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

Multiplybyto get
Volumecubic feet/minute0.472liter/second
gallons0.134cubic feet
cubic meters35.315cubic feet
Pressureinches mercury0.4912psi
inches water25.4mm water
psi27.68inches water
Densitypint water1.042pounds water
gallon water8.336pounds water
pounds water7000grains water
Temperaturedegrees Fahrenheit(degrees-32) x 0.556degrees Centigrade

For additional conversion tools visit the Kaeser Toolbox on our website 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!

Our Top 5 Compressed Air Blog Entries for 2019

By: Kaeser Compressors, Inc.

If you are looking for some quick tips to improve your compressed air system, consider starting with our most read blog posts from 2019.

#5 This is Why You Don’t Use PVC: Using PVC in a compressed air system poses significant safety risks. This post covers what you need to know if you are considering using it.

#4 Applying Motor Temperature Ratings: A perennial favorite, this blog post offers useful information to help you apply motor temperatures ratings. Motor temperature ratings are given by the type of insulation used on the wire as well as the utilization rate. These two parameters determine the expected lifetime of the motor windings.

#3 Some Like It Hot…Your Compressor Room Doesn’t: If you are having problems with compressor room overheating, read this post for tips on better temperature regulation.

#2 Choosing Between an Air-cooled or Water-cooled Compressor: This post outlines four questions to answer when deciding between an air-cooled and water-cooled compressor.

#1 The Art of Dryer Sizing: This post has been at the top of the list since it was published in 2015 and is the most viewed post again in 2019. Read this post to understand how temperature and pressure impact water content and to learn how to make sure dryers are properly sized.


Our most popular post published in 2019 was this summer’s How to Keep that Trusty Recip Going! If you have a reciprocating compressor because it’s a good fit for your shop, here are some tips to avoid some common issues as well as some maintenance tips to keep your recip unit going.

Do you have a topic you’d like us to cover in 2020? Let us know in the comments.

Winter is Coming (Here?) [Infographic]

According to the Farmers’ Almanac, the 2019-2020 winter season could be colder and snowier winter than usual for much of the United States. Is your compressed air system ready for the colder temperatures?

Here’s an infographic you might find helpful as you start preparing for the upcoming drop in temperatures. We also have these air system winterization tips in a video version on our YouTube channel.

Visit our website for more compressed air tips or contact one of our air system specialists for help with your compressed air system.

Five Tips for Receiving Commercial Freight [Infographic]

By Kaeser Compressors, Inc.

Most commercial freight is delivered via third party carriers , and even with the best of carriers, sometimes shipments are damaged. Your freight may shift, get dropped, or even be in a traffic accident. Damage may also occur at logistics centers en-route, where items are transferred from one truck to another.

Like most other suppliers, Kaeser’s freight terms are usually “FOB shipping point” –which means that once the item leaves our possession, we cannot ensure safe handling or monitor the condition the freight arrives in. In most cases the buyer is paying the freight, and has the obligation to inspect the shipment. Kaeser will assist you in case of damage, but accepting a damaged shipment without comment will limit your ability to make a claim. Here’s an infographic with some tips for receiving commercial shipments, whether from Kaeser or any other supplier.

We also have a new video with all of these tips on our YouTube channel! Click here to watch our Freight Tips Video.