Document k9jE9nnE44ex3MgKvZV8R08rn

HEATING VENTILATING AIR CONDITIONING GUIDE 1942 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. Substituting these values in Equation 1 and reducing: Da = 2.96 X 200 X 29.92 X 0.09\ 0.00126 X 100> X 960 X 0.016 X 200 960 ) 10* X 29.92 X 0.09 = 1.27 - 0.14 = 1.13 in. Fig. 4 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 Fig. 4. Typical Set of Operating Characteristics of a Natural Draft Chimney 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 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 1 and then plotting the results in. the manner shown in Fig. 4. 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 184 CHAPTER 9. CHIMNEYS AND DRAFT CALCULATIONS is practically possible. The following notes will serve as a guide for these selections: 1. The barometric pressure, represented by B0, is the actual pressure at the site of the chimney and not the pressure reduced to sea level datum. 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 = 0.131CO, + 0.095 O, + 0.083 N, (3) In this equation CO,, 0, 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. The density effect on the chimney gases due to superheated water vapor resulting from moisture and hydrogen in the fuel, or due to any air infiltrations in the chimney proper are disregarded. Though water vapor content is not disclosed by Orsat analysis, its presence tends to reduce the actual weight per cubic foot of chimney gases. 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 chimney gas temperature decreases from the breeching connection to the top of the stack. This drop in temperature depends upon the material and construction of the stack, its tightness or freedom from leaks, its area, its height, and the velocity of the gases through it: The same chimney will suffer different temperature losses depending upon the capacity under which it is working and the variable atmospheric conditions. No general equation covering all these variables has been suggested, but from observa tions on chimneys varying in diameter from 3 to 16 ft and in height from 100 to 250 ft the following equation was deduced*; --[(tr-*] (4) where r, = absolute temperature at the center of the connection from the breeching, degrees Fahrenheit. Ht, = the height of the stack above center line connection to breeching, feet. 5. The coefficient offriction between the chimney gases and a sooted surface has been taken by many workers in this field as a constant value of 0.016 for the conditions in volved. This value, of course, would be less fpr a new unlined steel stack than for a brick or brick-lined chimney, but in time the inside surface of all chimneys regardless of the materials of construction becomes covered with a layer of soot, and thus the coef ficient of friction has been taken the same for all types of chimneys and in general constant for all conditions of operation. - For reasons of simplicity and convenience to the reader, this constant value of 0.016 has been employed in the development of the various special equations and charts shown in this chapter. In important chimney design, especially when the construction or the materials are unusual, it is recommended that use be'made of Reynolds number* in determining the friction factor, /. 6. The length of the friction duct is the vertical distance between the bottom of the breeching opening and the top of the chimney. Ordinarily this distance is approximately equal to the height of the chimney above the grate level. 7. Assuming no air infiltration the amount of gases flowing and being discharged is, of course, equal to the amount of gases generated in the combustion chamber of the Notes on Power Plant Design, by E. F. Miller and Janies Holt (Massachusetts Institute of Technology, 1930F).or more complete discussion see Flow of Fluids in Closed Condu.its, by R. J.. S. Pigot.t (Mechanical Engineering, August, 1933). 185