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?
Table of Contents
- How air compressors work
- How Each Type of Air Compressor Works
- What is Air Displacement in Air Compressors?
- How do you use an air compressor for the first time?
- Does an air compressor need electricity?
- Are air compressors dangerous?
- Pumps vs. Compressors: Two Air Harnessing Instruments
- How Do Portable Air Compressors Work?
- How do gas powered air compressors work?
How 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 the compression of air which we then typically capture inside an air tank or sometimes referred to as a receiver.
Compressed air is then typically passed from the air tank to a tool or some mechanism which 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 an consistent level of pressure between an upper and lower limit set by the user (or sometimes the manufacturer).
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 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 electrcity as standard.
In remote or mobile circumstances, the air compressor can sometimes come with it’s own power source – a gasoline or diesel powered engine.
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 compressor work:
- Reciprocating Air Compressor
- Rotary Screw Air Compressor
- Axial Flow Air Compressor
- Centrifugal Air Compressor
- Rotary Vane Air Compressor
For an even more detailed explanation of each of these compressor types, click on the name of the compressor in the list above.
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 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 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 of both the parts and the 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 are 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 video from Gardner Denver which really shows off the inside of their rotary vane vacuum pump, but ultimately that’s just compression in reverse, so it still demonstrates the same working principles of a Rotary Vane Air Compressor. Gardner Denver need no introduction as one of the world leading names mission critical flow control products.
Here’s a great video from Air Link Compressors a supplier of compressors based on 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 a 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:
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.
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.
Does an air compressor need electricity?
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.
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 explosive, 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 are highly regulated in their 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
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:
- 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.
- 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 same way as any 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 poweered air compressors are made up of 2 ends:
- Engine End
- 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