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192 Chapter 10 1945 Guide --Stokers-in-this -class-feed-coal-to -the-furnace-intermittently- in-aceor-- -dance with temperature or pressure demands. A special control is used to insure sufficient stoker operation to maintain a fire during periods when no heat is required. Stoker-Fired Boiler and Furnace Units Boilers, air conditioners', and space heaters especially designed for stokers are available having design features closely coordinating the heat absorber and the stoker. Although efficient and satisfactory performance can be obtained from the application of stokers to existing boilers and furnaces, some of the combination stoker-fired units (Fig. 4) are more compact and attractive in appearance. Class 2 and 3 Stokers Stokers in this class are usually of the screw feed type without auxiliary Fig. 2. Underfeed Stoker, Bin Feed Type, Class 1 plungers or other means of distributing the coal. They are used exten sively for heating plants in apartments and hotels, also, for industrial plants. They are of the underfeed type and are available in both the hopper type, as illustrated in Fig. 5, and the bin feed type, shown in Fig. 6. These units also are built in plunger feed type with an electric motor or a steam or hydraulic cylinder coal feed drive. Stokers in this class are available for burning all types of anthracite, bituminous and lignite coals. The tuyere and retort design varies ac cording to the fuel and load conditions. Stationary type grates are used on bituminous models and the clinkers formed from the ash accumulate on the grates surrounding the retort. Anthracite stokers in this class are equipped with moving grates which discharge the ash into a pit below the grate. This ash pit may be located on one or both sides of the grate and on some installations is of sufficient capacity to hold the ash for several weeks' operation. / Automatic Fuel Burning Equipment 193 --Glass 4 Stokers-------- ------------------------ ------- ----------------------------------------- Stokers in this group vary widely in details of design and several methods of feeding coal are employed. The underfeed stoker is widely used, although a number of the overfeed types are used in the larger sizes. Bin-feed, as well as hopper models, are available in both under feed and overfeed types. Glass 5 Stokers The prevalent stokers in this field are: (1) underfeed side cleaning, (2) underfeed rear cleaning, (3) overfeed flat grate, and (4) overfeed inclined grate. Underfeed side cleaning stokers are made in sizes up to approximately 500 boiler horsepower. They are not so varied in design as those in the smaller classes, although the principle of operation is similar. A stoker of this type is illustrated in Fig. 7. The rear cleaning underfeed stoker is usually of the multiple retort design and is used in some of the largest industrial plants and central power stations. Zoned air control has been applied on these stokers, both longitudinally arid transversely of the grate surface. The overfeed flat grate stoker is represented by the various chain--or traveling-grate stokers. A typical traveling-grate stoker is illustrated in Fig. 8. Another distinct type of overfeed flat-grate stoker is the spreader (Figs. 9 and 10) type in which coal is distributed either by rotating paddles or by air over the entire grate surface. This type of stoker is adapted to a wide range of fuels and has a wide application on small sized fuels, and on fuels such as lignites, high-ash coals, and coke breeze. The overfeed inclined-grate stoker operates on the same general com bustion principle as the flat-grate stoker, the main difference being that rocking grates, set on an incline, are provided in the former to advance the fuel during combustion. Combustion Process In anthracite stokers of the Class 1 underfeed type, burning takes place entirely within the stoker retort. The refuse of combustion spills 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 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 strerigth and 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