Air compressors are the unsung heroes of industrial facilities, small industrial shops, and DIY garages everywhere. They run sophisticated machinery, power our pneumatic tools, help inflate a variety of things ranging from basketballs to bike tires, enable an artist to make delicate strokes with an airbrush, and can even remove dust from sensitive components inside a desktop computer.
Are all air compressors refillable? Compressors consisting of a tank and a built-in motor are refillable. In fact, they are self-refilling, meaning that the compressor motor will cycle on and off, replenishing the pressurized air in the tank as it is being used. While there are tankless air compressors for specific uses and applications, the overwhelming majority of air compressors have refillable tanks.
In this article, you’ll learn how air compressors supply the pressurized air we have all come to use, starting with the most common types of compressors. Air compressors are available in a wide range of sizes and are rated to supply specific amounts of compressed air for different applications. Read on to gain a deeper understanding and appreciation for how air compressors work.
How Do Air Compressors Work?
Although they come in many shapes and sizes, all air compressors perform the same essential function, which is to compress air into a highly pressurized form and safely store it until needed. Many air compressors have a feature that automatically cycles the compressor motor on and off to replenish pressurized air as needed. The pressure at which the compressed air is released (and utilized) can be controlled via an adjustable regulator.
How air compressors create, pressurized air is most commonly through mechanical means, typically a motor that is powered by electricity or diesel fuel. The motor drives mechanisms that take ambient air (this is the air that is all around us, in its natural state) and compress it through various means.
The compressed air is stored in a reinforced tank, usually taking the form of a cylinder. Smaller air compressors are designed to be portable, and are easily transported to job sites where pressurized air is needed, and feature cylinders that are oriented horizontally or vertically. Larger air compressor tanks, because of their size and weight, are set up in a fixed location and are usually bolted to the ground.
The Different Types of Air Compressors
There are two primary types of air compressors, and they refer to the scientific principle by which they pressurize ambient air. The first and most common type of air compressor is known as positive displacement, and it relies on squeezing air into a confined space to create pressurized air. The other type is known as a dynamic replacement, and it relies on compression achieved through the acceleration of air velocity followed by restriction of airflow.
Because air compressors supply the kinetic energy that is used across so many different applications, some ratings help identify a particular compressor’s capabilities and limitations. These ratings relate to vital aspects of compressor performance, and the most widely used specifications are:
Cubic feet per minute (CFM) – This is the most common metric used to express the capability of an air compressor. CFM refers to the volume of air that the compressor can supply. CFM is often stated as standard cubic feet per minute (SCFM), which is a universal measurement that takes into account atmospheric variations such as temperature and humidity.
All tools, devices, and machines requiring a compressed air hookup will state a minimum value for CFM, which is required at a bare minimum to operate that particular piece of equipment properly. As you can imagine, small pneumatic hand tools such as nailers and drills (particularly those that need compressed air sporadically) have low CFM values, while more significant pieces of equipment require more.
The following examples illustrate the CFMs (all at 90 psi) for common devices and tools:
| 7” angle grinder – 5 to 8 cfm | Brad nailer – < 1 cfm | Drill – 3 to 6 cfm |
| Framing nailer – 2.2 cfm | ½” impact wrench – 5 cfm | Orbital sander – 6 to 9 cfm |
| 3/8” ratchet – 5 cfm | Riveter – 4 cfm | Pneumatic saw – 5 cfm |
A popular rule of thumb when determining the proper size air compressor for your pneumatic needs is to add up the total cfm’s for all of your compressed air devices and then multiply this total by a factor of 1.5. For example, if you are planning to operate a riveter (4 cfm) and ratchet (5 cfm), add these values (9 cfm total) and multiply by 1.5 for a projected cfm demand of 13.5 to use when shopping around for an air compressor.
Pounds per square inch (PSI) – Where CFM measures the volume (or quantity) of compressed air, PSI refers to the force or velocity of the pressurized air as expressed by the amount of pressure it exerts per square inch of surface area. As with CFM, any piece of equipment requiring compressed air will specify the PSI range within which it can function normally
Because PSI is a measurement of pressure, it is particularly important in two regards. First, concerning the air compressor itself, the compressor will have a maximum PSI rating, which is the outermost limit of air pressure that its tank can safely hold. Fortunately, air compressor manufacturers incorporate safety features that cut-off the motor well before this maximum PSI is reached.
