Document 8Vwxnjo8qD15kX5wNgM65dGR5

388 CHAPTER 16 1950. Guide is not required to overcome the resistance of a fuel bed. Nevertheless, a draft in the firebox, of about 0.03 in. of water, is considered desirable so that any small openings in the firebox or flue passages will result in leak age of air inward, and not leakage of combustion products outward. This is not to be taken to condone leaks in fireboxes. Such leaks adversely affect plant efficiency. AVAILABLE DRAFT FOR THE DOMESTIC CHIMNEY The small domestic chimney does not appear to follow the same mathe matical relations as the larger industrial chimney, because of increased friction losses (apparently caused by the extremely low flue-gas velocities which create eddies and reversal of flow); hence, a slightly different method of determining the available draft is required. Chimneys:and Draft Calculations 389 water is equivalent to only 11 deg difference, in temperature. It would therefore be assumed that the temperature measurement with unshielded thermocouples indicated less ..than actual temperature in these cases. , No such observed differences was noted among the temperature measurements taken for the data in Fig. 4, which were made with a high-velocity, aspi rating shielded thermocouple. The accurate determination of the available chimney draft for any particular chimney is a complicated problem. . The following approximate method6 is based on the assumption that, for small residential chimneys 10 to 25 ft in height, and with a cross-sectional area of 35 to 55 sq in., the chimney efficiency will vary within only plus or minus 15 per cent among chimneys at the same flow and temperature conditions. The available draft may be expressed as: Z>. = nj>t (14) * Square Flue Liner 6} x 61 in. inside. " Barometric Pressure 29.02 in. Hg. Air Temperature 60F. The most accurate method, obviously, would be to use. actual test data of the particular chimney in question. Fig. 4 is a graphical presentation of the available chimney draft over a wide range of flows for a standard 13-ft brick chimney with an 8 x 8-in. nominal-size flue;6 whereas Tables 1 and 2 are the observed performances of various sizesand heightsof brick chimneys.7 These data are from two different sources, and have been corrected to different temperatures, but they may be made comparable by the following relation: 0, - Ox = S, - Si (13) where' Si = computed static draft at Ti and Bi~..................... St = computed static draft at Tj and Bt. 0r = observed draft at T\ and /l,. 0, = observed draft at T, and Bt. . It will be noted in Tables' 1 and 2 that the observed draft exceeded the computed static draft at some points. A draft difference of 0.004 in. of Fig. 5. Effect of Gas Flow on Chimney Efficiency" * Derived from temperature plots. Liner 8 x 8 in. outside, x 6} inside. Height 13 Ft. where n,, = chimney efficiency taken from Fig. 5 at the desired conditions of.tempera- ture and flow. . A = theoretical draft. It may be calculated from Equation 1, assuming-that the barometric pressure is 29.92 in. Hg,.and the ambient air temperature is 60 F. Example 2. Determine the available draft of a 13-ft chimney; inside diameter; 7 in.; inlet temperature, 205 F; flow rate, 275 lb per hr; temperature of air surrounding chimney, 60 F; and barometric pressure, 29.92 in. Hg. Solution. The theoretical draft from Equation 1 is: . Dt = 2.96 X 13 X 29.92 f~3 - = 0.0415 in. water. 520 665 -J' From Fig. 5, n. = 0.60 D. = 0.60 X 0.0415 D. = 0.0249 in. water at 29.92 in. Hg and 60 F. -r For calculations where the fuel rate, and the percentage C02 are the only known factors, Fig. 6 is included so that, the flue-gas rate can, easily be de termined for, coal, oil, and gas.-' By entering at percentage C02-moving vertically to the curve for the type of fuel, and then moving horizontally to