Accessories are installed inside or outside the boiler for their proper operation and to increase its efficiency.
Economizer – The flue gases from boiler carries heat up the chimney. The function of economizer is to recover some heat from the flue gases and utilize it to preheat the feed water to the boiler. The economizer is placed in between the exit from boiler and entry to the chimney. The economizer works as a heat exchanger device that heats water. In economizer the water flows in tubes and the flue gases pass over it outside the tubes. The heat is exchanged from flue gases to water.
Air pre-heater – the temperature of air is increased before it enters the furnace. It is generally placed after the economizer. Its purpose is to increase the thermal efficiency of the process. In tubular type air pre-heater the flue gases are passed inside the tubes and air to be heated is passed over the tube. The various types of air pre-heater are (i) Tubular type (ii) Plate type (iii) Storage type
Figure: Air pre-heater
Super heater – It increases the temperature of steam, generated by boiler, above its saturation point. It is done to reduce the losses due to condensation and to reduce the steam consumption by the turbine. It also eliminated the turbine blade erosion.
Feed water pump – The function of feed pump is to pump water from water space to a boiler. For medium size boilers, duplex pumps are used. Duplex pump is a double acting pump in which two cylinders are placed in a direction opposite to each other. The unit consists of one water cylinder and one steam cylinder. They work alternatively to ensure continuous supply of feed water. There are two types of feed water pumps used in boilers (i) Rotary pump (ii) Reciprocating pump
Injector – The function of injector is same as that of a boiler. It is used in vertical boilers and locomotive boilers. It is not used for high capacity boilers. It uses the venture effect of a converging diverging nozzle to convert pressure energy of fluid to velocity energy.
Steam separator – The steam from boiler to engine can be wet, dry or superheated. It is uneconomical if wet steam enters the engine so it is better to separate suspended water particles from steam before it enters the engine. This can be accomplished by using a steam separator. It is installed closed to the engine. The typical water separator is shown in the figure in which steam enters from Inlet and courses down. The steam strikes the baffle and is deflected up. The water particles having high density and hence greater inertia will strike the baffle plates and fall to the bottom of the separator.
Figure: Steam separator
Steam trap – The function of steam trap is to drain off condensate and non condensate gases without allowing the steam to escape through it. Mainly these are automatic valves.