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Plate type heat exchangers & control measures for motorships

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.

Plate heat exchanger It consists of five basic elements: the cover, the carrier rail, the heat transfer plates, the support column, and the tie bolts. The inlet and outlet for both fluids are usually located in the same cover. The fluids are separated by the heat transfer plates. Each plate contains a gasket that fits into grooves pressed in the plate and in the nozzle ports. The gasket prevents the two fluids from mixing. The gasket is vented to the atmosphere, which permits a leak to be promptly detected. The plates are sandwiched between a fixed cover and a movable cover by tie bolts. The top and bottom carrier rails align the plates to each other.

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. They are made up from an assembly of identical metal pressings (Figure 1 ) with horizontal or chevron pattern corrugations; each with a nitrile rubber joint.

Conventional plate type heat exchanger
Figure 1: Plate type heat exchanger

The plates, which are supported beneath and located at the top by parallel metal bars, are held together against an end plate by clamping bolts. Four branch pipes on the end plates, align with ports in the plates through which two fluids pass. Seals around the ports are so arranged that one fluid flows in alternate passages between plates and the second fluid in the intervening passages, usually in opposite directions.

The plate corrugations promote turbulence (Figure 2 ) in the flow of both fluids and so encourage efficient heat transfer. Turbulence as opposed to smooth flow causes more of the liquid passing between the plates to come into contact with them. It also breaks up the boundary layer of liquid which tends to adhere to the metal and act as a heat barrier when flow is slow.

The corrugations make the plates stiff so permitting the use of thin material. They additionally increase plate area. Both of these factors also contribute to heat exchange efficiency. Excess turbulence, which can result in erosion of the plate material, is avoided by using moderate flow rates. However, the surfaces of plates which are exposed to sea water are liable to corrosion/erosion and suitable materials must be selected.

Titanium plates although expensive, have the best resistance to corrosion/erosion. Stainless steel has also been used and other materials such as aluminium-brass. The latter may not be ideal for vessels which operate in and out of ports with polluted waters. The nitrile rubber seals are bonded to the plates with a suitable adhesive.

Removal is facilitated with the use of liquid nitrogen which freezes, makes brittle and causes contraction of the rubber seal which is then easily broken away. Other methods of seal removal result in plate damage. Nitrile rubber is suitable for temperatures of up to about 110 deg C. At higher temperatures the rubber hardens and loses its elasticity. The joints are squeezed when the plates are assembled and clamping bolts are tightened after cleaning.

Turbulence produced at plate type heat exchanger
Figure 2: Turbulence produced by plate corrugations
Overtightening can cause damage to the plates, as can an incorrect tightening procedure. A torque spanner can be used as directed when clamping bolts are tightened; cooler stack dimensions can also be checked.

plate type heat exchanger for ship machinery
Fig: plate type heat exchanger for ship machinery

Use of Titanium

The corrosion resistance of titanium has made it a valuable material for use in sea-water systems whether for static or fast flow conditions. The metal is light weight (density 4.5 kg/m3) and has good strength. It has a tolerance to fast liquid flow which is better than that of cupro-nickel. It is also resistant to sulphide pollution in sea water.

While titanium has great corrosion resistance because it is more noble than other metals used in marine systems, it does tend to set up galvanic cells with them. The less noble metals will suffer wastage unless the possibility is reduced by careful choice of compatible materials, coating of the titanium, insulation or the use of cathodic protection.

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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