Refrigeration oral questions
CoC Oral Exam Preparation (Part – 9): Refrigeration & Air Conditioner
Refrigeration:
What is a Refrigerant?
Refrigerants are used as working substances in a Refrigeration systems.
Fluids suitable for refrigeration purposes can be classified into primary and secondary refrigerants.
Primary refrigerants are those fluids, which are used directly as working fluids, for example in vapour compression and vapour absorption refrigeration systems.
These fluids provide refrigeration by undergoing a phase change process in the evaporator.
Secondary refrigerants are those liquids, which are used for transporting thermal energy from one location to other. Secondary refrigerants are also known under the name brines or antifreezes
Desirable properties of a refrigerant
Low boiling point (otherwise operation at high vacua becomes a necessity)
Low condensing pressure (to avoid heavy machine plant scantling and reduce the leakage risk)
High specific enthalpy of vaporisation ( to reduce the quatity of refrigerants in circulation and lower machine speeds, sizes etc.)
Low specific volume in vapour state (reduces size and increases efficiency)
High critical temperature (temperature above which vapour cannot be condensed by isothermal compression)
Non corrosive and non solvent (pure and mixed)
Stable under working conditions
Non flammable and non explosive
No action with oil ( the fact that most refrigerants are miscible may be advantageous e.e. the removal of oil films, lowering pour points etc, provided separators are fitted
Easy leak detect
Non toxic
cheap, easily stored and obtained
Refrigeration Working cycle:
The refrigeration cycle is shown in the Figure and can be broken down into the following stages:
Fig: Vapor Compression Refrigeration working cycle
Cycle 1-2:
Low-pressure liquid refrigerant in the evaporator absorbs heat from its surroundings, usually air, water or some other process liquid. During this process it changes its state from a liquid to a gas, and at the evaporator exit is slightly superheated.
Cycle 2-3:
The superheated vapour enters the compressor where its pressure is raised. The temperature will also increase, because a proportion of the energy put into the compression process is transferred to the refrigerant
Cycle 3-4:
The high pressure superheated gas passes from the compressor into the condenser. The initial part of the cooling process (3-3a) de-superheats the gas before it is then turned back into liquid (3a-3b). The cooling for this process is usually achieved by using air or water. A further reduction in temperature happens in the pipe work and liquid receiver (3b – 4), so that the refrigerant liquid is sub-cooled as it enters the expansion device
Cycle 4-1:
The high-pressure sub-cooled liquid passes through the expansion device, which both reduces its pressure and controls the flow into the evaporator
Thermostatic Expansion Valve (TEV)
The thermostatic expansion valve performs following functions:
1) Reduce the pressure of the refrigerant: The first and the foremost function of the thermostatic expansion valve is to reduce the pressure of the refrigerant from the condenser pressure to the evaporator pressure. In the condenser the refrigerant is at very high pressure. The thermostatic expansion valve has an orifice due to which the pressure of the refrigerant passing through it drops down suddenly to the level of the evaporator pressure. Due this the temperature of the refrigerant also drops down suddenly and it produces cooling effect inside the evaporator.
2) Keep the evaporator active: The thermostatic expansion valve allows the flow of the refrigerant as per the cooling load inside it. At higher load the flow of the refrigerant is increased and at the lower loads the flow is reduced. It won’t happen that the load on the evaporator is high and the flow of the refrigerant is low thereby reducing the capacity of the evaporator. The thermostatic expansion valve allows the evaporator to run as per the requirements and there won’t be any wastage of the capacity of the evaporator. The TEV constantly modulates the flow to maintain the superheat for which it has been adjusted.
3) Allow the flow of the refrigerant as per the requirements: This is another important function of the thermostatic expansion valve. It allows the flow of the refrigerant to the evaporator as per the load on it. This prevents the flooding of the liquid refrigerant to the compressor and efficient working of the evaporator and the compressor and the whole refrigeration plant.
