| 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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Types of Compressors
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