The other aspect of the PSI rating is concerning the particular piece of equipment to which the compressed air is being fed. It, too, should have a maximum PSI rating, which is essential to follow. Otherwise, damage to the equipment can result. Most, if not all, modern air compressors have air regulators that allow operators to adjust the PSI to safe levels for their devices and tools.
Horsepower (HP) – Both CFM and PSI refer to attributes of the compressed air held in the compressor. The horsepower, or HP, refers to the power of the motor that is responsible for driving the mechanisms that produce the pressurized air. As a general rule, the higher the HP,
the more powerful the motor and the higher the amount of CFMs that the compressor is capable of supplying.
Now that you are more familiar with the terminology used to rate air compressors, you can take a closer look at the two main types of air compressors, starting with positive displacement.
Positive Displacement Air Compressors
This air compressor category operates under the positive displacement theory, which means that air is pressurized in a sealed chamber by the volume being reduced. Positive displacement air compressors store the pressurized air they build up in tanks or cylinders that are usually differentiated by the number of gallons of compressed air they can hold.
The overwhelming majority of non-industrial air compressors are positive displacement air compressors. If you were to picture an air compressor in your mind, chances are it is a positive displacement type. If you have ever been inside a small industrial shop or in a commercial auto repair shop, you may have experienced the loud, running sound of an air compressor as it recharges the air pressure in the cylinder.
Positive displacement compressors are available in single-stage versions, which are sufficient to meet most household and light commercial demands, and multi-stage versions, which are designed to produce far greater amounts of compressed air within a shorter duty cycle.
As a general rule, the more powerful the air compressor, the greater the power (electricity) demands. Whereas a smaller compressor can run on standard 110-volt power, larger compressors will require either 220-volt or 240-volt power or even up to 480-volts with three-phase power.
Reciprocating Compressors
Of all the air compressors found in garages, small commercial shops, and light industrial operations, reciprocating air compressors are the most common by far. They can be as small as one-gallon models used primarily by hobbyists and artists and as large as 80 gallons, which would be sufficient to supply the compressed air needs for a small industrial facility or a busy auto body repair shop.
At the heart of a reciprocating compressor is a piston that sits inside a chamber. A motor on the compressor moves the piston inward and outward, and this cyclical action creates pressurized air. Ambient air is drawn into the chamber when the piston pulls away, and as the piston cycles back and pushes in, the air has nowhere to go. Therefore, there is a drastic reduction in the space occupied by the air that results in it being highly compressed.
As this process repeats, the volume of compressed air in the storage tank begins to build, and once it reaches a pre-set level determined by the manufacturer as the safety limit, the motor shuts off. This reserve of highly pressurized air is readily available for use by the operator. Once the tank pressure falls below a certain threshold, the motor will once again engage, and the duty cycle begins again. Because they generate lots of friction when they run, they are also very noisy.
Examples of Reciprocating (Piston Type) Air Compressor Sizes, Capacities and Specifications
The table below provides a summary of popular compressor sizes that are often utilized for various personal, commercial, and light industrial purposes:
| CYLINDER SIZE | CFM | MAX PSI | MOTOR HORSE POWER |
| 20 GALLON | 7.1 @90 PSI | 140 PSI | 2.0 HP |
| 30 GALLON | 5.7 @90 PSI | 155 PSI | 1.9 HP |
| 60 GALLON | 14.0 @90 PSI | 155 PSI | 4.7 HP |
| 80 GALLON | 22.1 @100 PSI | 175 PSI | 4.7 HP |
Rotary Compressors
Rotary air compressors work under the same scientific principle of positive displacement as reciprocating compressors, but instead of a piston cycling back and forth to compress air, this compressing action is typically performed by a pair of rotating screws. Rotary compressors are considered more efficient than reciprocating and are usually reserved for applications that require higher volumes of compressed air.
