never start a compressor without making sure that the shut-off valve between the compressor and the condenser is open.
When the compressor motor is first started, the operator should stand by the switch and start the motor in short intermittent spurts until it is certain that the compressor is operating properly. If possible, the compressor flywheel should be turned over several times by hand, to clear the cylinders of any oil that may have collected there during shipment or erection.
Inspect the oil level in the crankcase occasionally. The proper time to do this is immediately after the compressor has been shut down following a long period of operation.
The ideal oil level is from one half to three quarters up on the gage glass. The minimum oil level is one quarter up on the glass.
If the oil level is within the range of the glass, it will be indicated by a distinct line on the inside surface of the glass.
If no line is visible on the glass, then the level may be above or below it.
1. Describe Vapour Compression Cycle refrigeration system with a line diagram showing various pressures and temperature.12m
DESCRIPTION OF VAPOUR COMPRESSION CYCLE REFRIGERATION SYSTEM
Compressor
· During the suction stroke, as the internal pressure of the cylinder becomes lower than the suction inlet pressure, the suction valve opens to admit the refrigerant gas from the evaporator.
· The compressor receives the low pressure, low temperature superheated vapour from the evaporator.
· During the compression stroke the gas is compressed to a high pressure high temperature superheated vapour.
· Discharge valve opens when the cylinder pressure is higher than in the discharge side and the compressed gas is passed to the condenser.
· The compressor creates differential temperature which promotes heat transfer and increase in energy provides the driving force to circulate refrigerant through the system.
Condenser
· The compressed high pressure high temperature superheated vapour flows through the oil separator and discharges into the condenser where it is cooled with air or water as the cooling medium.
· The vapour in the condenser first give up its superheat and it is cooled from the discharged superheated temperature to saturation temperature corresponding to condenser pressure.
· The vapour then gives up its latent heat (Latent Heat of Condensation)and condenses back to liquid ,the condition where the liquid temp is below the condensing temp it is said to be sub cooled.
· The condition of liquid refrigerant leaving the condenser is high pressure high temperature sub cooled liquid which flows through the bottom of the receiver.
Receiver
· Receiver is a temporary storage space and a surge tank placed after the condenser in the refrigeration system which holds the reserve of liquid.
· Serves as a vapor seal to prevent vapor from entering the expansion valve
· The sub cooled liquid refrigerant is stored at high pressure in the receiver which should be capable of holding the total refrigerant charge and the level should never be more than 85% full, to allow for expansion and safety
TXV(Thermostatic expansion valve)
· Txv is a automatic device that meters the amount of refrigerant to match the evaporator load and to maintain a constant superheat of the refrigerant gas leaving the evaporator.
· The liquid refrigerant at high pressure flows from the receiver through the liquid line to the flow control valve.
· In the refrigeration system ,Expansion valve is placed ahead of the filter drier unit and before the evaporator inlet and it divides the high and low pressure sides of the refrigeration system.
Function of Expansion valve is to 1. Efficiently drop the pressure of the refrigerant by passing through a variable flow orifice, 2. Control the quantity of refrigerant flow to the evaporator to suit the rate of evaporation and maintains 3 to 5 degree of super heat temperature to the gas leaving the evaporator and entering the compressor.
Evaporator
· The liquid vapor refrigerant mixture then flows through the evaporator where it extracts heat from the refrigerated space. , and changes to a dry saturated vapour at approximately the same temperature and pressure as that at which it left the flow control.
· As it extracts heat the condition of refrigerant changes to dry saturated vapour(low pressure low temperature superheated vapour)recirculates back to the compressor and thus completes the cycle.
· The control system regulating the refrigerant flow(Expansion valve) is designed to ensure that the vapour leaving the evaporator is slightly superheated(3 to 5 degree superheat), thus ensuring that the compressor handles only dry vapour
2.) Describe with a neat sketch a drier unit as fitted to a large refrigeration system and state why is it required? 12m
FILTER DRIER
1.Gauge connection. 2. Cover. 3. Joint. 4. Withdrawing handle. 5.Spring. 6.Distance piece. 7. Division plate. 8. Felt washers. 9. Gauze plates.10.Casing.11.Inlet connection. 12. Oval flange.13.Charging connection. 14. Center bolt. 15. Drying agent. 16.Retaining sleeve. 17.Outlet connection.
FILTER DRIER
· In a refrigeration system it is essential to reduce the water content of refrigerant to a minimum by careful drying of components and by fitting filter drier units with drying agents in the system.
· The main purpose of filter drier unit is to remove the moisture from the refrigerant circulating in the system
· The filter drier units are placed in the liquid line after the receiver and ahead of the expansion valve
CONSTRUCTION:
· Filter drier consists of cylindrical steel casing one end fitted with flanged inlet connection for liquid refrigerant to enter and other end fitted with cover secured with bolts and nuts.
· Between the cylindrical body and the cover a joint is placed to seal the surface to prevent any leakage of refrigerant. A gauge connection is screwed on the top of the cover to fit the pressure gauge.
· On the cylindrical section of the casing ,acharging connection and a flanged outlet connection are fitted.
· The retaining sleeve cylindrical in construction and smaller in diameter than the outer casing can be easily lowered and retrieved easily.
· Both the sides of the retaining sleeve are enclosed by pair of gauze plates and felt washers so that drying agents are filled in this enclosed space.
· The centre bolt passes through both the gauze plates, holds the sleeve and the gauze plates together.
· One end of the centre bolt has a nut for securing and other end has a spring fitted, to have a control over tension of the Centre bolt while tightening. A handle is fitted to lift or lower the complete sleeve from the casing.
DESCRIPTION
· The filter can be either a compacted solid cartridge or bags of desiccant.
· The desiccant is a moisture absorbing substance which will eventually become saturated with moisture and have to be replaced.In rechargeable units, the
desiccant can be removed and the drier is refitted with a fresh charge.
· The common form of filter drier is a solid cartridge or capsule charged with a solid desiccant such as silica gel, activated alumina or zeolite (molecular sieve).
· The capsules must have strainers to prevent loss of the drying agent into the circuit, and therefore forms an effective strainer-drier to protect the valve orifice from damage by the fine debris.
REQUIREMENT OF FILTER DRIER IN REFRIGERATION SYSTEM
· The main requirement of filter drier in refrigeration is to remove moisture and to prevent other contaminants entering and blocking the expansion valve or maydeposit in the compressor, mechanical seal faces leading to damage and leakage.
· Moisture in the System comes with the ingress of air in the system.
· Moisture may freeze at theexpansion valve, giving some of the indication of under charging.
· Moisture will contribute tothe corrosion in the system and may cause lubrication problems and breakdown of thelubricating oil in the reciprocating compressor.
· Moisture can freeze to ice in the evaporator and cause blockage inside the evaporator coil
· Moisture can form acids by reaction with the freon refrigerants. This acid attacks the copper lines and deposits in the other parts of the system.
· Thus moisture causes adverse troublesome effects when it is deposited on the compressor mechanical seal faces leading to damage and leakage and the fine particles could possibly block the expansion valve orifice plate
3.Sketch a thermostatic expansion control valve as fitted in a refrigeration system, label all the parts and state the location in the refrigeration system.(12m)
TXV(THERMOSTATIC EXPANSION VALVE)
· TXVis a automatic device that meters the amount of refrigerant to match the evaporator load and to maintain a constant superheat of the refrigerant gas leaving the evaporator.
