ir compressor, with compound air
cylinder. The chief purpose of compounding is to reduce the maximum
strain. This construction also adds to isothermal economy. The large
cylinder to the left determines the capacity of the compressor, the air
being compressed first to a low pressure (ordinarily about 30 pounds per
square inch), afterward passing through an intercooler, by which its
temperature is reduced, and then it is compressed still higher, even to
5,000 pounds per square inch if desired. The terminal strain, which is
so severe in air compressors, is here considerably reduced, as in this
case it is only equal to the area of the initial air piston multiplied
by its low air pressure.

[Illustration: FIG. 9.]

Economical results are attained with this compressor at low cost of
construction. The fly wheels are small, and the bearings narrow, because
the maximum strain is less, and the momentum of the piston and other
moving parts is such that most of the high initial steam power is taken
up in starting these parts and is afterward given out at the end of the
stroke, when the steam pressure is low. The strains are direct, and
expensive foundations are not required. Fig. 11 illustrates the
Ingersoll-Sergeant Compound Straight Line Air Compressor. This differs
from the one just described chiefly in that it is single-acting, while
the other is double-acting.

[Illustration: FIG. 10.]

By single-acting is meant that the air cylinders compress their
respective volumes of air _once_ every revolution. The air is admitted
to the large cylinder through the piston, is compressed to about 30
pounds, and on the return stroke the pressure is raised to almost any
point required, and in proportion to the diameter of the smaller
cylinder. Though single-acting, the capacity of one of these compressors
is about equal to that of the double-acting machine of the same cost of
construction. The initial air cylinder is made large enough to
correspond with the capacity of the smaller double-acting cylinder. The
strains are equalized because the area of the large cylinder multiplied
by its low pressure is exactly equal to that of the small cylinder
multiplied by its high pressure. The maximum strains are reduced
considerably below those which exist in compressors that do not compound
the air.

[Illustration: FIG. 11.]

The advantage of the single-acting air cylinder over the double is that
it compresses a volume of free air only once ever