A. As the low pressure air piston 9 moves up, a partial vacuum is
created beneath it and air from the atmosphere enters the air inlet and
passage "r" past the lower receiving valve 38 and fills the lower end of
the cylinder with air at about atmospheric pressure (see Fig. 4). In the
meantime the air above the piston being compressed will hold the upper
receiving valve 37 to its seat, thus preventing a back-flow of air to
the atmosphere; at the same time the upper intermediate discharge valves
39 are forced from their seats, allowing the air from the low pressure
air cylinder to flow through passage "u" to the high pressure air
cylinder, the piston of which is now moving downward. The air beneath
the high pressure air piston 10 being compressed will hold the lower
intermediate discharge valves 40 to their seats, thus preventing the air
in the high pressure air cylinder flowing back to the low pressure air
cylinder. When the pressure in the high pressure air cylinder becomes
slightly greater than the main reservoir pressure, the final discharge
valve 42 will be forced from its seat and the air beneath the piston
allowed to flow to the main reservoir through passage "w". On the
opposite strokes of these pistons air is compressed in a similar manner,
but the opposite air valves are used.
[Illustration: Diagram of 8-1/2" Cross-Compound Compressor. The
High Pressure Steam (Low Pressure Air) Piston on Its Downward Stroke
Fig. 5.]