The whole of the fuel was heated up, but the heat absorbed in the
evaporation of the water lowered the temperature of the furnace,
and must be deducted from the heat of combustion.
Units.
Heat of combustion................................... 2,828,200
" " evaporation of 26.08 lb. water............... -29,888
---------
Available heat of combustion.............. 2,798,312
Dividing by 238.25 lb. gives the heat per 1 lb.
of fuel used................................... = 11,745 units.
And temperature of furnace:
11,745 units/(18.125 lb. × 0.238) + 530°......... = 3,253°
Temperature of chimney 700° + 460°............... = 1,160°
Maximum duty (3,253° - 1,160°)/3,253°............ = 0.643°
Work of displacing atmosphere by smoke at 700°:
Cubic feet.
Volumes of gases at 70°........................ = 228.3
" " " " 700°........................ = 499.8
-----
Increase of volume.................... 271.5
Units.
Work done=
(194.46 lb. × 271.5 cub. ft. × 144 sq. in. × 15 lb.)
/722 units ..................................... = 147,720
Maximum amount of work to be expected =
3,248,425 × 0.643.............................. = 2,101,700
Deduct work of displacing atmosphere............. = 147,720
---------
Available work........................ 1,953,980
Actual work done:
Units.
1,620 lb. of water raised from 60° and turned
into steam at 53 lb..... ...................... = 1,855,900
Loss by radiation and convection................. 70,430
10-1/2 lb. ashes left, say at 500°............... 1,129
---------
Total work actually done.............. 1,927,459
Unaccounted for.................................. 26,521
---------
Calculated available work........................ 1,953,980
The unaccounted-for work, therefore, amounts to only 1½ per cent.
of the calculated available work.
Sir Frederick Bramwell ingeniously arranged his data in the form
of a balance sheet, and showed 253,979 units unaccounted for; but
if from this we deduct the work lost in displacing the air, the
unaccounted-for heat falls to less than 4 per cent. of the total
heat of combustion. These results show how extremely accurate the
observations must have been, and that the loss mainly arises from
convection and radiation from the boiler.]
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.
" 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.