The basics and ABCs of compressed air

The ABCs of compressed air

You’ve probably heard this from us before, but at Kaeser we believe that the more you know about operating air systems, the more you’ll get out of them. We are committed to offering you the most current information you need to operate and maintain your compressed air system. We’ve shared some advanced tips for optimizing your air system, but in today’s post we’re going back to the basics with a glossary of terms used frequently in our industry.

Air flow: Volume of free air in cfm

Air receiver tank: Tank used for compressed air storage.

Artificial demand: Additional air consumption caused by excessive system pressure.

Capacity: The amount of air flow delivered or required under some specific conditions. May be stated as acfm, scfm, or cfm FAD (free air delivered).

Cubic feet per minute (cfm): The most common measure of air flow/volume in the US.

Cubic feet per minute, free air (cfm FAD): cfm of air delivered to some specific point and converted back to ambient air (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 a reference point and converted to a standard set of reference conditions (e.g., 14.5 psia, 68°F, and 0% relative humidity).

Demand: Flow of air under specific conditions required at a particular point.

Discharge pressure, rated: Air pressure produced at the compressor outlet.

Discharge pressure, required: Air pressure required from the compressor at the outlet.

Dual control: A type of individual compressor control in which the compressor runs at constant speed either fully loaded or fully unloaded (idling), and stops completely if it idles uninterrupted for a preset time to reduce energy consumption. Will automatically restart when line pressure drops below selected minimum pressure.

Duty cycle: Percentage of time a compressor unit operates during a specified period. Allowable duty cycle is the maximum recommended duty cycle for a compressor that does not compromise compressor performance or accelerate wear.

Dynamic control: Allows compressors to switch from “load” to “stop” at low motor temperatures, and to “idle” when the motor is hot. (The control may bypass “idle mode” if the motor temperature is low and more compressor starts are allowable.)

Flow meter: An instrument used to measure flow rate of a fluid or gas.

Load factor: The ratio of average compressor load to the maximum rated compressor load during a given period of time.

Modulation control: An individual compressor control system which will modulate the inlet air flow in response to variations in pressure near the discharge in order to maintain relatively stable system pressure. The compressor runs at a constant speed.

Pressure: Force per unit area.

Pounds per square inch (psi): Standard US metric for compressed air pressure.

Pounds per square inch absolute (psia): Absolute pressure above zero pressure.

Pounds per square inch gauge (psig): Pressure difference between absolute pressure (psia) and ambient pressure.

Pounds per square inch differential (psid): Pressure difference between two defined points in the system. May also refer to pressure drop between two points in a system.

Pressure dew point: Temperature at which water will begin to condense out of air at a given pressure. To ensure that no liquid water is present, the pressure dew point must be lower than the lowest temperature to which the compressor air will be exposed.

Pressure drop: Loss of pressure in a compressed air system due to friction or flow restriction.

Quadro control: An enhancement over Dual Control that includes an additional timer to fine-tune the “idle” period,while bypassing “idle mode” after periods of low air demand.

Vario control: Uses a “smart” timer to vary the idle time based on the frequency of motor starts — resulting in greater energy savings.

Variable speed drive/variable frequency drive: Air flow is controlled to maintain a specified discharge pressure by controlling electrical frequency (and therefore speed of the drive motor) in response to a pressure signal at the compressor discharge. This is the most advanced and energy-efficiency type of compressor control.

These are just a few of the most frequently used terms in the compressor world. Visit our Resource Center and download our e-books and guides to keep learning, or visit the Compressed Air & Gas Institute for more information.

Conversion formulas

Multiply by to get
Volume cubic feet/minute 0.472 liter/second
gallons 0.134 cubic feet
liters/minute 0.2642 gallons/minute
cubic meters 35.315 cubic feet
Pressure inches mercury 0.4912 psi
inches water 25.4 mm water
psi 27.68 inches water
bar 14.504 psi
Density pint water 1.042 pounds water
gallon water 8.336 pounds water
pounds water 7000 grains water
Power horsepower 0.7457 kilowatts
horsepower 2544.43 Btu/hour
Temperature degrees Fahrenheit (degrees-32) x 0.556 degrees Centigrade

Additional resources:

For additional conversion tools visit the Kaeser Toolbox and discover more ways to optimize your air system.
Our factory-trained air systems specialists would be happy to walk you through all these tools and answer any of your questions. All you have to do is ask!