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.