Document dnXRm0rb62eo5rkMgO5Bd5nLe

CHAPTER 17 DETERMINING INDUSTRIAL CHIMNEY SIZES If the required performance for a proposed chimney is known, andq/d chimney-gas velocity is assumed. Equation 3 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: D, H= 2.96B, /Wo _ WA \ To Tj 0.184fWJBpV* T'D (7) To solve a typical example: Proceed horizontally from a Weight Flow; Kate point to intersection with diameter line; from this intersection fol low vertically to chimney height line; from this intersection follow hori zontally to the right to Available Draft scale. Starting from a point of Available Draft, take steps in reverse order. The weight of gas per second, W = 12.075 D2VB0W. from which D = 0.288 /|f WTo B0W'V where H = required height of chimney above grate, feet. D = required minimum diameter of chimney, feet. V = chimney gas velocity, feet per second. D, = total required draft, inches of water. (8) For large chimneys, it is usual to assume that total construction cost ''.'Chimneys and Draft Calculations 421 is least when the product HD (height, X diameter) is minimum, , ,On this assumption, the product of Equations 7 and 8 can be differentiated, and the differential set equal to zero to find the minimum. Solution for velocity 'then yields the following equation: (9) where Ve = economical chimney gas velocity, feet per second. Equations 7, 8 and 9 can of course be simplified if values are assumed for some of the factors in it. Some typical figures for boiler plants are: Average chimney gas temperature 500 F........... ......................T0 = 960 F absolute Average atmospheric temperature 62 F........................................Ta = 622 F absolute Average coefficient of friction 0.016.................................................. / = 0.016 Average chimney, gas density, 0 F, 1 Atmosphere..We -- 0.09 lb per cu ft Barometer reading, sea level.........................................................Ba = 29.92 in. Hg When these values are substituted in Equations 7, 8 and 9, respectively, the results are: H = 1907), (10) D = 1.5W*11 (11) V. = 13.7W* (12) These equations should be used for general operating conditions only, or where the required data necessary for an exact determination are difficult or impossible to secure. Whenever it is possible to obtain accurate data, or the anticipated operating conditions are fairly well known, the required size should be determined from Equations 7, 8, and 9. Additional construction data for large industrial chimneys, whether brick, concrete, or steel, may be found in Kent's Mechanical Engineers' Handbook* or the Handbook of Building Construction.6 RESIDENTIAL CHIMNEYS A residential chimney, to provide satisfactory performance, must have adequate height and area, be of permanently tight construction, be as smooth as practicable internally, and be of such construction as to present no fire hazard to the building. The height of a residence or apartment chimney is usually limited by the height of the building, and by cost. The chimney height and location that are best suited to a building from an architectural standpoint, will sometimes be unsatisfactory for the proper operation of the heating equipment. Chimney height is likely to be critical n one-story ranch-type or rambler-type houses, and therefore, it is im portant to compare carefully the available draft of the chimney and the required draft of the heating appliance to determine whether or not they VV1" operate together satisfactorily. Most residential chimneys are constructed of brick with a clay flue liner, but recently several lightweight, prefabricated chimneys have been mar keted. These chimneys were primarily designed for use with gas equipent, but recently several have been approved by the National Board of Underwriters for use with all types of fuels. The advantages of the bghtweight, prefabricated chimney are ease of installation, somewhat lower Cost- and reduced weight on the supporting structure.