In order to give definite information concerning furnaces, fuels etc., the following data is quoted from a paper by Seth A. Moulton and W. H. Lyman before the Steel Heat Treaters Society in September, 1920. This considers a factory producing 30,000 lb. of automobile gears per 24 hr. The transmission gears will be of high-carbon steel and the differential of low-carbon steel, carburized. The heat-treating equipment required is: 1. Annealing furnaces 1,400 to 1,600 deg.F. 2. Carburizing furnaces 1,700 to 1,800 deg.F. 3. Hardening furnaces 1,450 to 1,550 deg.F. 4. Drawing furnaces 350 to 950 deg.F. All of the forging blanks are annealed before machining, about three-quarters of the machined gears and parts are carburized, all the carburized gears are given a double treatment for core and case, all gears and parts are hardened and all parts are drawn. The possible sources of heat supply and their values are as follows:-- 1. Oil 140,000 B.t.u. per gallon 2. Natural gas 1,100 B.t.u. per cubic foot 3. City gas 650 B.t.u. per cubic foot 4. Water gas 300 B.t.u. per cubic foot 5. Producer gas 170 B.t.u. per cubic foot 6. Coal 12,000 B.t.u. per pound 7. Electric current 3,412 B.t.u. per kilowatt-hour For the heat treatment specified only comparatively low temperatures are required. No difficulty will be experienced in attaining the desired maximum temperature of 1,800 deg.F. with any of the heating medium above enumerated; but it should be noted that the producer gas with a B.t.u. content of 170 per cubic foot and the electric current would require _specially_ designed furnaces to obtain higher temperatures than 1800 deg.F. TABLE 28.--COMPARATTVE OPERATING COSTS Assuming Cost of oil- and gas-fired furnaces installed as $100.00 per square foot of hearth Cost of coal-fired furnace installed as 150.00 per square foot of hearth Cost of electric furnace 100 kw. capacity installed as 90.00 per kilowatt Cost of electric furnace 150 kw. capacity installed as 70.00 per kilowatt Output 3,000 lb. charge, 8 hr. heat carburizing, 2 hr. heating only. Annual service 7,200 hr. Fixed charges including interest, depreciation, taxes, insurance and maintenance 15 per cent. Extra operating labor for coal-fired furnace 60 cts. per hour, one man four furnaces.