· The liquid refrigerant at high pressure flows from the receiver through the liquid line to the flow control valve.
· In the refrigeration system Expansion valve is placed ahead of the filter drier unit and before the evaporator inlet and it divides the high and low pressure sides of the refrigeration system.
FUNCTION OF EXPANSION VALVE:
1. Efficiently drop the pressure of the refrigerant by passing through a variable flow orifice
2. Control the quantity of refrigerant flow to the evaporator to suit the rate of evaporation and maintains 3 to 5 degree of super heat temperature to the gas leaving the evaporator and entering the compressor.
COMPONENTS
· Sensing bulb
· Capillary tube
· Membrane plate or diaphragm
· Equalization line (balance line)
· Inlet and outlet connection
· Valve stem
· Superheat spring
CONSTRUCTION
· The expansion valve consists of a gun metal housing with inlet and outlet connections both ends threaded
· A sensing bulb containing liquid refrigerant (same type as used in refrigeration system) mounted on the outlet pipe of the evaporator is connected to the top of the valve housing by a capillary tube
· The top part of the valve housing has a membrane plate(diaphragm) secured by the bellows
· The bellows is fixed to the push pins which moves the sleeve upwards or downwards depending upon the bulb pressure
· The sleeve retains the spring and the spring tension is adjusted by the adjusting spindle to maintain the super heat
· A valve disc is placed on the top of the sleeve regulates the refrigerant flow to the outlet connection
· Balance line or equalisation line from the evaporator outlet is connected to the bottom of the bellow to sense the evaporator pressure where a significant pressure drop exists across the evaporator
EXPANSION VALVE WORKING PROCEDURE
· The membrane plate which is subjected to different pressures (P1,P2,P3) will determine the opening or closing of the Expansion valve
Ø P1 – pressure in the sensing bulb
Ø P2- pressure in the evaporator
Ø P3 – pressure of the spring
The operation of expansion valve is subjected to three conditions
(1).Balanced Forces
(2).Increase In Super Heat
(3).Decrease In Super Heat
(1)BALANCED FORCES CONDITION
· In this condition ,the valve will assume a stable control position when the pressures (P1,P2,P3) are balanced
· The balance in pressures is given by,
P1=P2+P3
(2)INCREASE IN SUPER HEAT CONDITION
· In this condition ,the temperature of the refrigerant gas at the evaporator outlet (sensing bulb location)increases the saturation temperature corresponding to the evaporator pressure as it becomes superheated
· The pressure in the sensing bulb (due to higher temperature)increases above the combined pressures of evaporator and spring (P2 +P3) such that P1 is greater than (P2+P3)
· The pressure P1 acting on the top of the membrane plate moves the bellow downwards in which the valve stem will move in an opening direction and supply refrigerant to the evaporator
(3)DECREASE IN SUPER HEAT CONDITION
· In this condition ,the temperature of the refrigerant gas leaving the evaporator decreases and the corresponding pressure(P1) in the sensing bulb also decreases such that (P1) is less than (P2+P3)
· Since the bulb pressure P1 is less than the combined pressures of the evaporator and spring pressure (P2+P3),the bellow moves upwards such that the valve stem will move towards closing and gives restriction of the refrigerant flow to the evaporator
4. Sketch and Describe high pressure cut out fitted in a refrigeration system.On what condition the cut out activates and what action being taken ? (10m)
· HP cut out switch is a compressor safety device fitted in the refrigeration system
· HP cut out switch is fitted to the compressor outlet before the isolating valve in the system
THE FUNCTION OF HP CUT OUT SWITCH :
1.To cut out the compressor when the discharge pressure increases more than the desired working pressure
2.To prevent overloading of the compressor by avoiding damage to the components
COMPONENTS
· Bellows
· Pivot
· Main spring
· Switch arm
· Step
· Electrical contacts
· Switch arm catch
· Spring
DESCRIPTION OF HIGH PRESSURE CUT OUT SWITCH:
· The compressors are limited to maximum safe operating pressures and therefore have to be stopped before high pressures are reached
· HP cut out switch is set to stop the compressor motor at a pressure of about 90% of the maximum working pressure of the system
· The compressor outlet pressure (discharge pressure)is brought to one side of the bellows or diaphragm and is balanced by an adjustable spring on the other side
· The cut out point needs to be some 2 bar higher than the expected summer operating pressure
· A scale on the control indicates the pressure setting to commercial accuracy and is checked on commissioning the system.
· If the spring pressure is overcome the switch will open , breaking the electrical contact and stop the compressor
· Normally open contacts on the cut out then operates a warning alarm
· Excess pressure indicates any malfunction of part of the system – it may be condenser fault or incorrect closure of a valve
· One high-pressure switch may beset at a warning level and operate an alarm, without stopping the compressor.
· A second switch, set somewhat higher, will stop the equipment if this warning is ignored and if excessive pressures are reached.
· After the compressor is cut out ,the HP cut out switch should be reset manually and not automatically
WORKING OF HIGH PRESSURE CUT OUT SWITCH:
· The bellows connected by small bore pipe between compressor and condenser.
· Excess pressure expands the bellow and moves the switch arm catch around its pivot.
· The upper end slips to the right of the step and releases the switch arm so breaking electrical contact causing the compressor to cut out.
5. a)Explain the term “Superheat” with reference to the thermostatic expansion valve and “under-cool” in the condenser.
SUPERHEAT
· Superheat is referred to as the condition of refrigerant gas at outlet of the evaporator
· The degree of superheat is set by adjusting the spring force of the Thermostatic expansion valve.This setting is done at factory and not to be disturbed.
· The sensing bulb connected to the top of the expansion valve sensesthe gas temperature at evaporator outlet and controls the quantity of the refrigerant flow to maintain the Degree of superheat.
· To ensure no liquid passes through to the compressor, the expansion valve is set
so that the refrigerant gas at outlet from the evaporator has 3 to 5 degrees of superheat so that the compressor handles only dry vapour.
UNDERCOOL
· Therefrigerant gas at high pressure , high temperature super heated vapour is fed from the discharge side of the compressor to the “Condenser” .
· The vapour in the condenser first give up its superheat and it is cooled from the discharged superheated temperature to saturation temperature corresponding to condenser pressure.
· The vapour then gives up its latent heat (Latent Heat of Condensation)and condenses back to liquid
· The condition where the liquid temperature is below the condensing temperature it is said to be sub cooled.
· If the liquid temperature is sub cooled , flash of gas quantity will be reducedand refrigerating effect will be increased.
6. Explain the functions of the following with reference to the refrigeration system:-
a)Solenoid valve. b) High-pressure safety cut out. c) Oil pressure safety cut out.
a)SOLENOID VALVE
· The solenoid valve is a servo-controlled electromagnetic valve( electrically activated valve) that can either be open or closed, which providesautomatic opening and closing of liquid lines.
· Solenoid valve is fixed between the filter drier and expansion valve in the system and it is controlled by thermostatic switch.
