SECT. XI.]
PROPERTIES OF STEAM.
97
N
than in the single steam engines by the amount lost in cooling the inside of the
cylinder half the time; hence the value of i the length of the cylinder must be
increased one-half, besides doubling the area exposed in a given time. This will
render the equation for the loss of temperature, (art. 156.)
f = ‘14 (36 l + d)(T -_0
d v
With the proportions and temperatures of the example, (art. 157.) the loss by
cooling is about -fa of the power; therefore it is not this species of loss which
should prevent this simple kind of engine being employed for mines.
If water be applied to keep the cylinder tight, the additional loss from convert
ing this water into vapour will be considerable. If the mean temperature of this
water be 180°, the effect of each foot of area will be to abstract, or to destroy a
cubic foot of steam per minute, this being the quantity of evaporation from a foot
of surface of water sustained at that temperature. Therefore in an engine working
at the rate of 170 feet per minute, that is, expending 85 cubic feet of steam of
atmospheric density per minute, for each foot in area of the cylinder the loss will
be fa = fa of its power; hence, adding this to the cooling effect, we have fa + fa
= about -fa of the power.
162. In the common atmospheric engine where the injection is made within
the cylinder, the only person who had attempted to calculate the loss of force was
Smeaton; of which some account has been given by Mr. Farey, in Rees’s c Cyclo
paedia.’ The mode of calculation is not very clearly given, and it was formed at
a time when the properties of heat were less known.
163. Cylinders are usually made of the same thickness, or so nearly so as to
render the variation not worthy of notice; hence we will assume them to be of the
same thickness. The quantity of matter in them is cooled by the injection from
212° to about 150°, rarely lower, and in good engines not lower than 170° or
180°; the mean 160° may be taken for the effect. The specific heat of iron is
about 200 times that of steam, and calculating the mass of iron which must have
its temperature raised from 160° to between 160° and 212° by each cylinder
full of steam, we have the quantity which that of the steam must be lowered.
The surface of a cylinder is equal to its length, increased by half its diameter,
multiplied by its circumference = (/ + \ d) d -k ; and the thickness, with an allowance
equivalent to the escape of heat from the external surface, is one inch and a half
= one-eighth of a foot; and the mass of metal equivalent to the absorption of
heat is
(l + -J- d) d tt
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