The requisite data are furnished by the experiments conducted some years
since by President D.M. Greene, of Troy College, for the Bureau of Steam
Engineering, U.S. Navy.
According to these experiments, the heat which is lost per hour by
radiation through a metallic plate of ordinary thickness, exposed to dry
air upon one side and to the source of heat upon the other, for one
degree difference in temperature, is as follows:
Condition. Heat units.
Naked...................................... 2.9330672
Covered with hair felt, 0.25 inch thick.... 1.0540710
" " 0.50 " .... 0.5728647
" " 0.75 " .... 0.4124625
" " 1.00 " .... 0.3070554
" " 1.25 " .... 0.2746387
" " 1.50 " .... 0.2507097
If now t' = temperature of steam at the ordinate,
t = temperature of the surrounding atmosphere,
dS = surface of the cylinder included between each ordinate,
k = that figure from the table satisfying the conditions,
then the power loss (dR) per minute will be:
k (t'-t)dS
dR = ( -- ) ----------. (2)
60 33,000
To the same scale as the power gains, upon each ordinate, set off the
appropriate power loss, as calculated by this equation (2).
There will result the curve r, r, r, which determines the power which at
any point in the diagram is to be regarded as a loss, to be carried to
the debit side of the account. This curve of losses intersects the curve
of gains at a point (it is evident) where each equals the other.
Therefore this is the point at which expansion should cease, and this
absolute pressure is the economic terminal pressure, which determines the
number of expansions profitable under the given conditions.
In the foregoing example are taken k = 0.3070554, t' = 331.169, t = 60,
while the back pressure was taken at 7 pounds.
By way of further illustration, first let the back pressure be changed
from 7 to 5.
By equation 1 there will result a new curve of gains, W, W, W, a portion
only being plotted.
Second, let t' = 331.169 as before.
t = 150 instead of 60.
k = 0.2507097 instead of 0.3070554.
There will result the second curve of losses, R, R, R, intersecting the
second curve of gains at the point F, the new economic point for our new
conditions.
These two examples fully illustrate the whole subject, furnishing an easy
and, when carefully made, a very exact calculation and result.
The following are a few of the general conclusions to be drawn:
1. That radiation is a tangible and measurable cause, sufficient to
account for all losses heretofore ascribed to an intangible,
immeasurable, and wholly imaginary cause, viz., "internal evaporation and
re-evaporation."
2. In order to prevent the high initial temperatures now used becoming a
source of loss, it is necessary to prevent the quantity dS (t'-t)
becoming great, by making dS as small as possible. In other words, we
must compound our engines. Thus for the first time is pointed out the
true reason why compound engines are economical heat engines.
3. The foregoing reasoning being correct, it follows that steam jackets
are a delusion.
4. In order to attain economy, we must have high initial temperatures,
small high pressure cylinders, low back pressures from whatsoever cause,
high piston speeds, short rather than long strokes, to avoid the cooling
effects of a long piston rod; but especially must we have scrupulous and
perfect protection from radiation, especially about the cylinder heads,
now oftentimes left bare.
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