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354 CHAPTER 17 1949 Guide In the underfeed side-cleaning stokers the fuel is introduced at the front of the furnace to one or more retorts, and is advanced away from , the retort as combustion progresses, while finally the ash is disposed of at the sides. This type of stoker is suitable for all bituminous coals while in the smaller sizes it is suitable for small sizes of anthracite. In this type of stoker the fuel is delivered to a retort beneath the fire and is raised into the fire. During this process the volatile gases are released, are mixed with air, and pass through the fire where they are burned. The ash may be con tinuously or periodically discharged at the sides. The underfeed rear-cleaning stoker accomplishes combustion in much the same manner as the side-cleaning type, but consists of several retorts placed side'by side and filling up the furnace width, while the ash disposal is at the rear. In principle, its operation is the same as the side cleaning underfeed type. Overfeed flat-grate stokers receive fuel at the front of the grate in a 'Automatic Fuel Burning Equipment 355 as to maintain a balance between the load demand and the heat liberated by the fuel. Under such conditions no manual attention to the fuel bed should be required, other than the removal of clinker in stokers which operate on this principle of ash removal. As in all combustion processes, the maintenance of the correct proportions of air and fuel is essential. It is desirable to supply the minimum amount of air required to properly bum the fuel at the rate of feed. While there may be only slight variations in the rate at which the coal is being fed due to variations in the size or density of the coal, there may be wide variations in the rate of air flow as the result of changes in fuel bed re sistance. These changes in resistance may be caused by changes in the porosity of the fuel bed due to variations in size or friability of the coal, ash and clinker accumulation, and variations in depth of the fuel bed. Because of this variable fuel bed resistance, many bituminous stokers, even in the smaller domestic sizes, incorporate air controls which automatically com- layer of uniform thickness and move it horizontally to the rear of the furnace. Air is supplied under the moving grate to carry on combustion at a sufficient rate to complete the burning of the coal near the rear of the furnace. The ash is carried over the back end of the stoker into an ash pit beneath. This type of stoker is suitable |or small sizes of anthracite or coke breeze, and also for bituminous coals, the characteristics of which make it desirable to bum the fuel without disturbing it. This type of stoker requires an arch over the front of the fuel bed to maintain ignition of the incoming fuel and, frequently a rear combustion arch. In addition to the use of rocking grates, the overfeed inclined-grate stoker is provided with an ash plate on which ash is accumulated and dumped periodically. This type of stoker is suitable for all types of coking fuels but preferably for those of low volatile content. Its grate action keeps the fuel bed broken up thereby allowing free passage of air. Because of its agitating effect on the fuel it is not desirable for badly clinkering coals. It usually should be provided with a front arch to ignite the volatile gases. ,, Combustion Adjustments The coal feeding rate and air supply to the stoker should: be regulated so FlO. 10. OvEBFEED SpBEADEB StOKEB (PNEUMATIC TYPE) pensate for these changes in resistance and maintain a constant air fuel ratio. The efficiency of combustion may be determined by analyzing the flue gases as explained in Ghapters 11 and 16. It is desirable on most stoker installations to provide automatic draft regulation in order to reduce air infiltration and provide better control during the banking, or off, periods of the stoker. Furnace Design Although there is considerable variation in stoker, boiler, and furnace design, the stoker industry, from long-time experience, has established certain rules for the proportioning of furnaces for domestic, and com mercial stokers. The stoker installer and designer of stoker-fired equip ment should give careful consideration to these factors. The' Stoker Manufacturers Association has published standard recom mendations on setting heights for stokers having capacities up to 1200 lb of coal per hour2. The empirical formulas for determining these setting heights are: For burning rates up to 100 lb coal per hour H = 0.1125 B + 15.75 (1)