Document 8RRLRE81gZRwRLae9x0LYOOZ5

HEATING VENTILATING AIR CONDITIONING GUIDE 1941 boiler. The total products of combustion in pounds per .second for a grate-fired boiler may be computed'from the equation: a W 3600 where Cg = pounds of fuel burned per square foot of grate surface per hour. G ~ total grate surface of boilers, square feet. Cg X G = total weight of fuel burned per hour. ITtp = total weight of products of combustion per pound of fuel. (5) A similar computation may be made in the case of gas, oil, or stoker-fired fuel. Kig. 5. Chimney Performance Chart To solve a typical example: Proceed horizontally from a Weight Flow Rate point to intersection will diameter line; from this intersection follow vertically to chimney height line; from this intersection folio* horizontally to the right to Available Draft scale. Starting from a point of Available Draft, take steps it reverse order. Fig. 5 is a typical chimney performance chart giving the available draft intensities for various amounts of gases flowing and sizes of chimney. This chart is based on an atmospheric temperature of 62 F, a chimney gas temperature of 500 F, a unit chimney gas weight of 0.09 lb per cubic foot, sea level atmospheric pressure, a coefficient of friction of 0.016, and a friction duct length equal to the height of the chimney above the grate level. These curves may be used for general operating conditions. For specific conditions, a new chart should be prepared from Equation 1. It has been the usual custom, and still is to a lamentably great extent, to select the required size of a natural draft chimney from a table of 172 chimney sizes based only on boiler horsepowers. After the ultimate horsepower of the projected plant had been determined, the chimney size in the table corresponding to this figure was then selected as the proper size required. Generally, no further attempt was made to determine if the height thus selected was sufficient to help create the required draft demanded by the entire installation, or the diameter sufficiently large to enable the chimney quickly, efficiently, and economically to dispose of the gases. Since the operating characteristics of a natural draft chimney are similar in all respects to those of a centrifugal pump, or a centrifugal fan, it is no more possible to select a proper size chimney from such a table even with correction factors appended, than it is to select the proper size pump from tables based only on the amount of water to be delivered. DETERMINING CHIMNEY SIZES The required diameter and height of a natural draft cylindrical chimnev are given by the following equations: Or ________ H= 0.\MJWcBoV* (6) TCD The weight of gas per second, W = 12.075 D* VB0 Wc from which D = 0.288 J.JY-li T BoWcV (7) where H = required height of chimney above grate bar level, feet. D = required minimum diameter of chimney, feet (constant for entire height). V = chimney gas velocity, feet per second. D, = total required draft demanded by the entire installation outside of the chimney, inches of water. Equations 6 and 7 give the required size of a natural draft chimney with all of the operating factors taken into consideration. Values for all of the factors with the exception of the chimney gas velocity may be either' observed or computed. It is, of course, necessary to assume an arbitrary value for the velocity in order to arrive at some definite size. For any one set of operating conditions there will be as many sizes of chimney as there are values of reasonable velocities to assume. Of the number of sizes corresponding to the various assumed velocities, there is one size which will be least expensive. Since the cost of a chimney structure, regardless of the kind of material used in the construction, varies as the volume of material in the structure, the cost criterion then may be represented by the approximate equation: Q = rlHD (8) where Q = volume of material, cubic feet. t = average wall thickness, feet. 173 !