Cooler or Heat Exchanger on Ships
What is the purpose of charge air cooler on ships ?
To reduce air temperature & increase density of charge air.
More fuel can be burnt and more power can be obtained.
Reduce exhaust temperature and engine thermal load.
Increase scavenging efficiency, safe working temperature.
What are the cooler maintenance carried out for optimum efficiency ?
Cooler should be checked any deposit of lime, scale or oil sludge may be present in cooler; it should be cleaned.
Cooler of water side can be done with soft tube brush and oil side with carbon tetra chloride solution in reverse direction to normal flow with hand pump for about 4 hours.
After cleaning the cooler are hydraulically pressure tested normally 1.5 times the working pressure.
If 10% of the tubes have been leaked retubing in necessary. Normal leaking tubes may be stopped by plugging.
Corrosion can be protected by means of preservative coating (Anti corrosive paints) inside the shell and water boxes and by means of anodes such as zinc fitting inside water boxes.
Essential cooler for optimum efficiency can be maintained by controlling of temperature of fluid or sea water.
What is the purpose of division plate in cooler ?
Provided to increase numbers of pass.
This increase the cooling efficiency.
What do you know L.O cooler leakage ? How to do L.O cooler leakage occur on ships ?
When engine run,
Oil comes out at the cooling water overboard.
Sump tank oil level will fall down.
L.O pressure will drop
If L.O cooler leaks the engine should be stopped with permission from bridge.
The leakage can be detected by carrying out a hydraulic pressure test to the oil side.
After cooling down the engine stop main circulating L.O pump and main S.W cooling pump.
Close necessary valve of them
Open water box covers cooler both side.
Blank off discharge pipe of cooler oil side.
Connect hydraulic pump to inlet of cooler.
Apply oil pressure normally 1.5 times the working pressure. Then check the leakage at cooler both sides
Normally leaking tubes may be stopped by plugging
If 10% of the tubes have been leaked retubing is necessary.
Then the engine is put back normal running.
How do you know air cooler leakage ?
Check water level insight glass fitted at cooler drain pipe.
Drain the cooler / taste the water
If the water continuous comes out, the cooler is leakage. Also in the funnel white & dense smoke.
Then the engine should be stopped with permission from bridge.
Normal leaking tubes can be stopped by plugging.
Then the engine is put back normal running.
How to check cooler efficiency ?
Check sea water in/out temperature difference. Less difference means poor efficiency of cooler (must be high)
Check coolant medium in / out temperature (Difference temperature: should be low)
Feel over cooler shell, upper hot, middle warm, down cool is normal.
Check pump and by pass valve.
What is the purpose of baffle plate in coolers ?
To support the tube stack.
To guide the flow of fluid
To increase cooling surface area
To minimize the tube vibration
What are the causes of loss of vacuum in fresh water generator ?
Failure of ejector pump
Failure of ejector nozzle (fouling, erosion)
Malfunction of check valve (at ejector nozzle)
Defective vacuum breaker
Any air leakage into the system (At joint)
What will happen when vacuum reach 100% in fresh water generator ?
Increase the salinity because of agitation. At that time boiling rate is very high.
To control this condition, open the vacuum breaker to maintain 93% vacuum.
Why fresh water generator is fitted on ships ?
To produce the high purity distilled water from sea water
To provide make up water for boiler and portable water for drinking and domestic use. So can save cost.
What are the treatments for drinking purposes ?
Chlorination.
Ultra violet light sterilization.
Liberation of silver ions to the water.
What is meant by Osmosis ?
When different concentration solutions are separated by a semi-permeable membrane, water from less concentrated solution pass to the other solution through the membrane to equalize the concentration of the two solution.
It create hydraulic pressure gradient across the membrane as the volume and level of weaker solution fall and those of the stronger solution rise.
What is meant by Reverse Osmosis ?
The pressure greater than the osmotic pressure is applied to the side of higher concentration solution, the osmosis process is reversed.
Water from the stronger solution is forced back through the semi-permeable membrane to dilute the initially weak solution on the other side and further increase the concentration of the strong solution.
The total pressure required for this process consists of the osmotic pressure (between 4 bar for brackish water up to 28 bar for sea water) plus the system pressure losses and net driving pressures (around 25 bar).
