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.
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