Document 15xxNOJVxbK3e2Vy4d54Vn8Em

380 CHAPTER 16 1950 Guide where H = height of chimney, feet. Ba = existing barometric pressure, inches of mercury. Wo *= density of air at 0 F and 1 atmosphere pressure, pounds per cubic foot. Wc = density of flue gas at 0 F and 1 atmosphere pressure, pounds per cubic foot. To = temperature of air surrounding the chimney, Fahrenheit degrees absolute. To = average or effective temperature of the gases in the chimney, Fahrenheit degrees absolute. The quantity D,, found by the formula, is the pressure difference be tween the gas inside and air outside of the chimney, in inches of water, when'no flow occurs in the chimney. The quantity is variously known as POUNDS OF COAL BURNED PER SQ FT OF GRATE SURFACE PER HOUR Fiq. 1. Draft Required at Different Rates of Combustion for Various Kinds of Coal the theoretical draft, the static draft or the computed draft. It is very useful in predicting and analyzing chimney performance, but it is. seldom if .ever attained in an actual chimney because of the friction incident to gas flow and the effects of wind. One variable that will include all chimney losses is the chimney efficiency; hence,' it:is an excellent way of summing up chimney operational charac teristics.- The chimney efficiency, or percentage of theoretical draft ob tained, may be calculated as follows: . ... Efficiency final measured draft _________________(_2_)___ ideal draft calculated from the inlet temperature FACTORS AFFECTING REQUIRED DRAFT Before the proper chimney can be selected for an installation) the re quired draft of the combustion unit must be known. The required draft is, of course, equal to the' sum of all the resistances to gas flow from the ash pit door to, and including, the chimney connection. ' - Fig. 1 presents information on the fuel-bed draft loss for various kinds Chimneys"and Draft Calculations 381 of coal burned at different .rates.- -Rough generalizations can-be given for the losses in the flue passages of boiler or furnace, but, on account of the great differences in such devices, more reliable data on their flue gas volume, temperature and flue resistance should be obtained for design purposes from their respective' manufacturers: ' Flue gases encounter resistance to flow in breechings or smoke pipes, and this can probably-' be treated: with sufficient accuracy by means of the method used for air ducts. (See Chapter 31.) The friction in straight ducts can be estimated by means of the last term of Equations 3 and 4. Also, the temperature of flue gases falls during passage through breech ings or flue pipes. For uninsulated surfaces this probably can be ade quately estimated by assuming a loss of heat from' the flue gas of 3 Btu per (hr) (sq ft) (Fahrenheit deg temperature difference between the gases and surrounding air). INDUSTRIAL CHIMNEYS Chimneys can be classified as residential and industrial, the chief dif ference being their sizes and the types of draft. Chimneys over approxi mately I5 ft in diameter are in the mdustrial chimney.class, and their re quirements should be-treated accordingly. The majority of . industrial chimneys operate under induced or forced draft, resorting to natural draft operation only in the case of emergencies. They are built of brick, con crete, or steel, depending upon economy and the type of installation needed. Proper height is of importance because of removal of waste products, inas much as the products of combustion are often deflected downward around the chimney and, with the large amount" of gases that are exhausted to the . atmosphere through the industrial chimney, downwash can be very ob jectionable.1 AVAILABLE DRAFT FOR THE: INDUSTRIAL CHIMNEY The available draft, D, for large chimneys and stacks has been estimated with apparent satisfaction in the past by means of formulas which in effect deduct an estimated friction loss from a theoretical draft determined as in Equation 1. The friction loss can be estimated by means of one of the formulas available for ducts, such as the Fanning equation. This pro cedure results in formulas for the available draft as follows: For a cylindrical stack: fWp 1F.\ 0,00126IV* To D. = 2.96 H \T. T.) DiB,W. (3) and for a rectangular stack: Do = 2.96 HBo ^ Wo T.~ To) 0.0003881FT./L(s + y) (xy)`BoWo where Do -- available draft, incheB water gage. H = height of chimney above grate, feet. . ' Bo = existing barometric pressure, inches of mercury.' Wo = density of air at 0 F, 1 atmosphere pressure. Wo = density of flue gas at 0 F, 1 atmosphere pressure; To = temperature of atmosphere, Fahrenheit degrees, absolute. (4)