How Does an Air Compressor Work? Air Compressor 101

Whether you’re trying to figure out how to fix a problem with your air compressor, or just reading for interest – understanding how air compressors work can be an intriguing topic. Made more intriguing and interesting by the fact that not all air compressors work in the same way – mechanically at least.

So I’ll cover off at a high level, at least, how compressors work in principle, then give some more detail about how each of the types of compressors you’re likely to come across (or in fact own) work based on their mechanical configuration.

I’ll also point you in the direction of some more detailed resources we’ve put together that provide far more detail on the specifics of each type of air compressor. These are resources to tap into once we’ve covered the air compressor fundamentals in this article where we explain exactly; how does an air compressor work?

How Do Air Compressors Work?

In principle, air compressors turn rotary motion (kinetic energy) into increased pressure (potential energy). A power source drives the rotary motion, enabling air compression, which we then typically capture inside an air tank, sometimes referred to as a receiver.

Compressed air is then typically passed from the air tank to a tool or some mechanism that releases the potential energy in the compressed air and the air pressure in the air compressor tank reduces.

As the air in the compressor tank reduces, the air compressor pump works to replenish the air pressure in the air tank so as to keep a consistent level of pressure between an upper and lower limit set by the user (or sometimes the manufacturer) on the pressure switch.

One of the most important components of any air compressor system is its pressure regulator, which is used to control the flow of air that passes through the system. There would be no way to regulate the pressure and the intensity of the air that flows from the compressor tank into your pneumatic tools without this type of control function.

Regulators protect each tool from being over or underpowered and, by doing so, they help you to ensure that the quality of your pneumatic applications is at its optimum. Air compressor pressure regulators allow you to provide different levels of pressure to different types of pneumatic tools.

How Each Type of Air Compressor Works

There are 5 types of Air Compressor Mechanisms we tend to talk about, but that doesn’t mean to say that air compressors can’t be categorized in a multitude of ways. For the ultimate guide to the types of air compressors and air compressor categories, see this article.

I’ll explain how the following 5 types of compressors work:

• Reciprocating Air Compressor
• Rotary Screw Air Compressor
• Axial Flow Air Compressor
• Centrifugal Air Compressor
• Rotary Vane Air Compressor

Reciprocating Air Compressor

Reciprocating air compressors are positive displacement piston air compressors. The word reciprocates, can be defined as something moving back and forth.

Reciprocating compressors contain cylinders with one closed end and a piston that moves inside them. The cylinders are connected to valves that can be located underneath the compressor’s valve pockets.

A vacuum of sorts is created during the initial stage of compressing the air, this is done by air moving into the cylinders via a suction valve which is initiated by the movement of the piston.

The piston inside the cylinder then reverses its motion, hence, the term reciprocating being used to describe them. This reciprocation starts the air compressing process, increasing the pressure within the cylinder.

When the pressure has increased up to its design limit it then pushes the discharge valve opens and allows the air to flow out of the cylinder and into the storage tank.

Piston compressors come in two types, single-stage and two-stage:

Single Acting

Single-acting reciprocating air compressors only have a single side of the piston being used for the compression of air with the other side being connected to the crankshaft and not used for the compression.

Double Acting

Double acting reciprocating air compressors use both sides of the piston are used for the compression of air. Suction and compression take place simultaneously on each stroke of the piston.

Here’s a great animated video that shows the parts and functionality of a double-acting reciprocating (piston) compressor from Howdens, a multinational leader in the manufacture of industrial reciprocating compressors to API-618.

Rotary Screw Air Compressor

Rotary screw air compressors or twin-screw compressors as they are also sometimes known are some of the most popular mobile compressors available. They are commonly used as a replacement for piston compressors and make up a large portion of the compressor market.

The rotary screw compressor process is fairly different from the reciprocating piston compressor. It works by generating the compression continuously at the rotary screw drive air end of the compressor.

Air enters the unit at the inlet port of one of the two large screws rotating against one another, also known as the rotors or rotary screws.

This air then moves down the length of the screws and compresses the air towards the outlet due to the air gaps in the rotors getting smaller and smaller. These rotary screws are machined with small tolerances to very highly accurate finished products through which the vanes on the rotary screws can be as low as a thousandth of an inch apart.

Here’s a really great detailed explanation of how a rotary screw air compressor works produced by Doosan a worldwide manufacturer of portable power products who can be found all over the globe – including the USA!

Axial Flow Air Compressor

Axial-flow compressors are high-speed and very large-volume compressors that work, as you may have already guessed, by the airflow entering the compressor in the axial direction. They are a form of dynamic compressor that contains rotary and stationary components through which the air flows and then becomes compressed.

Axial flow compressors move airflow in the axis of rotation of the driving shaft. This driving shaft rotates the rotor blades around it causing an increase in kinetic energy and thus static pressure through a process called diffusion.