What are the semi permeable membranes used on ships ?
Hollow fine fiber (aromatic polyamide or cellulose acetate spurn to form hollow fiber)
Spirally wound (cellulose acetate for backish water and polymide or polysulphonate for sea water)
Fouling of main engine lube oil cooler may result in high lube oil temperature and low lube oil pressure. Lube oil coolers can be plate type or shell and tube type. However the reasons for poor cooler performance are:
Fouling of oil side, or
Fouling of cooling water side
As far as plate type coolers are concerned, cleaning is much easier. Shell side of lube oil coolers are normally designed with baffles and fins for enhanced heat exchange. Chemical cleaning is carried out regularly for the shell side. This removes sludge adhered inside and increases heat transfer efficiency.
Reasons for Poor Cooler Performance
Fouling of either lube oil side or cooling water side or both
Insufficient circulation of cooling water
Malfunctioning or improper adjustment of temperature controller / three way valve
Air lock inside the cooler
Choked cooling water inlet filter (for plate type coolers)
Broken or misplaced baffle plates (for shell and tube coolers)
How to Rectify
Keep the vents for cooler open, ensure vent is not choked and both lube oil and cooling water sides are free from air lock
Carry out chemical cleaning of cooling water side
Back flush the cooler
Check the cooling water pressure and quantity flowing through the cooler
Check adjustment of temperature controller / position of three way valve
Clean inlet filter for cooling water
Ensure baffle plates are in position
If necessary, take out tube nest out of the cooler for cleaning sludge (for shell and tube coolers)
Clean cooler plates manually (for plate type coolers)
Both plate type and shell and tube heat exchangers are normally used on ships. Both type of heat exchangers have advantages and disadvantages.
Plate Type Heat Exchangers
Advantages
Simple and Compact in size
Heat transfer efficiency is more
Can be easily cleaned
No extra space is required for dismantling
Capacity can be increased by introducing plates in pairs
Leaking plates can be removed in pairs, if necessary without replacement
Maintenance is simple
Turbulent flow help to reduce deposits which would interfere with heat transfer
Disadvantages
Initial cost is high since Titanium plates are expensive
Finding leakage is difficult since pressure test is not as ease as tube coolers
Bonding material between plates limits operating temperature of the cooler
Pressure drop caused by plate cooler is higher than tube cooler
Careful dismantling and assembling to be done
Over tightening of the clamping bolts result in increased pressure drop across the cooler
Joints may be deteriorated according to the operating conditions
Since Titanium is a noble metal, other parts of the cooling system are susceptible to corrosion
Shell and Tube Heat Exchangers
Advantages
Less expensive as compared to Plate type coolers
Can be used in systems with higher operating temperatures and pressures
Pressure drop across a tube cooler is less
Tube leaks are easily located and plugged since pressure test is comparatively easy
Tubular coolers in refrigeration system can act as receiver also.
Using sacrificial anodes protects the whole cooling system against corrosion
Tube coolers may be preferred for lubricating oil cooling because of the pressure differential
Disadvantages
Heat transfer efficiency is less compared to plate type cooler
Cleaning and maintenance is difficult since a tube cooler requires enough clearance at one end to remove the tube nest
Capacity of tube cooler cannot be increased.
Requires more space in comparison to plate coolers
Plate type heat exchangers were originally developed for the milk industry where daily cleaning is necessary. They were first used at sea, as coolers, in the 1950s.
The plates are metal pressings, corrugated with horizontal or chevron pattern corrugations. These make the plates stiffer and therefore permit the use of thinner material. They also increase the heat exchange area and produce a turbulent flow. All these factors contributes to the efficiency of heat transfer. Turbulence, as opposed to smooth flow, causes more of the liquid passing between the plates to come in contact with them. It also breaks up the boundary layer of the liquid which adheres to the metal and act as a heat barrier in smooth flow. However, the turbulence can cause plate damage due to erosion and materials normally used in tube coolers for seawater contact, may not be suitable in plate coolers.
Plate material for seawater contact is titanium. This is an expensive metal but apparently able to withstand the conditions of service. Aluminium-brass has been used with poor results. Possibly failure of Aluminium-brass has been due to the presence of organic sulphides and other chemicals in coastal and inland waters. Titanium is immune from this type of attack. However, other factors such as turbulence in plates coolers or changes in the materials of the seawater system may be responsible for early failures. Stainless steel has been used in plate coolers for duty with seawater, but proved unsuccessful although it is suitable for other applications.
