In the comparison above, the maximum evaporative power of wood has been used, 2½ lbs., and the ordinary power of coke, 8½ lbs. of water per pound of fuel.
320. In making coke in large quantities, the ovens should be at the mines, as we thus save transporting the extra weight of coal over coke.
The cost of making coke, exclusive of the cost of the coal, is approximately as follows:—
| 10 ovens capable of making annually 5,000 tons of coke, | $5,000 |
| Sheds, and apparatus to correspond, | 3,000 |
| In all, | 8,000 |
| Annual interest at 6 per cent., | 480 | |
| Annual cost of attendance, 2 men, | 1,000 | |
| The sum of which is, | $1,480 | |
| And the cost per ton, | 0.296 10 | |
or in round numbers, thirty cents per ton; and if coal is $1.50 per ton, adding twenty-five per cent. we have $1.87 as the cost of coal that will make one ton of coke, to which add the cost of making per ton, thirty cents, and we have as the whole cost of one ton of coke $2.17; and from the rule on page [327] we see that wood must not cost over $0.85 per cord to be as economical as coke at $2.17; of course inferior qualities of coal will give less good coke and change the comparison.
COMBUSTION.
321. The combustible element in all fuels is carbon; the heat necessary for steam producing, is obtained by combining the carbon of the fuel with the oxygen of the air, forming carbonic acid gas.
Carbonic acid gas consists of
| Oxygen | 16 | Parts by weight. |
| Carbon | 6 |