Document o9D2RbbOB7mE5dnBweY036rD7

HEATINC VENTILATJNC AIR CONDITIONING GUIDE 1941 354 CHAPTER 18. CRAVITY WARM AIR FURNACE SYSTEMS where G = grate area, square inch. p = combustion rate, pound coal per square foot of grate per hour. / = heating value of the coal, Btu per pound. Ei = efficiency at bonnet, ratio of heat delivered at bonnet to heat developed in furnace. Ei = efficiency of duct transmission, ratio of heat delivered at register to heat delivered at bonnet. 0.866 = factor of safety to allow for contingencies under service conditions such as accumulations of soot and ashes, ineffective firing methods, etc. H = total heat loss from structure. An addition of 2 per cent of the furnace capacity is proposed for each unit when the ratio of heating surface to grate area exceeds 20. This addition is based on tests4 conducted at the University of Illinois on seven types of furnaces having varying ratios of heating surface to grate area. This correction does not, however, apply to values of the ratio less than 15 nor greater than 30. By transposing the terms in Equation 7 and adding the correction term for ratios of heating surface to grate area other than 20 to 1, the following equation is obtained: 144 X H G = pXfXEiXE, X 0.866 [1 + 0.02 (R-20)] (8) in which R = ratio of heating surface to grate area. In the case of the Standard Code6 the numerical values used in Equa tion 8 were based on those determined from the tests conducted on the different types of furnaces. ' 144 X H G = 7.5 X 12,790 X 0.55 X 0.75 X 0.866 [1 + 0.02 (R-20)] (9) G = 0.004205 [1 + H 0.02 (R-20)] (10) As used in these calculations, H = Btu heat loss from the entire house per hour = summation of all room losses Hi + Hi + etc. + the Btu necessary to heat the outside air, if any, at intake. This outside air loss in Btu per hour will be approximately 1.27 times the cubic feet of air admitted through the intake per hour on a zero day. For systems which recirculate all the air this value will be zero. For systems which have an outside air intake, controlled by damper, this value might well be approxi mated, since this loss will probably be reduced to a minimum on a zero day. Assume for such cases that the building loss is increased by 25 per cent, and that there is the usual 25 per cent loss between furnace and registers. TYPICAL DESIGN The application of the preceding data to an actual example may be of assistance to the designer. Figs. 8, 9, 10 and 11 represent the plans of ^University of Illinois, Engineering-Experiment Station Bulletin No. 246, by A. C. Willard, A. P. Kratz, and S. Konzo, Chapter X, pp. 126-146. Loc. Cit. Note 3. 355 V i