Heat exchangers

Question 1

Sketch a cross-section of a horizontally mounted two pass, tubular heat exchanger suitable for lubricating oil, showing the flow path of the fluids

Answer 1

See heat exchangers word doc

Question 2

State four advantages and Disadvantages of plate type heat exchangers

Answer 2

Advantages

1. Plate cooler are smaller and lighter than a tube cooler for the same efficiency
2. Easy to dismantle and replace damaged plates or fit extra plates to increase capacity
3. Simple to clean because the Turbulent flow help to reduce deposits on plates which could interfere with heat transfer and there’s no extra space needed for dismantling
4. Cooling capacity can be easily modified by adding or removing plates

Disadvantages

1. Joints and plates are expensive
2. Leaks are difficult to locate because the plates cannot be pressurised and tested as easily as shell and tube type coolers
3. Joints can deteriorate and can be difficult to remove and bond new joints to the plates
4. over tightening of the clamping bolts can cause damage to the plates

Question 3

Describe with the aid of a sketch the operation of a plate type heat exchanger

Answer 3

The plate type heat exchanger is made up of several corrugated pressed plates separated by seals and held together in a frame with end plates at either end. The inlet and outlet branches for each liquid are attached to one plate. The arrangement of seals between the plates provides passageways between adjacent plates for the cooling liquid and the hot liquid. The plates have various designs of corrugations to aid heat transfer and provide support for the large flat surfaces. Plate coolers consist of a series of plates pressed and assembled in pairs. These are clamped and pressed together and sealed using nitrile rubber gaskets. The materials selected are based upon a number of factors dependent on liquid used. To aid with efficient heat transfer the material selected must have a high thermal conductivity. If the plates are going to be on contact with sea water a material that is non corrosive and non erosive must be used. The plates will be pressed to be as thin as possible to improve heat transfer but still be of sufficient strength. Corrugations will be utilised to strengthen the plates and increase the surface area of the plates to improve heat transfer. These corrugations will also promote turbulent flow, which is desirable to reduce the amount of deposits and scaling on the plates which would create a thermal barrier and lower overall efficiency. Turbulent flow also improves heat transfer. The material that fits all this criteria and requirements and which is used most often is titanium.

Question 4

List the maintenance procedure for a plate type heat exchanger

Answer 4

1. Heat exchanger is isolated
2. Ensure availability of tools
3. Ensure availability of PPE
4. Ensure understanding of the manufacturers manual
5. Inlet and outlet valves are closed
6. Cooler is drained, often done by lossening end cover
7. Plate stack length is measured at each clamp bolt
8. Clamp bolts are released, and plates spread out along the rail
9. Plates are cleaned with soft brush and water spray
10. Inspect the seals for damage
11. Slide plates together and replace clamp bolts
12. Clamp bolts are tightened evenly and gradually until the stack length measured previously is back to its starting point or by using a torque wrench
13. Flood up cooler and vent air and check for leaks
14. Cooler is put back into service

Question 5

List the maintenance procedure for a shell and tube type heat exchanger?

Answer 5

1. Heat exchanger is isolated
2. Ensure availability of tools
3. Ensure availability of PPE
4. Ensure understanding of the manufacturers manual
5. Inlet and outlet valves are closed
6. Tube side of cooler is drained
7. Both end covers are removed make sure that tube plate is still bolted in place
8. Tube insides are cleaned with water or cleaning brushes
9. Inspect all tubes for leaks
10. Check condition of sacrificial anodes and protective coatings
11. Replace end covers
12. Flood up cooler, vent air and check for leaks
13. Put cooler back into service
14. Shell side of cooler can only be chemically cleaned onboard and has to be sent ashore for any major cleaning