153 × 1½ = 229.5 cubic feet of air per pound of carbon. _Actual Volume_ _for One Pound Carbon_ _Per Cent_ _Cubic Feet_ _by Volume_ Carbon Dioxide 32 = 13.91 } Oxygen 16 = 7.00 } = 20.91 per cent Nitrogen 181.5 = 79.09 ----- ------ 229.5 = 100.00 For 100 per cent excess air the volume will be as follows: 153 × 2 = 306 cubic feet of air per pound of carbon. _Actual Volume_ _for One Pound Carbon_ _Per Cent_ _Cubic Feet_ _by Volume_ Carbon Dioxide 32 = 10.45 } Oxygen 32 = 10.45 } = 20.91 per cent Nitrogen 242 = 79.09 --- ------ 306 = 100.00 In each case the volume of oxygen which combines with the carbon is equal to (cubic feet of air × 20.91 per cent)--32 cubic feet. It will be seen that no matter what the excess of air supplied, the actual amount of carbon dioxide per pound of carbon remains the same, while the percentage by volume decreases as the excess of air increases. The actual volume of oxygen and the percentage by volume increases with the excess of air, and the percentage of oxygen is, therefore, an indication of the amount of excess air. In each case the sum of the percentages of CO_{2} and O is the same, 20.9. Although the volume of nitrogen increases with the excess of air, its percentage by volume remains the same as it undergoes no change while combustion takes place; its percentage for any amount of air excess, therefore, will be the same after combustion as before, if cooled to the same temperature. It must be borne in mind that the above conditions hold only for the perfect combustion of a pound of pure carbon. Carbon monoxide (CO) produced by the imperfect combustion of carbon, will occupy twice the volume of the oxygen entering into its composition and will increase the volume of the flue gases over that of the air supplied for combustion in the proportion of 100 + ½ the per cent CO 1 to ----------------------- 100 When pure carbon is the fuel, the sum of the percentages by volume of carbon dioxide, oxygen and one-half of the carbon monoxide, must be in the same ratio to the nitrogen in the flue gases as is the oxygen to the nitrogen in the air supplied, that is, 20.91 to 79.09. When burning coal, however, the percentage of nitrogen is obtained by subtracting the sum of the percentages by volume of the other gases from 100. Thus if an analysis shows 12.5 per cent CO_{2}, 6.5 per cent O, and 0.6 per cent CO, the percentage of nitrogen which ordinarily is the only other constituent of the gas which need be considered, is found as follows: 100 - (12.5 + 6.5 + 0.6) = 80.4 per cent.