Like any other tool you get what you pay for, bargains can be had for the oilless style, the noise will drive you crazy and they will wear out very quickly, on the other hand an oil-lubed unit will last for decades. A source of air in the shop is essential if you want to use a nailer, and once you have used one you will never want to go back to a hammer, even very low CFM models are usually adequate. Clean-ups are much easier using an air hose to blast sawdust out of the corners of machinery, and if your wheelbarrow tire is low you can top it up. There are many other tools that are run by air, rachets, drills, impact wrenches, chisels, nibblers, sanders and paint guns.
What do you need ?
For years I used a large stationary compressor in the shop to run my tools, when I bought an air nailer my thoughts about compressors changed, bigger is not necessarily better. Tiring of dragging long lengths of hose around the yard and into the house I bought a small "Hot Dog" style compressor with a 25' hose on it. This has become my "Go To" compressor, my big one is now only used for air hogs such as a drill or grinder. The little one runs my nailers, blows dust off tools, and pumps up my wheel barrow tire, in fact it does about anything the average homeowner would use a compressor for.
The two common diameters of hose for home users are 1/4" and 3/8", my advice is to get a 25 foot long 1/4" hose, select a type that is flexible, pay a bit more if you have to, it is so nice to use you will be glad of it in the long run. Any additional hose can be bargain 3/8" just put it between the compressor and the good 1/4" hose.
Nothing bothers me more than listening to a leaking air hose when I am working in the shop. This is usually caused by using cheap crappy quick connectors, spend a little bit more and get good ones. Take a look at the male ends, are they molded in two parts or do they appear to be turned in a lathe, the molded ones will leak most times.
Add a connector directly to the compressor so it will be convenient to add more hose and keep your good hose at the tool end.
More Hose or Extension Cord?
Even with a small portable 110 volt compressor there are times when the available power is not within reach of the project site, add more hose rather than using a long extension cord. For exceptional distances use your larger diameter hoses at the compressor and step it down toward the end with smaller diameter hoses.
What to look for in a Compressor
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 or Bar
Air pressure in America is usually rated in Pounds per Square Inch or PSI. Most air tools require 90 PSI to run properly.
In other countries it may be refered to as a Bar, one Bar being the atmospheric pressure at sea level, which equals 14.5 PSI.
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.
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.
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.
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.
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.
This chart is only a general guide, some brands of air tools may use more or less air than listed.
Twenty years ago this was an option, today there are so many reasonably priced units on the market that it is probably no longer practical. On the other hand it may be worth repairing a unit by replacing the pump if the rest of the compressor is still in good shape.
If you do decide to build your own there are two important considerations, first of all only use a certified air tank, don't take a chance on anything else. Secondly put a pressure relief valve on the tank, do this before even running the unit for the first test. Some years ago I sold a small refrigeration compressor and a 3 gallon tank, that I had picked up at a yard sale and never used, to a fellow that wanted to make a compresssor to blow the chips off his wood lathe. Several days later he phoned me to relate this experience, he installed a motor and the pump on the tank, put a pressure gauge on the tank and started it up to see how long it would take to get a usable amount of air in the tank, he did not have a pressure relief valve or automatic start and stop control for it yet. He was called away to the phone, the call took a while, when he returned to the shop the compressor was still running, the needle on the gauge was stopped at its limit of 250 pounds!
PAINTING AND FINISHING
1. All pipes, hoses, and fittings must have a rating of the maximum pressure of the compressor.
2. Air supply shutoff valves should be located (as near as possible) at the point-of-operation.
3. Air hoses should be kept free of grease and oil to reduce the possibility of deterioration.
4. Hoses should not be strung across floors or aisles where they are liable to cause personnel to trip and fall. When possible, air supply hoses should be suspended overhead.
5. Hose ends must be secured to prevent whipping if an accidental cut or break occurs.
6. Pneumatic impact tools, such as riveting guns or nailers, should never be pointed at a person.
7. Before a pneumatic tool is disconnected (unless it has quick disconnect plugs), the air supply must be turned off at the control valve and the tool bled.
8. Compressed air must not be used under any circumstances to clean dirt and dust from clothing or off a person' s skin. Shop air used for cleaning should be regulated to 15 psi unless equipped with diffuser nozzles to provide lessor pressure. When directed against the skin, even 30 psi of compressed air can be driven into the flesh or eyes. If there is a break in the skin, air can be driven into tissue, causing swelling and pain. Bubbles of air can be driven into blood vessels, which can be serious, even fatal.
9. Goggles, face shields or other eye protection must be worn by personnel using compressed air for cleaning equipment.
10. Static electricity can be generated through the use of pneumatic tools. This type of equipment must be grounded or bonded if it is used where fuel, flammable vapors or explosive atmospheres are present.
Low RPM (Quiet)
Cast Iron Pump
Flexible 1/4" Air Hose