Air compressors (electric) for home shops--A primer

Most single-stage compressors will provide 135 psi, maximum.  Some newer ones will go up to 155 maximum.  A two-stage compressor will give you 175.  Two-stage compressors use a first stage to compress the air "part-way".  The first stage empties into the second stage, which takes the partially-compressed air and squeezes it some more.

A given air tool will use a certain volume of compressed air.  Volume is measured in Standard Cubic Feet per Minute--SCFM.  The compressor must be able to supply both the pressure and the volume needs of the air tools connected to it.  Typical home shops use one tool at a time--if you are using more than one at a time, you need enough capacity to supply ALL the tools that would run at the same time. If the tools use more volume of air than the compressor can provide, the compressor motor will run constantly, and the pressure in the tank will drop below the recommended pressure for the air tool.  This results in the compressor running hotter and longer than it should.  You may exceed the duty-cycle rating of the compressor.  The air tool(s) will have less power and/or speed than they should.  None of this is good for the compressor or the tools.

You need a "reservoir" of compressed air.  Some very small compressors use a tiny tank, or no tank at all.  That's fine for portable use, when the carrying weight and bulk are more important than the actual performance of the unit.  Otherwise, any roll-around compressor is likely to have at least a 20-gallon tank, with 60 and 80 gallon storage tanks more suitable for permanent mounting in a home shop.  More is better.  Having a larger tank allows the compressor motor and pump to "rest" for a longer time before rechargings.  When the motor and pump rest, they are giving off heat without generating more heat--the temperature goes down.  That's good.

Some compressor pumps and motors are rated for continuous duty--it's okay if they don't shut off now and then.  Most compressor motors will need periodic rest breaks--perhaps 3 minutes of rest for every 10-minute period of time.  Since that leaves 7 minutes of running time in that 10-minute period, the duty cycle in this example is 7/10 or 70%.  If you buy an undersized compressor, it will need to run more than 70% of the time, and so will tend to overheat, leading to damage.

As with any electric motor, you are responsible for providing a suitable "plug in" that will supply the electricity needed to run the motor.  Some compressors (mostly the smaller ones) will run on 120 volts using a 15 amp fuse or circuit breaker.   Larger compressors may need 240 volts and a  20, 30, or even 50 amp breaker.  Worse, industrial-duty compressors may need "three phase" power which can be hugely expensive to have installed at your property.  If you have to update the wiring to your garage/shop to accomodate a larger compressor, don't forget to add the electrician's bill to the cost of the compressor to determine your final budget!  Do not forget to ask what sort of electrical power you will need to run the compressor!

There is no good standard for determining horsepower.  You can buy a shop-vac with an electric motor the size and weight of a grapefruit, and it will be proudly rated as having "6 horsepower".  Then you look at air compressors, and the motor will be ten times the size, twenty times the weight, and rated at "5 horsepower".  Blame the Federal Trade Commission for inaction.  Horsepower is Torque X Speed--unless it's calculated using mathematical formulas for the conversion of electrical power to horsepower.  And if the manufacturer calculates horsepower of an electric motor based on electrical consumption, and if they have no ethics, they can use the instantanious inrush current draw of a tiny motor and say it produces 6 horsepower (peak)  The better way would be to use the running, steady-state current draw, which when multiplied out using the mathematical formula, gives you a more realistic horsepower number.  Since horsepower is related to motor speed, if a motor runs twice as fast it can have half the torque and still keep it's rating.  In many respects, rated horsepower is a GAME that manufacturers play.  It is NOT a reliable rating of how much compressed air the thing will deliver.

The absolute worst compressors for noise are the cheap 'n' crappy "Oilless" or "Oil-Free" direct-drive designs.  They rattle like a 55 gallon drum half-full of steel balls, rolling downhill.  It's not my fault if you go completely barking insane--or deaf--from using one of these things.  You've been warned.  If there's a premium version of these that's appropriately muffled, I don't know about 'em.  Listen before you buy.  Painters can justify the oil-free designs, since there's no way oil can contaminate the air supply and ruin the paint job.  For anyone else, I strongly recommend a belt-drive compressor pump using an oil supply to cool and lubricate the pump.  Yes, you will need to check the oil level now and then, and even change the oil perhaps every year.  Be careful, some compressor pumps need special (that means EXPENSIVE) synthetic miracle oil or they burn up.  I'd avoid a compressor needing special synthetic oil, you can bet the oil has to be special because the pump isn't--maybe the pump is marginal for it's rated use, and the expensive oil is all that keeps it from Sudden Terminal Disassembly.  Or, the manufacturer has figured out a way to get you to buy their brand of special oil, so they keep making a profit off of you year after year.

With any electric compressor, you will want to look at the electric motor's specification plate.  Most compressor motors run at about 3,400 RPM.  It would be worth a bit of extra expense to buy a compressor that has an electric motor that runs at half that speed.  Less RPM, less noise, less wear.

You also want to know how fast the pump runs.  The direct-drive units are so noisy in part because the tiny pump runs very fast--at the same speed as the motor, and that's in the neighborhood of 3,400 RPM.  A belt-drive compressor may have a pump with twice the capacity but turning at about half the speed of the motor.  Again, less RPM = less wear and less noise.

