Document 2rVx1kDLopqBd1mbpD3G086

of and 1937American Society Heating Ventilating Engineers Guide, product of Equations 7 and 8 to 7/73,- differentiating this product with respect to V arid equating the resulting expression to zero. This pro cedure results in the following expression: Vt toKere.Vf = economical chimney gas velocity, feet per second. (10) Fig.. 7. Economical Chimney Sizes3 Diameter values also for gas temperatures of 4fX). 500 and 600 F. Equation 10 gives the economical velocity of the chimney gases for any set of operating conditions, and represents the velocity which will result in a chimney the size of which will cost less than that of any other size as determined by any other velocity for the same operating con ditions. After the value of the economical velocity has been determined, the corresponding height and diameter can then be determined from Equations 7 and 8, respectively, and the economical size will then be attained.- Equations 7, 8 and 10 may be simplified considerably for average operating conditions in an average size steam plant by assuming typical conditions. 472 Chapter 26--Chimneys and Draft Calculations Average chimney gas temperature, 500 F........................ Tc = 960 Mean atmospheric temperature, 62 F..... .............. --:Ta = 522 Average coefficient of friction. 0.016......................-.......... ~ 0.016 Average chimney gas density, 0.09............. ...................... Wc = 0.09 Sea level elevation, with barometer of 29.92.------------ .--Ba = 29.92 Substituting these values in Equations 10, 8 and 7, respectively, and reducing, the results are substantially: Ve = 13.7PF1/5 (n) D = 1.51F2/s (12) H = 190Zt (13) Fig. 7 gives the economical chimney sizes for various amounts of gases flowing and for required draft intensities as computed from Equations 11, 12 and 13. They are based on the operating factors used in reducing Equations 7, 8 and 10 to their simpler form. The sizes shown by the curves in the chart should be used for general operating conditions only, or for installations where the required data necessary for an exact deter mination are difficult or impossible to secure. Whenever it is possible to secure accurate data, or the anticipated operating conditions are fairly well known, the required size should be determined from Equations 7, 8 and 10. The recommended minimum inside dimensions and heights of chimneys for small and medium size installations are given in Table 1. GENERAL EQUATION The general draft equation for a steam producing plant may be stated as follows: >t -- Af = Af + Ab + ABd + Ac + ABr + Av + Ao + Ae + Ar (14) where Dt = theoretical draft intensity created by pressure transformer, inches of water. hi = draft loss due to friction in pressure transformer, inches of water. Af = draft loss through the fuel bed, inches of water. Ab = draft loss through the boiler and setting, inches of water. ABr = draft loss through the breeching, inches of water. Av = draft loss due to velocity, inches of water. ABd = draft loss due to bends, inches of water. Ac = draft loss due to contraction of opening, inches of water. Ao =.draft loss due to enlargement of opening, inches of water. Ae = draft loss through the economizer, inches of water. Ar = draft loss through recuperators, regenerators, or air heaters, inches of water: The left hand member of Equation 14 represents the total amount of available draft created by the pressure transformer, that is, the natural draft chimney, Venturi chimney, or fan, and is equal to the theoretical intensity less the internal losses incidental to operation. The right hand member represents the sum of all of the various losses of draft throughout the entire boiler plant installation outside of the pressure transformer itself. The left hand member expresses the available intensity and is 'analogous to the head developed by a centrifugal pump in a water works system, while the right hand member expresses the required draft in- 473