I have been working with electric power tools and pneumatic tools for over 30 years and know how to choose an Air Compressor. I have purchased many varieties of Air Compressor. I have also maintained these devices over a 30 year career.
Most online guides only go into the basics on what to look out for. These guides also give equal importance to the buying criteria. I have written about what to look out for in buying an air compressor. I’ve also paid close attention to what manufacturers like to brag about but actually doesn’t matter.
This guide is jargon-free and practical and will ensure you purchase the best Air Compressor whilst saving money.
What is an Air Compressor?
Air compressors are pneumatic devices that use a motor to reduce the volume of air to increase pressure. Most air compressors have a tank that pressurised air is stored in. The pressurised can be delivered to an object or device.
One example use is inflating tyres. Using pressurised air, a tyre will inflate much quicker compared to using a traditional manual pump. Another example is powering pneumatic tools. Pneumatic tools require pressurised air to function (compared with their electric tool equivalents). Read this on the advantages of pneumatic tools over their electric counterparts.
Air Compressor motors are rated in horsepower. This motor drives the cylinders that compress air. The relationship between horsepower and the output of a motor is complex. Consider the horsepower rating in context of other criteria mentioned below.
The next two sections are on pressure (PSI) and capacity (CFM). Horsepower is a measure of a motor’s efficiency at a given level of pressure and capacity.
For example, a 5HP and a 15HP air compressor could both operate at 75 PSI (pressure) and at a rate of 20 CFM (capacity). Performance is equal, yet the 5HP machine is operating more efficiently. This would save you money on fuel.
Horsepower alone shouldn’t be the only criteria to choosing the right air compressor. Also consider the CFM and PSI ratings alongside HP levels.
Top tip: most air compressors operate between 1.5HP and 6HP. Some larger stationary air compressors would be over 15HP. Look for 2HP at a minimum as standard AC cords require lower voltage to function.
An air compressor is all about air pressure as the name suggests. Air pressure is measured in pounds per square inch or PSI. Look for a bare minimum of at least 90 PSI even for the smallest of tasks.
Here is a chart detailing required PSI and various pneumatic tools:
|Up to 100||Portable tools (drill, impact wrench etc)|
|120 – 150||Medium tools (rim stripper, tire changer)|
|90 – 100||Hammers|
|Up to 100||Guns|
In order for a portable tool to function i.e. with a 100 PSI requirement, you’ll need a compressor with a higher shut-off pressure. A shut-off is a switch that shuts the air compressor off when the tank reaches a certain level.
A 100 PSI denominated air compressor will see at least 10 PSI to system loss. In this case, run at least 110 PSI. The consequence of having under-pressurised air is that your tools won’t work properly. Inflation functionality will also degrade.
Industrial-grade air compressors use a 2 stage shut off mechanism that can produce up to 175 PSI. This isn’t possible with home grade compressors. Single grade mechanisms only produce around 100 PSI.
See this video for a more technical explanation of PSI and measuring pressure:
The capacity of the air compressor measures the volume of pressurised air produced. This is measured in cubic feet per minute.
You’ll notice several CFM ratings because CFM is measured at a given PSI. Sometimes the printed CFM denotes the rating at 0 PSI. This rating also doesn’t consider the inefficiencies of the pump.
Like Horsepower, consider the CFM rating in conjunction with other elements and in this case the PSI.
If considering standard air compressors rather than industrial, a good metric to compare is CFM ratings at 100 PSI. As a rule of thumb look for at least 3 – 4 CFM at 100 PSI. This will cover most pneumatic tools.
Every air tool also comes with a recommended CFM set by the manufacturer. Take the highest CFM rating of the air tool and multiply this by 1.5 (bearing in mind the CFM rating at a given PSI). CFM fluctuates throughout operation. This method of calculation allows a margin for error.
Lastly, consider if you’ll be running multiple tools simultaneously. Some Air Compressors come with two air houses. Consider the CFM required for all the required tools.
Check out this video for a more technical explanation of CFM:
We measure Air Compressor tank sizes in US gallons. Tank sizes vary from 1 gallon to around 60 gallons. The tank size determines how long air tools can run before the motor in the compressor is required to create more compressed air.
