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Central cooling system & Scoop arrangement for motorships

Central cooling system: 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.

Figure 1 shows a complete central cooling system in which all components are cooled by fresh water. The three sections are (1) the sea-water circuit; (2) the high temperature circuit; and (3) the low temperature circuit.

The duty sea-water pump takes water from the suctions on either side of the machinery space and after passing through the cooler it is discharged straight overboard. The main and stand-by pumps would be of the double entry centrifugal type but, as an alternative, a scoop arrangement can be incorporated (Figure 2) with central cooling.

Central cooling system
Figure 1: Central cooling system

A main circulating pump must have a direct bilge suction for emergency duty, with a diameter not less than two thirds that of the main sea-water inlet. In motor ships a direct suction on another pump of the same capacity is acceptable. Materials for the reduced salt-water system for the central cooling arrangement will be of the high quality needed to limit corrosion/erosion problems.

Water in the high temperature circuit, is circulated through the main engine and auxiliary diesels by the pumps to the left of the engine in the sketch. At the outlet, the cooling water is taken to the fresh water distiller (evaporator) where the heat is used for the evaporation of sea water. From the outlet of the evaporator, the cooling water is led back to the suction of the high temperature pump through a control valve (C) which is governed by engine inlet temperature.

The control valve mixes the low and high temperature streams to produce the required inlet temperature, which is about 62 deg C Engine outlet temperature may be about 70 deg C For the low temperature circuit, the heat of the water leaving the central coolers is regulated by the control valve (F). Components of the system are arranged in parallel or series groups as required. The pressure control valve works on a bypass. The temperature of the water after the cooler may be 35 deg C and at exit from the main engine lubricating oil coolers it is about 45 deg C The fresh water in the closed system is treated with chemicals to prevent corrosion of the pipework and coolers. With correct chemical treatment, corrosion is eliminated in the fresh water system, without the need for expensive materials.

Scoop arrangement for motorship central cooling system
Figure 2: Scoop arrangement for motorship central cooling system

Scoop arrangement for a motor ship

A scoop (Figure 2) designed to supply sea-water circulation through the central coolers while the vessel is underway, may be installed instead of a conventional sea-water circulating pump. The scoop imposes some extra drag on the hull so that the power for sea-water circulation is supplied from the main propulsion instead of from the generators and electrical system.

Economic advantages are claimed for a correctly designed scoop but the arrangement is viable only for a simple straight through flow as for central coolers or the large condenser of a steam ship. The electrically driven pump is used only for manoeuvring or slow speeds. It is of smaller capacity than would be required for an ordinary circulating pump.

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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