TEV construction:
Small quantity of Vapour Refrigerant is sealed in a bulb or phial, and attached to Compressor suction pipe, just coming out from Evaporator.
Other end is connected by Capillary Tube to the chamber above Flexible Bellow in valve body.
The space below the Bellow is in communication with Evaporator outlet pressure (this is called Equalising Line)
If no further action is taken, pressure above and below the Bellow will be equalised and hence no superheat is obtained.
This is overcome by providing adjustable Bias Spring under the Bellow, and Bias Spring pressure is proportional to required superheat.
Operation:
Refrigerant Liquid from Condenser enters into TEV via Dryer, it expands to Evaporation Pressure, and some flash gas is formed.
Flash Gas amount varies between 25 – 35%, depending on refrigerant type, plant capacity and ambient temperature.
Mixture of this expanded gases and some part of liquid, passed into Evaporator, where complete Evaporation takes place.
Evaporator outlet pressure plus Spring pressure tends to close the valve, and is opposed by the pressure above the Bellow, trying to open it.
This pressure above the Bellow is in relation to temperature in Compressor suction pipe.
Equilibrium condition is reached, when Superheat is correct at phial attachment point.
Starved condition in Evaporator will result greater Superheat, so expansion of Vapour Refrigerant in phial will tend to open the valve further, to increase the flow.
Flooded condition in Evaporator will result lower Superheat, so contraction of Vapour Refrigerant in phial will tend to close the valve further, so decrease the flow.
Superheat Temperature adjusted at: 3 – 6°C, by Bias Spring pressure.
Why Equalising Connection is fitted?
In some plant having large Evaporator or Multi-circuit Evaporator, excessive pressure drop across Evaporator occurs, and always tend to starve the Evaporator and increase the Superheat.
To counteract this, if pressure drop across Evaporator, exceeds 3 bar, an Equalising Connection must be provided at TEV.
A direct connection between underside of Bellow and suction piping of Compressor, preferably between phial and Compressor.
Safety devices on Refrigeration Plant:
LP cut-out switch: Set at a pressure corresponding to 5°C below the lowest expected evaporating gauge reading.
HP cut-out switch: Set at a pressure corresponding to 5°C above the highest expected condensing gauge reading.
Lub Oil LP cut-out: Oil pressure usually set at 2 bar above crankcase pressure.
Condenser cooling water LP cut-out.
Liquid shock valve on Compressor cylinder head.
Bursting disc on cylinder head, between inlet and discharge manifold.
Bursting disc on Condenser, [if fitted].
Relief valve on Condenser.
Master solenoid valve: To prevent liquid being entered into Compressor, when the plant is standstill, especially in Large Plant.
Refer plant survey:
General examination of machinery and testing under working condition.
The log examined, to ascertain successful operation during voyages.
Compressor and prime mover to be open-up and examined.
Primary system to be leak-tested to their w. p. and brine cooling coils are to be hydraulically tested to 3 kg/cm².
Survey is done at 1 year from the date of installation, and special periodical surveys are to be carried out at 5 years intervals. ( 1+ 5 )
CFC: Chlorofluorocarbon
Due to damaging effects on OZONE layer and causing Global Warming, most CFCs are now replaced by HFCs,
HFC 134a has Ozone Depletion Potential, ODP ‘0’ and Global Warming Potential, GWP ‘0.28’.
Banned from 19 May 2005
Defrosting:
A method of removal of frost, built-up on Evaporator coils. Defrosting should be done before snow thickness exceeds ¼”.
Reasons for defrosting:
Affecting heat transfer properties.
Affecting air flow and circulation.
Liquid back to Compressor.
Defrosting Systems:
Water wash defrosting
Hot gas defrosting
Electric defrosting
Manual shut down defrosting
Warm brine defrosting
Various methods to defrost Brine System:
Hot brine thawing: Best and fastest method, used powerful brine heater with separate thawing system. Watertight trays under the pipes, collected 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.