· Thermostat is set to the desired temperature and given a 3 to 4 degree differential to prevent short cycling
· When the room temperature reaches the preset high level ,the thermostat
Switch energises the solenoid valve allowing liquid refrigerant to pass through the Expansion valve(Vice versa when the coil is de-energised)
· The malfunction of solenoid valve can occur due to burnt-out coil, a damaged diaphragm, and blockage by dirt particles.
b)HIGH PRESSURE SAFETY CUT OUT SWITCH:
· HP cut out switch is a compressor safety device fitted in the refrigeration system
· HP cut out switch is fitted to the compressor outlet before the isolating valve in the system
The function of HP cut out switch :
· To cut out the compressor when the discharge pressure becomes high more than the desired working pressure
· To prevent overloading of the compressor by avoiding damage to the components
· The control is set to stop the compressor motor at a pressure of about 90% of the
maximum working pressure of the system.
C) OIL PRESSURE SAFETY CUT OUT
· Oil pressure safety cut out is used to protect against too low oil pressure in forced lubrication system.
· It is a differential control using two bellows, one side represents low side pressure and the other responds to the oil pressure.
· The oil pressure must always be greater than the low side pressure for the oil to flow.
· If the oil pressure falls below a minimum value the control stops the compressor after a certain time has lapsed.
· The manual reset of oil safety cut out switch is required to put back the compressor to normal mode.
7.Describe the procedure of charging gas to a large refrigeration system after complete overhaul of the compressor, assuming the condenser/receiver is empty.
i. On a new Freon Installation.
ii. Before charging, the plant is pressure tested for any leaks with nitrogen and rectified if any with pressure of 200 lb/sq.in.
iii. Then the system is evacuated and kept under vacuum for several hours -this removes air in the system and causes any mixture to be evaporated and removed.
iv. The plant is allowed to stand in this condition for several hours. If no raise of pressureoccurs then the system can be changed.(if it is large unit using Brine as a secondary refrigerant ,then it should be charged dry without Brine)
v. The Refrigerant cylinder is in upright position is connected by a charging pipe and adapator (goose neck) to the charging valve. But before tightening the nut at the charging valve the bottle valve is crackopened so that the refrigerant will blow air out of the charging line.
vi. The gas cylinder should then be weighed so that an exact amount of refrigerant is charged into the system.
vii. The charging valve in the system and the cylinder valves are now opened and the gas is allowed to pass into evaporator.
viii. With the condenser water circulating, the compressor is now run in order to pump the gas from the evaporator to the condenser.
ix. When the required level seen (Three fourth gauge glass when the comp not running) the cylinder valve is closed and the compressor is allowed to stop.
x. Close the system charging valve
xi. Disconnect the cylinder and note the weight of the cylinder.
9. How will you detect the gas leak in the closed system of a refrigerated compressor using F-12,explain at least three methods.
METHODS FOR TESTING FOR LEAKS
(1)USE OF SOAP SUDS
· To prepare soap suds for testing, use a soap and water solution of about the consistency of liquid hand soap, which will lather freely, or work up a lather on the brush by rubbing the wetted brush on a cake of soap.
· A few drops of glycerine added to the solution will cause the lather to remain wet longer.
· When applying the soap suds, paint the soap lather on the joint all the way around, and examine the joint thoroughly for bubbles.
· When the joint is located so that a part of it is not visible, use a pocket mirror. It will sometimes take a full minute or more for bubbles to appear at a small leak.
· Questionable spots should be covered with lather and examined again.
(2)USE OF THE HALIDE TORCH
· Small Freon-12 leaks are detected by a special designed torch known as a "Halide Torch."
· Atmosphere suspected of containing Freon-12 gas is drawn through an exploring hose into the burner by an injector action.
· The air sample passes over a copper reactor plate in the burner chamber, which is heated to incandescence by the flame.
· If there is even a minute trace of Freon-12 present, the torch flame will turn from its normal blue or neutral colour to a characteristic green colour as it comes in contact with the reactor plate.
· The shade of green will depend upon the relative amount of Freon-12 present, being paler for small concentrations, and darker for heavier concentrations. Excessive quantities of Freon-12 will colour the flame a vivid purple, and may even extinguish it by crowding out the supply of oxygen in the air.
· A number of Halide Torches are available, most of which use acetylene gas or alcohol as a fuel.
(3)PRESSURE TESTING, PURGING AND DEHYDRATION
PRESSURE TESTING
· The system should be tested at a pressure of 10 bar, for the purpose of locating leaks. (All parts have been previously shop tested to confirm with standard specifications).
PURGING
· Complete all piping connections, and open all liquid, discharge, and suction shut-off valves.
· Connect a Freon-12 service cylinder to the charging connection in the liquid line, elevate the rear end of the Freon-12 drum so as to be sure that the outlet is below the liquid level, and slightly open the valve on the Freon-12 service cylinder, introducing enough refrigerant to raise the pressure in the system to 1bar to2 bar.
· Then close the valve and disconnect the refrigerant drum and test the system for leaks at this pressure with a Halide Torch.
· Connect a drum of dry nitrogen to the valve at the charging connection, and introduce enough nitrogen gas to raise the pressure in the system to the required test pressure.
· Test the entire system for leaks with a Halide Torch.The amount of Freon-12 refrigerant in the system will act as an indicator.
· When it is found that the system is perfectly tight, release the pressure on the system by opening the "purge" valve.
DEHYDRATION
· Using an approved vacuum pump (preferred).
· Connect the suction side of the vacuum pump to the liquid charging valve , allowing the pump to discharge to the atmosphere.
· Open all valves on the system to be evacuated.
· Run the vacuum pump until the lowest vacuum it will produce is obtained, then stop the pump, and close the liquid charging valve.
10. Describe the procedure of charging gas to a large refrigeration system, when condenser gas level is lower than normal.
TOPPING UP OF REFRIGERANT:
1. Connect the Freon gas cylinder and gas charging valve with a special flexible hose and purge the air from the hose. (If liquid is to be charged connect after the condenser and if gas has to be charged connect to the suction side of the compressor)
2. Open the cylinder valve and gas charging valve and run the compressor.
3. Allow the liquid / gas to flow in to the system. Check the liquid level in the condenser whether it has reached to the normal level, then close
freon gas bottle valve and gas charging valve.
4.Release the charging hose of pressure and remove from place.
PRECAUTIONS:
1. Liquid refrigerant must never be charged directly into the compressor suction.
2. Properly tested charging hoses are used, and that non return valves are fitted when
charging into the high pressure side of the system.
11. Write short notes on maintenance of the following refrigeration system equipments:- a)Condensers. b) Oil separators c)Filters and driers.4m each.
IN GENERAL ,THE FOLLOWING WORK SHOULD BE DONE AT REGULAR INTERVALS:
a) MAINTENANCE OF SHELL AND TUBE CONDENSER:
1. To prevent fouling of water tubes with scale or marine growth,this reduces the heat
transfer capacity of the condenser,the tubes should be cleaned by ‘tube brush’.
Alternatively, a ready mixed inhibitive scouring acid can be used, with subsequent
neutralization. Such agents must be suitable for use with the tube materials, and
Applied in strict accordance with the chemical manufacturer’s instructions and
environmental protection.
2. The end cover should be inspected for wastage, caused by erosion and corrosion
especially the division plate. Zinc anodes to be renewed.
3. The tube ends on both sides of the end plate to be leak tested using soap solution
or any leak detector.
4.Finned coils, especially outdoor condensercoils, if fitted must be cleaned frequently.
