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THE BASICS

A basic compressor will have an air pump, a tank, controls for starting and stopping the pump to maintain pressure in the tank, a pressure gauge and a safety valve. See drawing and description of basic components.

In addition you will require a hose and accessories which may include a quick coupler, blow gun, tire inflator and assorted air tools.

HORSE POWER (HP)

The horse power rating used to be a way of determining what size of compressor you would need, unfortunately the marketing guys are now exaggerating the horse power ratings to sell more compressors. Check out how much electrical power it takes it to run, a true 5 HP motor will draw about 24 amps on a 220 volt circuit, (single phase power), whereas most overrated hobby compressors will run on a normal 15 amp 110 volt circuit which is only capable of running about 2 real HP.

Gas engine HP is rated differently than electric motor HP, you would require a 10 HP gas motor for the same performance as a 5 HP electric motor.

PRESSURE (PSI)

Air pressure is usually rated in Pounds per Square Inch or PSI. Most air tools require 90 PSI to run properly.

CUBIC FEET PER MINUTE (CFM)

Air Tools require a certain volume of air to run them. The volume of air that a compressor produces is rated in CFM (Cubic Feet per Minute). CFM ratings tend to be exaggerated just like HP ratings, but you should get 3-4 CFM per each real HP at 90 PSI. The higher the pressure of the air that's already been squeezed into the tank, the harder the pump has to work to squeeze more air into it. So the same pump becomes less efficient at higher pressures. That's why a compressor might be rated at 7.6 CFM @ 40 PSI but will only crank out 5.6 CFM @100 PSI.

TANK SIZE

Usually measured in US Gallons, this is not as important as it might seem. A compressor tank doesn't produce air, it only stores air. It is much more important to have a big enough pump and motor, because if you are producing as much air as you want to use, you'll never run out of air no matter how small the tank is. If you want to run a tool steady, such as a sander, it is important to be producing as much air as you need. A smaller tank is more portable and gets up to pressure quicker, whereas a compressor with a large tank doesn't start and stop as often and cools the air a bit better.

ONE OR TWO STAGE

Single stage compressors have one or more cylinders, and each cylinder pumps air directly into the tank. Two stage compressors have at least 2 cylinders, and the air is pumped from one cylinder into another and then into the tank. The main reason for buying a two stage compressor would be if you need high pressure, but not too many applications need high pressure. Therefore, you probably don't need a two stage compressor. For most applications you would be better off to get a good quality single stage compressor than a low cost two stage compressor.

DIRECT DRIVE OR BELT DRIVE, OIL OR OIL-LESS

There are some very good compressors in both belt drive and direct drive versions, the problem is that most of the direct drive compressors you'll see were designed to keep the cost down, and of course that also means low quality. Direct drive compressors are directly connected to the motor shaft and therefore turn the same speed as the motor. Engineers have designed some compressors to spin twice as fast so they could get more air out of them, and keep the price low but the life expectancy is cut down to about 1/4 of the low RPM compressors and the noise is almost unbearable.

If you don't need portability or if you need higher air volume, your best value will be a belt drive, oil lubricated compressor. You should avoid high speed aluminum pumps, they have very low life expectancies and were built for low price, just like some of the direct drive compressors.

Most belt drive compressors are splash lubricated which means they have dippers on the bottom of the connecting rods to splash oil around in the crankcase. As long as there is enough oil in the crankcase, splash lubricated pumps should last a long time. Some of the best compressors have positive pressure lubrication, like in an automotive engine they use an oil pump to force oil to the bearing journals. This system allows compressors to be built to run at 100% duty cycle.

DUTY CYCLE

The duty cycle is expressed as a percentage and tells you the number of minutes the compressor is allowed to run out of a 10 minute period. For example, a low quality hobby compressor usually has a 50 % duty cycle, which means it shouldn't run more than 5 minutes out of any 10 minute period. If you exceed the duty cycle the pump will get too hot and won't last as long as it should. Most industrial compressors have at least a 75% duty cycle and some are as high as 100%, which means you can run them continuously.

MOISTURE PROBLEMS

When air is compressed it gets hot, and when it cools water condenses out of it. The harder your compressor works the hotter it will get and the more moisture problems you will have. Sometimes the best way to solve moisture problems is to get a bigger compressor. Humidity can also cause you to have inconsistent moisture problems. A common moisture trap provides a low spot for water to collect but it's designed to collect droplets of water not water vapor so if the air is hot it will carry moisture through and then the water vapour will condense in the hose. The trick is to cool the air before it gets to the moisture trap because if it cools after the trap, more moisture will condense out. In a body shop they will usually use refrigerated air dryers to cool the air and remove the moisture. It is important to have the moisture trap mounted at the end of the line as far from the compressor and as close to the tool as possible.

 RUNNING OFF A GENERATOR

There is often a problem using an electric compressor powered by a generator. Most electric tools have a "start-up surge" they need more current to start than they do to run. Electric compressors use about three times as much power to start as they do to run. A compressor that runs on 15 AMPS may need 45 AMPS (or more!) to start. The reason compressors don't blow your house's circuit breakers is that the demand is for a very brief time (thousandths of a second). The power lines have all the power your compressor needs, so the compressor starts, and the "spike" is so short in duration that your circuit-breaker doesn't have time to react. But your generator probably doesn't have that starting power available, so it will make a valiant effort, fail, and shut down - and your compressor won't start at all. Make very sure your generator is big enough to handle that huge start-up surge or consider using a gas powered compressor.

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