lubrication oil, and remove oil after completion
of the compression process.
AIR COMPRESSORS AND
There are basically two types of compressors:
positive displacement and dynamic. This section
will discuss the reciprocating air compressors,
the rotary air compressors, the helical screw
compressors classed as positive displacement
compressors, and the dynamic centrifugal com-
General auxiliary equipment will also be
discussed. Auxiliary equipment consists of any
device(s) that may be added to the system to
improve its efficiency or provide a specific func-
tion. It provides a safe condition under which the
compressor system will be operating.
The most commonly used stationary air com-
pressors are the reciprocating, positive displace-
ment design. They may be single acting or
double acting, single stage or multistage, and
horizontal, angle, or vertical in design.
In a single-stage unit there is but one com-
pressing element; it compresses air from the
initial intake pressure to the final discharge
pressure in one step. A multistage machine has
more than one compressing element. The first
stage compresses air to an intermediate pressure,
then one or more additional stages compress it
to the final discharge pressure.
In the reciprocating compressor the compres-
sion cycle is composed of three phases: intake,
compression, and discharge.
During the intake stroke the downward move-
ment of the piston creates a partial vacuum
inside the cylinder. The spring-operated intake
valve is forced open by the differential pressure
between free air on one side and the partial
vacuum inside the cylinder. As the valve opens,
air fills the cylinder. The piston now moves into
the compression stroke, forcing the intake valve
closed and raising the pressure of the air trapped
in the cylinder. When the pressure of this air
is great enough to overcome the force of the
spring-operated discharge valve, the valve opens
and the compressed air is discharged from the
Compressors are classified as low pressure,
medium pressure, or high pressure. Low-pressure
compressors provide a discharge pressure of 150
psi or less. Medium-pressure compressors provide
a discharge pressure of 151 psi to 1,000 psi. Com-
pressors that provide a discharge pressure above
1,000 psi are classified as high pressure. Note that
compressors are classified at different pressures
than those for classifying total compressed air
systems discussed earlier.
Most low-pressure air compressors are of the
two-stage type with either a vertical or a vertical
W arrangement of cylinders. Two-stage, V-type,
low-pressure compressors usually have one
cylinder that provides the first (low-pressure) stage
of compression and one cylinder that provides the
second (high-pressure) stage, as shown in figure
11-3. W-type compressors have two cylinders for
the first stage of compression and one cylinder
for the second stage. This arrangement is il-
lustrated in figure 11-4.
Compressors may be classified according to
a number of other design features or operating
Medium-pressure air compressors are of the
two-stage, vertical, duplex, single-acting type.
Many medium-pressure compressors have dif-
ferential pistons, as shown in figure 11-5. This
type of piston provides more than one stage of
compression on each piston.
ROTARY AIR COMPRESSORS
Rotary sliding vane compressors are machines
in which longitudinal vanes slide radially in a
slotted rotor that is mounted eccentrically in a
cylinder. The rotor is fitted with blades or vanes
that are free to slide in and out of longitudinal
slots and maintain contact with the cylinder walls
by centrifugal force. In operation, as the blades
are forced outward by centrifugal force, compart-
ments are formed in which air is compressed (fig.
11-6). Each compartment varies from a maximum
volume on the suction side of the revolution to
a minimum volume on the compression half of
the revolution. This gives a positive displacement-
type suction and pressure effect.
Another type of rotary compressor is the twin-
lobe unit sometimes referred to as a blower