Document 9YLaN3wEeJZejo5KZmmGdRr5

American Society of Heating and Ventilating Engineers Guide, 1937 tensity and is analogous to the total dynamic head in a water works system. For a general circulation of gases where 7?a = 'Dt Da = available draft intensity, inches of water. Dr = required draft, inches of water. (15) The draft loss through thefuel bed (h?), 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 Table 1. Recommended Minimum Chimney Sizes for Heating Boilers and Furnaces3 Warm Am Furnace Capacity inSqIn. or Leader Pips Steam Boiler Capacity Sq Ft or Radi* ATIOIf 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 Hot Water Heater Capacitt Sq Ft or Radi ation 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 Nominal Dimen sions or Fire Clay Lining in Inches Rectangular Flue Actual Inside Dimensions of Fire Clay Tuning in Inches Actual Area 8q In. 8^x13 7 xllH 81 13x13 11)4x11)4 127 8)4x18 6)4x16)4 110 13x18 11)4x16)4 . 183 18x18 15)4x15)4 248 20x20 17)4x17)4 298 20x24 24x24 * 17x21 21x21 24 x24b 357 441 576 24 x 28b 28 x 28b 30 x 30b 28 x 32b 672 784 900 896 Round Flub Inside Diametcof fining in Inches Actual Area Sq In. 10 79 12 113 15 177 18 254 20 314 22 380 24 452 27 573 Hkiiht in Ft Above Grate 35 40 45 50 55 60 65 UUoTM8B^Mi^(Mt'Tofh19M)S'H V'E' ^ f Minimum Requirements for the Heating and Venti^Dimensions are for unlined rectangular flues. 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 474 Chapter 26--Chimneys and Draft Calculations 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 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 'pounds of coal burned per"sq ft of grate surface per hour Fig. 8. Draft Required at Different Rates of Combustion for Various Kinds of Coal. 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. 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 case 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. . 475