Document 0JqZX99X0e7Z3QbDnozNynaod

HEATINC VENTILATING AIR CONDITIONING GUIDE 1941 tion, together with the reduced heat losses up the chimney by reason of decreased gas velocity, results in fuel economy, with consequent lower costs of plant operation. CHARACTERISTICS OF CHIMNEYS In order to analyze the performance of a natural draft chimney, it may be advantageous to compare its general operating characteristics with those of a centrifugal pump and also of a centrifugally-induced draft fan, there being a similarity among the three. Figs. 2, 3 and 4 show the general operating characteristics of a typical centrifugally-induced draft fan, a typical centrifugal pump, and a typical natural draft chimney, respectively. The draft-capacity curve of the chimney corresponds to Fig. 3. Operating Characteristics of Typical Centrifugal Pump the head-capacity curve of the pump and also to the dynamic-headcapacity curve of the fan. When the gases in the chimney are stationary, the draft created is termed the theoretical draft. When the gases are flowing, the theoretical intensity is diminished by the draft loss due to friction, the difference between the two being termed the total available draft. If pressures at the bases of a column of air and a column of chimney gas, each of height Hfeet; and da and dQ represent the respective densities of the air and the gas in pounds per cubic foot, then the theoretical draft Dt in pounds per square foot is: Dt = doH - dcH Expressing the densities under standard conditions of pressure and temperature, and assuming that the absolute pressure of the gas is the same as that of the air, the theo retical draft becomes: * = 15.38 Expressed in inches of water this is: Wo Wc\ "^TER 8. CHIMNEYS AND DRAFT CALCULATIONS The friction loss in the chimney may be determined from the Fanning equation* Head lost in feet of fluid = *1x2--g in'which R is the inside perimeter ofthe cross-section in feet, A the cross-section area in'square feet, and V the velocity of fluid m feet per second. theSluosbsstoitfuhtienagdfionr inVchitessvoafluweaitnertethrmissboecf oImVeasn:d the cross-section area, and expressin6g For cylindrical stacks Al = 0.01936 and lor a rectangular stack of sides x and y in feet, AAll = 0.00597 lLW>J-x + *y*dc y) Substituting for dc its value: 460 Bp We 29.92 Tc rives for a cylindrical stack. jy TcfL Al = 0.00126 Dl Bq Wc nd.for a rectangular stack, Al = 0.000388 xy* B0 Wc The'available draft then is, for a cylindrical stack: = sIL (1) ind for a rectangular stack: v A AAAOOO TW r fr /v where Da = available draft, inches of water. H = height of chimney above grate bars, feet. B0 = barometric pressure corresponding to altitude, inches of mercury. Wo = unit weight of a cubic foot of air at 0 F and sea level atmospheric pressure, pounds per cubic foot. Wc = unit weight of a cubic foot of chimney gases at 0 F and sea level atmospheric pressure, pounds per cubic foot. To = absolute temperature of atmosphere, degrees Fahrenheit. Tc = absolute temperature of chimney gases, degrees Fahrenheit. W = weight of gases generated in the combustion chamber of the boiler and passing through the chimney, pounds per second. / = coefficient of friction. L = length of friction duct of the chimney, feet. - The first term of the right hand expression of Equation 1 represents the theoretical draft intensity, and the second term, the loss due to friction; 169