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American Society of Heating and Ventilating Engineers Guide, 1935 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 bed, which draws in more secondary air through 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 Fig. 1. Relation Between CO, and Excess Air in Gases of Combustion size of slot opening is dependent on whether the ashpit damper is open or closed. It is better to have too much tfiarrtoo little secondary air; the opening is too small if there is a puff of flame when the firing door is opened. Fig. 2, 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. Bituminous coals require a large amount of secondary air during the period subsequent to a firing in order to consume the gases and to reduce the smoke. The smoke produced is a good indicator, arid 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 dosed all the time. 446 - From U.'S. Bureau of Mines. Fig. 2. 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 3. For temperatures between very fnild 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 bed and more frequent cleanings to keep its resistance down. Insufficient draft also restricts the control 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, and it is often possible to produce a higher rate by increasing the thickness of the fuel bed. Combustion of-Anthracite1 An anthracite fire should never be poked, as this serves to bring ash to the surface of the fuel bed where it melts into clinker. Egg size is suitable for large firepots (grates 24 in. and over) if the fuel "See reports published by The Anthracite Institute Laboratory, Primos, Pennsylvania..447