With diesel machinery the amount of heat carried away by the exhaust gases varies between 20 to 25 per cent of the total heat energy supplied to the engine. Recovery of some of this heat loss to the extent of 30 to 50 percent is possible by means of an exhaust gas boiler or water heater.
The amount of heat recovered from the exhaust gases depends upon various factors, some of which are, Steam pressure, temperature, evaporative rate required, exhaust gas inlet temperature, mass flow of exhaust gas, condition of gas exchange surfaces, etc.
Composite boilers are often used in conjunction with Diesel machinery, since if the exhaust gas from the engine is low in temperature due to slow running of the engine and reduced power output; the pressure of the steam can be maintained by means of an oil fired furnace. Steam supply can also be maintained by these types of boilers when the engine is not in operation.
The Cochran boiler whose working pressure is normally of the order of 7 bar is available in various types and arrangements, some of which are:
Simple pass composite, i.e., one pass of the exhaust gases and two uptakes, one for the oil fired system and one for exhaust system. Double pass composite, i.e., two passes for the exhaust gases and two uptakes, one for the oil fired system and one for the exhaust system. Double pass exhaust gas, no oil fired furnace and a single uptake. Double pass alternatively fired, two passes from the furnace for either exhaust gases or oil fired system with one common uptake.
The material used is good quality low carbon boiler steel plate. The furnace is pressed out of a single plate and is seamless.
"REED'S GENERAL ENGINEERING KNOWLEDGE FOR MARINE ENGINEERS", by Leslie Jackson and Thomas D. Morton