| = | 5,150° - 560° | = 0.891; |
| 5,150° |
that is to say, under the extremely favorable if not impracticable conditions assumed, there must be a loss of 11 per cent. Next, to give a numerical value to the potential energy, H, to be derived from a pound of carbon, calculating from absolute zero, the specific heat of carbon being 0.25, and absolute temperature of air 520°:
| Units. | ||
| 1 lb. of carbon × 0.25 × 520 | = | 130 |
| 12.2 of air × 0.238 × 520 | = | 1,485 |
| Heat of combustion | = | 14,544 |
| 16,159 | ||
| Deduct heat equivalent to work of displacing atmosphere by products of combustion raised from 60° to 100°, or from 149.8 cubic feet to 161.3 cubic feet, | 32 | |
| Total units of heat available | 16,127 | |
Equal to 16.69 lb. of water evaporated from and at 212°. Hence the greatest possible evaporation from and at 212° from a lb. of carbon—
| W = | 16,159 u. × 0.891 - 32 u. | = 14.87 lb. |
| 966 u. |
I will now take a definite case, and compare the potential energy of a certain kind of fuel with the results actually obtained. For this purpose the boiler of the eight-horse portable engine, which gained the first prize at the Cardiff show of the Royal Agricultural Society in 1872, will serve very well, because the trials, all the details of which are set forth very fully in vol. ix. of the Journal of the Society, were carried out with great care and skill by Sir Frederick Bramwell and the late Mr. Menelaus; indeed, the only fact left undetermined was the temperature of the furnace, an omission due to the want of a trustworthy pyrometer, a want which has not been satisfied to this day.[2] The data necessary for our purpose are:
| Steam pressure 80 lb. temperature | 324° = 784° | absolute. |
| Mean temperature of smoke | 389° = 849° | " |
| Water evaporated per 1 lb of coal, from and at 212° | 11.83 lb. | |
| Temperature of the air | 60° = 520° | absolute. |
| Temperature of feed water | 209° = 669° | " |
| Heating surface | 220 square feet. | |
| Grate surface | 3.29 feet. | |
| Coal burnt per hour | 41 lb. |
The fuel used was a smokeless Welsh coal, from the Llangennech colleries. It was analyzed by Mr. Snelus, of the Dowlais Ironworks, and in Table II. are exhibited the details of its composition, and the weight and volume of air required for its combustion. The total heat of combustion in 1 lb of water evaporated:
= 15.06 × (0.8497 + 4.265 × (0.426 - 0.035/8))
= 15.24 lb. of water from and at 212°
= 14,727 units of heat.
TABLE II.—PROPERTIES OF LLANGENNECH COAL.