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Heating Ventilating Air Conditioning Guide 1938 where Dt = theoretical draft intensity created by pressure transformer, inches of water. hf = draft loss due to friction in pressure transformer, inches of water. hF -- draft loss through the fuel bed, inches of water. Hb = draft loss through the boiler and setting, inches of water. hBr = draft loss through the breeching, inches of water. kv = draft loss due to velocity, inches of water. ftBd = draft loss due to bends, inches of water. he = draft loss due to contraction of opening, inches of water. ho = 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 tensity and is analogous to the total dynamic head in a water works system. For a general circulation of gases Da = Dt where Da = available draft intensity, inches of water. Dt 83 required draft, inches of water. (15) The draft loss through thefuel bed(hp), 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 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. 8 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 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 190 Chapter 10. Chimneys and Draft Calculations broken up, 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 Table 1. Recommended Minimum Chimney Sizes for Heating Boilers and Furnaces .. Warm Am Furnace Capacity in Sq In. or Leader Pipe Stkam Boiler Capacitt Sq Ft op Radi* atton Hot Water Heater Capacitt Sq Ft or Radi ation Nominal Dimen sions or Fire Clat Lining in Inches Rectangular Flub Actual Inside Dimensions of FireClay lining in Inches Actual Area 8q 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 xllH 81 13x13 lMxiiM 127 8^x18 6Mxl6K 110 13x18 UMxi6M 183 18x18 15Jxl59* 248 20x20 17Mxl7M 298 20x24 24x24 17x21 21x21 24 x 24b 357 441 576 24 x 28b 28 x 28b 672 784 30 x 30b 900 28 x 32b . 896 Round Flub Inside Diam eter of Lining in Indies Actual Area Sq In. 10 79 Height in Ft Abotb Grate 35 12 113 15 177 18 254 20 314 22 380 24 452 40 45 50 55 60 65 27 573 .This table is taken from the A.S.H.V.E. Code of Minimum Requirements for the Heating and Ventilation of Buildings (Edition of 1929). ^Dimensions are for unlined rectangular dues. 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. In powdered-fuel and oil-fired installations, there will be no draft loss through the fuel bed since there is none and, consequently, this factor becomes zero in the general draft equation. All other factors being constant, the height of the chimney in installations of this character will be less than the height in coal-fired installations, and in the rasp of me chanical draft installations the driving units need not be as large since the head against which the fan is to operate is not as great in the former as in the latter. 191