Document qdLDwyz7VRme1araoyJKOw4rG

402 CHAPTER 19 1949-Guide 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 draft 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 point of maximum draft and zero capacity is called shut-off draft, or point of im pending delivery, and corresponds to the point of shut-off head of a centrif ugal 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 2 and then plotting the results in the manner shown in Fig. 1: Fig. 2 is a typical chimney performance chart giving the available draft for various gas flow rates and sizes of chimney. This chart is based on an Fiq. 1. Typical Set of Operating Characteristics op a Natural Draft-Chimney 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 leitel. These curves may be used for general operating conditions. For specific conditions, a new chart may be prepared from Equation 2 or 3. DETERMINING CHIMNEY SIZES If the required performance for a proposed chimney is Known andif a chimney-gas velocity is assumed, Equation 2 can be transposed to yield the necessary height and an equation can be developed for the required diame ter. These operations result in the following equations: /Wo \To Ut IF.\ 0.184/tF,,i7F* T.) T.D (6) Chimneys and Draft Calculations 403 The weight of gas per second, W = 12;075 D'VBoWe from which n 0.288 i /. WT' where y BoW.V H * required height of chimney above grate,feet. D = required minimum diameter of chimney, feet. V *= chimney gas velocity, feet per second. Dr -- total required draft, inches of water. (7) >4->-- 7 & X/'S'vAiflp i ' p \ // 'y/ Vy ' $-\-- 1 0 c si //V/ ^& . 0 \0 0 ^ 5J '/z& ^ ^ 4 c | / ^7 5^ N\y 1 O' \\ | ---o D~4n~Y$, ^ 0^ O O O ^ \ O \Y | /r'i . 'x 0 ' Available Draft per FL of Height, in. of Water Fig. 2. Chimney Performance Chart To solve a typical example: Proceed horizontally from a Weight Flow Rato point to intersection with diameter line; from this intersection fol low vertically to chimney height line; from this intersection follow hori. xontally to the right to Available Draft scale. Starting from a point of Available Draft, take stepsinreverse order. ` For large chimneys, it is usual to assume that total construction cost is least when the product HD (height X diameter) is minimum. On this assumption, the product of Equations 6 and 7 can be differentiated and the differential set equal to zero to find the minimum. Solution for velocity then yields the following equation: Vo where'. V, economical chimney gas velocity, feet per second. (8)