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.

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