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 compression. On the contrary, it is a most fallacious system, because it introduces all the disadvantages of water injection without its isothermal influence.  Water, by mere surface contact with air, takes up but little heat, while the air, having a chance to increase its temperature, absorbs water through the affinity of air for moisture, and thus carries over a volume of saturated hot air into the receiver and pipes, which on cooling, as it always does in transit to the mine, deposits its moisture and gives trouble through water and freezing.  It is, therefore, of much importance to bear in mind that unless water can be introduced during compression to such an extent as to keep down the temperature of the air in the cylinder, it had better not be introduced at all.

If too little water is introduced into an air cylinder during compression, the result is warm, moist air, and if too much water is used, it results in a surplus of power required to move a body of water which renders no useful service.  The following table deduced from Zahner’s formula gives the quantity of water which should be injected per cubic foot of air compressed in order to keep the temperature down to 104 degrees Fah.