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HEATING VENTILATING AIR CONDITIONING GUIDE 1944 over the edge of the retort into an ash pit or receptacle from which it may be removed either manually or automatically. Larger underfeed anthracite stokers operate on the same principle except that the retort is rectangular and the refuse, spills over only one or two sides of the grate. Anthracite for stoker firing is usually the No. 1 buckwheat or No. 2 buckwheat size. Because the majority of the smaller bituminous coal stokers operate on the underfeed principle, a general description of their operation is given. When the coal is fed into the retort, it moves upward toward the zone of combustion and is heated by conduction and radiation from the burning 'V CHAPTER 10. AUTOMATIC FUEL BURNING EQUIPMENT 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 continuously or periodically discharged. 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 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 fuel in the combustion zone. As the temperature of the coal rises, it gives off moisture and occluded gases, which are largely non-combustible. When the temperature increases to around 700 or 800 F the coal particles become plastic, the degree of plasticity varying with the type of coal. A rapid evolution of the combustible volatile matter occurs during and directly after the plastic stage. The distillation of volatile matter con tinues above the plastic zone where the coal is coked. The strength arid porosity of the coke formed will vary according to the size and character istics of the coal. While some of the ash fuses into particles on the surface of the coke as it is released, most of it remains on the hearth or grates and as this ash layer becomes thicker with time, that portion exposed to the higher temperatures surrounding the retort fuses into a clinker. The temperature in the fuel bed, the chemical composition and homogeneity of the ash, and the time of heating govern the degree of fusion, . Most bituminous coal stokers of Classes 1, 2, 3 and 4 require manual removal of the ash in clinker form. 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 206 Fig. 10. Overfeed Spreader Stoker (Pneumatic Type) at a sufficient rate to coiriplete 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 for small sizes of anthra cite or coke breeze and also for bituminous coals, the characteristics of which make it desirable to burn 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. Frequently, a rear combustion arch is also required. 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 for 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 as to maintain a balance between the load demand and the heat 207