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Page 7
14,544 units
T = 520� + ------------------ = 5,150� absolute.
13.2 lb. X 0.238
The lower temperature, _t_, we may take as that of the feed water, say
at 100� or 560� absolute, for by means of artificial draught and
sufficiently extending the heating surface, the temperature of the
smoke may be reduced to very nearly that of the feed water. Under such
circumstances the proportion of heat which can be realized is
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 X 0.25 X 520 = 130
12.2 of air X 0.238 X 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�, } 32
or from 149.8 cubic feet to 161.3 }
cubic feet, /
------
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--
16,159 u. X 0.891 - 32 u.
W = --------------------------- = 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]
[Footnote 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 wood........................... 26.08
Ash.............................................. 10.53
Hydrogen, oxygen, nitrogen, and sulphur.......... 7.18
------
Total non-combustible..................... 43.79
Carbon, being useful combustible................. 194.46
------
Total fuel................................ 238.25
Air per pound of carbon................................ 17-1/8 lb.
Time of experiment..................................... 4 h. 12 min.
Water evaporated from 60� into steam at 53 lb. pressure 1,620 lb.
Heat lost by radiation and convection.................. 70,430 units.
Mean temperature of chimney............................ 700� F.
" " " air................................ 70� F.
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