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Shell and tube heat exchangers for engine cooling

What is a heat exchanger ? A device that transfers heat through a conducting wall from one fluid to another. Heat exchangers are used to transfer heat from a hotter fluid (liquid or gas) to a colder fluid. This broad definition covers a wide range of equipment, including boilers, condensers, distilling plants, and ventilation cooling coils.

Shell-and-tube heat exchanger is fabricated from round tubes that are settled in, and run parallel to a shell. Heat is transferred between the fluids by passing through the walls of tubes. This type of exchanger consists of six basic elements: the bonnet, tubsheet, shell, tubes, baffles or support plates, and the tie rods.

Shell and tube heat exchangers for engine cooling water and lubricating oil cooling (Figure 1) have traditionally been circulated with sea water. The sea water is in contact with the inside of the tubes, tube plates and water boxes.

A two-pass flow is shown in the diagram but straight flow is common in small coolers. The oil or water being cooled is in contact with the outside of the tubes and the shell of the cooler. Baffles direct the liquid across the tubes as it flows through the cooler. The baffles also support the tubes and form with them a structure which is referred to as the tube stack.

Conventional tube type cooler
Fig 1 :Conventional tube type cooler

The usual method of securing the tubes in the tube plates is to roll-expand them. Tubes of aluminium brass (76% copper; 22% zinc; 2% aluminium) are commonly employed and the successful use of this material has apparently depended on the presence of a protective film of iron ions, formed along the tube length, by corrosion of iron in the system.

SHELL-AND-TUBE HEAT EXCHANGER
Fig: SHELL-AND-TUBE HEAT EXCHANGER

Unprotected iron in water boxes and in parts of the pipe system, while itself corroding, does assist in prolonging tube life. This factor is well known (Cotton and Scholes, 1972) but has been made apparent when iron and steel in pipe systems have been replaced by non-ferrous metals or shielded by a protective coating.

The remedy in non-ferrous systems, has been to supply iron ions from other sources. Thus, soft iron sacrificial anodes have been fitted in water boxes, iron sections have been inserted in pipe systems and iron has been introduced into the sea water, in the form of ferrous sulphate.

The latter treatment consists of dosing the sea water to a strength of 1 ppm for an hour per day for a few weeks and subsequently dosing again before entering and after leaving port for a short period. Electrical continuity in the sea-water circulating pipework is important where sacrificial anodes are installed.

Metal connectors are fitted across flanges and cooler sections where there are rubber joints and 'O' rings, which otherwise insulate the various parts of the system. Premature tube failure can be the result of pollution in coastal waters or extreme turbulence due to excessive sea-water flow rates. To avoid the impingement attack, care must be taken with the water velocity through tubes.



For aluminium-brass, the upper limit is about 2.5 m/s. Although it is advisable to design to a lower velocity than this to allow for poor flow control - it is equally bad practice to have sea-water speeds of less than 1 /sec.

A more than minimum flow is vital to produce moderate turbulence which is essential to the heat exchange process and to reduce silting and settlement in the tubes. Naval brass tube plates are used with aluminium-brass tubes.

Cooler expansion arrangement
Fig :Details of Cooler expansion arrangement

The tube stacks are made up to have a fixed tube plate at one end and a tube plate at the other end (Figure above ) which is free to move when the tubes expand or contract. The tube stack is constructed with baffles of the disc and ring, single or double segmental types. The fixed end tube plate is sandwiched between the shell and water box, with jointing material, Synthetic rubber 'O' rings for the sliding tube plate permit free expansion.

The practice of removing the tube stack and replacing it after rotation radially through 180 degrees, is facilitated by the type of cooler described. This may prolong cooler life by reversing the flow so that tube entrances, which are prone to impingement damage, become outlets. Cooler end covers and water boxes are commonly of cast iron or fabricated from mild steel.

Unprotected cast iron in contact with sea water, suffers from graphitization, a form of corrosion in which the iron is removed and only the soft black graphite remains. The shell is in contact with the liquid being cooled which may be oil, distilled or fresh water with corrosion inhibiting chemicals. It may be of cast iron or fabricated from steel.

