at a temperature of zero, it will reach about 100 degrees when the gauge
pressure is 14.7 pounds. We find this by following down the first line
intersected by the adiabatic curve to the point where the zero heat
curve intersects this same line, the reading being given in figures to
the left immediately opposite. If the air had been admitted to the
compressor at 60 degrees, it would register about 176 degrees at 14.7
pounds gauge pressure. If the air were 100 degrees before compression,
it would go up to about 230 degrees at this pressure. Following this
adiabatic curve until it intersects line No. 5, representing a pressure
of five atmospheres above a vacuum (58.8 lb. gauge pressure), we see
that the total increase of temperature on the zero heat curve is about
270 degrees, for the 60 degree curve it is about 370 degrees, and for
the 100 degree curve it is about 435 degrees. The diagram shows that
when a volume of air is compressed adiabatically to 21 atmospheres (294
lb. gauge pressure), it will occupy a volume a little more than
one-tenth; the total increase of temperature with an initial temperature
of zero is about 650 degrees; with 60 degrees initial temperature it is
800 degrees, and with 100 degrees initial it is 900 degrees. It will be
observed that the zero heat curve is flatter than the others, indicating
that when free air is admitted to a compressor cold, the relative
increase of temperature is less than when the air is hot. This points to
the importance of low initial temperature.

We have now seen that the economical production of compressed air
depends upon the following conditions:

(1) A low initial temperature.

(2) Thorough cooling during compression.