These rotary compressor screws are mirror images of each other and do not come into contact at all. As they rotate, the air that is trapped between their threads gets squeezed and pushed along the screws through a narrow opening. This is a continuous operation, and because there is a mere fraction of the amount of friction compared to a reciprocating (piston) compressor, rotary compressors are far quieter during operation.
Where reciprocating compressors are designed for cycling on and off as needed to replenish the compressed air in the tank and therefore are limited to specific applications, rotary compressors are designed for extended duty cycles and can even run continuously for very long periods. As such, they are well suited for environments requiring a steady, continuous supply of compressed air.
Rotary screw air compressors are distinguishable from reciprocating compressors in that they typically have a horizontally oriented cylinder upon which is mounted on a huge metal cabinet (resembling an extensive air conditioning or refrigeration unit) which houses the rotary screw assembly, motor, and other components. As they are engineered for more demanding applications, rotary screw compressors can typically deliver anywhere from 10 to over 60 cfm.
Dynamic Displacement Air Compressors
Positive displacement air compressors rely on physically squeezing ambient air into a tighter, more confined space to generate pressurized air, and this compressed air is stored in a reinforced tank. In contrast, dynamic displacement air compressors speed up an air mass and then achieve increased air pressure by subsequently restricting the airflow.
Where positive displacement air compressors can serve commercial and industrial purposes, they are typically limited to applications not exceeding 3,000 cfm because of their duty cycle limitations (particularly in the case of reciprocating compressors). Such is not the case for dynamic displacement air compressors as they are specifically engineered for big jobs.
Centrifugal Compressors
This is the more common type of dynamic displacement air compressor, and it is typically found in large industrial applications. Centrifugal compressors are typically rated to deliver between 400 to 15,000 cfm and are engineered to perform lengthy duty cycles, sometimes lasting as long as six to twelve months at a time. However, centrifugal compressor systems are large, expensive, and require extensive cooling systems because of the heat that is generated.
At the heart of a centrifugal compressor is an impeller (a fan-shaped device with blades that is designed to direct air or fluid from its center to its outer perimeter) that accelerates air fed from an intake toward diffuser plates and a spiral-shaped funnel (known as a volute) that restricts the airflow and increases its pressure in the process.
Many centrifugal compressors consist of multiple stages, with each compressing air further and further. Typical applications for this type of compressor include petrochemical refineries, chemical plants, and other extensive institutional facilities involved in the processing, refinement, and production of various types of fuels and gases.
Because of the enormous scale of the work they perform, centrifugal compressors are tankless, meaning they supply compressed air on demand and continuously without interruption.
Axial Compressors
Another type of dynamic replacement compressor is known as an axial compressor. It operates on the same basic principle of accelerating an air mass and subsequently restricting its flow to generate an exponential increase in pressure. Axial compressors are highly specialized and are not commonly used with industrial applications outside of perhaps power generation plants and the like.
Instead, they are most commonly utilized with gas turbines found in jet engines, ocean vessel engine rooms, and turbochargers. Axial compressors can generate an incredible amount of compressed air, with a range of 8,000 to 13 million cfm.
Are Inflators and Pumps the Same as Air Compressors?
By definition, an air compressor compresses ambient air into a confined space, thereby increasing its pressure, or achieving the same effect by restricting the airflow of an accelerated air mass. Put another way, compressed air is highly pressurized, and a large volume of it can move in a short period. Most compressors store an ample reserve of compressed air sufficient to meet most pneumatic requirements.
In contrast, an electric pump simply moves air from one location to another with the aid of a small motor and fan. The same can be said for an electric inflator. While both devices may be sufficient to inflate an air mattress or inflate a bicycle or automobile tire, they can nither supply the bare minimum cfm nor adequate psi to operate pneumatic tools for longer than brief, momentary spurts. Tire inflators may be a great addition to an emergency kit, but their functionality is limited at best.
Compressed Air – The Fourth Utility
Pressurized, or compressed air, is sometimes referred to as the fourth utility. Unlike electricity, gas, and water, we are all capable of generating it ourselves with the right air compressor. Compressed air is the ultimate facilitator – it enables the use of so many powerful devices, and it is the gateway to being able to perform a host of jobs and tasks you never thought possible. Hopefully, if there is one takeaway from reading this article, it is that when it comes to compressed air, even a little goes a long way.