| Absolute pressure.
Total pressure measured from absolute zero. |
| Aftercooler. Heat
exchanger for cooling air discharged from air compressor. |
| Atmospheric pressure.
Pressure above absolute zero at a specific location and
altitude. |
| Brake horsepower (bhp).
Horsepower delivered to the output shaft of the drive motor.
Unit of comparison between motors. Total package bhp is the sum
of all motor shaft outputs, including compressor and cooling
fans |
| Capacity. The
amount of air flow delivered or required under some specific
condition. May be in acfm, scfm, etc. |
| Cubic feet of air per minute (cfm).
Volume rate of air flow. |
| Cubic feet of air per minute,
free air (cfm(free air)). Cfm of air delivered to
some specific point and converted back to ambient (free air)
conditions. |
| Actual cubic feet per minute (acfm).
Flow rate of air measured at some reference point and based on
actual conditions at that reference point. |
| Inlet cubic feet per minute (icfm).
Cfm flowing through the compressor inlet filter or inlet valve
under rated conditions. |
| Standard cubic feet per minute
(scfm). Flow of free air measured at some reference
point and converted to a standard set of reference conditions
(e.g., 14.7 psia, 60 degrees F, and 0% relative humidity). |
| Demand. Flow of air
under specific conditions required at a particular point. |
| Discharge pressure, rated.
Air pressure produced at a rated reference point. |
| Discharge pressure, required.
Air pressure required at the system inlet. |
| Displacement.
Amount of air (in cfm) displaced by the compressor piston under
no load, discharging directly to the atmosphere. |
| Dual control.
Load/unload control system that maximizes compressor efficiency
by matching air delivery and air demand. Compressor is normally
operated at full load or idle, and is stopped and restarted
automatically depending on demand. |
| Free air. Air at
ambient conditions of temperature, humidity, and atmospheric
pressure at any specific locations. |
| Inlet pressure. The
total pressure at the inlet flange of the compressor. |
| Load factor. Ratio
of the average compressor load to the maximum rated compressor
load during a given period of time. |
| Modulating control.
System will run the compressor at varying loads to accommodate
demand variations. Running a compressor at less than full load
results in a drop in compressor operating costs. |
| Pressure. Force per
unit area. |
| Pounds per square inch (psi).
Force per unit area exerted by compressed air. |
Pounds per square inch
absolute (psia).
Pressure above absolute vacuum. Atmospheric pressure is stated
in psia. |
| Pounds per square Inch gauge
(psig). Pressure at some reference point as measured
with a gauge. |
| Pounds per square inch
differential (psid). Pressure difference between two
points. |
| Pressure dew point.
Temperature at which water will begin to condense out of air at
a given pressure. |
| Pressure drop. Loss
of pressure in a compressed air system due to friction or
restriction. |
| Receiver tank. Tank
used for storage of air discharge from a compressor. |
| Rules
of Thumb |
| Air compressors normally deliver 4 to 5 cfm per
horsepower at 100 psig discharge pressure. |
| Depending on the size of the system, compressed air
costs about 25 to 30 cents per thousand cubic feet of
free air ingested by the compressor (including operating
and maintenance costs). |
| Every 2 psi pressure drop costs 1% of compressor
horsepower in efficiency. |
| Power cost for each 1 horsepower operating constantly
for one year at 10 cents per kwh is about $750. |
| A 50 horsepower compressor rejects heat at
approximately 126,000 Btu per hour. |
| Size air receivers for about 1 gallon capacity for
each cfm of compressor capacity. |
Typical compressor discharge air temperature before
aftercooling:
Rotary screw-175 degrees F; Single stage recip.-350
degrees F; Two stage recip.-250 degrees F.
The water vapor content at 100 degrees F of saturated
compressed air equals about 2 gallons per hour for
each 100 cfm of compressor capacity.
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|
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| Types
of Compressors / Evaluating
a Compressed Air System / Selecting
an Air Compressor / Maintenance
/ Troubleshooting |
