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360 CHAPTER 14 1956 Guide tion efficiency is not always obtained when sufficient excess air is supplied to eliminate incomplete combustion entirely, because the air supply must not only be adequate but also be properly mixed with the combustible gases. Even with appliances connected to an effective flue every reason able precaution should be taken to insure as complete combustion as possible at all times. This is of vital importance in unvented equipment such as a gas space heater, for example. It is considered good practice to supply from 20 to 50 percent excess air, the exact amount depending on the kind of fuel burned and especially on the type of equipment in which it is utilized; Flue Gas Analysis As radiation, convection, and conduction losses from common types of central heating appliances are largely employed in heating occupied spaces, flue gas losses logically become the item of principal concern. Another reason for their importance is the fact that they are usually larger than all other heat losses combined. These important considerations and others indicate not only the desirability but absolute necessity of a reasonably accurate method for determining flue losses if it is desired to compute even approximate operating efficiencies. Customary procedure in arriving at flue gas losses is to make an analysis of a representative sample of flue gases and to measure their temperature. This information will enable the observer to compute the amount of flue gases produced, the excess air, the actual quantity of air supplied for com bustion, and the flue losses. While the.analysis of flue gases has been well described in several governmental and other scientific publications, the subject is of such importance that it warrants brief repetition here. Carbon dioxide and oxygen content are of principal interest in determining flue losses. Either or both of these values may be employed in such calcula tions. While the former constituent is probably most commonly employed, special considerations may make the latter of greater interest. Fortun ately, both can be determined readily by use of an Orsat gas analysis ap paratus, a device of reasonably simple construction and design. For field testing and burner adjustment, simple portable devices are available for determining carbon dioxide only. See Chapter 52 for details regarding the operation of the Orsat apparatus. The weight of dry flue gas per pound of fuel burned is widely used in combustion loss calculations. For solid fuels this item may be determined by application of Equation 10. llCO, + 8Q, + 7(CO+Ag Pounds dry flue gas per pound fuel 3 (COa + CO) X 1 Values for COz 02, CO, and V2 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. Total dry gas volumes of flue gases resulting from the combustion of one cubic foot of gaseous fuels for various percentages of COz may be deter mined by application of Equation 11; Cubic feet dry flue gases per cubic foot fuel gas = Cubic feet COt produced per cubic foot of gas burned X 100 Percent COt by analysis. After obtaining the quantity of flue gases from Equations 10 or 11, the excess air quantity may be determined by subtracting the quantity of dry Fuels and Combustion 361 flue gases which would result from perfect combustion^ Computations of flue losses are described in the next section on Heat Balance. Application of the preceding equations and tables are illustrated by Ex amples 1 and 8. Example l: The analysis of the flue gases resulting from the burning of a natural gas is 10.0 percent COt, 3.1percent Oj, and 86.9 percent N, by volume. The analysis of the fuel.is 90 percent CH,, 5 percent Nj, and 5 percent CjJs by volume. Find U the maximum theoretical percent COj and the percent excess air. Solution: From Equation 9: From Equation 8, (10.0X100) 100 - ( \0.21/ 118% CO, Percent Excess Air (11.8 - 10.0) X 90 .10 16.2 Example 2: For the analyses in Example 1 find, per cubic foot of fuel gas, the cubic feet of dry air required for combustion, the cubic feet of each constituent in the flue gases, and the total volume of dry and wet flue gases. Solution: From Equation 3 (or Table 7) the volume of dry air required for combus tion is: (9.53) (CH,) + (16.68)(CX/.) = 9.53 X 0.90 + 16.68 X 0.05 = 9.41 cu ft/cu. ft gas. * From Table 7, the constituents per cubic foot of gas are: Nitrogen, JVS: From methane = (0.9 CH.i) (9.53 -- 2.0) = 6.78 From ethane = (0.05 Cjff.) (16.68 - 3.0) = 0.68 Nitrogen in fuel = 0.05 Nitrogen in excess air = 0.791 X .162 X 9.41 = 1.20 Total Nitrogen ' 8.71 cu ft Oxygen, Of. Oxygen in excess air = 0.209 X .162 X 9.41 = 0.32 cu ft Carbon dioxide, COf. From methane = (0.9 CU,)(1.0) = 0.90 From ethane = (0.05 CtHti (4.0/2.0) = 0.10 Total Carbon Dioxide 1.00 cu ft Water vapor, HtO (does not appear in Orsat analysis): (0.9 Cff)(2.0) = 1.8 (0.05 Cy?t) (6-0/2.0) = .15 Total water vapor = 1.95 cu ft Total volume of dry gas per cubic foot of gas: 8.71 + 0.32 + 1.00 = 10.03 cu ft . Total volume of wet gases per cubic foot of gas (neglecting water vapor in combus tion air): 10.03 + 1.95 = 11.98 cu ft The cubic feet of dry flue gas per cubic foot of fuel gas may also be computed from Equation II as follows: (1.00). (100) 10.0 10.0 cu ft heat balance The usual practice in analyzing the performance of heating appliances is to make an accounting, insofar as possible, of the disposition of all heat units available in the quantity of fuel burned. This accounting is called