In plate type heat exchangers, the joint material is normally ‘nitrile rubber’ which is bonded to the plate with suitable adhesive such as ‘Plibond’. Other joint materials for higher temperatures are available, such as compressed asbestos fiber (CAF). The nitrile rubber is suitable for temperatures about 100 deg. Celsius. At higher temperatures the rubber hardens and looses its elasticity. The rubber joints are compressed when the cooler is assembled and clamping bolts tightened.
Over tightening can cause damage to the chevron corrugated plates. Also pressure drop across he cooler increases due to over tightening. At the same time loosely tightened plates results in inefficient sealing between plates. Dimensions are checked during tightening and loosening of plate cooler . Joints must be adequately clamped to prevent leakage.
All liquids inlets and outlets are a the fixed end plate. The movable ends sits in the horizontal carrying bars and he plates are also located and supported by these. The flow ports at the corners of the plates are arranged so that the cooling liquid and the liquid being cooled pass between alternate pairs of plates. Best efficiency is obtained by liquids moving in opposite directions. Joint leakage is visible externally except for the double joint at the ports on one side of the plate. A drain hole act as a tell-tale for his section.
Shell and Tube Heat Exchangers for engine jacket water and lubricating oil cooling are normally circulated with seawater. The seawater is in contact with the inside of the tubes and the water boxes at the cooler ends. The oil or water being cooled is in contact with the outside of the tubes and he shell of the cooler. Baffles direct the liquid across the tubes as it flows through the cooler. he baffles also support the tubes.
Tubes of Aluminium-brass (76% copper; 22% zinc; 2% aluminium) are commonly used. Ordinary brasses and other cheap materials have been used with unsatisfactory results. Successful use of aluminium-bass has apparently depended on the presence of a protective film formed along the tube length by corrosion of iron in the system. Thus unprotected iron in water boxes and other parts, while itself corroding, has prolonged tube life. This was made apparent when steel was replaced by other corrosion resistant materials or protected more completely. The remedy in these systems is to fit sacrificial soft iron or mild steel anodes in water boxes or to introduce iron in the form of ferrous sulphate fed into the seawater. The latter treatment consists of dosing the seawater to a strength of 1 ppm for an hour per day over a few weeks and subsequently to dose before entering and after leaving port for a short period.
Early tube failures may be due to pollution in coastal waters or to turbulence in some cases. Many coolers are fitted with tubes of 70/30 Cupro-Nickel. More expensive materials are available. Tubes are expanded into tube plates and may be further bonded by soldering.
Tube Plates
Naval brass tube plates are used with aluminium bass tubes. Tube stacks are made up to have a fixed tube plate at one end and a tube plate at the other end which is free to move with the expansion of the tubes. Other materials found in service are gunmetal, aluminium bronze and sometimes special alloys.
Tube Stack
The tube stack is fitted with alternate disc and ring baffles. The fixed end tube stack is sandwiched between the casing and the water box. If the joints leak at the other end the special ‘tell-tale’ ring will allow the liquids to escape without mixing. The joint rings are of synthetic rubber.
Water Boxes & Covers
Easily removable covers on water boxes permit repairs and cleaning to be carried out. The covers and water boxes are commonly of cast iron or fabricated from mild steel. Where they have been coated with rubber or a bitumastic type coating, the iron or steel has been protected but has provided no protection for tubes and tube plate. Uncoated ferrous (iron) materials in water boxes provided a protective film on the tubes as the unprotected iron itself corrodes, products of corrosion coating the tubes. The iron also gives some measure of cathodic protection.
Water boxes of gunmetal and other materials are used but these, like the coated ferrous metals, give no protection. Soft iron or mild steel anodes can be fitted in the water boxes and provided they cause no turbulence, will help to give cathodic protection and a protective film.
Shell
The shell or cylinder is fabricated or cast. It is in contact with the liquid being cooled. This may be oil, with which there is no corrosion problem, or water, which is normally inhibited against corrosion. The material is not critical (provided it is not reactive with any inhibiting chemicals) because it is not in contact with seawater.