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Circulating systems for motorships, heat exchangers & control of temperatures
The heat produced by running machinery, must be removed to ensure the satisfactory functioning of the equipment.
Cooling is achieved primarily through circulation of water, oil and air but the abundant supply of sea water is normally reserved for use as an indirect coolant because the dissolved salts have a great potential for depositing scale and assisting in the setting up of galvanic corrosion cells. Pollution of coastal areas by industrial and other wastes has added to the problems of using sea water as a coolant.
Circulating systems for motorships
The usual arrangement for motorships (Figure below) 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.
Fig :Conventional sea-water circulation system
The supply for other auxiliaries and equipment may be derived from the main sea-water system also. There may be two sea-water circulating pumps installed as main and stand-by units, or there may be a single sea-water circulating pump with a stand-by pump which is used for other duties.
The latter may be a ballast pump fitted with a primer and air separator. Ship side valves, can be arranged with high and low suctions or fitted to water boxes. High suctions are intended for shallow water to reduce the intake of sediment.
Low suctions are used at sea, to reduce the risk of drawing in air and losing suction when the ship is rolling.
A water box should be constructed with a minimum distance of 330 mm between the valve and the top, for accumulation of any air which is then removed by a vent. A compressed air or steam connection is provided for clearing any weed. Ship side valve bodies for the sea-water inlet must be of steel or other ductile metal. Alternative materials are bronze, spheroidal graphite cast iron, meehanite or another high-quality cast iron. Ordinary grey cast iron has proved to be unreliable and likely to fail should there be shock from an impact or other cause. Permissible cast irons must be to specification and obtained from an approved manufacturer.
Bronze has good resistance to corrosion but is expensive and therefore tends to be used for smaller ship side valves. Steel is cheaper, but prone to corrosion, It may be cast or fabricated. Unprotected steel valve casings and pipes will, in the presence of sea water and bronze seats, valve lids and spindles, waste due to galvanic corrosion.
However, the presence of corroding iron or steel confers benefits in sea-water systems. The metal acts as a sacrificial anode and additionally delivers iron ions which are carried through and give protection to other parts of system where they deposit.
The fresh-water circuit comprising jacket water circulating pumps, fresh-water coolers, cylinder jackets, cylinder heads, exhaust valves (if fitted), turbo-blowers and a branch to an evaporator, is under positive head, and therefore in a closed system with a header tank. It is normal for there to be a blanked connection between the sea-water system and engine jacket water circuit, for use in an emergency.
If the engine pistons are fresh-water cooled, the circuit may be in parallel with the jacket circuit but it is more likely to be separate. Main and stand-by piston cooling water circulating pumps are mounted directly on the drain tank so that with flooded suctions no primer is required. The piston cooling system embraces a separate cooler, the inlet manifold, telescopic pipes, pistons, outlet manifold, drain tank and pumps.
The engine system temperatures are kept as high as practicable. The system shown has salt-water bypass valves on oil and water coolers for temperature control. These are valves controlled by thermo-pneumatic devices. It is usual to make provision for warming the fresh circulating water before the main engines are started, either by steam or by circulating from the auxiliary jacket water cooling circuit.
The auxiliary sea-water cooling circuit for generator diesel prime movers may have its own sea inlet and pumps for circulation, with a cross connection from the main sea-water circulation system. Air compressors together with the inter- and after-coolers may be supplied with sea-water cooling in parallel with the main system or alternatively, there may be crankshaft-driven pumps. Charge air coolers are sea-water circulated.
The jacket water system for generator diesel prime movers is similar to that for the main engines, usually with a separate header tank. Pumps for the services are duplicated or cross connected. Sea-water pipes for circulation of cooling water, together with those for bilge and ballast systems, are prone to internal wastage from corrosion and erosion. External corrosion is also a problem in the tank top area. Steel pipes additionally suffer from rusting.
Control of temperature in heat exchangers
The three basic methods for controlling the temperature of the hot fluid in a heat exchanger when the cooling medium is sea-water, are:
- to bypass a proportion or all of the hot fluid flow,
- to bypass or limit the sea-water flow;
- to control sea-water temperature by spilling part of the sea-water discharge back into the pump suction.
The last of these methods could be used in conjunction with one of the other
two and it was resorted to when sea water was used for direct cooling of diesel engines. It enabled the sea water to be passed through jackets at a temperature warmer than that of the sea. Very cold sea water would cause severe thermal stress. The temperature of sea water for direct cooling was kept to between 40 deg and 49 deg C, the upper limit being necessary to limit scale formation.
Automatic control equipment for the system shown above, is based on using a control valve to bypass the sea water at the outlet side of the heat exchanger. This ensures that the heat exchanger is always full of sea water and is particularly important if the heat exchanger is mounted high in the sea-water system and especially if it is above the water line. Pneumatically operated valves may be fitted for temperature control, through bypassing the sea water, The flow of hot fluid through a heat exchanger may be controlled by a similar bypass or by a control valve of the Walton wax-operated type, directly actuated by a temperature sensor.
Summarized below various circulating systems for motorships, some of the basic procedure of heat exchangers & control of temperatures:
- Sea water circulation-systems
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....
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Shell and tube heat exchangers for engine cooling water and lubricating oil cooling
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....
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Plate type heat exchanger
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....
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Details of charged air cooler
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....
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Maintenance of heat exchangers
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....
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Central cooling system & Scoop arrangement for motorships
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.....
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Circulating systems for steamships
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 ....
- Closed feed system and feed heating for motor ships
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,....
- Marine condenser assembly
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)....
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Three stage air ejector with internal diffusers
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. ...
- Pressure governor for motor ships
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....
- Liquid ring pump- Nash rotary liquid ring pumps
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...
- The Weir electro-feeder - a multi-stage centrifugal pump
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....
- Feed water heaters for motor ships
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....
- Devaporizer & turbo-feed pump
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. ...
- Typical de-aerator & Cascade trays
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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