Document KJpjzK0OnrBG9xMgbJdp3231w

364 CHAPTER 19 1946 Guide or where the required data necessary for an exact determination are difficult or impossible to secure. Whenever it is possible to secure accurate data, or the anticipated operating conditions are fairly well known, the required size should be determined from Equations 6, 7 and 8. FACTORS AFFECTING REQUIRED DRAFT The foregoing considerations deal with chimney size selection when the required draft and flue gas volume and temperature are known. The Height of Chimney, ft. Chimneys and Draft Calculations ______________________ '_________________ 365 hour per square foot for each Fahrenheit degree temperature difference between the gases and surrounding air. DOMESTIC CHIMNEYS The height of a chimney for a residence or apartment is generally limited by the height of the building since it is desirable to have the chimney architecturally congruous with the building. The height desir able from an architectural viewpoint and the location of the chimney may be disadvantageous to the operation of the boiler or furnace and it is therefore important that the manufacturer of the fuel burning appliance to be installed be consulted in regard to the adequacy of the chimney. Fig. 3. Economical Chimney SizEs 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 41.) The friction in straight ducts can be estimated by means of the last terms of 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 quately estimated by assuming a loss of heat from the flue gas of 3 Btu per orPOUNDS 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 in order to provide a satisfactory performance must have adequate height.and area, be of permanently tight construction and should be as smooth internally as practicable. 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 chimneys3 are given in Tables 1 and 2. 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 the formula: