aust from
other auxiliaries for heating the feed, which exhaust, if an injector
were used, would be wasted, as has been pointed out.

In locomotive practice, injectors are used because there is no exhaust
steam available for heating the feed, this being utilized in producing a
forced draft, and because of space requirements. In power plant work,
however, pumps are universally used for regular operation, though
injectors are sometimes installed as an auxiliary method of feeding.

Table 18 shows the relative value of injectors, direct-acting steam
pumps and pumps driven from the engine, the data having been obtained
from actual experiment. It will be noted that when feeding cold water
direct to the boilers, the injector has a slightly greater economy but
when feeding through a heater, the pump is by far the more economical.

Auxiliaries--It is the general impression that auxiliaries will take
less steam if the exhaust is turned into the condensers, in this way
reducing the back pressure. As a matter of fact, vacuum is rarely
registered on an indicator card taken from the cylinders of certain
types of auxiliaries unless the exhaust connection is short and without
bends, as long pipes and many angles offset the effect of the condenser.
On the other hand, if the exhaust steam from the auxiliaries can be used
for heating the feed water, all of the latent heat less only the loss
due to radiation is returned to the boiler and is saved instead of being
lost in the condensing water or wasted with the free exhaust. Taking
into consideration the plant as a whole, it would appear that the
auxiliary machinery, under such conditions, is more efficient than the
main engines.

[Illustration: Portion of 4160 Horse-power Installation of Babcock &
Wilcox Boilers at the Prudential Life Insurance Co. Building, Newark,
N. J.]

STEAM

When a given weight of a perfect gas is compressed or expanded at a
constant temperature, the product of the pressure and volume is a
constant. Vapors, which are liquids in aeriform condition, on the other
hand, can exist only at a definite pressure corresponding to each
temperature if in the saturated state, that is, the pressure is a
function of the temperature only. Steam is water vapor, and at a
pressure of, say, 150 pounds absolute per square inch saturated steam
can exist only at a temperature 358 degrees Fahrenheit. Hence if the
pressure of saturated steam be fixed, its temperature is also f