Document jBww8kJp3oYXzzzRm4yK5DggZ

HEATING VENTILATING AIR CONDITIONING GUIDE 1940 not be confused with mechanical draft systems that create draft mechani cally, but which must also be automatically controlled. The use of such a device to provide a more uniform and dependable control of draft than could be maintained by manually operated dampers, will produce better combustion of fuel. This higher efficiency of combus 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. 1, 2 and 3 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 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. The general equation for this net total available draft intensity of a natural draft chimney with a circular section is as follows: where Da = 2.96HB0 (Wo \T0 _ Wc\ To) 0.00126W*TcfL DiB0Wc (1) 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. To = 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. D = minimum diameter of chimney, feet. The first term of the right hand expression of Equation I 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, B0 = 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. 182 CHAPTER )Q. CHIMNEYS AND DRAFT CALCULATIONS Substituting these values in Equation 1 and reducing: D, = 2.96 X 200 X 29.92 X (9 0863 0.09\ 0.00126 X 100 X 960 X 0.016 X 200 V 522 960) 10s X 29.92 X 0.09 - = 1.27 - 0.14 = 1.13 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 no 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 draft- capacity curve corresponds to the head-capacity curve of centrifugal Fig. 4. Relation Between Barometric Pressure.and Altitude 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 of zero draft and maximum capacity' is 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 I 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: 1. The barometric pressure varies inversely as the altitude of the plantabove sea level. Fig. 4 gives the barometric pressure corresponding to various elevations as computed from the equation: where 90 09 Bt = 62,737 l.o.g. to&^op- Ei = altitude of plant above sea level, feet. 183 - v (2)