Document V3zgKQknZ7GZg4rgvLbj9rGd8

American Society of Heating and Ventilating Engineers Guide, 1934 s 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 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 and thus automatically causes a larger draft above the fuel Chapter 27--Fuels and Combustion Bituminous coals, require a large amount of secondary air during the period subsequent to a firing, to consume the gases and to 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. The following suggestions will be helpful: 1. In cold weather, with high combustion rates, the secondary air damper should be half open all the time. 2. In very mild weather, with a very low combustion rate, the secondary air damper should be closed all the time. Fig. 2. Relation Between CO, and Excess Air in Gases of Combustion bed, which 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. 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 open 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 opened. 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 its being partly closed. The same openings are satisfactory for anthracite. 368 From U. S. Bureau of Mines. Fig. 3. Relative Amount of Fire Do<5r Slot Opening Required in a Given Furnace to Give Equally Good Combustion for High Temperature Coke of Various Sizes When Burned at Various Rates 3. For temperatures between very mild and very cold, the secondary air damper should be in an intermediate position. 4. For ordinary house operation, secondary air is needed after each firing for about one hour. Draft Required The draft required to effect a given rate of burning the fuel as measured at the smokehood is dependent on the following factors: 1. Kind and size of fuel. 2. Combustion rate per square foot of grate area per hour. 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 369