Document 20ykEGgv6Vpj4EaxnKVMRMM5

AMERICAN SOCIETYOF HEATING & YENTILAIIN6 ENGINEERS GDIDEJ923 a gas steam boiler, giving an evaporation of 465 lb. of water (from and at 212 deg. fahr.) with a consumption of 1,000 cu. ft. of gas: . B.t.u. in steam lb. water x 970.4 E ciency -- B.t.u. in gas ~ cu. ft. x B.t.u. per cu. ft. 451,236 With gross value : qqq = 82.04 per cent 451,236 With net value : 500 qqq = 90.25 per cent Furthermore, it will be seen that the maximum efficiency possible, based upon the gross heat value of the above gas (without condensing the water vapor in the flue gas), would be or 90.90 per cent. 05U Efficiencies of 80 per cent based on the gross heat value of gas fuel, are obtainable in properly designed vented appliances. It should be remembered that the efficiency of a gas appliance in actual operation closely approaches that determined in laboratory tests. WASTE FROM USING GAS IN GOAL BURNING APPARATUS In coal stoves or coal furnaces the path traveled by the flame is short and the radiating surface is relatively small. For this reason, the use of gas in such stoves is always wasteful and will require about three times as much gas for the same heating service as would be required if the gas were used in a properly built natural-gas furnace. Even with perfect combustion in the fire pot of a coal stove or coal furnace, the waste will usually be about 75 per cent. Properly built natural-gas furnaces have a longer fire travel and much more radiating surface than coal furnaces, and are, therefore much more efficient.1 Hot air furnaces, designed for use either with manufactured or natural gas, are available in sizes ranging from 5,000 cu. ft. to 20,000 cu. ft. rated capacity. House heating boilers, of the cast-iron sectional type, designed especi ally for gas fuel, are available in single units ranging from 200 sq. ft. to 7,500 sq. ft. rated capacity (steam). Typical gas warm-air furnace and boiler are pictured in Figs. 48 and 49, respectively. RATINGS Gas appliance manufacturers have followed the custom established by coal appliance manufacturers in rating their product in terms of cu. ft. (warm air) and sq. ft. of radiation (water and steam). Gas appliance ratings in these terms are generally emperical, however; they are selected " Waste and Correct Use oCsNatural Gas in the Home"; Sec. 29. Technical Paper 257, Department of the Interior, Bureau of Mines; by Samuel S. Wyer. 92 AMERICAN SOCIETY OF HEATING & VENTILATING ENGINEERS GDIBEK23 to conform in size with a coal appliance of equivalent power under ordi nary operating conditions. The capacity of a gas furnace or boiler is subject to much smaller fluctuations than a boiler employing solid fuel; the element of length of firing period does not enter into consideration; heating surfaces remain clean for longer period of time. Although most coal boilers are rated on an 8-hr. firing period (with anthracite coal and clean heating surfaces) for catalog purposes, the architect or heating engineer makes allowances for less favorable conditions when deciding upon appropriate equipment. Gas appliance manufacturers give to their product a rating comparable to that of a.coal boiler on the 8-hr. basis. In other words, where a 2,400 sq. ft. coal boiler would be chosen, a 2,400 sq. ft. gas boiler would gener ally be appropriate. It is a common practice for gas boiler manufacturers also to give their boilers a rating in terms of "Available B.t.u." (B.t.u. per hour available in the steam or water at the boiler outlets). A gas furnace or boiler will show essentially the same capacity or efficiency with any gas fuel, provided steps are taken to furnish proper burner equipment and air regulation for each type of fuel. Most manu factured gases, although of lower heating'value than natural gases, if burned in sufficient volume, will produce equivalent results. The heat ing value of a mixture of gas and air (air jirst-sufficient to completely burn the gas) is almost the same for any typical commercial gas, as is shown in Table 50. 93