The premium compressors will use a motor that turns about 1,700 RPM, and a pump that turns 800-900 RPM.  The motors and pumps will be huge by comparison to stuff that turns 3,400 rpm.  The extra money buys long-term reliability in addition to lower noise levels.

If you have a stationary compressor, you may want to think about locating it away from where you do your work--perhaps in it's own room; or maybe you can build some insulated 2X4 walls around the compressor area to reduce the noise level in your workshop.  In many cases, a portable compressor can just be wheeled "outside, around back" so you don't have to listen to the thing in your shop.  The neighbors may object, though...

If you have a portable compressor--and you actually roll it around--your installation is easy.  Park it near an electrical outlet, or buy a PROPERLY HUGE extension cord.  Connect your air hose to the tank outlet.  Connect your air tool to the air hose using a quick-coupler set.  Enjoy.  Don't forget to drain the water out of the tank when you're done.  When I had a "4-horse, 20 gallon" compressor, I disconnected the built-in air pressure regulator because I never needed anything less than "all it could give".  I could run an air hammer for about five seconds, then I had to stop and let the compressor "catch up".  That got really annoying.

I then went to a "5-horse, 60 gallon" stationary compressor.  This was much better.  I used 1/2 inch air hose 50 feet long.  That would reach anywhere in my garage, but it sometimes had to snake around vehicles, engines-on-stands, other equipment, etc, and so there was always a mess of hose laying on the floor to trip over.  I had a second air hose that I could add to the regular hose, for filling tires far out in the driveway.  Since it was a 135-psi single stage compressor, I could thread the air hose directly into the tank, and then reduce size to the more common 3/8 inch just before the quick-coupler near the air tool.  Pretty much all my air tools except the hand-held die grinder worked acceptably well.  The compressor still worked awful hard to keep up, and I had water at the air tools if the ambient humidity was high.

My final, best system involves using an 80 gallon, 175 psi two-stage compressor, with metal pipe to plumb the compressed air to near where I want to use my air tools.  You might plumb in several attaching points at various places in your shop, so that you can keep the hose length to a minimum.   Be sure to install water traps and drains at all of the system "low points".  Air tools and paint sprayers shouldn't have to deal with water in the air supply.  If you install water traps, and drain them often, you won't rust the inside of your tank and pipes, which means that 1) the system probably won't weaken and explode on you someday, and 2) the air tools won't have to deal with tiny rust flakes in the air supply, either.

A compressor will vibrate when running.  You will need a short piece of large-diameter, high-grade hose or a generous loop of metal tubing (I use 8 feet of 3/4" copper tube, coiled into a double loop) to allow for some movement of the compressor.  I'm using about 50 feet of 3/4" black iron pipe which when I bought it was about a dollar a foot, plus the elbows, tees, reducer bushings, and quick-couplers to make it all work.  (About $100 altogether) The copper tube connects the compressor to a combination pressure regulator and compressed air filter mounted to the wall.  The regulator/filter connects to the iron pipe with a tee  One leg of the iron pipe goes down to a water trap/drain, with the other leg going up near the ceiling.  From there, I have a 90 degree elbow, and a straight run of 20 feet of iron pipe, but it's angled upwards a couple of degrees so that any water will run back towards the compressor and get caught in the water trap/drain.  At the end of the first 20 feet, I have a tee, with one leg continuing along the ceiling, and one leg going down to a conveniently-located quick-coupler, below that another water trap/drain.  At the farthest end of the pipe, there's a second drop to a quick-coupler and below that, another water trap/drain.  Every low point has a water drain.  The only water drains that have ever had water is the one on the bottom of the compressor tank, and the one built-into the regulator/air filter unit.  I NEVER have water at the tool now.

To set the regulator pressure, I went to the farthest end of my compressed air plumbing, and plugged in an air hose, pressure gauge, and my die grinder.  The die grinder is the most "air hungry" tool I own.  Then, with the compressor tank fully charged, I adjusted the pressure regulator so that the guage AT MY DIE GRINDER showed 90 PSI when I pulled the trigger.  (The guage on the pressure regulator was showing about 120, and there's 175 psi in the tank of the compressor.)  I now know that my air tools will run at full power.

The metal pipe will help to shed heat.  When the air is compressed, temperature goes up.  When the compressed air is released, temperature goes down.  This is the same heat/cool cycle that makes air conditioners and other refrigerated appliances work.  Problem is, you're compressing air that most likely has some amount of humidity in it.  When the air is cooled, the humidity condenses out and becomes liquid water.  You must trap and drain the water before it goes through your air tools, and you don't want it inside your tank and pipes because it will rust the metal.  Thus the reason for all the water traps and drains.

You must supply air tools with some amount of oil.  I choose to do that by dumping in a small quantity of oil from a squeeze bottle, directly into the air tool before each day's use.  If you use an automatic oiler mounted in the plumbing somewhere, you better not connect a paint sprayer to any plumbing or hose downstream from the oiler--your paint spraying air will be contaminated.

What you use for a compressed air system is up to you and your budget--but to recap:  You need compressed air, not horsepower.  You need clean, pure air without water or rust flakes.  And you need to supply your compressor with the appropriate kind of electricity.  Just keep in mind that buying the system once is cheaper than buying it AGAIN because what you bought the first time is too stinking small.

Please also take a look at my guide to Snap-On Counselor automotive oscilloscopes .

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