Direct expansion grid system: Hot gas defrosting.
Battery cooling system: Water spray, electrical or steam heater.
Brine cooling: Hot brine thawing.
Cargo Fridge Defrosting:
In Battery System, hot brine passing brine heater is used.
Steam is released to brine heater and brine flow is restricted by brine inlet valve, until brine temperature has risen above 0°
Brine temperature of 43°C is suitable for defrosting.
Why Cold Room is defrosted and how many methods of defrosting?
Coil Room is required to defrost to gain more Heat Transfer Efficiency.
Methods of Defrosting are: ( i) Plant stopped and manual watering (ii) Hot gas circulating (iii) Electric Heater.
Faults in Shipboard Refrigeration Systems
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 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 may be blocked at the strainer.
Partial blockage of refrigerant at the filter or drier or evaporator may cause undercharging.
Action:
Identify and rectify the leakage of refrigerant from the system.
Clean the filter and drier.
Charge the system with fresh refrigerant as required.
2. Overcharge of Refrigeration System
Indication:
• 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.
• High pressure switch of the refrigerant compressor activates and stops the compressor.
• The suction and the discharge pressures are high.
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 office on the regulator.
Action:
• 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.
• Purge the air from the system and maintain effective cooling.
• Remove ice from the regulator by using any of the defrosting methods.
3. 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.
Action:
– Renew silica gel in case of minor moisture.
– collect refrigenant and remove all air and moisture by vacuum pump if the amount is huge.
4. Air in the System
Indication:
• 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 leaks in to the suction line because the working pressure of the Freon-12 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.
5. Oil in the Refrigeration System
Indication:
• Temperature is not dropping in the cold rooms as normal, due to fact that oil act as insulation in the evaporator.
• It may cause excessive frost on the suction line.
• Refrigerant compressor runs for the extended period of time.
• Lubricating oil level in the compressor will drop.
• Refrigerant level will fall if oil has caused blockage.
Causes:
• This may happen if the oil separator is not working properly.
• Oil may carry over from the compressor and may not come back to the compressor due to blockage in the system.
• Defective piston rings or worn out liner of the compressor may cause the oil to carry over along with the refrigerant.
• Compressor may take high capacity current during starting.
Action:
• Check the oil separator for proper functioning.
• Check the drier for proper cleaning and if its require cleaning clean it
• Evaporator coil should be drained to remove any trace of oil.
• If there is oil in the cooling coils, increase the condenser and evaporator temperature differentials and remove excess frost on the suction pipe.
• Heat pipes with blow torch.
6. Flooding of Refrigerant in the System
This is seen as liquid getting back to the suction of the refrigerant compressor. It may be due to a faulty or incorrectly adjusted expansion valve and also due to solenoid valve leakage. It may also result from overcharging of the refrigeration system. Flooding may lead to an iced up evaporator.
7. Evaporator Coil Icing:
Icing of the evaporation coils which may happen due to:
1. Cause:Too low temperature setting
Action: Increase the coil temperature by adjusting TEV or it’s sensor.
2. Cause: The coil capacity is less
Action: Install large capacity evaporator coils
3. Cause: Defrost is not operational
Action: Check if the defrost system is functioning at regular intervals.
8. Compressor Start and Stops Frequently:
If while maintaining the correct temperature of the ship’s provision room or reefer cargo, the reefer compressor is frequently cutting-in and out, then such problem needs to be sorted out immediately. The most normal causes for such operation are:
1. Cause: Wrong Setting of Cutouts: It may be because the high pressure (HP) cutout is set too high or LP cutout is set too low
Action: Check and change the setting to advisable limit
2. Cause: Differential Setting Span is Small: The low pressure (LP) cut out is provided with starting and stopping pressure setting. If the setting span is too small, it will lead to frequent cut-in and cut-out of the compressor
Action: Change the setting and increase the span between starting and stopping compressor pressures.
