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Piping Material
The purpose of the compressed air
piping system is to deliver compressed air to the points of usage.
The compressed air needs to be delivered with enough volume,
appropriate quality, and pressure to properly power the components
that use the compressed air. Compressed air is costly to
manufacture. A poorly designed compressed air system can increase
energy costs, promote equipment failure, reduce production
efficiencies, and increase maintenance requirements. It is
generally considered true that any additional costs spent improving
the compressed air piping system will pay for themselves many times
over the life of the system.
Compressor Discharge
Piping
Discharge piping from a compressor
without an integral aftercooler can have very high
temperatures. The pipe that is installed here must be able to
handle these temperatures. The high temperatures can also cause
thermal expansion of the pipe, which can add stress to the pipe.
Check the compressor manufacturer's recommendations on discharge
piping. Install a liquid filled pressure gauge, a thermometer, and
a thermowell in the discharge airline before the aftercooler.
Proper support and/or flexible discharge pipe can eliminate
strain.
Condensate
Control
Condensation control must be
considered when installing a compressed air piping system. Drip
legs should be installed at all low points in the system. A drip
leg is an extension of pipe below the airline, which is used to
collect condensation in the pipe. At the end of the drip leg a
drain trap should be installed. Preferably an automatic drain will
be used (see drain
valves section for a complete description of the type of drain
valves available).
To eliminate oil, condensate, or cooling water (if
the water-cooled aftercooler leaks), a low point drain should be
installed in the discharge pipe before the aftercooler. Be sure to
connect the aftercooler outlet to the separator inlet when
connecting the aftercooler and the moisture separator together. If
they are not connected properly, it will result in either poor
aftercooling or poor separation.
The main header pipe in the system should be sloped downward in
the direction of the compressed air flow. A general rule of thumb
is 1" per 10 feet of pipe. The reason for the slope is to direct
the condensation to a low point in the compressed air piping system
where it can be collected and removed.
Make sure that the piping following the aftercooler slopes
downward into the bottom connection of the air receiver.
This helps with the condensate drainage, as well as if the
water-cooled aftercooler develops a water leak internally. It would
drain toward the receiver and not the compressor.
Another method of controlling the condensation is to take all
branch connections from the top of the airline. This eliminates
condensation from entering the branch connection and allows the
condensation continue to the low points in the system.
Pressure Drop
Pressure drop in a compressed air
system is a critical factor. Pressure drop is caused by friction of
the compressed air flowing against the inside of the pipe and
through valves,
tees, elbows and other components that make up a complete
compressed air piping system. Pressure drop can be affected by pipe
size, type of pipes used, the number and type of valves, couplings,
and bends in the system. Each header or main should be furnished
with outlets as close as possible to the point of application. This
avoids significant pressure drops through the hose and allows
shorter hose lengths to be used. To avoid carryover of condensed
moisture to tools, outlets should be taken from the top of the
pipeline. Larger pipe sizes, shorter pipe and hose lengths, smooth
wall pipe, long radius swept tees, and long radius elbows all help
reduce pressure drop within a compressed air piping system.
In recent years several manufacturers have developed piping
systems especially for compressed air (fig. P1-2). These compressed
air piping systems typically have smooth walls, are lightweight,
and reduce the installation costs associated with copper and
threaded pipe. Follow the manufacturer's recommendations for
installing these systems. 
Loop Pipe System
The layout of the system can also
affect the compressed air system. A very efficient compressed air
piping system design is a loop design. The loop design (fig. P1-3)
allows airflow in two directions to a point of use. This can cut the overall
pipe length to a point in half that reduces pressure drop. It also
means that a large volume user of compressed air in a system may
not starve users downstream since they can draw air from another
direction. In many cases a balance line is also recommended which
provides another source of air.
Reducing the velocity of the airflow through the compressed air
piping system is another benefit of the loop design. In cases where
there is a large volume user an auxiliary receiver can be
installed. This reduces the velocity, which reduces the friction
against the pipe walls and reduces pressure drop. Receivers
should be positioned close to the far ends or at points of
infrequent heavy use of long distribution lines. Many peak demands
for air are short-lived, and storage capacity near these points
helps avoid excessive pressure drop and may allow a smaller
compressor to be used.
Piping materials
Common piping materials used in a
compressed air system include copper, aluminum, stainless steel and
carbon steel. Compressed air piping systems that are 2" or smaller
utilize copper, aluminum or stainless steel. Pipe and fitting
connections are typically threaded. Piping systems that are 4" or
larger utilize carbon or stainless steel with flanged pipe and
fittings.
Note: Plastic piping may be used on compressed air systems,
however caution must used since many plastic materials are not
compatible with all compressor lubricants. Ultraviolet
light (sun light) may also reduce the useful service life of some
plastic materials. Installation must follow the manufacturer's
instructions.
It is always better to oversize the compressed air piping system
you choose to install. This reduces
pressure drop, which will pay for itself, and it allows for
expansion of the system.
Corrosion-resistant piping should be used with any compressed air
piping system using oil-free compressors. A non-lubricated system
will experience corrosion from the moisture in the warm air,
contaminating products and control systems, if this type of piping
is not used.
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