Note 1 — THE GENERATION OF STEAM, AND THE THERMODYNAMIC PROBLEMS INVOLVED.1
Reading edition
Lecture delivered at the Institution of Civil Engineers, session 1883-84. For the illustrations we are indebted to the courtesy of Mr. J. Forrest, the secretary.
[2]
In the fifty-second volume of the Proceedings (1887-78), page 154, will be found a remarkable experiment on the evaporative power of a vertical boiler with internal circulating pipes. The experiment was conducted by Sir Frederick Bramwell and Dr. Russell, and is remarkable in this respect, that the quantity of air admitted to the fuel, the loss by convection and radiation, and the composition of the smoke were determined. The facts observed were as follows:
Steam pressure 53 lb= 300.6° F.
lb.
Fuel—Water in coke and wood26.08
Ash10.53
Hydrogen, oxygen, nitrogen, and sulphur7.18
———
Total non-combustible43.79
Carbon, being useful combustible194.46
———
Total fuel238.25
Air per pound of carbon17-1/8 lb.
Time of experiment4 h. 12 min.
Water evaporated from 60° into steam at 53 lb. pressure1,620 lb.
Heat lost by radiation and convection70,430 units.
Mean temperature of chimney700° F.
Mean temperature of air70° F.
No combustible gas was found in the chimney.
I will apply Carnot's doctrine to this case.
Potential energy of the fuel with respect to absolute zero:
Units.
239.25 lb. × 530° abs. × 0.238= 30,053
194.46 lb. × 17-1/8 × 530° × 0.238, the weight and heat of air 420,660
194.46 × 14,544 units heat of combustion of carbon 2,828,200
————
Total energy3,278,813
Heat absorbed in evaporating 26.08 lb. of water in fuel -29,888
————
Available energy3,248,425
Temperature of furnace—
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 combustion2,828,200
Heat of evaporation of 26.08 lb. water-29,888
———
Available heat of combustion2,798,312
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
Volumes of gases at 700°= 499.8
———
Increase of volume271.5
Work done= Units.
(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 work1,953,980
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 convection70,430
10½ lb. ashes left, say at 500°1,129
————
Total work actually done1,927,459
Unaccounted for26,521
————
Calculated available work1,953,980
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.