3. Cause: Defective Valves: If the compressor discharge valve is leaky or the line solenoid valve is not closing properly, this will lead to variation in sensor pressure and result in frequent cut-in and cut-out of compressor
Action: Replace all the defective valves
4. Cause: Clogged Suction Filters: Compressor is provided with a filter in the suction line. If that is clogged, it will lead to frequent LP cut out
Action: clean the filter.
9. Compressor Starts But Stops immediately
When the compressor in the reefer circuit starts and suddenly stops, it can be because of the following reasons:
1. Cause: Low pressure cut out gets activated
Action: Ensure that all the suction line valves are in open condition, the refrigeration is properly charged and the low pressure cut out is not defective.
2. Cause: Defective oil pressure cut out
Action: Check for proper functioning of oil pressure cutout and replace the defective cutout.
3. Cause: Defrosting timer is getting activated frequently
Action: If the defrost timer is getting activated frequently, leading to cutout of compressor, check and repair defrost timer.
4. Cause: The lube oil level is below required level
Action: This can be because of leakage of lube oil from seal or carry over of oil. Rectify the leakage and refill the oil level.
5. Cause: Foaming of oil leading to reduced oil pressure
Action: Ensure no foaming takes place, renew the oil if required.
6. Cause: Motor overload cutouts are activating
Action: Ensure that electrical motor trips are working properly.
10. Excessive icing up at Compressor suction:
Causes:
Abnormal operation of TEV.
Overcharge of the system.
Moisture in the system owing to dirty Dryer.
Defective Suction valve:
Indication:
Continuous running of Compressor.
Insufficient cooling effects.
Noisy operation.
High suction pressure.
11. Defective Discharge valve:
Indication:
Continuous running of Compressor.
Insufficient cooling effects.
Noisy operation.
High suction pressure during running.
Low discharge pressure during running.
Suction pressure rises faster after Compressor is shut-down.
Warm cylinder head.
12. Choked Expansion valve:
Causes:
Due to dirt and freeze-up of water present in system.
Effects:
Starved Evaporator
High superheat temperature.
Rapid Condenser pressure rise can cause stopping of Compressor,
Remedy:
Clean Expansion valve and filter
Renew Dehydrator.
Secondary Refrigerant:
Calcium Chloride Brine ( 3 ½ lb. of Ca Cl₂ + 1 gal. of water ) with density of 1.25 is widely Sodium Dichromate or lime added to maintain
pH values of 8.0 – 8.5.
Sodium Chloride Brine.
Why LP Cut-off fitted?
Fitted as safety control and it protect against: (a) Extreme compression ratio. (b) Freezing up of Evaporator. (c) Entrance of air and water vapor resulting from LP side leakage.
Fridge compressor sump oil filling:
Stop condition: (i) Tight shut both inlet and outlet valves of compressor. (ii) Open filling plug and fill to required level. (iii) Air purge to be done when plant resume.
During running: (i) Make vacuum pressure in crankcase and suck oil itself. (ii) Ensure oil pipe immersed in oil to prevent air ingress.
What is made of fridge filter dryer?
Ans: (i) Activated Alumina (Aluminium Oxide) (ii) Silica Gel (Thorzone)
Safety Devices fitted on Fridge Compressor:
Safety Head or Unloader.
Bursting Disc in Compressor.
LP and HP Gauges and Cut-out.
LO Low Pressure Cut-out.
Condenser cooling water Low Pressure Cut-out
Charging of Refrigeration Plant:
There are two methods for charging reefer plants:
Liquid charging and
Gas charging.
Now a day’s gas charging is preferred over liquid charging because it is more safe and simple.