5.The water side of heat exchangercoils should be cleaned of any accumulations of scale or algae assoon as a change in working conditions shows that they are gettingdirty.
b)MAINTENANCE OF OIL SEPARATOR:-
1. Gas from the system to be pumped down to the condenser
2. Circuit breaker of the compressor switched off and sign board “men at work” to be placed.
3. Close the compressor discharge valve and release the pressure by slackening the drain plugof the oil separator.
4. Open the float valve and take out the float out of the chamber carefully and clean internal thoroughly with a clean rag and blow dry air such that the partition mesh is cleaned.
5. Overhaul the oil return needle valve and fit back the float as original with new gaskets
and seal rings.
6. After tightening all the fittings, open the compressor discharge valveone turn and close back to pressurize the oil separator for leak test.
7. Once leak test issatisfactory, the suction and discharge valves of the compressor are fully opened andthe system set back to normal.
c)MAINTENANCE OF FILTER AND DRIER:-
FILTERS IN REFRIGERATION CIRCUIT
1. The filters are removed from location by shutting the inlet and outlet valves of the equipment.
2. The filters are cleaned using electro cleaner and later flushed with low pressure air.
3. The filters are fitted back to its original location by purging the air by opening the valves.
FILTERS IN AIR CIRCULATION SYSTEM.
1. Accumulated dirt on air filters will increase the resistance and lead to reduced air flow.
2. Under no circumstances the fans should run without filters in place, or dirt will be deposited in inaccessible parts of the plant.
3. The provision of a manometer across the filter to indicate the pressure drop will give a positive indication of the need to clean or replace the filter.
12. Explain the causes and rectification of the following defects, when observed during the running of the refrigeration compressor: (4m) each.
a) Too high delivery pressure.
b) Too high suction pressure.
c) Compressor starting and stopping too often on low-pressure switch.
a)TOO HIGH DELIVERY PRESSURE:
CAUSES
1. Air or non-condensable gasin the system.
2. The cooling water is toowarm or an insufficient
quantity is passing through thecondenser.
3. Temp, of cooling water is too high.
4. Fouling of condenser.
5. Condenser cooling water valve inoperative.
6. Mud or scales block thecondenser tubes.
7. Too much liquid R12 orR22 in the receiver and someliquid in the condenser,(overcharge of refrigerant).
RECTIFICATION
1. Purge the foul gas from the condenser.
2. Inspect and clean the water valve and water filter, and make sure that the water valve is opened wide enough.
3. Clean the condenser tubes.
4. Drain off the excess R12 or R22 in an empty cylinder.
b)TOO HIGH SUCTION PRESSURE.
CAUSES
1. Too much liquid refrigerants being fed through the expansion valve.
2. Leaky suction valves.
RECTIFICATION
1. Adjust the expansion valves and check the sensing bulb.
2. Regulate expansion valve, check bulb attachment.
3. Remove head, examine valve discs, replace if worn/broken .
4. Remove the cylinder covers, check all the valves, repair or renew and test.
c)COMPRESSOR STARTING AND STOPPING TOO OFTEN ON LOW-
PRESSURE SWITCH.
CAUSES
1. Evaporator Coils clogged with frost.
2. Liquid, suction or expansion valve filters clogged.
3. Sensing bulb on expansion valve has lost charge.
4. The delivery valves of the compressor leaking.
5. leaky solenoid Valve .
6. Expansion valve chocked with ice.
RECTIFICATION
1. Defrost coils.
2. Pump down and clean filters.
3. If leaking, remove cylinder head, examine, replace if necessary.
4. Detach thermal bulb from suction line and hold in palm of one hand, with the other hand gripping the suction line; if flooding through is observed, bulb has not lost its charge. If no flooding through is noticed, replace expansion valve.
14.Explain the causes and rectification of the following defects, observed during the running of the refrigeration compressor:-
a) Too low suction pressure.
b) Too low delivery pressure.
c) Compressor stopping too often on high-pressure switch.
a) TOO LOW SUCTION PRESSURE
CAUSES
1. Condition may be due to restricted liquid line and expansionvalve or suction screens.Insufficient gas in system.
2. Too much oil circulating in system.
3. Improper adjustment of expansion valves.
4. Blockage in liquid pipe, expansion valve or suction filters.
RECTIFICATION
1.Evacuate, remove , inspect and clean filters.
2. Charge with more R12 or R22.
3. Inspect to see whether oil has accumulated anywhere in the system.
4. Set the expansion valves to feed more refrigerant.
B)TOO LOW DELIVERY PRESSURE:
CAUSES
1. Too much cooling water to the condenser.
2. Cooling water is too cold.
3.Liquid R12 or R22 is coming back from the evaporator.
4. Leaky delivery valve.
RECTIFICATION
1. Regulate the water supply and adjust Reduce the water supply.
2. Adjust the expansion valve and check that the expansion valve bulb in contactwith evaporator pipe and fastened correctly.
4. Remove the cylinder cover, inspect the valve plates andpiston rings; renew them if necessary
c)COMPRESSOR STOPPING TOO OFTEN ON HIGH-PRESSURE SWITCH
CAUSES
Condition may be due to
1. Insufficient water flowing through condenser.
2. clogged condenser.
3. High pressure cutout incorrectly set, adjusted incorrectly.
4. System overcharged with refrigerant.
RECTIFICATION
1. Determine if water has been turned off.
2. Adjust water regulating valves.
3. Check for scaled or fouled condenser and clean the condenser.
4. Check setting of high pressure cutout.
5. High pressure cutout may be tripping due to insufficient condenser capacity because condenser tubes are submerged.
15. State how each of the following faults identified and rectified for a vapor compression refrigeration machine. 3m each
d) Air in the system.
e) Moisture in the system.
f) Undercharge.
g) Overcharged.
a)AIR IN THE SYSTEM
INDICATION:
1. This may cause the refrigeration compressor to overheat, with a high discharge pressure and normal condensing temperature.
2. There are possibilities of small air bubbles in the liquid sight glass of the condenser.
3. Condensing pressure of the refrigerant in the condenser will be high.
4. If there is excessive air, it may reduce the cooling capacity of the system, making the compressor to run for the extended period of time.
5. It may cause the gauge pointer of the condenser to jump indefinitely.
CAUSES:
1. During charging, air may enter into the system.
2. If Freon-12 is used air may leak into the suction line because the working pressure of the Freon-12 refrigerant is less than the atmospheric pressure.
3. During standby mode air may enter into the system
4. Air enters through the system valve glands during pumping down operation
ACTION:
1. Air in the system can be removed by collecting the system gas in the condenser, leaving the condenser cooling water on and venting out the air from the top of the condenser because air will not be condensed in the condenser but remains on top of the condenser above the liquid refrigerant.
Connect the collecting cylinder to the purging line of the condenser, open the valve, and collect air in the cylinder.
2. After purging the air from the system don’t forget to shut the purging valve.
3. Check the level of the refrigerant in the system. If required, charge the system with fresh refrigerant.
4. Restart the compressor with all safety precautions.
b)MOISTURE IN THE SYSTEM
INDICATION
The presence of moisture is indicated in the sight glass fitted in the liquid line
1. Moisture in the System comes with the ingress of air in the system.
CAUSES
1. Moisture will tend to develop inter crystalline embrittlement of brass or copper bellows, such as used in the high pressure control switch; and leads to corrosion on compressor valves, pistons, shaft, etc
2. Moisture may freeze at theexpansion valve, giving some of the indication of under charging. Moisture will contribute tothe corrosion in the system and may cause lubrication problems and breakdown of thelubricating oil in the reciprocating compressor.
