FIG. 100.—Gas fired furnace.
FIG. 101.—Car door type of annealing furnace.
Oil furnaces are also made for both high- and low-pressure air, each having its advocates. The same people also make gas-fired furnaces.
Several types of furnaces for various purposes are illustrated in Fig. 100 and 101. The first is a gas-fired hardening furnace of the surface-combustion type.
A large gas-fired annealing furnace of the Maxon system is shown in Fig. 101. This is large enough for a flat car to be run into as can be seen. It shows the arrangement of the burners, the track for the car and the way in which it fits into the furnace. These are from the designs of the Industrial Furnace Corporation.
Before deciding upon the use of gas or oil, all sides of the problem should be considered. Gas is perhaps the nearest ideal but is as a rule more expensive. The tables compiled by the Gilbert & Barker Manufacturing Company and shown herewith, may help in deciding the question.
| Heat units per thousand cubic feet 1,000,000 | |
|---|---|
| Natural gas | 1,000,000 |
| Air gas (gas machine) 20 cp | 815,500 |
| Public illuminating gas, average | 650,000 |
| Water gas (from bituminous coal) | 377,000 |
| Water and producer gas, mixed | 175,000 |
| Producer gas | 150,000 |
Since a gallon of fuel oil (7 lb.) contains 133,000 heat units, the following comparisons may evidently be made. At 5 cts. a gallon, the equivalent heat units in oil would equal:
| Heat units per thousand cubic feet at $0.375 | |
|---|---|
| Natural gas | at $0.375 |
| Air gas, 20 cp | at 0.307 |
| Public illuminating gas, average | at 0.244 |
| Water gas (from bituminous coal) | at 0.142 |
| Water and producer gas, mixed | at 0.065 |
| Producer gas | at 0.057 |