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316 CHAPTER 16 1943 Guide hour or two after firing, if, k>me. secondary. air is admitted through the slots of the fire door. However, unless the slots are closed when secondary air is'no longer beneficial, the decrease in efficiency during the remainder, of the firing cycle because of excess air may more than offset the gain resulting from the secondary air at the'beginning of the firing period. Unless the secondary air can be readjusted between firings, it is probable that a greater average efficiency will be obtained for domestic hand-fired devices by leaving the secondary air slots closed at all times. There is usually an appreciable amount of air leakage around the firing door and secondary air slots of domestic furnaces and boilers. When attention is given between firings the efficiency of combustion can be appreciably raised by admitting secondary air over a. bituminous coal fire to bum the gases and reduce the smoke. The smoke produced is a good indicator, and that opening is best which reduces the smoke to a minimum. Too much secondary air will cool the gases below the ignition point, and prove harmful instead of beneficial. Secondary air that enters the combustion chamber too. far removed from the zone of combustion will also be harmful, for the oxygen in the secondary air will not react with any unburned gases unless the mixture is subjected to high temperatures. The air requirements of oil and gas burners are discussed in Chapter 17, Automatic Fuel Burning Equipment. DRAFT REQUIREMENTS The draft required to effect a given rate of burning the fuel is dependent on the following factors: 1. Kind and size of fuel. 2. Grate area. 3. Thickness of fuel bed. . 4. Type and amount of ash and clinker accumulation. 5. Amount of excess air present in the gases. 6. Resistance offered by the boiler passes to the flow of the gases. 7. Accumulation of soot in the passes. : Insufficient draft will necessitate additional manipulation of the fuel bed and more frequent cleanings to keep its resistance down.- Insufficient draft also restricts the control that can be accomplished -by adjustment of the dampers. The quantity of excess air present has a marked effect on the .draft required to produce: a given rate.of burning. If the.excess is caused by holes in the fuel bed, or an extremely thin fuel bed, it.is often possible to produce a higher rate of burning by increasing the thickness of the bed. The thickness of the fuel,bed should not, however, be increased, too. much because the increased draft resistance will reduce the rate of primary air supply and the rate of burning. For amount of draft required see Chapter 19, Chimneys and Draft Calculations. DRAFT REGULATION Because of the varying heating load demands present in 'most instal lations it is necessary to vary the rate of fuel burning. The maintenance of the proper air supply for the various rates of burning is accomplished Fuels and.Combustion 317 by regulation-of the drafts. Correct and incorrect methods of draftregulation are. shown in Fig. 3. The air enters through the ashpit draft door, firing door, and by leaks in the setting,, whereas the gases, leave only, through the uptake.. By throttling the gases with, the damper in..the uptake all the air entering by each of the three intakes is reduced in the same proportion, thus maintaining about the same per cent of excess air. If the ashpit draft door is closed, the air. admitted, through; the ashpit is reduced, while it is increased through the other two intake openings, resulting in an increase of excess air. A considerable increase in the efficiency of hand-fired furnaces and boilers can be realized, by regulating the air supply with the damper in the smokepipe instead of the ashpit damper. Use. of the ashpit damper is required, of course, for low rates of combustion. Methods of control of. draft conditions when burning oil or gas are noted in Chapter 17, Automatic Fuel Burning Equipment. Fig. 3. Correct and Incorrect Methods of Draft Regulation in a Hand-Fired Furnace FURNACE VOLUME The principal requirements for a hand-firedfurnace are that it shall have enough grate area and correctly proportioned' combustion space. The amount of'grate area' required is dependent upon the desired combustion rate. The furhace volume is influenced by the kind of coal used. Bituminous coals, on account of their long-flaming characteristic, require more space in which to bum the gases of combustion completely than do the coals low in volatile matter. For burning high volatile coals provision should be made for mixing the combustible gases thoroughly, so that combustion is complete before the gases come in contact with the relatively-cool heating surfaces.-. An abrupt change in the direction of flow tends to mix the gases of combustion more thoroughly. Anthracite requires com paratively little combustion space. COMBUSTION OF GAS The majority of gas burners utilized in central domestic heating plants are, of the Bunsen - type and operate with a non-luminous flame. In this