Document bOLBZDG1gx8GBbkdn3wRR633O

336 CHAPTER 16 1949 Guide . Since the calculation is usually made from Orsat'analysis, Equation 9 will be found to be a convenient statement of this relationship. "("-?) Per cent excess air ' f CO\ N, X 0.264 -( ft- Y ) (9) : In this formula the symbols represent volumetric percentages of the flue gas constituents as determined by analysis. Due to the different carbon-hydrogen ratios of the different fuels the maximum COt attainable varies. Representative values for complete combustion of several fuels are given in Table 9. To produce heat efficiently with any of the common fuels the following requirements must be observed: 1. Adequate heat absorbing surface is necessary. 2. The heat transfer surfaces must be clean. 3. A minimum of excess air should be used. 4. The combustion air and the combustible gases produced by the'fuel must be well mixed. 5. The quantity of combustible gases escaping to the stack must be kept small. If insufficient heating surface is provided in a. heating appliance, or if the heat transfer surfaces are covered with soot, ash or scale, the flue gas temperature will be excessive and the amount of sensible heat passing upthe stack will be .unnecessarily large. Too much excess air dilutes the flue gases excessively and increases the sensible flue gas loss,, while a defi ciency of air will cause some combustible gases to pass out of the appliance unbumed. The highest combustion efficiency is.not always, obtained by supplying enough excess air to reduce the incomplete combustion loss to zero, but the incomplete combustion loss should be kept small. If the secondary air is not well mixed with the combustible gases, some, incom plete combustion may still occur. 'Unnecessary secondary air also dilutes the flue gases and increases the sensible heat escaping up the chimney. Some excess air is always required in the practical operation of heating plants. It is considered good practice, under usual operating conditions, to supply from 25 to 50 per cent excess air, depending upon the fuel used. HEAT BALANCE In analyzing the. performance of a heating appliance, it is frequently desirable to make an accounting, insofar as possible, of the disposition Table 9. Repbesentative Maximum COj Value Fuel Theoretical CO* 21.00 20.20 18.20 15.00 16.50 . 12.00 " " 11.00 COt Usually Attained In Practice 12-14 12-14 13 10.5 ' ' 13.5 9.7 8.5 Etiels and Combustion . . 337 of all the..heat- units in the fuel used.. Such, an accounting-is sometimes called a heal balance. The several components of the heat .balance may either, be .expressed in terms.of .Btu per pound of fuel used or as .a perr centage of the calorific .value of the fuel. The components of the heat balance, are listed in items 1 to. 7. 1. Useful heat transferred to heating medium and usually evaluated by determin ing 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 = wicp-ff, - t.) 3. Heat loss in water vapor formed by the combustion of hydrogen. Q ff A* = --*a091^ + 0.455 i.-fj (10) (11) . 4. Heat loss in water vaporin the air supplied for combustion. h 0.456 M to. (f,- U) 5. Heat loss from incomplete combustion. h, = 10143 . C\|c-o---,---+ -c--o- |J 6. Heat loss from unburned carbon in the ash or refuse. A` = 14093.(l^-C) (12) K(13') (14) . 7. Radiation and all. other unaccounted -for losses. Since the radiation and convection losses from a heating appliance are not usually' determined by direct measurement, they, together with any other-losses not meas ured, are determined by subtracting the total of items 1 to 6 inclusive from the heat of combustion of the fuel. Frequently, when there'is CO in the flue gases there also will be small'amounts of unburned hydrogen and hydrocarbon gases in the products of combustion'. .. The loss represented by.-these unburned gases may easily be as large as that resulting from the presence of carbon monoxide. In this event item 7 of the. heat balance would also include this unmeasured loss. Symbols used in Equations 10 to 14 inclusive.are: hi = heat loss in the.dry chimney gases, Btu per pound of.fuel. . h*." heat loss in water vapor from combustion of hydrogen, Btu per pound of fuel. . - . I';. , ... .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. ' fe. weight of dry flue gas per pound of fuel (from Equation 6), pounds. c,, =? mean specific heat of flue gases at constant.pressure (ei ranges from 0.242 to 0.254 for flue gas temperatures from'300'F to 1000 Fj*, Btuper pound. 1 tt = temperature of flue gases at exit of heating device, Fahrenheit degrees. .temperature of combustion air^Fahrenheit degrees. t , . H-%*~ percentage of.-hydrogen in fuel by weight from ultimate analysis' of.fuel'; .'as-fired; ' . . . i \ v : V; s. -