Document zbkr25GgNwMRXM4dkpkogLqw7

HEATINC VENTILATING AIR CONDITIONING GUIDE 1942 The left hand member of Equation 13 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- Table 1. Recommended Minimum Chimney Sizes for Heating Boilers and Furnaces Warm Are Furnace Capacity in Sq In. of Leader Pipe Steak ' Boiler Capacity Sq Ft op Radi ation Hot Water Heater Capacity Sq Ft or Radi ation Nominal Dimen sions or Fire Clay Lining in Inches Rectangular Flue Actual Inside Dimensions of Fire Clay Lining la Inches Actual Area Sq In. 790 1000 590 690 900 900 1,100 1,700 1.940 2,130 2,480 3,150 4,300 4,600 5,000 5,570 5,580 6,980 7,270 8,700 9.380 10,150 10,470 973 1,140 1,490 1,490 1,820 2,800 3,200 3,520 4,090 5,200 7,100 7,590 8,250 9,190 9,200 11,500 12,000 14,400 15,500 16,750 17,250 8^x13 7 xllJi 81 13x13 8^x18 13 x 18 li^xiiM 6J^x 16J4 11Kx16M 127 110 183 18x18 15%xl5J 248 20x20 17)ixl7M 298 20x24 24x24 17x21 21x21 24 x 24b 357 441 576 24 x 28b 28x286 30 x 30b 28 x 32b 672 784 900 896 Round Flub Inside Diam eter of lining in Inches Actual Area Sq In. Height in Ft Above Grate' 35 10 79 12 113 15 177 18 254 20 314 22 380 24 452 40 45. 50 55 60 65 27 573 1 This table la taken from the A.S.H.V.E. Code of Minimum Requirements for the Heating and Venti lation of Buildings (Edition of 1929). bDimensions are for unlined rectangular flues. tensity and is analogous to the total dynamic head in a water works system. For a general circulation of gases where Da = Vt Da = available draft intensity, inches of water. Dr = required draft, inches of water. (14) The draft loss through thefuel bed (A#), or the amount of draft required to effect a given or required rate of combustion, varies.between wide limits and represents the greater portion of the required draft.. In coal-fired 190 CHAPTER 9. CHIMNEYS AND DRAFT CALCULATIONS installations, the draft loss through the fuel bed is dependent upon the following factors: (1) character and condition of the fuel; clean or dirty; (2) percentage of ash in the fuel; (3) volume of interstices in the fuel bed, coarseness of fuel; (4) thickness of the fuel bed, rate of combustion; (5) type of grate or stoker used; (6) efficiency of combustion. There is a certain intensity of draft with which the best results will be obtained for every kind of coal and rate of combustion. Fig.' 7 gives the intensity of draft, or the vacuum in the combustion chamber required to burn various kinds of coal at various rates of combustion. Expressed in other words, these curves represent the amount of draft required to force the necessary amount of air through the fuel bed in order to effect various rates of combustion. It will be noted that the amount of draft increases as the percentage of volatile matter diminishes, being comparatively low POUNDS OF COAL BURNED PER SQ FT OF GRATE SURFACE PER HOUR Fig. 7. Draft Required at Different Rates of Combustion for Various Kinds of Coal for the lower grades of bituminous coals and highest for the high grades and small sizes of anthracites. Also, when the interstices of the coal are large and the particles are not well broken up, as with bituminous coals, much less draft.is required than when the particles are small and are well broken lip, as with bituminous slack and the small sizes of anthracites. In general, the'draft loss through the fuel bed increases as: (1) the per centage of volatile matter diminishes; (2) the percentage of fixed carbon increases; (3) the thickness of the bed increases; (4) the percentage of ash increases; (5) the volume of the interstices diminishes. In making the preliminary assumptions for the draft loss through the fuel bed, due allowances should be made for a possible future change in the grade of fuel to be burned and also in the rate of combustion. A value should be selected for this loss which will represent not only the highest rate of combustion which will be encountered, but also the grade of coal which has the greatest resistance through the fuel bed and which may be burned at a later date. 191