Marine Auxiliary Machinery 7th-edition - McGeorge H. D. [1995, PDF]

Main propulsion services and heat exchangers

Main propulsion services and heat exchangers
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 1.1) 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. 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
Main propulsion services and heat exchangers 3
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:
1 to bypass a proportion or all of the hot fluid flow,
2 to bypass or limit the sea-water flow;
3 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