Document RK3gjXYZZ6nJLXrNLeE246D8

American Society of Heating and Ventilating Engineers Guide, 1936 the head-capacity curve of the pump and also to the dynamic-head capacity 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. The general equation for this net total available draft intensity of a natural draft chimney with a circular section is as follows: 0a = 2.96000 fFA 0.00126 W'TcfL Tc) D'B0WC (1) where 0a = available draft, inches of water. H = height of chimney above grate bars, feet.. Bo = 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 6 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 = amount 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. 0 = minimum diameter of chimney, feet. , The first term of the right hand expression of Equation 1 represents the theoretical draft intensity, and the second term, the lossdue to friction. Example 1. Determine the available draft of a natural draft chimney 200 ft-in height and 10 ft in diameter operating under the following conditions: atmospheric tempera ture, 62 F; chimney gas temperature, 500 F; sea level atmospheric pressure, Bo = 29.92 in. of mercury; atmospheric and chimney gas density, 0.0863 and 0.09, respectively; coefficient of friction, 0.016; length of friction duct, 200 ft. The chimney discharges 100 lb of gases per second. Substituting these values in Equation 1 and reducing: ^ 0a = 2.96 X 200 X 29.92 X 0.09\ 0.00126 X 100*. X 960 X 0.016 X 200 960/ 10s X 29.92 X 0.09 = 1.27 - 0.14 = 1.13 in. Fig. 3 shows the variation in the available draft of a typical 200 ft. by 10 ft chimney operating under the general conditions-noted in Example 1. When the chimney is under static conditions and ho gases are flowing, the' available draft is equal to 1.27 in. of water, the theoretical intensity. As the amount of gases flowing increases, the available intensity decreases until it becomes zero at a gas flow of 297 lb per second, at which point the draft loss due to friction is equal to the theoretical intensity. The draftcapacity curve corresponds to the head-capacity curve of centrifugal pump characteristics and.the dynamic-head-capacity curve of a fan. The point of maximum draft and zero capacity is called shut-off draft, or point of impending delivery, and corresponds to the point of shut-off head of a centrifugal pump. The point, bf zero draft and maximum capacity is 454 Chapter 26--Chimneys and Draft Calculations called the wide open point and corresponds to the wide open point of a centrifugal pump. A set of operating characteristics may be developed for any size chimney operating under any set of conditions by substituting the proper values in Equation 1 and then plotting the results in the manner shown in Fig. 3. . In substituting the values for the various factors in Equation 1, care should be exercised that the selections be as near the actual conditions as is practically possible. The following notes will serve as a guide for these selections: Fig. 4. Relation Between Barometric Pressure and Altitude 1. The barometric pressure varies inversely as the altitude of the plant above sea level. Fig. 4 gives the barometric pressure corresponding to various elevations as computed from the equation: . i = 62,737 logio (2) where Ei = altitude of plant above sea level, feet. In general, the barometric pressure decreases approximately 0.1 in. of mercury per 100 ft increase in elevation. 2. The unit weight of a cubic foot of chimney gases at 0 F and sea level barometric pressure is given by'the equation: Wc = .0.131 C0 + 0.095 Oi + 0.083 AT, . ^ (3) In this equation COs, Ot and Nt represent the percentages of the parts by volume of the carbon dioxide, oxygen and nitrogen content, respectively, of the gas analysis. For ordinary operating conditions, the value of Wc may be assumed at 0.09. 3. The atmospheric temperature is the actual observed temperature of the outside air at the time the analysis of the operating chimney is made. The mean atmospheric temperature in the temperate zone is approximately 62 F. . 4. The chirpney gas temperature does not vary appreciably from the gas temperature as jt leaves the breaching and enters the chimney. For average operating conditions, the chimney gas temperature will vary, between 500 F and 650 F except in the case when economizers and recuperators are used, when th,e temperature-will vary between 300 F and 450 F. If a chimney has been properly constructed, properly lined and'has no air infiltrationidiie.to open-joints, the temperature of the gases throughout the chimney will not differ appreciably from the foregoing figures. In most up-to-date heating plants, the temperature may be read from instruments or ascertained from a pyrometer. The .455