3. Moisture can freeze to ice in the evaporator and cause blockage inside the evaporator coil.
4. Moisture can form acids by reaction with the freon refrigerants. This acid attacks the copper lines and deposits in the other parts of the system.
5. Thus moisture causes adverse troublesome effects when it is deposited on the compressor mechanical seal faces leading to damage and leakage and the fine particles could possibly block the expansion valve orifice plate
ACTION
1.In a refrigeration system it is essential to reduce the moisture content of refrigerant to a minimum by careful drying of components and by fitting filter drier units with drying agents in the system.
2.The operation of filter drier in refrigeration is to remove moisture and to prevent other contaminants entering and blocking the expansion valve or maydeposit in the compressor, mechanical seal faces leading to damage and leakage
3. The system is installed with a drier to absorb any moisture and also filter fine particles of debris in the gas using silica gel as an agent. If moisture found on the higher side even after renewing silica gel a few times, then the gas has to be renewed.
C) UNDERCHARGE OF REFRIGERATION SYSTEM
Under charging means either the system is charged with less gas or gas leaked out from the system. The system runs with less amount of gas in circulation.
INDICATION:
1. Compressor is running hot and performance of the compressor falls off due to high superheat temperature at the suction side of compressor.
2. Suction and discharge pressure of the compressor is low.
3. Large vapor bubbles in the liquid sight glass.
4. Low gauge readings in the condenser.
5. Ammeter reading for the compressor motor is lower than normal.
6. Rise in room temperature which is to be cooled.
7. Compressor is running for extended period of time.
CAUSES:Leakage of refrigerant at the
1.Shaftseal
2. Flangecouplings and joints
3.Valveglands etc.
4.Leakage of gas through the condenser tubes due to corrosion.
ACTION:
1. Identify and rectify the leakage of refrigerant from the system.
2. Clean the filter and drier.
3. Charge the system with fresh refrigerant if required.
d)OVER - CHARGE OF REFRIGERATION SYSTEM
INDICATION:
1. The liquid level in the condenser is too high (high condenser gauge reading). This will reduce the available condensing surface, with corresponding increase in the saturation temperature and pressure.
2. High pressure switch of the refrigerant compressor activates and stops the compressor.
3. There is possibility of excessive liquid refrigerant getting to the evaporating, giving icing at the compressor suction, and a pressure drop across the expansion valve.
4. The cold room temperature may rise, if the evaporator is flooded.
CAUSES:
1. It may be due to the reason that excessive refrigerant has been charged in the system.
2. High delivery pressure may also be due to air/incondensable gases presentin the system.
3. It may also be due to the formation orifice on the regulator.
ACTION:
1. Remove the refrigerant from the system. This is done by connecting a cylinder to the liquid line charging valve, starting the compressor, and then operating the charging valve.
2. Purge the air from the system and maintain effective cooling. While purging there is always some gas will pass along with air hence the regulation says the purging has to be collected in recycling cylinders and same has to be recorded.
3. Remove ice from the regulator by using any of the defrosting methods.
17.What are the readings taken and recorded of a domestic refrigeration system during watch keeping on board a ship?(4m)
WATCH KEEPING DUTIES.
The following are the readings taken for a refrigeration system and recorded in the
Engine log book every four hours of watch keeping.
ITEM CHECK PROCEDURES
1.COMPRESSOR
· Check Suction(pressure,temperature), discharge(pressure,temperature),and compressor oil level.
· Check for abnormal noise , vibration and the condition of drive belt
· Lube oil pressure and foam formation in crankcase .
· Temperature of lube oil in crankcase.
2. COMPRESSORS, FANS, PUMPS…….Check no failure alarms in alarm condition.
3. DOMESTIC REFRIGERANT CHAMBERS.…………..Check temperatures,and condition of evaporator coil for frosting.
4. CONDENSER AND PIPING………….
· Check water pressure gauge that water flows satisfactorily
· Check refrigerant liquid level, condenser cooling water temperature(inlet and outlet)condenser cooling water temperature inlet and outlet difference should be atleast 8 to 10 degrees
· Examine for leaks.
5. SIGHT GLASS IN REFRIGERANT LIQUID LINE
· Check that full flow of liquidand the refrigerant level in the receiver.
· Check that only liquid, and not a liquid/gas mixture is going to the expansion valves.
· Check the water indicator (coloured ring in contact with the liquid), when water is detected it changes colour( typically from pink to blue).
6.REFRIGERATION BLACKBOARD……..Check any special instructions from Chief Engineer
The general condition to be checked for any leakage and any trace of oil (first indication for any refrigerant leakage.)
The completion of the refrigerator Logbook should be the last task of a watch keeper
before going off duty, to ensure that he is leaving all temperatures in order.
18.Explain the following with reference to the refrigeration compressor:-
a) Sketch a mechanical shaft seal, method of lubrication and name its parts.8m
b) How is gas leakage detected through shaft seal and what is its effect on environment?2m
c) What factors can cause the shaft seal failure in service?2m
a) MECHANICAL SHAFT SEAL
· The compressors having external drive require a shaft seal or gland where shaft passes out of the crankcase (open drive compressors) are either belt driven or directly coupled to the shaft of the electric motor or other prime mover
· The main function of the mechanical seal:
1.To prevent gas leakage through the shaft from the system
2.To prevent air entry through the shaft into the system
SKETCH
COMPONENTS
· Neoprene ring
· Rotating metal ring
· Fixed carbon ring
· Spring
· Bellows
· Seal mounting plate
DESCRIPTION
· The usual form of shaft seal for open drive compressors comprises of a fixed carbon ring in contact with a highly polished metal facing ring
· The carbon ring is spring loaded to maintain contact under all working crankcase pressures and prevents gas leakage
· The metal facing ring firmly rotates along with the shaft which compresses a spring loaded bellow against the collar to form gas tight seal
· Neoprene seal firmly (holding the carbon ring) is fitted circumferentially over the shaft to prevents any gas leakage past over the shaft
· Where the shaft comes out of the crankcase ,there exists a small clearance which is completely covered by a spring loaded bellow
· The complete shaft seal is firmly secured with a seal mounting plate and is bolted to the end cover of the crankcase with proper gasket placed in between to prevent gas leakage
· Lubricating oil under pressure is fed to the shaft seal arrangement to remove heat(rubbing surfaces) and to reduce friction (contact surfaces)
· Since the starting torque of the compressor is too high ,the shaft moves forward orbackward from its original position is called the ‘END CLEARANCE’
· This torque is balanced by the seal spring force to keep in equilibrium position
b) How is gas leakage detected through shaft seal and what is its effect on environment?
· The gas leakage can be deducted by carrying out one of the following methods- Electronic leak detectors, Leak detector torch(Halide torch), Sulphur candles, Litmus paper and soap solution.
· If gas leaks thro shaft seal there will be traces of oil around seal area.
· The leakage of refrigerant gas such as Carbon Dioxide (CO2),methane (CH4),nitrous oxide creates a green house effect by trapping heat in low atmosphere leading to global warming
· Global warming accelerates ozone depletion
c) What factors can cause the shaft seal failure in service?
