Document VjNRQLgwg7eBLwL7LweLB5YMw

-3 CHAPTER 14 1956 Guide 362 Table 9. Approximate Maximum Theoretical COt Values, and COt Values for Various Fuels with Different Percentages of Excess Air ' Pebcent COi at Given Excess Aih Valdes Type of Fuel Ultimate Percent COt . 20% j 40% 60% Coke Anthracite Bituminous Coal No. 1 and 2 Fuel Oil No. 6 Fuel Oil Natural Gas Carburetted Water Gas Coke Oven Gas 21.0 20.2 18.2 15.0 16.5 12.1 17.2 112 .. 17.5 16.8 15.1 12.3 13.6 9.9 14.2 9.2 15.0 14.4 12.9 10.5 11.6 8.4 12.1 7.8 13.0 12.6 11.3 9.1 10.1 7.3 10.6 6.8 Mixed Gas (Natural and Carbu retted Water Gas) Propane Gas (Commercial) Butane Gas (Commercial) 15.3 13.9 14.1 12.5 11.4 11.6 10.5 9.6 9.8 9.1 8.4 8.5 a heat balance. Various components of this balance are generally expressed in terms of Btu per pound of fuel burned, or as a percentage of its calorific value. Components of special interest are listed as items 1 to 7 inclusive. 1. Useful heat transferred to heating medium, usually computed by. determining the rate of flow of the heating fluid through the heating device, and the change in enthalpy of the fluid (heat added) between the inlet and outlet. 2. Heat loss in the dry chimney gases. hi = WjCp (fs - f,,). 3. Heat Iobs in water vapor formed by the combustion of hydrogen. , hi = Qj~Ht (1089 - t. + 0.455 Q (12) (13) 4. Heat loss in water vapor in the air supplied for combustion. h, = 0.455 M w. (it - <,) 5. Heat loss from incomplete combustion. - 10143<7(a5^co) (14) (15) 0. Heat loss from unburned carbon in the ash or refuse. (16) 7. Radiation and all other unaccounted for losses. Radiation and convection losses from a heating appliance are not usually deter mined by direct measurement. For this reason they, together with any other losses not measured, are determined by subtracting the total of items 1 to 6 from the heat of combustion of the fuel. If the heating appliance is located within the heated space, however, radiation and convection losses may be considered as useful heat rather than lost heat. They may, therefore, be omitted from calculations of heat * A value of 14800 applies is calculating ash pit loss; in calculating heat of formation of carbon compounds use' 14093 Btu per lb. Fuels and Combustion 363 losses, or added to item 1. If there is CO in the flue gases, small amounts of un burned hydrogen and hydrocarbons will probably also be present. The small loses due to incomplete combustion of these latter gases would also be included in item 7. Symbols used in Equations 12 to 16 inclusive are: hi = heat loss in the dry chimney gases, Btu per pound of fuel. ht = heat loss in water vapor from combustion of hydrogen, Btu per pound of fuel. hi = heat loss in water vapor in combustion air, Btu per pound of fuel. hi = heat loss from incomplete combustion of carbon, Btu per pound of fuel. hi = heat loss from unburned carbon in the ash, Btu per pound of fuel, ujj = weight of dry flue gas per pound of fuel (from Equation 10), pounds. c,, = mean specific heat of flue gases at constant pressure (cp ranges from 0.242 to 0.254 for flue gas temperatures from 300 F to 1000 F)!, Btu per pound. t, = temperature of flue gases at exit of heating device, Fahrenheit. U = temperature of combustion air, Fahrenheit. Hi = percentage of hydrogen in fuel by weight from ultimate analysis of fuel burned. 1091.8 = enthalpy of saturated water vapor at a temperature of 70 F, Btu per pound. M = humidity ratio of combustion air, pounds of water vapor per pound ol dry air. ' to. = weight of combustion air per pound of fuel used, pounds, from Equations 2, 4, 5, 6, 7 and 8. CO, COi = percentages of CO, COt in flue gases by volume. C -- weight of carbon burned per pound of fuel corrected for carbon in ash, pounds. WC,, - W. 100 W (17) where C,, = percentage of carbon in the fuel by weight from the ultimate analysis. Wi -- weight of ash and refuse, pounds. Ci = percent of combustible in ash by weight (combustible in ash is usually considered to be carbon). W = weight of fuel used, pounds. Flue gas losses for solid and liquid fuels, listed as items 2, 3 and 4 of the heat balance, may be determined with sufficient precision for most purposes from curves shown in Fig. 62, if COt content and temperature of flue gases are known. Values of the losses plotted for fuel oil were com puted from the ultimate analysis of a typical fuel oil used in domestic burners, while those presented for the several ranks of coal were computed from the typical ultimate analyses shown in Table 1. The curves for medium volatile bituminous coal may be used for high volatile bituminous coal with negligible error. Utilization of gaseous fuels, for numerous reasons, is generally a more sim ple process than is the case with either solid or liquid fuels. Accordingly, the determination of a practical heat balance is also a more simple procedure in that items 5 and 6 do not generally apply to gas installations. A series of typical alignment charts has been combined in Fig. 7 for use in determin ing flue losses of items 2, 3 and 4 from common types of gas burning appli ances. To determine flue losses place a straight edge extending from the corrected temperature reading to the percent COt recorded. Percent flue loss is indicated where the straight edge interesects the flue loss column.