Air compressors may be driven in various ways, but the most commonly used are those which are directly connected to a steam engine, thus doing away with intermediate machinery. When the air piston draws in a charge of air, the air fills the cylinder at atmospheric pressure, or a little below, and on the return stroke of the piston it has to be compressed to the same pressure as in the receiver before it can lift the delivery valve, and as the valve is held to its seat by a spring, and also by its own weight, the pressure has to be considerably above that of the receiver before the valve will lift. To overcome this the valves are operated by mechanical means, which lifts them at a point of the stroke, when the pressure in the cylinder corresponds with that of the receiver.
Fig. 363.—See page [71].
This arrangement avoids pounding of the valves as well as the noise caused by the air when rushing at much higher pressure from the cylinder into the receiver.
For the sake of economy, air compressors are compounded, as for example, by drawing the air into a large cylinder and compressing it to a certain stage, whence it passes into a smaller cylinder, which compresses it to a much higher pressure.
In a simple compressor, for very high pressures, there is at the end of the stroke a large volume of air left in the clearance space, which expands on the return stroke, to atmospheric pressure, before another charge of air can be drawn in.
But in the compound compressor, the air is delivered from the low pressure receiver to the high pressure cylinder far above atmospheric pressure, thus the remaining air need not expand so much and allows the cylinder to take a larger volume of air. The load is also distributed more evenly.
The following are valuable “points” relating to the care and management of air compressors.