(1)MOISTURE
· Moisture cause to form acids by reaction with the freon refrigerants.
· This acid attacks the copper in the lines and deposits in other parts of the system.
· This can become particularly troublesome when it is deposited on the compressor mechanical seal faces leading to damage and leakage.
(2)METAL PARTICLES
Metal particles which is generated during operataion of the compressor by friction between componets , due to corrosion in the condenser/receiver,loosly packed dissicant in drier units . If these particles are not filtered effectively in the gas path at various places like,filterdrier,exp valve filter and compressor suction mixes with lubricating oil in the crank case compressor. When these comes in contact with mechanical seal causes damage to the sealing faces leading to leakage of gas.
(3)DRIVING BELT ALIGNMENT
· The belt is to be checked for tension and alignment of motor to compressor at regular interval. If the belt tension is high or misalignment, it can cause breakage of the shaft seal.
20. a)Explain the properties of a refrigeration system lubricating oil.(4m)
b)Sketch and describe reciprocating compressor lubricating oil system.
a)REFRIGERATION SYSTEM LUBRICATING OIL SHOULD POSSESS THE FOLLOWING PROPERTIES:
1)GOOD CHEMICAL STABILITY :There should be no chemical reaction with the refrigerant or materials in the system.
2)GOOD THERMAL STABILITY :They should not form hard carbon deposits at hot spots in the compressor(In valves and discharge ports)
3)LOW VISCOSITY: This is the ability of an oil to maintain good lubrication properties at high temperature and good fluidity at low temperature i.e. to provide a good lubricating film at all times.
4)LOW WAX CONTENT: Particularly important in the case of CFC and HCFC plants, operating at low evaporating temperature, as separation of wax particles from the refrigerant -oil mixture may cause problem by blocking expansion and regulating valves.
5)LOW POUR POINT AND FLOW POINT:Ability of the lubricating oil to remain in a fluid state at the plant’s lowest evaporating temperature.Lubricating oils with a low pour point are easier to
circulate through the refrigerating plant’s low pressure side.
6)MOISTURE FREE AND ANTI- FOAM CHARACTERISTICS:Any moisture added with lubricating oil may cause corrosion or would form ice and choke the expansion valve. The lubricating oil containing a certain amount of dissolved refrigerant will be diluted during the idling of the compressor and causes oil foaming
b)RECIPROCATING COMPRESSOR LUBRICATING OIL SYSTEM:
DESCRIPTION
· In a reciprocating compressor lubricating oil system, a gear pump or a lobe type of pump in which the inner rotor having 3 lobes is rotated inside an outer rotor having 4 lobes, the pockets of increasing and decreasing area ,thus giving a pumping action.
· The pump which is driven by the crank shaft takes oil suction from the crankcase through a suctionstrainer(filter).
· The discharged oil from the pumppasses through a fine filter and is branched to the capacity control mechanism(unloader) and another line which lubricates main bearings, bottom end bearings and the gudgeon pin bush through drilled passage of crankshaft and connecting rod.
· The oil also lubricates the shaft seal(mechanical seal) and then returns back to the crankcase through an orifice to maintain the line pressure.
· A pressure gauge is fitted to the discharge side of the pump for checking the
lubricating oil pressure and sight glass is fitted in the crank case for checking the crankcaselub oil level.
· In case of excessive high back pressure in the line, there is a pressure reliefvalve fitted on the discharge side of the pump which returns the oil back to the crankcase.
· To protect against too low oil pressure , the lubricating oil line is connected to a differential oil pressure switch which cuts off the compressor in the event of too low oil pressure.
21.Explain briefly how does air enters into the closed system of refrigeration compressor?(2m)
AIR ENTERS THE SYSTEM :
1. During charging of refrigerant and lubricating oil and after renewal of any components like solenoid valve, exp valve, drier air may enter into the system.
2. If Freon-12 is used air may leak into the suction line because the working pressure of the Freon-12 refrigerant is less than the atmospheric pressure.
3. During standby mode air may enter into the system
4. Through the system valve glands and leaky compressor seal during pumping down operation
The air oxidizes the lubricant oil and chemically reacts with the additives forming sludge in the presence of moisture affecting theviscosity.
Air in the system will cause the system delivery pressure to increase
22.What action will you take if the moisture could not be removed from the gas by renewing ‘silica gel’, due to contamination with excess moisture?(4m)
THE FOLLOWING TO BE CARRIED OUT IN ORDER TO RENEW CONTAMINATED REFRIGERANT IN THE CONDENSER:-
1. The pumping down of the gas to be carried out to collect the complete system gas to the condenser and later close the condenser outlet valve.
2. Switch of the circuit breaker.
3. The empty freon gas cylinders must be cooled in a fish room to bring the temperature approx. -18 deg.C.
4. The cooled cylinders are connected to the charging valve between condenser and filter drier unit.
5. Open the empty cylinder valve and condenser outlet valve, the liquefied gas from the condenser is transferred to the bottle due to the differential pressure.
6. Renew bottles to collect all the gas present since single bottle would not be enough to collect the gas.
7.The reason for more gas bottles is that temperature of bottles rise during collection
process equalizing the pressure of the condenser and bottle.
8. Renew silica gel on completion of removing the entire contaminated refrigerant.
9. Charge fresh Freon gas to the system to normal condenser level.
10. It may be required to renew silica gel couple of times even after renewing the
refrigerant, due to left over moisture adhering to the walls of condenser.
29. Explain the functions with a neat sketch the following with reference to the refrigeration system:-
b) Thermostatic expansion valve. e) Compressor shaft seal.
a)THERMOSTATIC EXPANSION VALVE
· TXV is a automatic device that meters the amount of refrigerant to match the evaporator load and to maintain a constant superheat of the refrigerant gas leaving the evaporator.
· In the refrigeration system ,Expansion valve is placed ahead of the filter drier unit and before the evaporator inlet and it divides the high and low pressure sides of the refrigeration system.
FUNCTION OF EXPANSION VALVE is to
1. Efficiently drop the pressure of the refrigerant by passing through a variable flow orifice,
2. The pressure of the liquid is reduced to the evaporating pressure so that the temperature of the refrigerant entering the evaporator is below that required in the refrigerated space.
3. Control the quantity of refrigerant flow to the evaporator to suit the rate of evaporation and maintains 3 to 5 degree of super heat temperature to the gas leaving the evaporator and entering the compressor.
b) MECHANICAL SHAFT SEAL
· The compressors having external drive require a shaft seal or gland where shaft passes out of the crankcase (open drive compressors) are either belt driven or directly coupled to the shaft of the electric motor or other prime mover
THE MAIN FUNCTION OF THE MECHANICAL SEAL:
1.To prevent gas leakage through the shaft from the system
2.To prevent air entry through the shaft into the system
SKETCH
DESCRIPTION
· The usual form of shaft seal for open drive compressors comprises of a fixed carbon ring in contact with a highly polished metal facing ring
· The carbon ring is spring loaded to maintain contact under all working crankcase pressures and prevents gas leakage
· The metal facing ring firmly rotates along with the shaft which compresses a spring loaded bellow against the collar to form gas tight seal
· Neoprene seal firmly (holding the carbon ring) is fitted circumferentially over the shaft to prevents any gas leakage past over the shaft
· Where the shaft comes out of the crankcase ,there exists a small clearance which is completely covered by a spring loaded bellow
· The complete shaft seal is firmly secured with a seal mounting plate and is bolted to the end cover of the crankcase with proper gasket placed in between to prevent gas leakage
· Lubricating oil under pressure is fed to the shaft seal arrangement to remove heat(rubbing surfaces) and to reduce friction (contact surfaces)
· Since the starting torque of the compressor is too high ,the shaft moves forward orbackward from its original position is called the ‘END CLEARANCE’
· This torque is balanced by the seal spring force to keep in equilibrium position
30.Describe the danger of “Contamination” with reference to sump oil in the refrigerated compressor and its effect on the system?6m
DANGERS OF OIL CONTAMINATION AND ITS EFFECTS ON THE SYSTEM
In the refrigeration systems some oil is always carried over from the compressor into the condenser by the refrigerant gas, and circulates through the system.
