Choosing the right compressor

How to Choose the Right Compressor for Your Needs

Choosing the right compressor can be a daunting task, given the number of choices there are today. Picking the wrong compressor can mean more than just purchasing mistake. A compressor that is too small can overheat and fail prematurely. Your air tools may also not have the amount of available air to function correctly. All of which can leave you frustrated and out of pocket for the wrong choice.

Understanding How a Compressor Works

To choose the right compressor first, we need to understand how a compressor works. There are many different sizes and types of compressors out there to choose from. And other size compressors can use different types of motors and pumps to generate compressed air.

All compressors are basically pumps that take air from the atmosphere and compresses it into a storage tank. Like fluid pumps, there can be many different ways to achieve this, depending on the volume and pressure requirements of the compressor.

Small Compressors

Most small compressors use a piston-style pump and a direct drive motor to make compressed air. Similar to how the engine in your car works, the piston in the compressor is moved down, drawing air into the chamber. As the piston moves up, the inlet valve closes, and the exhaust valve opens, compressing the air into the storage tank as the piston moves up.

Choosing the right compressor, small pancake  compressor
Small Porter cable Pancake Compressor

The pump on a small compressor is usually driven by a direct drive motor. This means that one revolution on the motor shaft equals one rotation of the pump. Using a direct drive motor makes it cheaper to produce the compressor. However, it also limits how much torque can be applied to the pump.

These types of compressors are typically slower to build pressure. And have a limited amount of air they can supply at a constant rate and pressure. Depending on your requirements it could be the right compressor for you.

Medium-Sized Two-Stage Compressors

Two-stage compressors use two pistons instead of one. One piston will also be larger than the other. The larger piston will draw in air from the atmosphere. As the piston moves up, it forces the air into the second smaller piston, further compressing the air while pushing it into the storage tank.

It is possible to have a direct drive motor on a two-stage compressor, and most use a separate motor and belt drive. Usually, a two-stage compressor is driven by a separately mounted electric motor. The motor will use a belt drive system that allows for the use of different sized pulleys. This results in more available torque (twisting power) at the pump, which means that it can produce higher pressures without straining the motor.

cutaway of a two stage compressor
Two-Stage Compressor

Large Two-Stage Compressors

A large two-stage compressor is generally used in a workshop environment. These styles of compressors will often have four or six pistons. And just like in a car engine will have larger displacements (pumping capacity). They will still work in the same way as the medium-sized two-stage compressors.

A large two-stage compressor will typically use a 3 phase electric motor. It will generally be a much larger motor and will use a multiple belt drive pulley. This is required due to the increased torque needed to drive the pump.

Rotary Vane Type Compressors

A rotary vane type of compressor does not have any pistons. Instead, the air is drawn into a round chamber by a rotating offset crankshaft. Mounted in the rotating crankshaft is a series of vanes. The vanes are allowed to move in out in their mounting slots. As the pump rotates, the air is drawn into the large portion of the drum by the vanes. The air is then compressed as the crank rotates, and chamber size is reduced by the vanes pressing against the wall of the outer drum. Finally, the compressed air is forced out of an exhaust port, and the cycle continues.

Basic Rotary vane compressor sectional view
Rotary Vane Compressor sectional view

These styles of compressors are generally quite large and are used in busy shop environments. This style of compressor will normally run continuously throughout the day. They are capable of supplying large amounts of compressed air, both in volume and pressure.

A rotary vane compressor will usually be driven by a large 3-phase electric motor. They can also utilize a belt and pulley drive system. Or they can use a direct drive motor. This allows the manufacturer to use an energy-efficient motor while still providing the torque required to drive the compressor.

Screw Type Compressors

These compressors use a series of rotating screws. As the screws turn against each other, the air is forced through the compressor and into the tank. Screw-type compressors will be used in big production shops where a large air volume is required to run many air tools.

Screw compressors tend to run continuously and are powered by 3-phase electric motors. They are similar in design principle to the rotary Vane type compressors.

Choosing the right compressor, Screw compressor housing cutaway
Cutaway view of the IR brand screw compressor. It shows the screws in place in the compressor housing

Calculating Your Air Requirements

Now we know a little bit about the different types of compressors, we can begin to figure out the best compressor for our needs. The first thing that I look at is how many CFM’s, (Cubic Feet per Minute), your largest air tool requires to run. If I’m buying a compressor for a workshop, I will need to add up how many air tools will run at once. Then calculate the amount of CFM’s I will need to support them.

