most systems the compressor is part of the system with
distribution lines leading from the compressor to the
devices to be operated.
Compressed air systems are categorized by their
operating pressure as follows:
High-pressure (HP)3,000 to 5,000 psi
Medium-pressure (MP )151 to 1,000 psi
Low-pressure (LP)150 psi and below
Figure 3-51.Intake and compression strokes in a
HEAVY-DUTY AIR COMPRESSORS
Compressors are used in pneumatic systems to
provide requirements similar to those required by pumps
in hydraulic systems. They furnish compressed air as
required to operate the units of the pneumatic systems.
Even though manufactured by different
companies, most compressors are quite similar. They
are governed by a pressure control system that can be
adjusted to compress air to the maximum pressure.
The compressor unit may be of the reciprocating,
rotary, or screw design.
The reciprocating compressor is similar to that of
an automotive engine. They may be air- or liquid-
cooled. As the pistons move up and down, air flows into
the cylinder through the intake valve. As the piston
moves upward, the intake valve closes and traps air in
the cylinder. The trapped air is compressed until it
exceeds the pressure within the collecting manifold, at
which time the discharge valve opens and the
compressed air is forced into the air manifold (fig. 3-
51). The reciprocating compressor is normally
connected to the engine through a direct coupling or a
clutch. The engine and compressor are separate units.
The rotary compressor has a number of vanes held
in captive in slots in the rotor. These vanes slide in and out
of the slots, as the rotor rotates. Figure 3-52 shows an end
Figure 3-52.Compression cycle in a rotary compressor.