Document dn96pnd4jJ3x3BYpa98j18rXQ

442 CHAPTER 17 1957 Guide an outdoor temperature of 0 F since this is likely to be the more critical , condition with respect to chimney draft. The available draft for outdoor temperatures between 0 F and 60 F can be obtained by interpolation from Figs. 4 and 5 with only slight error. Fig. 6 is a graphical representation of the available draft for a 13-ft brick chimney with a~ nominal 8 X 8-in. flue liner7 over a wide range of mass flow rates and for inlet flue-gas temperatures ranging from 200 to _1000 F. This family of curves is a typical group of performance curves showing that, there is a certain mass flow rate that produces a maximum available draft for any flue-gas temperature. The following approximate method may alternately be used to determine the available draft for small residential chimneys, from 10 to 25 ft in height Chimneys and Draft Calculations 443 H = chimney height, feet; i 1 T\ = chimney inlet temperature, Fahrenheit degrees,' absolute. T. = ambient air temperature, Fahrenheit degrees, absolute. The clay-lined brick cliimney is the most commonly used chimney, but recently other building, materials have been used for reasons of economy or convenience. Investigations, have, established that, the results shown in Figs. 4 and 5 for brick chimneys can be used, with slight error, for chimneys made of shale tile, concrete block, or cinder block. " SHORT CHIMNEYS The application of heating systems to one-story houses without base ments requires special consideration because the chimneys in such houses FLUE-CAS FLOW-POUNDS PER HR Fig. 6. Available Chimney Draft for 13 Ft Brick Chimney*- b * Square Flue Liner 6J x 6} in. inside. b Barometric Pressure 29.92 in. Hg. Air Temperature 60F. and with internal cross-section areas from 35 to 55 sq in., with a maximum probable error of 15 percent at the same flow and temperature con ditions. This method, is based on the chimney efficiencies7 shown1 m Fig. 7 and the ideal draft computed from the chimney inlet temperature; The available draft may be expressed as: ' D. = nj), (13) where 7l0 = chimney efficiency taken from Fig. 7 at the desired conditions of tempera ture and flow. j Di = ideal draft, calculated from Equation 14, assuming that the barometric pressure is 29.92 in Hg, and the ambient temperature is 60 F. D, 0.2554BJB (14); where Ba = barometric pressure, inches of mercury. Fig. 7. Effect of Gas Flow on Chimney Efficiency* * Derived from temperature plots. Liner 8 x 8 in. outside, 61 x 6} inside. Height 13 ft. ?re often raade so low in height for architectural reasons that they produce insufficient draft for some kinds of heating systems. Gas-burning devices no not necessarily require a chimney for proper combustion of the gas, but the chimney must be of adequate size and height to carry the flue ^se out of the building since it is undesirable and contrary to American Standards to have the products of combustion' discharged in the living 9>a?- If a natural-draft oil-burning device is operated with insufficient raft because of a short ehimney, smoky combustion usually occurs, pulsations are possible in the combustion chamber, the chimney or smoke- pipe may become blocked with soot, and the capacity of the heating equip- ay be reduced to the point of inadequacy. Insufficient draft is ton ^ *nar*ly the cause of smoky combustion in coal-burning heaters, but snort a chimney can cause slow pickup of the fire and insufficient heating capacity of the device. - on the amount of draft usually required for present-day heating 15 ra*'et* output, any chimney with an' effective height of less than chim a"ove the center line of the thimble should be regarded as a short sbrt chimneys every precaution should be taken to attain. Practiw * P?ssjt,'e average temperature in the chimney and the lowest are tufi e friction loss. Short chimneys of conventional construction ely to produce from 60 to 90 percent of the ideal draft and, therefore,