Axial compressors are typically made up of many alternating rows of rotating (rotors) and stationary (stators) blades. The blades are similar to that of an aircraft wing as they have small airfoil cross-sections.

Centrifugal Air Compressor

A centrifugal flow compressor is a form of dynamic compressor that contains a radial design.

Centrifugal Compressors have a radial design that operates at a constant pressure rather than a constant flow like displacement compressors. Their performance can be affected by numerous external conditions that may be difficult to control like the change in inlet temperature.

Centrifugal air compressors are reliable, efficient, and compact devices that allow for the management of air compressor capacity at constant pressures due to their modern controls. They typically use gears but some modern configurations of the centrifugal compressor have started to adopt very high-speed electric motors to drive the impellers.

These configurations are suited to applications that require oil-free air as they operate without a gearbox and its accompanying compressor oil-lubrication needs.

Here’s a really great visual representation of how a centrifugal air compressor works from FS Elliot, who is a global manufacturer of Centrifugal Manufacturers based in the US!

Rotary Vane Air Compressor

By rotating a rotary vane shaft, the vanes of the compressor draw in free air and compress it as they rotate, generating compressed air power. Read more about rotary vane air compressors here.

Here’s a great video from Air Link Compressors a supplier of compressors based in the UK. It’s more a “moment in the life of an air particle flowing through a rotary vane compressor” – but it gives a great view of how they work!

What is Air Displacement in Air Compressors?

Air displacement is the foundation of each varying type of air compressor. For the air to be compressed, the internal mechanisms within each compressor move to push air through its chamber. There are two common types of air displacement used for this purpose:

Positive Displacement

The majority of air compressors use this method. This is when air is pulled into a chamber, and then the machine reduces the volume inside the chamber to compress the air. It can then be moved into the storage tank through a valve and stored for use later on.

Dynamic Displacement

Can sometimes be referred to as non-positive displacement. This method uses an impeller with rotating blades to suck air into the chamber. The blade’s motion creates energy in the air, and this energy then causes air pressure to build up.

Dynamic displacement is used with turbo-compressors, as they can generate a large volume of air at a significantly fast rate. Turbochargers in cars often use dynamic displacement air compressors too.

How Do You Use an Air Compressor for the First Time?

Like when using any mechanical instrument or tool for the first time, compressors should be used with care. Ensure that the maximum pressure is set correctly and to a safe level by monitoring pressure on the gauge as the air compressor fills the air tank.

How Does an Air Compressor Know When to Stop and Start?

These air pressure limits will often be referred to as “cut in” and “cut off” pressures. These are the pressures at which the air compressor pump and motor are provided the signal to either “start pumping” or “stop pumping”.

These signals are sent to the compressor motor by an air compressor pressure switch, which is a vital component in the air compressor control system for safety and efficiency.

How Does an Oilless Air Compressor Work?

First of all, it’s important to understand that air oil-less air compressors aren’t entirely free of oil, they just use it differently. In an oil-lubricated air compressor, the oil is used to lubricate the piston for air pressurization, and so, the oil often contaminates the compressed air produced and needs filtering out.

With an oil-less air compressor, the oil is kept in a separate compression chamber by the chamber houses. Other materials lubricate the piston so it can move freely within the compressor cylinder.

Some oil-free have a different type of lubrication built-in to their systems. Instead of using oil, they will contain PTFE lining inside the cylinder to help reduce friction and heat. These types of machines are typically maintenance-free and work great on smaller air compressors or ones that are not used so often.

For more information visit our Oilless Air Compressors vs. Oil Compressors guide.

How Does an Air Compressor Produce Clean Air?

For an air compressor to provide clean air to your tools it must undergo filtering, sometimes even a series of filtering. To begin, when air is sucked in to the compression chamber there will be a filter at the intake to filter out any contaminants in the atmospheric air.

Atmospheric air, whether that’s in your garage or in a factory setting is bound to contain contaminants like dirt or dust, which are impossible to avoid. So the only way to initially protect your system is by filtering these contaminants out.

Within the compressed air system, if the compressor undergoes oil lubrication of some sort then this will need separating from the airstream along with water from moisture build-up. Both can be removed using various types of filters, separators, or coolers.

Most air compressor systems in an industrial setting are fitted with an aftercooler as well as a water separator, in order to separate as much condensation water as possible from the compressed airflow. With the correct choice and sizing of the water separator, it may be possible to achieve an efficiency of 80-90%. The remaining water flows with the compressed air as water mist into the air receiver and can then be drained out from there or removed further down the line in additional filters.

Oil in the form of droplets can be separated partly in an aftercooler or oil separator filter. Oil vapor must be removed using activated carbon oil filters and they are quite effective at reducing the quantity of oil to 0.003 mg/m3.

