Document LK2D8zwke9y65y7GOXp2ve1G5

180 Chapter 9 1945 Guide 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. 4 presents information on the fuel-bed draft loss for various kinds of coal burned at different rates and 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 terms of foregoing Equations 2 and 3. Also, the temperature of flue gases falls during passage through breech ings or flue pipes. For uninsulated surfaces this probably can be ade- POUNDS OF COAL BURNED PER SQ FT OF GRATE SURFACE PER HOUR ' Fig. 4. Draft Required at Different Rates of Combustion . for Various Kinds of Coal quately estimated by assuming a loss of heat from the flue gas of 3 Btu per hour per square foot for each degree Fahrenheit temperature difference between the gases and surrounding air. DOMESTIC CHIMNEYS Experience has shown that the chimney sizes given in Table 1 are in general adequate for heating boilers and furnaces with the capacities indicated, when such devices are hand fired and that chimneys of these sizes are likely to be architecturally congruous with houses or buildings which require heating devices of such capacities. Sometimes it is desirable to use a smaller or shorter chimney than that indicated in Table 1. In such cases it is necessary to estimate whether a chimney of limited height can be made to produce sufficient draft in the particular case or whether mechanical draft must be employed. It should be remembered that mechanically fired devices, oil burners and stokers, are equipped with blowers so that, with these devices, the chimney is.not required to overcome the resistance of a fuel bed. Never theless, a draft in the fire box, of about 0.03 in. of water is considered Chimneys and Draft Calculations 181 -Table 1. -Recommended-Minimum_Chimney^Sizes.eor__. Heating Boilers and Furnaces3 Warm Air Furnace Capacity in Sq In. op Leader Pipe Steam Boiler Capacity Sq Ft of Radi ation Hot Water Heater Capacity Sq Ft of Radi ation Nominal Dimen sions op Fire Clay Lining in Inches Rectangular Flue Actual Inside Dimensions of Fire Clay Lining in 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%xl3 7x11% 81 13x13 8^x18 13x18 llHxllU 6Mxl6M ll)xl6k( 127 110 183 18x18 15Mxl5H 248 20x20 17Mxl7M 298 20x24 24x24 17x21 21x21 24x24b 357 441 576 24x28b 28x28b 30x30b 28x32b 672, 784 900 896 Round Flub Inside .Diam eter of Lining - in Inches Actual Area Sq In. 10 79 12 113 15 177 18 254 20 314 - 22 380 24 452 27 573 Height in Ft Above Grate 35 40 45 50 55 60 65 "This table ts 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. desirable so that any small openings in the fire box or flue passages will result in leakage of air in, and not leakage of combustion-products out, of such parts. This is not to be taken to condone leaks in fire boxes. Such leaks adversely affect plant efficiency. OBSERVED TEST PERFORMANCE The observed performances of some brick chimneys3 are given in Tables 2 and 3. The tests on which these data are based were made at various outside temperatures as shown and, to make them comparable among themselves, the observed drafts were corrected to 32 F, 1 atmosphere pressure, by tJie formula: where 0, = 0, + 5, - 5, (12) Si = computed static (theoretical) draft, experimental conditions. * 5, = computed static (theoretical) draft, standard conditions. Oi = observed draft, experimental conditions. Oi = observed draft corrected to standard conditions. It will be noted that the observed draft exceeded the computed static draft during some observations on the shorter chimneys.. This is mainly attributed to the draft producing effect of the hot gases immediately above the chimney. By- means of a manometer it was found that a ia erfrnl-ln(~<-- of Some Experimental Chimneys, by R. S. Dill. P. R. Achenbach and J. T. Duck (A.S.H.V.E. Transactions, Vol. 48. 1942, p. 351).