1. The presence of oil in the circulating refrigerant reduces the heat transfer capacity
being greatest in the evaporator, since oil becomes more viscous and tends to
congeal at low temperature.
2. Temperature will not drop in the cold rooms as normal , due to fact that oil act as insulation in the evaporator.
3. It may cause excessive frost on the suction line.
4. Refrigerant compressor runs for extended period of time.
5. Lubricating oil level in the compressor sump will drop.
6. Oil contamination with refrigerant may happen if the oil separator is not working properly.
7. Oil my carry over from the compressor and may not return back to the compressor due to blockage in the system.
8. Defective piston rings or worn out liner of the compressor may cause the oil to carry over along with the refrigerant.
9. Compressor may take high capacity current during starting.
31. How is Freon gas affect the “ozone” and the climate?4m
OZONE EFFECTS
· Release of CFC refrigerants into the atmosphere is harmful to the environment.
CFCs released into the atmosphere are broken down by photolysis to release
chlorine atoms, which catalytically destroy ozone, the stratospheric gas that acts
as a filter of ultra violet light from the sun.
· CFC’S, along with other ‘greenhouse gases’, inhibit the release of heat radiated
from the earth, thereby contributing to global warming, leading to rapid rise in sea
level.
· ClF3 →F3+Cl
· Cl + Ozone(O3) → ClO +O2
· ClO + O →Cl+O2
CLIMATIC EFFECTS
· The increased UV light on earth as a result of ozone depletion will cause skin cancer, suppression of human immune system and increase in cataracts
· Damage to aquatic organism and the earth’s food chain
· Changing weather patterns leading to decrease in grain and crop production
· Increase in green house gases, creating smog and global warming
33. How is the reciprocating compressor lubricated?4m
DESCRIPTION
• In a reciprocating compressor lubricating oil system, a gear pump or a lobe type of pump in which the inner rotor having 3 lobes is rotated inside an outer rotor having 4 lobes, the pockets of increasing and decreasing area ,thus giving a pumping action.
• The pump which is driven by the crank shaft takes oil suction from the crankcase through a suction strainer(filter).
• The discharged oil from the pumppasses through a fine filter and is branched to the capacity control mechanism (unloader) and another line which lubricates main bearings, bottom end bearings and the gudgeon pin bush through drilled passage of crankshaft and connecting rod.
• The oil also lubricates the shaft seal(mechanical seal) and then returns back to the crankcase through an orifice to maintain the line pressure.
• A pressure gauge is fitted to the discharge side of the pump for checking the lubricating oil pressure and sight glass is fitted in the crank case for checking the crank case lub oil level.
• In case of excessive high back pressure in the line, there is a pressure relief valve fitted on the discharge side of the pump which returns the oil back to the crankcase.
• To protect against too low oil pressure , the lubricating oil line is connected to a differential oil pressure switch which cuts off the compressor in the event of too low oil pressure.
34) What are the uses of psychometric charts and when do you use them?4m
· A psychometric chart shows vertical lines representing dry bulb temperatures, and dotted lines represent relative humidity, Moisture content in air indicated in y axis, Specific enthalpy corresponding to dry temp .
· To obtain the relative humidity, a pair of wetdry bulb temperatures is marked on the chart, and the line of relative humidity passing through this point is the required relative humidity.
36. Why is expansion valve used in the refrigeration or air conditioning system?
· TXV is a automatic device that meters the amount of refrigerant to match the evaporator load and to maintain a constant superheat of the refrigerant gas leaving the evaporator.
· In the refrigeration system ,Expansion valve is placed ahead of the filter drier unit and before the evaporator inlet and it divides the high and low pressure sides of the refrigeration system.
FUNCTION OF EXPANSION VALVE is to
1. Efficiently drop the pressure of the refrigerant by passing through a variable flow orifice,
2. The pressure of the liquid is reduced to the evaporating pressure so that the temperature of the refrigerant entering the evaporator is below that required in the refrigerated space.
3. Control the quantity of refrigerant flow to the evaporator to suit the rate of evaporation and maintains 3 to 5 degree of super heat temperature to the gas leaving the evaporator and entering the compressor.
42. What are the safety features incorporated in a vopur compression cycle.2m
THE FOLLOWING ARE THE SAFETY FEATURES FITTED IN A VAPOR COMPRESSION SYSTEM
· High-pressure cut out switch
· Low-pressurecut in/cut out switch
· Differential Oil pressure safety cut out switch
· Pressure control on the condenser
· Refrigeration compressor abnormal alarm and stop .
· Refrigeration compressor motor overload stop
· Safety spring on the discharge valve of reciprocating compressor.
44.What are the different types of Freon gas used for refrigeration system?4m
· Refrigerants R12, R22, R502 and R717 (ammonia), are the substances in generaluse.
· The suitable alternative refrigerant for R12 and R502 currently limited toR22,ammonia(R717) and R134a.
· R22 will continue to be the first choice for all newmarine installations.
· R134a has been developed as an alternative for R12, But it has a drawback in
being un suitable for use with mineral oil, and is expensive.
45. What is the use of the compressor in the vapour compression system?2m
COMPRESSOR
· The compressor receives the low pressure, low temperature superheated vapour from the evaporator.
· During the suction stroke, as the internal pressure of the cylinder becomes lower than the suction inlet pressure, the suction valve of the compressor opens to admit the refrigerant gas from the evaporator.
· During the compression stroke the gas is compressed to a high pressure high temperature superheated vapour.
· Discharge valve opens when the cylinder pressure is higher than in the discharge side and the compressed gas is passed to the condenser.
· The compressor creates differential temperature which promotes heat transfer and increase in energy provides the driving force to circulate refrigerant through the system.