One thing is that an oil-free compressor eliminates the need for an oil filter, meaning the compressor can work at a lower discharge pressure, thereby reducing the resulting energy consumption. It has been proved in many cases that oil-free compressors are the best solution for clean air. For projects that involve spray painting, clean air is vital to ensure that the work is not affected.

How Are Air Compressors Powered?

Air compressors can be powered by mains electricity straight out of the socket at home. Whether that’s 120v in the US or 240v in the UK. In an industrial setting, you’ll find air compressors powered by 3 phase electricity as standard.

In a single-phase power supply found in homes, the power is supplied through two wires called the phase and neutral and generally the voltage supply is 230V. In a three-phase power supply found in industrial settings, the power is supplied through three wires (well, four if you include the neutral wire) and the voltage supply is 415V.

For an equal amount of power, the single-phase supply requires more wire than that of a three-phase supply. The efficiency of a three-phase supply is significantly higher than the single-phase supply, along with a greater power transfer capability which makes it better suited to industrial settings where the machinery, etc. requires greater amounts of power to operate.

In remote or mobile circumstances, the air compressor can sometimes come with its own power source – a gas-powered air compressor (gasoline) or diesel-powered engine.

Does an Air Compressor Need Electricity?

The environment will influence your decision as to whether you’ll use an electrical air compressor or a diesel air compressor, or possibly both. Consistent outdoor work usually demands a diesel air compressor, and many people who use an electric system still keep a diesel compressor around or have access to one in case of an emergency or extended power outage, which is always possible!

Not all air compressors need an electricity supply to operate. In some instances, gasoline (gas) or diesel-powered air compressors are needed, such as on building sites where utilities and mains electricity are yet to be installed.

The important consideration really is the comparable cost of the two. It’s estimated that a diesel air compressor costs over twice as much to run than the same type but electric. Electric air compressors provide many benefits over diesel compressors, not just saving money on energy but reducing maintenance frequency and costs, increasing reliability and productivity, better quality compressed air, few moving parts, and much more!

How Long Can an Air Compressor Run?

How long an air compressor can run typically depends on its duty cycle rating or recommended duty cycle period. All air compressors will come with a rating, typically described as a percentage. This percentage represents the amount of time an air compressor can run during its cycle period.

For example, let’s say we have a 30 minute cycle period with a 50% duty cycle. The air compressor can run for 15 minutes but then must idle for 15 minutes to ensure that it does not overwork or overheat, which will cause premature wear.

If another compressor had a 60 minute cycle period with a 75% duty cycle, then the compressor could operate for 45 minutes and then would need 15 minutes rest before working again. It’s important to obey duty cycle recommendations by the manufacturer to protect your compressed air system.

For more information visit our Air Compressor Duty Cycle – What Is It, How It Works guide!

On the other hand, if for some reason your compressor is overheating due to a number of reasons, your thermal overload protector on the motor may reset and cause the compressor to switch off until it is cool enough to continue. The same can be said for the circuit breaker, if this begins to draw more amperage than it’s intended to do, it’ll trip so that the compressor doesn’t cause further damage.

These potential issues, along with many others, can affect the length in which your air compressor can successfully run. Therefore, staying on top of your air compressor and conducting preventive maintenance are crucial to ensure the optimum running performance of the system.

Why Is a Tank Important When Using an Air Compressor?

Air compressor tanks aren’t just accessories, they’re important additions to any compressed air system to provide immediate value, offering multiple advantages.

An air compressor tank can help you with maintaining constant pressure by dampening the pulsations created in the pressure when pistons operate. Tanks help to provide a steady flow of compressed air at constant pressure, making it easier to work with this uniform airflow and reducing wear on the machine.

Air compressor motors activate automatically when the system senses a need for more compressed air. If you are running a compressed air system without a tank, your motor will have to work overtime to keep up with the demand.

Air compressor tank receivers provide a simple goal for the system to reach, reducing cycles and preventing excessive loading or unloading of air into the system. You’ll also be able to wind up saving on your energy costs since you’re powering up the compressor less frequently.

Of course, let’s not forget the fact that air compressor tanks are air reservoirs used to store your compressed air for use. The tanks come in many sizes and are suitable to be used with almost any compressor type. Having this reservoir of compressed air can help you meet peak demand while also ensuring there is enough supply for dependable and continuous air delivery when needed.

Finally, air compressor tanks can help to remove condensation. As atmospheric air contains water in vapor form, when it is compressed, the vapors cool and condensate in the system. Tanks allow the condensation to be drained before it is allowed to enter your air stream.

Are Air Compressors Dangerous?

Compressed air is dangerous, and by virtue, air compressors can be dangerous if not operated with competence.

Compressed air is “potential energy” that has been accumulated, in this instance in the air tank of an air compressor.

Explosions can result from overpressurization and, much like other types of explosives, release huge amounts of potential energy almost in an instant.

