By Increase of Temperature alone.
13 Final Temperature if Water is used in Compression. Fah.
14 Percentage of Water to Air Required.
15 Foot Pounds to Compress One Pound Air. Dry.
16 Foot Pounds to Compress One Pound Air. With sufficient Moisture.

The first advantage is by far the most important one, and is really the
only excuse for water injection in air compressors. We have seen (table
3) that the percentage of work of compression which is converted into
heat and loss when no cooling system is used is as follows:

Compressing to 2 atmospheres loss 9.2 per cent.
" " 3 " " 15.0 " "
" " 4 " " 19.6 " "
" " 5 " " 21.3 " "
" " 6 " " 24.0 " "
" " 7 " " 26.0 " "
" " 8 " " 27.4 " "

We see that in compressing air to five atmospheres, which is the usual
practice, the heat loss is 21.3 per cent., so that if we keep down the
temperature of the air during compression to the isothermal line, we
save this loss. The best practice in America has brought this heat loss
down to 3.6 per cent. (old Ingersoll Injection Air Compressor), while in
Europe the heat loss has been reduced to 1.6 per cent. Steam-driven air
compressors are usually run at a piston speed of about 350 feet per
minute, or from 60-80 revolutions per minute of compressors of average
sizes, say 18" diameter of cylinder. Sixty revolutions per minute is
equal to 120 strokes, or two strokes per second. An air cylinder 18" in
diameter filled with free air once every half second, and at each stroke
compressing the air to 60 pounds, and thereby producing 309 degrees of
heat, is thus, by means of water injection, cooled to an extent hardly
possible with mere surface contact. The specific heat of water being
about four times that of air, it readily takes up the heat of
compression.

A properly designed spray system must not be confused with the numerous
devices applied to air cylinders, by means of which water is introduced.
In some cases the water is merely drawn in through the inlet valves. In
others it passes through the center of the piston and rod, coming in
contact with the interior walls of the air cylinder between the packing
rings. Introducing water into the air cylinder in _any other way, except
in the form of a spray, has but little effect in cooling the air during
compressi