46. a)How many filters are fitted on the gas side in the refrigeration system and where are they fitted?2m
b) How does the expansion valve work and control the superheat in the Evaporator? 10m
a)FILTERS FITTED ON THE GAS SIDE OF THE REFRIGERATION SYSTEM
THREE FILTERS ARE INSTALLED IN REFRIGERATION PLANT:
1. Filter drier in the high pressure side of refrigerant.
2. Compressor suction filter.
3. Expansion valve filter.
b) EXPANSION VALVE WORKING AND CONTROL OF SUPERHEAT IN THE EVAPORATOR
· The membrane plate which is subjected to different pressures (P1,P2,P3) will determine the opening or closing of the Expansion valve
Ø P1 – pressure in the sensing bulb
Ø P2- pressure in the evaporator
Ø P3 – pressure of the spring
The operation of expansion valve is subjected to three conditions
(1).Balanced Forces
(2).Increase In Super Heat
(3).Decrease In Super Heat
(1)BALANCED FORCES CONDITION
· In this condition ,the valve will assume a stable control position when the pressures (P1,P2,P3) are balanced
· The balance in pressures is given by,
P1=P2+P3
(2)INCREASE IN SUPER HEAT CONDITION
· In this condition ,the temperature of the refrigerant gas at the evaporator outlet (sensing bulb location)increases the saturation temperature corresponding to the evaporator pressure as it becomes superheated
· The pressure in the sensing bulb (due to higher temperature) increases above the combined pressures of evaporator and spring (P2 +P3) such that P1 is greater than (P2+P3)
· The pressure P1 acting on the top of the membrane plate moves the bellow downwards in which the valve stem will move in an opening direction and supply refrigerant to the evaporator
(3)DECREASE IN SUPER HEAT CONDITION
· In this condition ,the temperature of the refrigerant gas leaving the evaporator decreases and the corresponding pressure(P1) in the sensing bulb also decreases such that (P1) is less than (P2+P3)
· Since the bulb pressure P1 is less than the combined pressures of the evaporator and spring pressure (P2+P3),the bellow moves upwards such that the valve stem will move towards closing and gives restriction of the refrigerant flow to the evaporator
47.What is the term “carry over” and what are the dangers of it in a refrigerated system?2m
CARRY OVER
· The vapour in the condenser first gives up its super heat and then cooled from thedischarge temperature to the saturation temperature corresponding to condensing pressure.
The vapour then gives up its latent heat and it condenses back to liquid.
· This process will not takeplace as the quantity of sea water flow decreases due to the fouling of the condenser.
· At this condition, discharge temperature and pressure of refrigerant gas from the compressor will be higherthan normal.
· The high temperature and pressure of the discharge gas carries more quantity of oil to the oil separator and to the condenser(term called ‘carry over’)
DANGERS OF CARRY OVER IN REFRIGERATION SYSTEM
· The oil “carry over” passed to the condenser cannot be retrieved and would enterinto the evaporator.
· The oil in the evaporator thus affects heat transfer and in extreme condition does not allow the refrigerant gas to circulate through the system
· The desired room temperature cannot be maintained and the gas flow is restricted and blocked due to oil in the evaporator
· Thus the complete refrigeration system stops functioning normally
48. What are the min and max temperature of all the cold rooms on board ship?2m
THE FOLLOWING ARE THE TEMPERATURES OF THE COLD ROOMS ON BOARD SHIP:-
· Vegetable room +4(minimum) to +8(maximum)
· Handling room+4(minimum) to +8(maximum)
· Meat room -12 (maximum)to -16(minimum)
· Fish room -15 (maximum)to -18(minimum)
49. How do you identify the condenser is dirty and what action will you take?
IDENTIFICATION OF DIRTY CONDENSOR
· The discharge temperature and pressure of the refrigerant gas from the compressor will be higher than normal.
· Fouling of inside of the water tubes with scale or marine growth,thus reduces the heat transfer capacity of the condenser. The difference in temperature between inlet and outlet will be lower than normal.
· The tubes,end cover and end cover division plate would be wasted, caused by erosion and corrosion.
· Thus the refrigerant will not be cooled to form liquid in the condenser and thusaffecting the performance of the system.
· The high pressure cut out switch prevents the compressor from normal running operation
The condenser cooling water side to be cleaned and necessary repair to the damaged end cover has to be carried out
THE FOLLOWING ACTIONS ARE CARRIED OUT TO CLEAN THE CONDENSER:-
· The Sea water valves to the condenser is to be isolated and both the side end covers are opened.
· The tubes of the condenser should be cleaned by ‘rodding’ through with special brushes.
· Alternatively, a ready mixed inhibitive scouring acid can be used, with subsequent neutralization
· The tubes end cover and end cover division plate should be inspected for wasting, caused by erosion and corrosion.
· Corrosion plugs(sarificial anodes) should be replaced as required
· The water leaving the condenser should also be tested regularly, to check for refrigerant leaks between the water tubes and refrigerant side.
· Upon completion the end covers are fitted with new gaskets.
50.Explain the term “Liquid knock” in a refrigeration system.2m
· The liquid refrigerant stored at high pressure in the receiver, flows through the Thermostatic expansion valve, which automatically controls the rate of flow to the evaporator to suit the rate of evaporation.
· The pressure of the liquid is reduced to the evaporating pressure so that the
temperature of the refrigerant entering the evaporator is below than that required in the refrigerated space.
· The liquid vapour refrigerant mixture then flows through the evaporator, extracts heat from the refrigerated space and changes to a dry saturated vapour at approx the same temperature and pressure at which it left the flow control valve.
· The compressor which receives the low pressure, low temperature superheated vapour from the evaporatorand during the compression stroke the gas is compressed to a high pressure, high temperature, superheated vapour.
· If the condition of the refrigerant leaving the evaporator and entering the compressor exists in liquid state “Liquid knock” can take place in the compressor during compression stroke and this can cause serious damage to the compressor.
· The reasons for liquid refrigerant the entering the compressor
· Expansion valve malfunction, incorrect setting, sensing bulb wrongly placed
· Heavy frosting in the evaporator causing less heat transfer
51. What are the two ways of charging gas to the refrigerated compressor?
· Charging of refrigerant in the refrigerated system can be done of low pressure or high pressure side.
· While filling refrigerant, filling connection should be connected to cylinder via hose. Non return valves are used for charging in high pressure line.
· Low pressure side should be charged ONLY WITH GAS.
· High pressure side should be charged ONLY WITH LIQUID REFRIGERANT.
PROCEDURE FOR FILLING REFRIGERANT IN HIGH PRESSURE LINE:
1.Before filling refrigerant through hose, air should be removed by slackening threads of hose to filling line and crack open cylinder valve to 15* till a flash off takes place.
2. Close the outlet valve of the receiver.
3. Open the filling connection valve and cylinder valve and run the compressor.
4. Allow refrigerant to pass through filter drier, solenoid valve and expansion valve.
5. As refrigerant keeps on filling, it condenses in condenser and gradually raises the receiver level. After desired level is reached, close the filling valve and open outlet of receiver.
PROCEDURE FOR CHARGING REFRIGERANT IN LOW PRESSURE LINE:
1. Connect the cylinder to the charging valve in suction line.
2. Open the charging valve and crack open the cylinder valve.
3. Run the compressor to draw refrigerant gas into the system.
52.What safety precautions will you take when working with a refrigerated compressor during troubleshooting or repairs?2m
THE FOLLOWING SAFETY PRECAUTIONS ARE TO BE FOLLOWED DURING OVERHAULING OF COMPRESSOR:-
· Pump down the entire refrigerant gas to the condenser.
· Ensure that the entire system valves are shut and isolated.
· Isolate the compressor from its electric motor and switch off the circuit breaker.
· Place a safety board stating "MEN AT WORK" at the circuit breaker.
· Release the pressure on the compressor by slackening the oil filling plug with
great care due to high internal pressure.
· Correct tools are to be used with proper torque while opening, tightening and holding bolts.
· Wear safety goggles and gloves.