_Compressed air_ is air compressed by mechanical force into a state of
more or less increased density. _The power obtained from the expansion_
of greatly compressed air in a cylinder, on being set free is used
in many applications as a substitute for steam or other force as in
operating drills, shop tools and _engines_ which are driven by the
elastic force of compressed air.
_A compressor_ is a machine usually driven by steam by which air is
compressed in a receiver so that its expansion may be utilized as a
source of power at distances where an ordinary engine could not be
conveniently used.
The compressor proper comprises two sets of valves, usually designed
to be opened automatically by excess of pressure under them and to
be closed by gravity or by the action of springs when the pressures
become equal. The inlet valves open just after the piston commences
its stroke, when the expansion of the compressed air remaining in the
cylinder behind the piston has lowered the pressure above the valves.
They close at the end of the intake stroke, just as the piston comes to
rest. The outlet valve lifts during the compression stroke, at about
the time the rising pressure in the cylinder becomes equal to that in
the outlet passage above the valves; and they close when the flow of
air ceases as the piston completes its stroke.
Any of the accurately fitted steam engine valve gears may be used for
compressors, observing only that the compressor is in every way a
reversed steam engine.