Document 3731ypp7a4NDqqkJKQr29vBzD

358 CHAPTER 14 1956 Guide Where approximate results only are desired, values appearing in Table 8 may be substituted for Equations 2, 3, and 4; or the air required for perfect combustion may be estimated by assuming that 0.9 cu ft air is required per 100 Btu of fuel. Where extreme precision is involved it is suggested that the reader refer to scientific literature published on the subject of combustion and related topics by the various industries concerned. If approximate values for the oretical air requirements suffice, or if complete information on the fuel is not available, the following values should also be found helpful: 1. Solid Fuels (Pounds air per pound fuel). Anthracite, 9.6; Semi-Bituminous, 11.2; Bituminous 10.3; Lignite 6.2; and Coke 11.2. Table 8. Approximate Aib Requirements for Theoretically Perfect Combustion of Fuels* Type op Fuel Aib Required fob Perfect Combustion Lbs per Lb Fuel Cu Ft per Unitb Fuel Approxi mate . Preci sion, Per Cent Exceptions . Solid Btu per lb X 0.00073 Btu per lb X 0.0097 Liquid Btu per lb X 0.00071 Btu per lb X 0.0094 Gas Btu per lb X 0.00067 Btu per cu ft X 0.0089 3 Fuels containing more than 30% water 3 Results low for gasoline and kerosene 5 Gases of 300 Btu per cu ft or less * Values in table taken from page 276 of Gaseous Fuels, 1948, published by American Gas Association. b Units for solid and liquid fuels in pounds, for gas in cubic feet. 2. Fuel Oil (Pounds air per gallon): Commercial Standard No. 1, 102.6; No. 2, 105.5; No. 5, 112; No. 6, 114.2. 3. Gaseous Fuels (Cubic feet of air per cubic foot): Natural, 10.0; Mixed Natural and Manufactured, 8.0; Manufactured, 4.7, Propane, 23.8, Butane, 31.0. COMBUSTION EFFICIENCY FROM THE FLUE GAS ANALYSIS Excess Air A commonly employed index of efficiency of combustion is the relation existing between the amount of air theoretically required for perfect com bustion and the amount of air actually supplied. Since the difference between air supplied for combustion and theoretical air required is charac terized as excess air, its percentage may be calculated by use of the follow ing equation, P,,ercent excess a.ir = I/A---i-r--s--u-p-p- l-i-e--d------ -T--h-e--o--r:-e-t-i-c--a--l-a---ir\) X 1.,00 y Theoretical air j .(5..) The amount of dry air supplied per pound of fuel burned may be obtained from Equation 6 which has reasonable precision for most solid and liquid fuels. Values for C02, CO and Nn are percentages by volume from the flue gas analysis, and C is the weight of carbon burned per pound of fuel,'cor rected for carbon in the ash. Pounds dry air supplied per pound of fuel .= 3.04N, (CO, + CO) X C (6) Because excess air calculations are almost invariably made from Orsat Fuels and Combustion 359 analysis results, and theoretical air requirements are not always known, another convenient method of expressing the relation , of Equation 5 is as follows: 100(0, - CO/2) Percent excess air = N, X 0.264 -- (O, -- CO/2) (7) As measurement standards for gaseous fuels are almost universally ex pressed in cubic feet. Equation 8 may be employed for computing excess air on a percentage basis for gases.21 where Percent excess air (17 - CO,) X 100 CO, (8) V = ultimate carbon dioxide, percent of flue gases resulting from perfect com bustion. CO, = carbon dioxide content of flue gases, percent. p = dry products from perfect combustion, cubic feet per cubic foot of gas burned. A = air theoretically required for complete combustion, cubic feet per cubic foot of gas burned. As the ratio of P/A is appropximately 0.9 for most city gases, a value of 90 may be substituted for 100 in Equation 8 for rough.calculation. Carbon-hydrogen ratios of different fuels vary considerably, hence the maximum -or ultimate C02 attainable also varies. Where they are un known, theoretical maximum C02 values,may be calculated from a flue gas analysis by use of Equation 9. Maximum theoretical % CO, = flue gas sample X 100 -tO, in same sample 100 021 rrA Approximate maximum C02 values for perfect combustion of several common types of fuel are shown in Table 9 together with values of C02 that will be attained with different amounts of excess air. Desirable values to be attained in practice depend upon the fuel, the method of firing, and other considerations. In general, fuels burned in suspension, such as gas, oil-, and pulverized coal, can be burned with a lower amount of excess air than fuels burned on grates. To produce heat efficiently by burning any common fuel a number of basic requirements must be met: (1) adequate heat absorbing surface of proper shape and construction is necessary in the appliance, (2) heat trans fer surfaces must be clean, (3) a minimum amount of excess air must be present, (4) air employed for combustion and combustible gases must be properly mixed, and (5) flue gas losses must be reduced to a safe minimum. If insufficient heating surface is employed, or if heat transfer surfaces are covered with soot, ash, or scale, flue losses will generally be excessive due to the large amount of sensible heat escaping to the chimney. Too much excess air dilutes flue gases excessively and increases sensible flue gas loss. On the other hand, a deficiency of excess air will in all probability cause incomplete combustion, and some of the combustible gases will pass from the appliance without being completely burned. Highest combus-