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Heat exchangers
The document discusses heat exchangers used in marine engineering. It describes the two main types - shell and tube heat exchangers and plate heat exchangers. Shell and tube heat exchangers consist of tubes bundled inside a shell, with water boxes at each end to distribute the cooling medium. Plate heat exchangers are made of pressed plates with corrugations to aid heat transfer, held within a frame. The document provides details on construction, materials used, temperature control, corrosion control and cleaning methods for both types of heat exchangers.
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Heat exchangers
- 1. M a ri n e E n g i n e e r i n g K n o w l e d g e U E 2 3 1 | Y A S S E R B . A . F A R A G20 October 2020
- 2. M a ri n e E n g i n e e r i n g K n o w l e d g e U E 2 3 1 | Y A S S E R B . A . F A R A G20 October 2020 Reynold’s No= Density x Velocity of fluid flow X Pipe diameter Kinematic Viscosity Reynolds No < 2000 the fluid is Laminar Reynolds No > 2500 the fluid is Turbulent Types of Flow 369 For Water
- 3. M a ri n e E n g i n e e r i n g K n o w l e d g e U E 2 3 1 | Y A S S E R B . A . F A R A G20 October 2020 Flow Pattern Heat exchanger is classified as heater and cooler from the function point of view, and as shell and tube and plate type from structure point of view. The heat exchanger mediums could be two liquids or liquid and air or steam and liquid or electricity and fluid. One of cooler type may be immersed type as duct cooler or keel cooler. 370
- 4. M a ri n e E n g i n e e r i n g K n o w l e d g e U E 2 3 1 | Y A S S E R B . A . F A R A G20 October 2020 Where : Q is total heat transfer K is the Thermal conductivity x is the wall thickness θ is the logarithmic mean temperature difference Heat transfer 371
- 5. M a ri n e E n g i n e e r i n g K n o w l e d g e U E 2 3 1 | Y A S S E R B . A . F A R A G20 October 2020 Shell & tube type
- 6. M a ri n e E n g i n e e r i n g K n o w l e d g e U E 2 3 1 | Y A S S E R B . A . F A R A G20 October 2020 Construction 373
- 7. M a ri n e E n g i n e e r i n g K n o w l e d g e U E 2 3 1 | Y A S S E R B . A . F A R A G20 October 2020 Shell & Tube Finned Tubes In recent designs of tube heaters and coolers the guided flow concept has been introduced, i.e. a secondary heating or cooling, surface in the form of radial fins integral with the tubes between which flow is guided radially, alternately out and in from section to section. 374 The joint arrangements at the tube plate ends are different. At the fixed end, gaskets are fitted between either side of the tube plate and the shell and end cover. At the other end, the tube plate is free to move with seals fitted either side of a safety expansion ring.
- 8. M a ri n e E n g i n e e r i n g K n o w l e d g e U E 2 3 1 | Y A S S E R B . A . F A R A G20 October 2020 Tubes • Made of aluminum brass (76 per cent copper; 22 per cent zinc ; 2 per cent aluminum) are commonly used . • The successful use of aluminum brass 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 has been 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 sea water by dosing the sea water 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. 375 • Naval brass tube plates are used with aluminum brass tubes. Other materials found in service are gunmetal, aluminum bronze and sometimes special alloys . Tube Plates
- 9. M a ri n e E n g i n e e r i n g K n o w l e d g e U E 2 3 1 | Y A S S E R B . A . F A R A G20 October 2020 Tubes configuration 376
- 10. M a ri n e E n g i n e e r i n g K n o w l e d g e U E 2 3 1 | Y A S S E R B . A . F A R A G20 October 2020 WATER BOXES AND 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 the tubes and tube plate. Uncoated ferrous (iron) materials in water boxes provide a protective film on the tubes as the unprotected iron itself corrodes, the products of corrosion coating the tubes. The iron also gives some measure of cathodic protection. 377 • Headers or water boxes surround the tube plates and enclose the shell. They are arranged for either a single pass or a double pass of cooling liquid. • The tube bundle has baffles fitted which serve to direct the liquid to be cooled up and down over the tubes as it passes along the cooler. The baffles also support the tubes. • Should either liquid leak past the seal it will pass out of the cooler and be visible. There will be no intermixing or contamination. • 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.