Gas Charging of Refrigeration Plant:
For gas charging, a special T piece valve block with mounted pressure gauge is provided to combine three connectors inter-connecting:
-Vacuum pump
-Charging Cylinder
-Charging Point
Following steps are to be taken for charging gas into the reefer plant:
1. Connect gas bottle or charging cylinder, vacuum pump and charging point in the reefer system to the valve block.
2. The discharge of the vacuum pump is to be connected in the empty recovery bottle
3. First open the valve between vacuum pump and charging bottle located in the valve block without opening the main valve of the charging cylinder. This will remove all the air inside the pipe. Once vacuum is reached, close the valve of charge cylinder in the valve block
4. Now open the valve of the charging point pipe in the valve block and run the vacuum pump until the vacuum is reached. This will remove the trapped air from this pipe. Then shut the valve in the valve block
5. Now keep the system idle for 5 minutes to check there is no pressure drop. This will ensure there are no leakages in the system
6. Now open charging bottle pipe valve and the charging point pipe valve located in the valve block. This will set the line for charging. Ensure that the vacuum pump valve is shut7. Now open the main valves in the charging cylinder and charging point of the reefer system
8. Do not overfill the system. Make sure the receiver has 5 % space for expansion
Ensure that no refrigerant is leaked out in the environment as these effects the ozone layer in the atmosphere.
Gas bottle is kept on weighing scale for measuring the amount of charged supplied to the system.
Air Conditioning:
Relative Humidity:
Ratio of amount of water vapour in given volume of air, to maximum amount of water vapour that can be present before precipitation occurs.
Control of temperature:
Comfortable temperature range is about 22°C and 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 pre-set 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.
Ozone Depletion:
Ozone gas layer is a region of the atmosphere, 12 – 30 miles above Earth’s surface.
This layer moderates the climate, and protects life on Earth from ultraviolet rediation
Release of industrial waste and other process breakdown ozone layer and so disturb natural balance.
Chlorofluorocarbons, CFCs, at ground level, rise and broken down by sunlight, whereupon chlorine reacts with and destroys ozone molecules.
Single chlorine atom may destroy 10 – 100,0000 ozone molecules.
Ozone Depletion Substance (ODS):
CFC 11 1.0 Halon1211 3.0 (Used in portable extinguishers)
CFC 12 1.0 Halon1301 10.0 ( Used in fixed installation)
CFC 115 0.6 HCFC 22 0.05
CFCs: Chlorofluorocarbon Refrigerant:
Chlorofluorocarbon Refrigerants includes:
CFC11, CFC12, CFC22, CFC 115,
CFC500, CFC502, CFC 503 and CFC 504. (8 Types)
Difference between Air Cond. and Fridge:
Air Cond. controls Humidity, Temperature and Flow Rate of fresh air.
Fridge cools down the provisions.
Air Reducing Valve:
Fitted on compressed Air Bottle outlet.
Reduced compressed air is used for control of Reversing Mechanism in unidirectional gear drive engines, ship whistle, automatic controls and air motors.
High-pressure air enters under the valve.
The spring, acting on the valve spindle, opens the valve and the air passes to the reduced pressure
Compression given to the spring controls the amount of opening of the valve.
If the opening increases, the higher pressure obtained on other side, acts to close down the valve to normal lift, and hence correct reduced pressure maintained.
A Relief Valve is fitted on low-pressure side to prevent excessive pressure rise on reduced air system.
Where fitted Dehumidifier and its function.
Fitted at discharge side of Reducing Valve on control air line.
Main function is to prevent oil and condensate water passes through control air line.
Psychrometric chart
This chart is used for finding the relative humidity of air which has been measured using a ‘wet and dry bulb’ thermometer. This is a pair of thermometers, one of which has its bulb wrapped in a damp cloth. The drier the air,the greater the evaporation of water off the cloth and therefore the lower the reading on the ‘wet bulb’ thermometer.
Dew point:
When a mixture of dry air and water vapour has a saturation temperature corresponding to the partial pressure of the water vapour it is said to be saturated. Any further reduction of temperature (at constant pressure) will result in some vapour condensing. This temperature is called the dew point, air at dew point contains all the moisture it can hold at that temperature, as the amount of water vapour varies in air then the partial pressure varies, so the dew point varies.