Document mBVmo86wR0gNbzjbqpe7M237Q

378 CHAPTER 14 1958 Guide fuel gases vary. While personal errors involved in C02, temperature, and chart determinations, would doubtless more than offset any inaccuracies due to universal use of the alignment charts shown, precise laboratory work may require a more exact method. For more complete information the reader is referred to Combustion, 3rd Edition, and Gaseous Fuels, (published by American Gas Association) and particularly to tables covering various properties of different commercial gases included in these publications. CONDENSATION AND CORROSION Sulfur dioxide or sulfur trioxide, formed by the combustion of sulfur in fuels, are the principal corroding substances in flue gases. They become active whenever sufficient moisture is present for the formation of sulfurous or sulfuric acid,20 and they lower the dew points of flue gases appreciably. Therefore, unless heating equipment is designed for operation at flue gas temperatures below the dewpoint, which is seldom the case, it is always ad Table 10. Average Flue Gas Dew-Point for Various Fuels* Type op Fuel Average Dew-Point Temperature, F Anthracite........................................................................................... Semi-Bituminous Coal................................................................ Oil.......................................................................................................... Natural Gas........................................................................................ Manufactured Gas........................... ............................................... Propane Gas (2500 Btu/cu ft)................ .......................... Butane Gas (3200 Btu/cu ft)....................................................... Butane-Air Gas Mixture (535 Btu/cu ft)................................ 68 84 93 111 127 137 119 124 121 visable to maintain temperatures above this value in all parts of the ap pliance. Excessive spot temperatures in the combustion chamber or else where, on the other hand, are also destructive in that they may result in rapid oxidation of ordinary heating surfaces. American Standard Require ments for gas furnaces, floor furnaces, and recessed heaters, for example, specify that minimum spot heating surface temperatures during normal operation must neither fall below 178 F (50 F above average dewpoint) nor exceed 830 F to 1230 F on any portion of the heating surface, depend ing on the type and thickness of the metal. In any event it is usually desirable to maintain flue temperatures within the limits indicated not only throughout the appliance, but in its connecting vent, flue, or chimney as well. Otherwise, excessive condensation and corrosion problems, with resultant customer dissatisfaction, will in all probability be the result. Average dewpoint temperatures of flue gases resulting from the combustion of various fuels, when burned with the amount of excess air normally supplied to insure complete combustion, are shown in Table 10. SOOT The deposit of soot on the flue surfaces of a boiler or heater acts as an insulating layer over the surface, and reduces the heat transmission to the water or air. The Bureau of Mines Report of Investigations No. 327221 shows that the loss of seasonal efficiency is not so great as has been be lieved, and usually is not over 6 percent because the greater part of the Fuels and Combustion 379 heat is transmitted through the combustion chamber surfaces. The Bureau of Standards Report BMS 54" points out that, although the de crease in efficiency of an oil fired boiler, due to soot deposits, is relatively small, the attendant increase in stack temperature may be considerable. The soot accumulation clogs the flues, reduces the draft, and may pre vent proper combustion. Soot can probably be most effectively removed by a jet of compressed air, by means of a brush, or a vacuum cleaner. However, it has been found that copper chloride, lead chloride, tin chloride, zinc chloride, common salt and some other salts are. partially effective in Table 11. Permanent Openings for Air Supply to Spaces Containing Fuel Burning Equipment Fuel Burning Equipment Free Area op Ventilating Opening* Air Inlet ! Air Outlet Gross o u tp u t capacity exceeding 250,000 B tu per h r Equipment in a large enclo sure having adequate air in filtration None required None required Equipment in a large enclo sure having unusually tight construction Total free area not less than sq in. per (1000 Btu) (hr) furnace input Total free area not less than H sq in. per (1000 Btu) (hr) furnace input Equipment in a small enclo sure in a building having adequate air infiltration Opening in party wall or door, above draft hood, with free area of not less than 1 sq in. per (1000 Btu) (hr) furnace input Opening in party wall or door, below combustion air inlet to appliance with free area of not less than 1 sq in. per (1000 Btu) (hr) furnace input Equipment in a small enclo sure in a building of unusu ally tight construction Grilled opening connecting directlv with outdoors, through duct if necessary; free area not less than 1 sq in. per (1000 Btu) (hr) fur nace input Grilled opening connecting directly with outdoor air, through duct if necessary. Area and. location same as above. Equipment in an enclosure having adequate air infiltra tion Total free area equivalent to not less than the combined area of the flue outlets of all fuel burning equipment in the space None required Equipment in an enclosure having unusually tight con struction Total free area equivalent to not less than twice the com bined area of the flue outlets, of all fuel burning equip ment in the space None required Gross o u tp u t capa c ity not exceeding 250,000 B tu per h r 1 " Operation of exhaust fans, kitchen ventilation systems, fire places, etc. may create conditions requiring special consideration. removing soot from furnaces and boilers when properly used.23 A discus sion of instruments and methods of evaluating smoke will be found in Chapter 52. AIR SUPPLY TO FUEL BURNING EQUIPMENT All rooms or spaces containing boilers, furnaces, water heaters or any other fuel burning equipment must be provided with a constant supply of combustion air at adequate static pressure to insure proper combustion in the fuel burners. Additional air is required to replace air entering chim neys through draft hoods and barometric draft dampers, and also to proVlde ventilation in closely confined boiler and furnace rooms. Experience has shown that the ventilating air openings required in tlft F moms depend not only on the size of the heating plant but also on he type of fuel being burned, the relative size of the equipment room, the