In single retort stokers the coal is commonly fed from a hopper at the stoker front into a centrally situated trough or retort by means of a pusher driven by a steam actuated piston or by an electric motor. In small underfeed stokers a slowly revolving screw or worm is commonly used to feed the coal from the hop per to the retort. Each new charge of coal introduced serves to push an equal volume of fuel out of the top of the retort and onto the grate bars which extend transversely from the sides of the retort. Alternate grate bars are movable and are connected to the stoker drive so as to impart a slight slicing motion which serves to maintain a porous fuel bed, to distribute the burning fuel across the stoker and to carry the ash and refuse to dump trays placed along each furnace side wall. Air for combustion is supplied under pressure by a fan or blower and is admitted to the fuel bed through openings along the sides of the retort and between the grate bars.
The multiple retort underfeed stoker comprises a series of retorts (usually inclined) mounted side by side across the furnace width. Coal is fed from a hopper, fixed at the stoker front, by rams connected to a crank shaft and driven through proper reducing gearing by an electric motor or other means. Secondary rams, situated in the bottom of the retorts, serve to break up the coke masses, to distribute the fuel over the stoker and to deposit the ash and refuse either on dump grates or into a pit at the rear where grinder rolls crush the refuse and discharge it into a pit below. Air for combustion is supplied under pressure by a fan, through nozzles or tuyeres arranged in longitudinal rows between adjacent retorts.
Mechanical stokers are usually controlled by regulators which are set to maintain a predetermined steam pressure and which are actuated by changes in this pressure. A drop in steam pres
sure causes an increase in stoker and fan speed so that more coal and air are admitted to correct the falling steam pressure. An increase in steam pressure causes a decrease in the amount of coal and air admitted. Stokers so equipped maintain constant steam pressure automatically and are called "automatic mechani cal stokers." Mechanical stokers are principally used for firing stationary steam boilers and metallurgical furnaces. However, types have been developed and satisfactorily applied to loco Superheaters are used to increase the temperature of steam without increasing its pressure. The furnace gases furnish the heat, either in the furnace, among the boiler tubes or in the ex haust to the stack where some lost heat can be recovered. For a discussion of superheaters and superheat see BOILERS ; STEAM.
Re/ieaters are used to prevent loss of superheat and the forma tion of moisture when steam is used in turbines. (See BOILERS.) Air Preheaters.—The exhaust gases coming directly from the furnace or from the economizers are used in air heaters or pre heaters to heat the air that is to be used for combustion in the furnace, thus making use of otherwise lost heat. (See BOILERS.) Feed Water water fed to a boiler can be con verted into steam it must first be heated to the temperature cor responding to the pressure in the boiler. This feed water is heated in open heaters, closed heaters or economizers, either to that or to a lesser temperature, depending on the plant. The first two types of feed water heaters use live or exhaust steam to obtain heat, the last uses the exhaust gases from the furnaces. (See