For this reason, pressure equipment, such as the air tanks and accessories you attach to an air compressor is highly regulated in its design and manufacture. There are standards all across the world for the safe design and manufacture of pressure equipment for containing all forms of fluid and gas – not just limited to compressed air.

Here’s an image of the destruction caused by an air compressor tank rupture

How Does an Air Compressor Keep Cool?

It’s important to keep an air compressor cool for maximum performance. Heat management systems have become increasingly popular within industrial settings to help monitor the compressor’s heat dispersal system to ensure that it has a trouble-free operation.

There are a number of cooling systems available to help compressors. Liquid cooling is a method where a coolant is circulated through the unit and is most commonly found in larger compressors.

Air-to-liquid heat exchangers are generally utilized in rotary screw compressor models that use lubricants. These systems cool hot oil by passing air over it. Liquid-to-liquid heat exchangers use oil and engine coolant to utilize a shell and tube or plate heat exchanger with separate paths for each liquid. A liquid-to-air heat exchange is then initiated to help cool down the engine coolant, and these systems are typical for rotary screw compressors that use oil lubricants.

A final, and very simple cooling mechanism is natural air convection. This method involves the hot air generated by the compressor being dispersed away from the system by means of cooling fans! For further information on cooling methods visit the following pages on our site:

• Air Compressor Heat Exchanger Guide
• Intercooler vs Aftercooler In Compressed Air System
• Air Compressor Aftercooler Guide to Compressed Air Coolers
• Air Compressor Intercooler Guide to Compressed Air Intercoolers

How Loud Is an Air Compressor When It’s Working?

In general, most air compressors operate within 40 dB to 100 dB, which I know is a very big range. But, there are loud air compressors and ultra-quiet ones available on the market. Some old air compressors get louder with age and the more they deteriorate, whilst modern-day manufacturers focus on technologies that will make their compressors as quiet as possible.

For more information on quiet air compressors, or should I say how to quiet an air compressor, visit our guides here:

• How To Quiet An Air Compressor
• How To Quiet An Oilless Air Compressor

Pumps vs. Compressors: Two Air Harnessing Instruments

Many people may believe that “pumps” and “compressors” are the same thing, that’s not the case. The key difference between the two helps better explain air compressors:

• A pump takes liquids or gasses and moves them between places
• A compressor takes a gas, squeezes it down to a far smaller volume and resulting in higher pressure, and then sends it elsewhere

The most significant difference as you can tell is that a pump can work with liquids, whilst a compressor can not. Liquids are far more difficult to compress!

It’s possible to find a pump within an air compressor. For instance, we will take a look at a reciprocating air compressor, as previously described… they use pumps to perform the compression stage. The functions of pumps and compressors can overlap with one another in machines where the pressure rises with each machine revolution.

Air pumps generally fall into two categories:

1. Reciprocating pumps: They move back and forth (reciprocating motion). An example would be a bicycle tire pump, where the cylinder pulls outside air in with a back-and-forth motion to force it into the tire, and therefore pump it.
2. Rotary pumps: also referred to as centrifugal pumps, they spin! These pumps use an impeller which is simply an enclosed propeller. The blades on it move incoming fluid and send it through an outlet at high speeds. This type of pump uses its motorized energy to pull fluids from one place to another, and should not be confused with a turbine as such, which captures fluids already in motion.

How Do Portable Air Compressors Work?

Portable air compressors work much in the same way as an air compressor, however, the power source to run the compressor tends to vary. Many portable air compressors are still powered by mains electricity, but some are also powered by gasoline or diesel to make them truly “mobile” without the need for a source of electricity – they bring their own!

How Do Gas Powered Air Compressors Work?

Gas-powered air compressors are made up of 2 ends:

1. Engine End
2. Air End

The engine end, as you may have guessed is made up of a gasoline or diesel-powered engine which provides the rotary power required to rotate the compressor pump via belts or pulleys.

The air end, as you may now appreciate is the end that has the rotary power applied to it in order to generate compression which serves to fill your air tank with compressed air.

Here’s a great video talking you through the basics of a Gasoline Powered Air Compressor

How Do Air Compressor Quick-Connects Work?

Air compressor quick-connects, or otherwise referred to as quick-release couplings allow you to easily connect and disconnect airlines from either the air compressor or air tool. They are commonly used instead of other types of coupling due to their ease.

Quick-couplings are designed to seal when disconnected via an internal o-ring, eliminating the need for a shut-off valve. These connections are typically made up of a male part and a female part which contain internal pressures when properly connected.

This connection also allows the joint to love, meaning there will be less kinking and twisting of the hose, which is very admirable in busy workspaces and keeps the work area safe.

FAQs (Frequently Asked Questions)

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If you have any questions regarding how air compressors work, please leave a comment below, with a photo if applicable, so that someone can help you!