The weight of flue gases resulting from the combustion of a pound of dry coal will be the sum of the weights of the air per pound of coal and the combustible per pound of coal, the latter being equal to one minus the percentage of ash as found in the boiler test. The weight of flue gases per pound of dry fuel may, however, be computed directly from the analyses, as shown later, and the direct computation is that ordinarily used. The ratio of the air actually supplied per pound of fuel to that theoretically required to burn it is: N 3.036(---------)×C CO_{2}+CO ------------------ (13) C O 34.56(- + H - -) 3 8 in which the letters have the same significance as in formulae (11) and (12). The ratio of the air supplied per pound of combustible to the amount theoretically required is: N ------------------ (14) N - 3.782(O - ½CO) which is derived as follows: The N in the flue gas is the content of nitrogen in the whole amount of air supplied. The oxygen in the flue gas is that contained in the air supplied and which was not utilized in combustion. This oxygen was accompanied by 3.782 times its volume of nitrogen. The total amount of excess oxygen in the flue gases is (O - ½CO); hence N - 3.782(O - ½CO) represents the nitrogen content in the air actually required for combustion and N ÷ (N - 3.782[O - ½CO]) is the ratio of the air supplied to that required. This ratio minus one will be the proportion of excess air. The heat lost in the flue gases is L = 0.24 W (T - t) (15) Where L = B. t. u. lost per pound of fuel, W = weight of flue gases in pounds per pound of dry coal, T = temperature of flue gases, t = temperature of atmosphere, 0.24 = specific heat of the flue gases. The weight of flue gases, W, per pound of carbon can be computed directly from the flue gas analysis from the formula: 11 CO_{2} + 8 O + 7 (CO + N) ---------------------------- (16) 3 (CO_{2} + CO) where CO_{2}, O, CO, and N are the percentages by volume as determined by the flue gas analysis of carbon dioxide, oxygen, carbon monoxide and nitrogen. The weight of flue gas per pound of dry coal will be the weight determined by this formula multiplied by the percentage of carbon in the coal from an ultimate analysis. [Graph: Temperature of Escaping Gases--Deg. Fahr. against Heat carried away by Chimney Gases--In B.t.u. per pound of Carbon burned.[31] Fig. 20. Loss Due to Heat Carried Away by Chimney Gases for Varying Percentages of Carbon Dioxide. Based on Boiler Room Temperature = 80 Degrees Fahrenheit. Nitrogen in Flue Gas = 80.5 Per Cent. Carbon Monoxide in Flue Gas = 0. Per Cent] Fig. 20 represents graphically the loss due to heat carried away by dry chimney gases for varying percentages of CO_{2}, and different temperatures of exit gases.