*TM 5-810-4/AFM 88-8, Chap. 3
locating the air intake at the coolest air source, usually the
The intake for a compressor will be located either outdoors
north side of the building.
or indoors, whichever provides better air quality. Elevation
of the compressor relative to sea level is required to deter-
2-3. Intake pipe materials.
mine atmospheric pressure and density of intake air. Air
The inside of intake piping must be smooth and not subject
quality will be judged by its temperature, humidity, and
to rusting or oxidation. Rust that flakes off will enter and
cleanliness. Indoor sound levels are lower when the air
damage the compressor. Acceptable intake air piping mater-
intake is located outside the building, especially with a re-
ials include plastic, copper, stainless steel, aluminum, or
ciprocating compressor. Where practicable, an outside air
galvanized steel. On metallic piping, mechanical couplings
intake should be located on the coolest side of the building.
will be used. Welded joints must be avoided since weld
at least 6 feet above the ground or roof. For reciprocating
beads can break free, enter, and damage the compressor.
units, the intake will be located at least 3 feet away from any
wall to minimize the pulsating effect on the structure, and an
2-4. Critical pipe lengths.
intake filter silencer or an intake pulsation damper should be
Resonance of intake piping with reciprocating air compres-
provided. A compressor intake must not be located in an
sors is prevented by avoiding certain pipe lengths. These are
enclosed courtyard. Intake pipes must be positioned to
called "critical pipe lengths," and are a function of the air
prevent entrance of snow or rain water, and must be far
temperature and the speed of the compressor in revolutions
enough from steam, gas, or oil engine exhaust pipes to insure
per minute (rpm). Critical pipe lengths must be verified with
intake air that is free of moisture or pollution. Protection by
a hood or louvers should be considered when the intake is
subject to adverse weather conditions. It is desired that the
2-5. Intake air filter.
intake air filter be located on the compressor and piped from
The selection of the filter type is based on whether the air
the enclosed filter hood to the outside. This method prevents
compressor to be used is lubricated or nonlubricated, and on
ingestion of foreign material to the internals of the
the quality of ambient air.
compressor should the piping have a poor joint or other leak
a. Viscous impingement filters have an efficiency of 85
upstream of the intake filter.
to 90 percent of particle sizes larger than 10 microns. This
type of filter is acceptable for lubricated reciprocating com-
2-2. Intake temperature.
The density of air varies inversely with its temperature; an
b. Oil bath filters have an efficiency of 96 to 98 percent
increase in delivery of approximately I percent is gained for
of particles sized larger than 10 microns. This type of filter
every 5 degrees F reduction of intake temperature. Table 2-I
is more expensive, and for the most part no longer recom-
shows the effect of inlet or initial temperature on air
mended by compressor manufacturers, but may be considered
compressor delivery and demonstrates the importance of
for lubricated reciprocating compressors operating