Document n9n9md1GpRJa16deGNGmq7VKw

American Society of Heating and Ventilating Engineers Guide, 1932 in combustible products passing to the stack unburned. An excess of air absorbs heat from the products of combustion and results in a greater loss of sensible heat to the stack. Total Air Required. The theoretical amount of air required per pound of fuel for perfect combustion is dependent upon the analysis of the fuel; however, for estimating purposes the theoretical air required for different grades of fuel may roughly be taken from Table 1. An excess of about 50 per cent over the theoretical amount is considered good practice under usual operating conditions. The amount of excess air, based upon the laws of combustion, can be determined by its relation to the percentage of C02 (carbon dioxide) in the products of combustion. This relationship is shown by the curves Chapter 16--Fuels bed and diameter of fire pot. The ratio of the secondary to the primary air increases with decrease in the size of the fuel pieces, with increase in the depth of the fuel bed, and with increase in the area of the fire pot; the ratio also increases with increase in rate of burning. Size of the fuel is a very important factor in fixing .the quantity of secondary air required for non-caking coals. With caking coals it is not so important because small pieces fuse together and form large lumps. Fortunately a smaller size fuel gives more resistance to air flow through the fuel bed arid thus autoiriatically a larger draft above the fuel bed, which thus draws in more secondary air for the same slot openings. In spite of this, a small size fuel requires a larger opening of the door slots; for a certain size for each fuel no slot opening is required, and for larger sizes too much excess air gets through the fuel bed. Fig. 2. Relation Between CO, and Excess Air in Gases of Combustion (Fig. 2) for high and low volatile coals and for coke. In hand-fired fur naces with long periods between firings the combustion goes through a' cycle in each period and the quantity of excess air present varies. Secondary Air. The division of the total into primary and secondary air necessary to produce the same rate of burning and the same excess air depends on a number of factors which include size of fuel, depth of fuel Table 1. Pounds of Air per Pound of Fuel as Fired Anthracite CoKB Ssmi-Bituminous Bituminous Lignite 9.6 11.2 11.2 10.3 6.2 Fig. 3. Relative Amount of Fire Door Slot Opening Required in a Given Furnace to Give Equally Good Combustion for High Temperature Coke of Various Sizes When Burned at Various Rates It is impossible to establish a single rule for the correct slot opening for all types and sizes of fuels and for all rates of burning. Furthermore, the size of slot opening is dependent on whether the ashpit damper is operi or closed. It is better to have too much than too little secondary air; the opening is too small if there is a puff of flame when the firing door is operied. Fig. 3 taken from the U. S. Bureau of Mines, Report of Investigations No. 2980, shows the relationship of the slot opening, for a domestic fur nace, to the size of coke and the rate of burning; these openings are with the ashpit damper wide open, and would be less if the available draft permits of it being partly closed. The same openings are satisfactory for anthracite; Bituminous coals require a large amount of secondary air during the period subsequent to a firing, to consume the gases and to reduce the 253