Now it appears, from the result of Mr. Watt's ingenious
experiments on the quantity of latent heat in steam, that the
quantity of heat necessary to change any given quantity of
water ALREADY BOILING HOT to steam, is about five times and a
half greater than would be sufficient to heat the same quantity
of water, from the temperature of freezing, to that of boiling
water.
But we have just observed, that 20 lbs. of ice-cold water may be
heated to the boiling point, with the heat generated in the
combustion of 1 lb. of pine-wood; it appears therefore that 20 lbs.
of boiling water would require 5 1/2 times as much, or 5 1/2 lbs.
of wood to reduce it to steam.
And if 20 lbs. of boiling water require 5 1/2 lbs. of wood,
21/64 of a pound of water boiling hot will require 0.09023 of a
pound of wood to reduce it to steam.
If now, to this quantity of fuel,-- -- 0.09023 lb.
we add that necessary for heating the
loaf to the temperature of boiling
water, as above determined, -- -- -- 0.07335 lb.
--------
this gives the total quantity of fuel
necessary for baking one of these loaves
of bread,-- -- -- -- -- -- -- -- 0.16353 lb.
Now as these loaves, when baked into bread, weighed 2 lbs. 5
1/2 loths = 2 11/64 lbs. each and required, in being baked,
the consumption of 0.16353 of a pound of wood, this gives for
the expence of fuel in baking bread 0.07532 of a pound of
pine-wood to each pound of rye bread; which is about 13 1/4
lbs. of bread to each pound of wood.
But we have seen, from the results of the before-mentioned
experiments, that when the bread was baked under circumstances
the most favourable to the economy of fuel, no less than 80 lbs.
of pine-wood were employed in heating the oven to bake 304 lbs.
of bread, which gives less than 4 lbs. of bread to each pound
of wood; consequently, TWO THIRDS at least of the heat
generated in the combustion of the fuel must, in that case,
have been lost; and in all the other experiments the loss of
heat appears to have been still much greater.
A considerable loss of heat in baking will always be inevitable;
but it seems probable, that this loss might, with proper
attention to the construction of the oven, and to the
management of the fire, be reduced at least to one half the
quantity generated from the fuel in its combustion. In the
manner in which the baking business is now generally carried
on, much more than three quarters of the heat generated,
or which might be generated from the fuel consumed, is lost.
APPENDIX, No VIII.
The following Account of the Persons in the House of Industry
in Dublin, the 30th of April 1796, and of the Details of the
Manner and Expence of feeding them, was given to the Author,
by order of the Governors of that Institution.
Public-domain text, read in full here on John Shaqi.
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