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How to choose an air compressor

There is a lot of confusion out there about selecting the right size air compressor to run your air tools. Basically, there are several factors to look at when choosing the proper air compressor for your shop or garage.You should understand these factors enough to be able to talk to your local salesman and select the right compressor.

The first criteria to consider is horse power rating. Many people assume the higher the horsepower rating on the box, the better the air compressor. However, these ratings are not a true representation of the power the compressor has. You will need to look at the power the compressor draws in electricity.For example, a 5HP unit may need around 15 amps from a normal 110 volt circuit.This rating will only give you approximately 2HP.The 5HP rating the manufacturer is toting is inflated.

In order to really get 5HP you will need at least 24 amps from a 220 volt circuit.If you power tools say that you need a 5HP compressor to run, then you need to get the industrial unit that runs at 24 amps.Otherwise you will not have enough air to run your impact wrench or other air tools.

If you are wondering why the stores can sell these air compressors without the true 5HP rating, it is because most users don't use impact wrenches.If you are using the compressor to blow up a tire or to inflate a raft, you don't need 5HP.However, if you are using tools that have ratings for 5HP then you need to pay the money to get a quality compressor.

Next, you will need to figure out how much PSI you will need.PSI stands for `pounds per square inch'.Most of the commonly used air tools require about 90 PSI to operate correctly.Just like 5HP may not be truly 5HP, you can be deceived by the ratings for PSI also.

You will still need a compressor with a higher shut-off pressure than 90 PSI if you are using tools rated for 90 PSI.Most air compressors that you find at the local hardware store are "single-stage" and shut off at 125 - 135 PSI.You may think this is more than adequate for your tools.

Generally, these light duty compressors shut off at 100 psi this doesn't account for the pressure that you will lose in the line to your power tool.The little light duty compressor will barely run an impact wrench.A small compressor may be fine for light duty garage use.But if you really intend use your air tool, more is definitely better.
Many industrial compressors are "two-stage," which means they build up to shut-off pressure in two stages. The first stage builds to about 90 PSI and the second stage builds to 175 PSI.This gives you the pressure needed to run some of your more demanding tools for a longer period of time.

You will next want to look at the `cubic feet per minute' (CFM).Cubic feet per minute is a measurement of volume. Basically it is how much air is being moved. Air tools require a certain amount of air volume to run on. PSI is just part of equation.

Don't be confused by the different CFM ratings at different pressure charts. Each manufacturer tries to make their product look better by giving higher CFM ratings at different pressures.The only real concern is how much CFM you will get at 90 PSI. Remember 90 PSI is what most air tools require to operate. Generally, air tools require 4 - 6 CFM.A good rule of thumb on air compressors is you should get 3 -4 CFM per real HP at 90 PSI.

The final factor to consider is tank size.For example, 30 gallon is a common tank size.So the question remains, how large a tank will you need?First of all, don't confuse a large tank with more run time for your air tools. If you use your impact intermittently, a large tank is fine.

However if you have a need for continuous use, you will need a small tank with big enough pump and motor. If the pump and motor are powerful enough, you shouldn't run out of air. You can save some dollars by purchasing an air compressor with a large tank and smaller motor for intermittent use. If you need to run a 1" impact wrench (about 20 CFM) intermittently, and have a small compressor with a large tank, you might have enough air stored in the tank to do the job. However, if you are constantly running your air tool, you will need to invest in a more powerful air compressor to do the job.

In summary, you need to evaluate what you will be using your compressor for.Look at motor size (HP), pressure (PSI), air volume (CFM), and tank size. You should be able to select the air compressor armed with this information.

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