Marine Heat Exchangers
Marine heat exchangers are the majority common way to cool a boat’s engine, using the lake, river or ocean water in which the boat floats. Since this water may be caustic a sealed mixture of distilled water and antifreeze may cool the engine. Heat from the water-antifreeze mixture is then transferred to the ocean (or lake or river) water, which flows, into a heat exchanger. The water-antifreeze mixture runs through the heat exchanger dumping heat, but remaining separate from corrosive salts and chemicals found in the water the boat is floating in. If the ocean water eventually corrodes and ruins the heat exchanger it can be replaced at a fraction of the cost of replacing the engine. To protect the marine heat exchanger from corrosive salts, a sacrificial zinc anode is screwed into the heat exchanger. This anode must be periodically replaced as part of regular maintenance. Because the water the boat floats in may be contaminated with floating particles such as wood or Styrofoam balls the well designed boat will have a filter (often stainless steel mesh) to remove these particles before they are moved toward the heat exchanger. This filter must be periodically cleaned or else the flow of water to the heat exchanger will become obstructed and the engine will over heat.
Marine Uses :
Large ships usually carry evaporating plants to produce fresh water, thus reducing their reliance on shore-based supplies. Steam ships must be able to produce high quality distillate in order to maintain boiler-water levels. Diesel engined ships often utilise waste heat as an energy source for producing fresh water. In this system, the engine cooling water is passed through a heat exchanger, where it is cooled by concentrated sea water (brine). Because the cooling water (which is chemically treated fresh water) is at a temperature of 70-80 degrees C, it would not be possible to flash off any water vapour unless the pressure in the heat exhanger vessel was dropped. To alleviate this problem, a brine-air ejector venturi is used to create a vacuum inside the vessel. Partial evaporation is achieved and the vapour passes through a demister before reaching the condenser section. Sea water is pumped through the condenser section to cool the vapour sufficiently to precipitate it.
The distillate gathers in a tray, from where it is pumped to the storage tanks. A Salinometer monitors salt content and diverts the flow of distillate from the storage tanks if the salt content exceeds the alarm limit. Sterilisation is carried out after the evaporator. Evaporators are usually of the shell-and-tube type (known as an Atlas Plant) or of the Plate Type (such as the type designed by Alfa Laval). Temperature, production and vacuum are controlled by regulating the system valves. Sea water temperature can interfere with production, as can fluctuations in engine load. For this reason, the evaporator is adjusted as seawater temperature changes, and shut down altogether when the ship is manoeuvring. An alternative in some vessels, such as naval ships and passenger ships, is the use of the Reverse Osmosis principle for fresh water production instead of evaporators.
A water-jacketed exhaust manifold is necessary on marine engines to reduce the temperature of the engine-room air space and the exhaust pipe. If the exhaust manifold is in the sea-water circuit it should be installed with the sea-water inlet at the back and the outlet at the front on the top to ensure that it operates completely full of sea-water. If the manifold is in the fresh-water circuit a small by-pass hole must be provided in the thermostat to ensure that some water is circulating through the manifold at ail times.
Our development is to combine a water jacketed exhaust manifold with the heat exchanger and header tank. This arrangement is particularly suitable for small series-produced engines; the manifold is cooled by fresh water and as a result a keel-cooled engine can be made by omitting the heat exchanger tube stack and the sea-water pump. On installation the fresh-water outlet from the manifold would be connected to the keel pipes and the return taken back to the engine fresh-water pump. Heat exchanger/manifold assemblies are heavier than ordinary marine manifolds and must therefore be supported on the underside using the fixing lugs provided.
When automotive engines are being converted for marine use the existing centrifugal-type pump should be retained for the fresh-water circuit and an additional pump fitted for the sea-water circuit. The sea water pipe bore should be chosen so that the velocity does not exceed 2 m/sec on the suction side and 3 m/sec on the discharge side of the pump. If the engine is being used to drive auxiliary equipment in a ship and the sea water supply is taken from the ship’s main, ensure that the recommended flow rate cannot be exceeded.