Specifications

Introduction
All air tools, spray guns, sanders,
etc., must be supplied with air
which is elevated to higher
pressures and delivered in
sufficient volume. The air
compressor compresses air for
use in this equipment and is a
major component of a spray
painting system. This chapter
will examine the various types
available.
Compressed air is measured on
the basis of volume supplied per
unit of time (cubic feet per minute,
or cfm) at a given pressure per
square inch (psi), referred to as
delivery.
Displacement is the output of air
by a compressor at zero pressure,
or free air delivery.
1. What is an air compressor?
An air compressor is a machine
designed to raise the pressure of
air from normal atmospheric
pressure to some higher pressure,
as measured in pounds per square
inch (psi). While normal
atmospheric pressure is about
14.7 pounds per square inch, a
compressor will typically deliver air
at pressures up to 200 psi.
When selecting a compressor:
Rule of thumb
The cubic feet per minute delivered
by an electrically powered 2 stage
industrial air compressor is 4 times
the motor's horse power rating.
(CFM=4xHP)
2. What types of compressors are
most common in spray finishing
operations?
There are two common types; the
piston-type design and the rotary
screw design.
Because most commercial spray
finishing operations consume large
quantities of compressed air at
relatively high pressures, the piston
type compressor is the more
commonly used.
3. How does a piston-type
compressor work?
This design elevates air pressure
through the action of a
reciprocating piston. As the piston
moves down, air is drawn in
through an intake valve. As the
piston travels upward, that air is
compressed. Then, the now-
compressed air is discharged
through an exhaust valve into the
air tank or regulator.
Piston type compressors are
available with single or multiple
cylinders in one or two-stage
models, depending on the volume
and pressure required.
Figure 1 - Piston Type Air
Compressor
4. How does a rotary screw
compressor work?
Rotary screw compressors utilize
two intermeshing helical rotors in a
twin bore case. Air is compressed
between one convex and one
concave rotor. Trapped volume of
air is decreased and the pressure is
increased.
Figure 2 – Rotary Screw Air
Compressor
5. What is a single stage
compressor?
This is a piston-type compressor
with one or more cylinders, in
which air is drawn from the
atmosphere and compressed to its
final pressure with a single stroke.
All pistons are the same size, and
they can produce up to 125 psi.
6. Where are single stage
compressors used?
The application of this compressor
is usually limited to a maximum
pressure of 125 psi. It can be used
above 125 psi, but above this
pressure, two stage compressors
are more efficient.
7. What is a two-stage
compressor?
A compressor with two or more
cylinders of unequal size in which
air is compressed in two separate
steps.
The first (the largest) cylinder
compresses the air to an
intermediate pressure. It then
exhausts it into a connecting tube
called an intercooler.
From there, the intermediate
pressurized air enters the smaller
cylinder, is compressed even more
and is delivered to a storage tank
or to the main air line.
Two-stage compressors can deliver
air to over 175 psi.
They are normally found in
operations requiring compressed
air of 125 psi or greater.
8. What are the benefits of two-
stage compressors?
Two-stage compressors are usually
more efficient. They run cooler and
deliver more air for the power
consumed, particularly in the
over-100 psi pressure range.
7. Air Compressors
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