Understanding a Compressors Rating

Every compressor built will have an operating specification. The specifications will be shown on the compressor’s information page. Being able to understand these specifications will help us choose a suitable compressor.

Compressors will be listed in the following manner: “5Hp, 230Volt, 80 Gallon, Two-Stage compressor”

So, let us break down to what it all means.

“5HP” is the power rating of the electric motor that drives the compressor

“230 Volt” is the line voltage required to run the motor

“80 Gallon” is the size of the storage air tank

“Two-Stage” compressor is the type of pump used.

Though this information does give us a lot of good information, there is one other specification we need to know, flow capacity. Flow capacity is listed in the “Specifications,” and it will look something like this:

“CFM @ 90 PSI = 14.3”

This specification is important because this is what the compressor is capable of supplying air continuously.

Air Tool Consumption Rates

So, every air tool has an air consumption rate. This is the amount of air the tool needs to operate.

For example:

HVLP Spray Gun will need 10 – 14 CFM @ 40 PSI

½ Impact Gun will need 4 -5 CFM @ 90 PSI

Air Ratchet will need 3 – 4.5 CFM @ 90 PSI

Sand Blaster will need 18 – 35 CFM @ 90 PSI

Air Powered Sander will need 2 – 4 CFM @ 90PSI

Almost every air compressor will have a storage tank attached to it. The storage tank is a reserve supply of air, so when you pull the trigger of your air tool, you have an immediate response. Once that storage tank pressure drops to a preset pressure, the compressor will start. This means that you are no longer using stored air pressure, and you are relying on the compressor itself to keep up with your air supply demand.

Load requirements

Before you purchase a compressor, you should first calculate what your expected CFM requirements will be. A compressor used at home for occasional auto work or home maintenance can be pretty small. However, if you plan to do some serious work or when setting up a professional shop, you will need a lot bigger compressor.

A one-person shop using basic air tools will only need a compressor to keep up with its most significant air tool consumption. So, choosing a compressor with a CFM rating of 6 or better at 90 PSI would probably be ok. However, if you have multiple people using the air supply, you will need to add up all the possible tools used at one time.

A shop with three people using standard air tools will need a compressor that can supply at least 15CFM @ 90PSI (largest air tool 5CFM x 3 = 15CFM)

Large Air Consumption Tools

Tools like sandblast cabinets are heavy consumers of air. These tools require a large compressor that can keep up with the air demand. Even the smallest sandblaster will have a large air consumption. If you plan to run a sandblaster in your shop, you should look at the Vane or Screw Type compressors. These compressors types can keep up with heavy demands. It is more important to choose the right compressor when using large air-consuming tools.

For example, a shop with five employees doing mixed types of work using air tools and a sandblaster will need a possible air supply of 55 CFM @ 90 PSI. (4 x standard air tools @ 5CFM + Sand Blaster @35CFM = 55CFM

Final considerations

When it comes to my final compressor selection, I will calculate the requirements then add a buffer. For example, if I need 5CFM, I will start looking for a compressor capable of 7.5CFM @ 90 PSI or better. This way, the compressor should be easily able to keep up with my usage requirements. A CFM buffer can be more significant with smaller compressors. As the compressor gets bigger, the buffer can be a little smaller.

Selecting an air compressor with a larger tank will also give you a buffer. Using the stored pressure, then allowing the compressor to recharge will also work. This will enable you to choose a compressor with a CFM capacity close to your actual requirements.

Choosing the Wrong Compressor Versus the Right Compressor

Choosing a compressor that is too big is not usually an issue unless you need that compressor to be portable. In which case, it will be much harder to move the compressor around. However, using a compressor that is too small is a different story.

A compressor that is too small and can’t keep up with your demand will cause the compressor to run continuously. This can result in the compressor overheating and failing prematurely. It will take longer for you to accomplish the task you are trying to do. Having to stop and wait for the compressor to catch up is frustrating and time-consuming.

The best way to choose a compressor is to calculate your load requirements first. Then select the best compressor for your budget in a size that is ideally slightly larger than the highest estimated consumption rate.

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