- 11. M a ri n e E n g i n e e r i n g K n o w l e d g e U E 2 3 1 | Y A S S E R B . A . F A R A G20 October 2020 Shell & Tube Type Baffles Shells Tube plates Tubes stack C.W inlet C.W outlet Sacrificial anodes Always Ensure laminar flow is maintained during operation! 378
- 12. M a ri n e E n g i n e e r i n g K n o w l e d g e U E 2 3 1 | Y A S S E R B . A . F A R A G20 October 2020 Plate type
- 13. M a ri n e E n g i n e e r i n g K n o w l e d g e U E 2 3 1 | Y A S S E R B . A . F A R A G20 October 2020 The plate-type heat exchanger is made up of a number of pressed plates surrounded by seals and held together in a frame. The inlet and outlet branches for each liquid are attached to one end plate. Construction 380
- 14. M a ri n e E n g i n e e r i n g K n o w l e d g e U E 2 3 1 | Y A S S E R B . A . F A R A G20 October 2020 Construction 381
- 15. M a ri n e E n g i n e e r i n g K n o w l e d g e U E 2 3 1 | Y A S S E R B . A . F A R A G20 October 2020 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 surface. A double seal arrangement is provided at each branch point with a drain hole to detect leakage and prevent intermixing or contamination. Plate Type 382
- 16. M a ri n e E n g i n e e r i n g K n o w l e d g e U E 2 3 1 | Y A S S E R B . A . F A R A G20 October 2020 Joints 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. The nitrile rubber is suitable for temperatures up to about 100°C. At high temperatures the rubber hardens and loses its elasticity . The rubber joints are compressed when the cooler is assembled and the clamping bolts tightened . Overtightening can cause damage to the chevron corrugated plates so the cooler stack must be tightened, and dimensions checked, during the process 383
- 17. M a ri n e E n g i n e e r i n g K n o w l e d g e U E 2 3 1 | Y A S S E R B . A . F A R A G20 October 2020 Plates 384
- 18. M a ri n e E n g i n e e r i n g K n o w l e d g e U E 2 3 1 | Y A S S E R B . A . F A R A G20 October 2020 Plate Type 385
- 19. M a ri n e E n g i n e e r i n g K n o w l e d g e U E 2 3 1 | Y A S S E R B . A . F A R A G20 October 2020 Compact and space saving, virtually no head room required. Easily inspected and cleaned, all the pipe connections are at the frame plate hence they don't have to be disturbed when plates are dismantled. Variable capacity, plate number can be altered to meet capacity requirements. With titanium plates there is virtually minimum corrosion or erosion risk Turbulent flow (which is erosive) which takes place between the plates will increase heat transfer and enable fewer plates to be used. The major advantage over tube type coolers is that their higher efficiency is reflected in a smaller size for the same cooling capacity. Advantages 386
- 20. M a ri n e E n g i n e e r i n g K n o w l e d g e U E 2 3 1 | Y A S S E R B . A . F A R A G20 October 2020 HEs Selection In selection of a heat exchanger certain points should be considered some are: 1. Quantity of fluid, maximum to minimum to be cooled. 2. Range of inlet and outlet temperature of fluid to be cooled. 3. As above for the cooling medium. • Specific heat of the mediums. • streamline of turbulent flow. • Type of medium, corrosive or non corrosive for safety. 4. Operating pressures. 5. Maintenance fouling cleaning access. 6. Position in system and associated pipe work. 7. Cost, material 387
- 21. M a ri n e E n g i n e e r i n g K n o w l e d g e U E 2 3 1 | Y A S S E R B . A . F A R A G20 October 2020 Temperature Control
- 22. M a ri n e E n g i n e e r i n g K n o w l e d g e U E 2 3 1 | Y A S S E R B . A . F A R A G20 October 2020 Temperature control Temperature control of coolers is usually achieved by adjusting the cooling liquid outlet valve. The inlet valve is left open and this ensures a constant pressure within the cooler. This is particularly important with sea water cooling where reducing pressure could lead to aeration or the collecting of air within the cooler. Air remaining in a cooler will considerably reduce the cooling effect. Methods of controlling temperature of hot liquid when the cooling medium is Sea water are basically are: 1. Bypass a proportion or all of the hot fluid flow. 2. Bypass or limit the sea water flow (outlet v/v only) 3. By spilling part of the sea water discharge back into the pump suction 389
- 23. M a ri n e E n g i n e e r i n g K n o w l e d g e U E 2 3 1 | Y A S S E R B . A . F A R A G20 October 2020 Temperature control M/E Cooler P & I controller Temperature sensor 3-way valve C.W inlet C.W outlet Oil inlet Oil outlet By pass 390
- 24. M a ri n e E n g i n e e r i n g K n o w l e d g e U E 2 3 1 | Y A S S E R B . A . F A R A G20 October 2020 Corrosion Control 1. By using Zinc anodes to avoid galvanic corrosion 2. Using iron headers to have a protective film on the tube stack 3. Avoiding aeration by controlling the flow by throttling on the liquid outlet. 4. Controlling the flow velocity and maintain laminar flow inside the H.E 5. Periodic maintenance and cleaning 6. Avoiding overheating, and hence avoiding scales formation. 7. If the H.E to be reserved for long period, it should be filled with fresh water 8. Tubes to be made of aluminium brass. 391
- 25. M a ri n e E n g i n e e r i n g K n o w l e d g e U E 2 3 1 | Y A S S E R B . A . F A R A G20 October 2020 Cleaning Methods 1. Mechanical Cleaning : by soft brushes ( Avoid damage the protective Film of the inside of the tubes or plates) 2. Chemical Cleaning: By an Acid (Hydrochloric Acid) advised by the maker (depending on the cooled liquid and cooling medium) 392