Tank size is less important than you would think. The tank doesn’t create the air but rather simply stores compressed air. The motor and pump (responsible for the HP, PSI and CFM of the machine) are more indicative of the compressors performance.
Some air tools do require a continuous flow of pressurised air whereas others require air in short bursts. For those tools requiring a continuous flow of air, a larger tank is beneficial. This means the amount of times a motor has to cycle on and off is reduced. This increases the overall lifespan of the machine.
Another example is inflating a large bouncy castle with a 2 gallon compressor. The motor would be continuously running. This is bad for fuel efficiency but also labours the motor.
Sometimes it’s ok to consider a small tank compressor with a decent motor and pump. This should be considered in conjunction with compressor requirements. Smaller tanks also reach pressure quicker.
Single and double stage compressors
We have already mentioned the single (also called a piston compressor) and double stage compressor. The main difference is the number of times that air gets compressed between the inlet valve and the tool nozzle.
With a single-stage compressor, the cylinders pump air to the tank via a single stroke with a piston. The compressed air then moves to the storage tank and serves as energy for tools and other purposes.
With two-staged compressors air is compressed twice which doubles air pressure. A smaller piston compresses the air for a second time before being cooled and delivered to the tank.
Two-staged compressors produce more pressure. The high pressure of the two-stage compressors is better suited for heavy-duty tasks or industrial purposes. Factories, workshops and manufacturing plants would use two-stage compressors. They also cost far more.
For the independent craftsmen a single-stage compressor provides adequate power for a variety of inflation and power tool requirements.
This important element gets ignored the most. If not paid close attention to, you may purchase a device you think is capable of certain tasks only to find out later it offers a 50% duty cycle. It’s important to understand what a duty cycle is to avoid making the wrong purchase.
A ‘duty cycle’ is the amount of time a compressor will deliver pressurised air within a ‘cycle time’. Think of cycle time as an amount of time the air compressor states it will run for. Confusingly, add a percentage to that time…
For example, consider a compressor with a cycle time of 10 minutes and a duty cycle of 50%. This means the compressor will run for a maximum of 5 minutes within that 10 minute time frame.
It is normally represented as a percentage. So pay very close attention to cycle time as well as duty time.
Efficiency is also a significant factor here. A lower powered Air Compressor can have a higher duty cycle than a more powerful one. You can end up with better performance from the lower powered machine.
My recommendation is to look for a machine with at least a 75% duty cycle. Some machines even go as high as 100%. The decision should be made on the inflation or tool requirements.
Oil or Oil-less
Air Compressors will come as oil filled or oil-less. Both have pros and cons and I’ll explain what the differences are and what to look out for.
The way Air Compressors work is by drawing air in with a piston. Air is then compressed into the tank. The piston chamber needs lubrication in order to function for which oil is used. This is an oil compressor. In an oil-free compressor the cylinder housing the piston is pre-lubricated for permanent lubrication. Oil compressors need regular oiling (like a car engine) whereas oil-less does not require lubrication.
Oil compressors require more maintenance than oil-less. Oil must be changed regularly depending on frequency of usage. Oil-lubricated compressors also weigh more than oil-free compressors even with the same design.
The main factor to consider between the two is what type of jobs will be completed with the compressor. Whilst oil-free compressors can produce 2 – 5 CFM of air flow, oil-lubricated compressors will produce 10 – 25 CFM of air flow. This is enough power for heavier duty tools and is mainly used for contractors rather than the DIYer.
In summary, oil-less compressors are cheaper, lighter and easier to maintain than oil-lubricated compressors. However oil-lubricated compressors produce more power.
I have provided a jargon-free guide to the main considerations in your purchase of an air compressor. Consider these elements together alongside your requirements. It’s easy to spend far more than is necessary in your purchase of an air compressor. Pay close attention to your requirements and buy an appropriate machine.
Also be wary of brands claiming certain technical aspects in isolation. Consider these claims alongside the comprehensive list of specifications relevant for the purchase of an air compressor.