Manufacturers recommend that coolers be arranged vertically. Where horizontal installation is necessary, the sea water should enter at the bottom and leave at the top. Air in the cooler system will encourage corrosion and air locks will reduce the cooling area and cause overheating. Vent cocks should be fitted for purging air and cocks or a plug are required at the bottom, for draining. Clearance is required at the cooler fixed end for removal of the tube stack,


Summarized below various circulating systems for motorships, some of the basic procedure of heat exchangers & control of temperatures:
  1. Sea water circulation-systems

  2. The usual arrangement for motorships has been to have sea-water circulation of coolers for lubricating oil, piston cooling, jacket water, charge air, turbo-charger oil (if there are sleeve type bearings) and fuel valve cooling, plus direct sea-water cooling for air compressors and evaporators....

  3. Shell and tube heat exchangers for engine cooling water and lubricating oil cooling

  4. Shell and tube heat exchangers for engine cooling water and lubricating oil cooling have traditionally been circulated with sea water. The sea water is in contact with the inside of the tubes, tube plates and water boxes....

  5. Plate type heat exchanger

  6. The obvious feature of plate type heat exchangers, is that they are easily opened for cleaning. The major advantage over tube type coolers, is that their higher efficiency is reflected in a smaller size for the same cooling capacity....

  7. Details of charged air cooler

  8. The charge air coolers fitted to reduce the temperature of air after the turbo-charger and before entry to the diesel engine cylinder, are provided with fins on the heat transfer surfaces to compensate for the relatively poor heat transfer properties of air....

  9. Maintenance of heat exchangers

  10. The only attention that marine heat exchangers should require is to ensure that the heat transfer surfaces should remain substantially clean and flow passage generally clear of obstructions. Indcation that fouling has occured is given by a progressive increase in the temperature difference between the two fluids, and change of pressure....

  11. Central cooling system & Scoop arrangement for motorships

  12. The corrosion and other problems associated with salt water circulation systems can be minimized by using it for cooling central coolers through which fresh water from a closed general cooling circuit is passed. The salt water passes through only one set of pumps, valves and filters and a short length of piping.....

  13. Circulating systems for steamships

  14. The main sea-water circulating system for a ship with main propulsion by steam turbine is similar to that of a motorship with a central cooling system. The difference is that the sea water passes through a ....

  15. Closed feed system and feed heating for motor ships

  16. To ensure trouble-free operation of water-tube boilers the feed water must be of high quality with a minimal solid content and an absence of dissolved gases. Solids are deposited on the inside surfaces of steam generating tubes,....

  17. Marine condenser assembly

  18. A condenser is a vessel in which a vapour is deprived of its latent heat of vaporization and so is changed to its liquid state, usually by cooling at constant pressure. In surface condensers, steam enters at an upper level, passes over tubes in which cold sea water circulates, falls as water to the bottom and is removed by a pump (or flows to a feed tank)....

  19. Three stage air ejector with internal diffusers

  20. A steam-jet ejector may be used to withdraw air and dissolved gases from the condenser. In each stage of the steam-jet ejector, high pressure steam is expanded in a convergent/divergent nozzle. ...

  21. Pressure governor for motor ships

  22. The main feature of the governor is that if the pump loses suction the steam ports are opened wide, allowing the pump to accelerate rapidly to the speed at which the emergency trip acts....

  23. Liquid ring pump- Nash rotary liquid ring pumps

  24. Nash rotary liquid ring pumps, in association with atmospheric air ejectors, may be used instead of diffuser-type steam ejectors and are arranged as shown...

  25. The Weir electro-feeder - a multi-stage centrifugal pump

  26. A multi-stage centrifugal pump mounted on a common baseplate with its electric motor. The number of stages may vary from two to fourteen depending upon the capacity of the pump and the required discharge pressure....

  27. Feed water heaters for motor ships

  28. Surface or direct contact feed heaters, play an important part in the recovery of latent heat from exhaust steam. Direct contact feed heaters are also known as de-aerators....

  29. Devaporizer & turbo-feed pump

  30. If the de-aerator cannot be vented to atmosphere or to a gland condenser satisfactorily, a devaporizer is connected to the vapour outlet condensing the vapour vented with the non-condensable gases and cooling these gases before they are discharged. ...

  31. Typical de-aerator & Cascade trays

  32. Normally, the de-aerator is mounted directly on a storage tank, into which the de-aerated water falls, to be withdrawn through a bottom connection by a pump or by gravity. The tank usually has a capacity....



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