The heat produced by running machinery must be removed to ensure the satisfactory functioning of the equipment. Cooling is achieved primarily through the circulation of water, oil and air but the abundant supply of seawater 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 seawater as a coolant.
Circulating systems for mentorships
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The usual arrangement for motor ships has been to have seawater circulation of coolers for lubricating oil, piston cooling, jacket water, charge air, turbocharger oil (if there are sleeve-type bearings) and fuel valve cooling, plus direct seawater cooling for air compressors and evaporators.
The supply for other auxiliaries and equipment may be derived from the main seawater system also. There may be two seawater circulating pumps installed as main and standby units, or there may be a single seawater circulating pump with a standby pump which is used for other duties. The latter may be a ballast pump fitted with a primer and air separator. Shipside 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 weeds. Shipside valve bodies for the seawater inlet must be of steel or other ductile metal. Alternative materials are bronze, spheroidal graphite cast iron, methane 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 corrosion resistance 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 seawater and bronze seats, valve lids and spindles, waste due to galvanic corrosion.
However, the presence of corroding iron or steel confers benefits in seawater systems. The metal acts as a sacrificial anode and additionally delivers iron ions which are carried through and give protection to other parts of the system where they deposit. The freshwater circuit comprising jacket water circulating pumps, freshwater coolers, cylinder jackets, cylinder heads, exhaust valves (if fitted), turbo blowers and a branch to an evaporator, is under a positive head, and therefore in a closed system with a header tank.
It is normal for there to be a blanked connection between the seawater system and engine jacket water circuit, for use in an emergency. If the engine pistons are freshwater cooled, the circuit may be in parallel with the jacket circuit but it is more likely to be separate. Main and standby 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 saltwater bypass valves on oil and water coolers for temperature control. These are valves controlled by thermopneumatic 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 seawater cooling circuit for generator diesel prime movers may have its own sea inlet and pumps for circulation, with a cross connection from the main seawater circulation system. Air compressors together with the inter and aftercoolers may be supplied with seawater cooling in parallel with the main system or alternatively, there may be crankshaft-driven pumps.
Charge air coolers are seawater 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. Seawater pipes for the 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.
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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 seawater, are:
1 to bypass a proportion or all of the hot fluid flow,
2 to bypass or limit the seawater flow;
3 to control seawater temperature by spilling part of the seawater 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 seawater was used for direct cooling of diesel engines. It enabled the seawater to be passed through jackets at a temperature warmer than that of the sea.
Very cold seawater would cause severe thermal stress. The temperature of seawater for direct cooling was kept between 40° and 49' 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 seawater at the outlet side of the heat exchanger.
This ensures that the heat exchanger is always full of seawater and is particularly important if the heat exchanger is mounted high in the seawater system and especially if it is above the water line. Pneumatically operated valves may be fitted for temperature control, by bypassing the seawater,
The flow of hot fluid through a heat exchanger may be controlled by a similar bypass or by a control valve of the Walton-operated type, directly actuated by a temperature sensor.
Shell and tube coolers
Shell and tube heat exchangers for engine cooling water and lubricating oil cooling have traditionally been circulated with seawater. The seawater is in contact with the inside of the tubes, tube plates and water.
two-pass flow is shown in the diagram but the 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. The usual method of securing the tubes in the tube plates is to roll and 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.
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 nonferrous metals or shielded by a protective coating.
The remedy in nonferrous 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 seawater, in the form of ferrous sulphate.
The latter treatment consists of dosing the seawater 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 seawater 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 seawater flow rates. To avoid the impingement attack, care must be taken with the water velocity through tubes.
For aluminiumbrass, 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 seawater 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. The tube stacks are made up of a fixed tube plate at one end and a tube plate at the other end 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 and synthetic rubber 'O' rings for the sliding tube plate to 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 common of cast iron or fabricated from mild steel. Unprotected cast iron in contact with seawater, 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 seawater should enter at the bottom and leave at the top.
Air in the cooler system will encourage corrosion and airlocks 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.
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