*TM 5-810-4/AFM 88-8, Chap. 3
CHAPTER 3
AIR COMPRESSORS
3-1. Application.
rotary compressor, the oil-free rotary lobe compressor is
Whenever it is economically feasible, a central compressed
available from 100 to 500 cfm. Oil-free rotary screw and
air system will be utilized to serve multiple points of use.
rotary lobe compressors can be used for baseload or partial
The air pressure in the receiver will be in the range of 80 to
load.
125 pounds per square inch guage (psig). Compressors and
d. Rotary sliding vane. Air volumes range up to approxi-
all accessories will conform to American Society of
mately 3,000 cfm. Such compressors can be oil-injected, oil-
Mechanical Engineers (ASME) B19.l and B19.3, ASME
flooded, or oil-free types. This type of compressor has low
Boiler and Pressure Vessel Code Section VIII, PTC-9 &
operating cost, no pulsation, and is free from vibration. This
PTC-10, and Instrument Society of America (ISA) S7.3, as
permits installing the compressor directly on the simplest
applicable. Oxygen must not be handled in the presence of
foundation.
hydrocarbon lubricants. Where lubricating oils cannot be
e. Centrifugal. Air volumes range from approximately 1
tolerated at the point of use, oil-free air compressors will be
.200 cfm to approximately 18,000 cfm. A blowoff silencer
used. Oil-free air is required for such end uses as food
is needed for noise control. Centrifugal compressors require
handling, medical and dental applications (consult TM 5-
no lubrication in contact with the airstream and therefore
838-2 or AFR 88-50, as applicable, and NFPA 99),
provide oil-free air.
chemical processing, and instrument air for pneumatic
f. Tank-mounted compressor. A considerable field prob-
controls. Oil-free air can be obtained by using a centrifugal
lem has been experienced with tank rupture at the weldment
compressor, which is not lubricated due to its configuration;
points. To alleviate the potential problems of tank rupture at
a water-sealed rotary compressor: or a reciprocating
weldment points, tank-mounted air compressors are required
nonlubricated air compressor using carbon or Teflon for
to be factory-assembled units with tanks constructed in
piston and packing rings. For isolated cases where oil-free
accordance with ASME Boiler and Pressure Vessel Code
air is required on a compressed air system coalescing filters
Section VIII.
may be used to remove solids, moisture, and oil from the air
stream.
3-3. Capacity.
Total air requirement will not be based upon the total of
3-2. Types.
individual maximum requirements, but upon the sum of the
An analysis will be made for each compressor selection to
average air consumption of air operated devices. Determina-
insure that the best value is obtained. Comparisons of such
tion of the average air consumption is based on the concept
items including, but not limited to, brake horsepower (bhp)
of load factor (the ratio of actual air consumption to the
per 100 cubic feet per minute (cfm), unloaded horsepower,
maximum continuous full-loaded air consumption). The
expected compressor life, and expected operation and
Compressed Air and Gas Institute (CAGI) Compressed Air
maintenance costs, should be made between the different
and Gas Handbook explains the procedure for using load
types of compressors before final selection is made. The
factor to determine compressor capacity. After making the
following basic types are available:
calculation, add 10 percent to the estimated consumption for
a. Reciprocating. Air volumes range up to approximately
leakage. The total is the compressor capacity required for
6,000 cfm. The need for shielding or baffling structures
design. More capacity may be added to allow for future
around the reciprocating compressor to meet noise attenua-
growth of the facility or serviced area over the next 2 years.
tion requirements requires investigation. These positive dis-
placement compressors are available with oil-lubricated and
3-4. Multistaging.
oil-free cylinders.
Multistage compression can be used to reduce power losses
b. Liquid sealed rotary. This type of unit provides oil-
associated with the air temperature rise during compression.
free, positive displacement. non-pulsating operation. The
Also, compression efficiency will be increased using multis-
compressors will have enclosed rotors with conical porting
taging. The air compressor unit, however, will increase in
for adjustment of internal clearance. Air volumes range from
cost and will be a more complicated machine. Before select-
50 cfm to approximately 300 cfm. This type of air compres-
ing compressor staging, an economic evaluation should be
sor is recommended for health care facilities.
performed with consideration given to the required air pres-
c. Rotary helical screw. Oil lubricated rotary helical
sure levels and the intended compressor use. When using
screw compressors have an air volume range from 22 to
multistage compression, intercoolers should always be used
3,100 cfm. This type of compressor serves best as a baseload
to improve the efficiency of the air compressor unit.
machine. Oil-free rotary helical screw compressors have an
air volume range from 400 to 12,000 cfm. Another type of
3-5.
Number.
3-1
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