Document MMJ1zBz62pmGn7xXxJ8NY5x

T HEATINC VENTILATINC AIR CONDITIONING CUIDE 1944 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 - - *v 2D 30 35 40 45 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 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 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 chimneys are given in Tables 2 and 3*. The tests on which these data are based were made at various outside Observed Performance 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). 192 CHAPTER 9. CHIMNEYS AND DRAFT CALCULATIONS temperatures as shown and, to make them comparable among themselves, the observed drafts were corrected to 32 F, 1 atmosphere pressure, by the formula: where Si = computed static (theoretical) draft, experimental conditions. St *** computed static (theoretical) draft, standard conditions. 0i = observed draft, experimental conditions. Os -- observed draft corrected to standard conditions. Table 1. Recommended Minimum Chimney Sizes for Heating Boilers and Furnaces3 Warm Air Furnace Capacity in Sq In. of Leader Pipe Steam Boiler Capacity Sq Ft of Radi ation Hot Water Heater Capacity Sq Ft of Radi ation Nominal Dimen sions of 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^x13 7x11>4 13x13 HMxiiK 8^x18 6&xl6K 13x18 11Kx16M 18x18 15Jxl5J 20x20 17^xl7M 20x24 24x24 17x21 21x21 24x24> 24x28b 28x28b 30x30b 28x32b 81 127 110 183 248 298 357 441 576 672 784 900 896 Round Flue Inside Diam eter of Lining in Inches Actual Area Sq In. 10 79 Height in Ft Above Grate 35 12 113 40 15 177 45 18 254 50 20 314 22 380 24 452 55 60 65 27 573 This table is 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. 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 measurable draft existed in this gas column for some distance above the chimney top. However, the temperatures in the chimney were measured with unshielded thermocouples and the actual gas temperatures may have been higher for this reason than the observed temperatures on which the computations of draft were based. The tests were made in calm weather. 193