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°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°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.

COST OF VARIOUS TYPES OF FURNACES
Class fuel Fuel per charge Unit fuel cost Installation cost Efficiency per cent Fixed charges Cost per charge
1 2 3 4 5 6 7
Carburizing
1 Oil 52.0 gal. $0.15 gal. $2,400.00 12.6 $.40 $8.20
2 Natural gas 4.4 M 0.50 M 2,400.00 18.8 0.40 2.60
3 City gas 8.3 M 0.80 M 2,400.00 17.0 0.40 7.04
4 Water gas 18.7 M 0.40 2,400.00 16.4 0.40 7.88
5 Producer gas 37.3 M 0.10 M 2,400.00 14.5 0.40 4.13
6 Coal 814.0 lb. 6.00 ton 3,600.00 9.4 0.60 3.98
7 Electricity 500.0 kw-hr. 0.015 kw. 9,000.00 53.0 1.50 9.00
Heating
1 Oil 30.8 gal. 0.15 gal. 2,400.00 21.4 0.10 4.72
2 Natural gas 2.61 M 0.50 M 2,400.00 32.0 0.10 1.40
3 City gas 4.9 M 0.80 M 2,400.00 28.8 0.10 4.02
4 Water gas 11.1 M 0.40 M 2,400.00 27.6 0.10 4.54
5 Producer gas 22.1 M 0.10 M 2,400.00 24.6 0.10 2.31
6 Coal 348.0 lb. 6.00 ton 3,600.00 22.0 0.15 1.38
7 Electricity 329.0 kw-hr. 0.015 kw. 10,500.00 81.75 0.44 5.38

This shows but two of the operations and for a single furnace. The total costs for all operations on the 30,000 lb. of gears per 24 hr. is shown in Table 29.

TABLE 29.—COMPARATIVE ANNUAL PRODUCTION COSTS FOR 30,000 POUNDS OUTPUT IN 24 HOURS
No. Equipment Installation cost Annual operating expenses Total Cost per lb. metal, cents
Fixed charges Heat Labor
12 34 56 78
I Oil $179,000.00 $26,850.00 $156,000.00 $105,000.00 $287,850.00 $3.19
II Oil and electric 213,000.00 31,950.00 142,770.00 97,000.00 271,720.00 3.02
III Natural gas 117,000.00 17,550.00 44,250.00 97,000.00 158,800.00 1.78
IV (A) Natural gas containing furnaces 120,000.00 18,000.00 41,000.00 94,000.00 153,000.00 1.70
V Natural gas and electric 181,000.00 27,150.00 73,820.00 90,000.00 190,970.00 2.13
VI City gas 122,000.00 18,300.00 123,200.00 94,000.00 235,500.00 2.62
VII City gas and electric 182,000.00 27,300.00 128,820.00 90,000.00 246,020.00 2.74
VIII Water gas 214,000.00 18,600.00 104,000.00 94,000.00 216,600.00 2.41
IX Water gas and electric 238,000.00 27,450.00 117,420.00 90,000.00 234,870.00 2.62
X Producer gas 246,000.00 18,900.00 69,300.00 90,000.00 178,200.00 1.98
XI Producer gas and electric 255,000.00 27,750.00 92,520.00 90,000.00 210,270.00 2.34
XII Coal and electric 194,000.00 29,100.00 87,220.00 90,000.00 206,320.00 2.30
XIII Electric 257,000.00 38,550.00 135,000.00 84,000.00 257,550.00 2.86

NOTE.—Producer plant fixed charges are included in the cost of gas and are charged as "heat" in column 5, so they are omitted from column 4.

CHAPTER XII

PYROMETRY AND PYROMETERS

A knowledge of the fundamental principles of pyrometry, or the measurement of temperatures, is quite necessary for one engaged in the heat treatment of steel. It is only by careful measurement and control of the heating of steel that the full benefit of a heat-treating operation is secured.