, it would have to be
superheated to a point where its total heat is 1199 + 168 = 1367
B. t. u. or, at 200 pounds gauge pressure, superheated approximately 325
degrees if the heat given up to the cylinder walls were the same as for
the saturated steam. As superheated steam conducts heat less rapidly
than saturated steam, the amount of heat imparted will be less than for
the saturated steam and consequently the amount of superheat required to
prevent condensation will be less than the above figure. This, of
course, is the extreme case of a simple engine with the range of
temperature change a maximum. As cylinders are added, the range in each
is decreased and the condensation is proportionate.

The true economy of the use of superheated steam is best shown in a
comparison of the "heat consumption" of an engine. This is the number of
heat units required in developing one indicated horse power and the
measure of the relative performance of two engines is based on a
comparison of their heat consumption as the measure of a boiler is based
on its evaporation from and at 212 degrees. The water consumption of an
engine in pounds per indicated horse power is in no sense a true
indication of its efficiency. The initial pressures and corresponding
temperatures may differ widely and thus make a difference in the
temperature of the exhaust and hence in the temperature of the condensed
steam returned to the boiler. For example: suppose a certain weight of
steam at 150 pounds absolute pressure and 358 degrees be expanded to
atmospheric pressure, the temperature then being 212 degrees. If the
same weight of steam be expanded from an initial pressure of 125 pounds
absolute and 344 degrees, to enable it to do the same amount of work,
that is, to give up the same amount of heat, expansion then must be
carried to a point below atmospheric pressure to, say, 13 pounds
absolute, the final temperature of the steam then being 206 degrees. In
actual practice, it has been observed that the water consumption of a
compound piston engine running on 26-inch vacuum and returning the
condensed steam at 140 degrees was approximately the same as when
running on 28-inch vacuum and returning water at 90 degrees. With an
equal water consumption for the two sets of conditions, the economy in
the former case would be greater than in the latter, since it would be
necessary to add less heat to the water returned to the boiler to raise
it to the steam temperature