I started my woodworking career with an electric quarter sheet sander, quickly graduated to an electric random orbit disc sander, and finally realized that I could substantially shorten sanding time with an air palm sander. I settled on a 5″ Dynabrade sander and a Sears 3HP air compressor. It took me less than an hour to realize my mistake: the little compressor I bought couldn’t start to keep up with the air demands of the air sander. You would run out of air pressure almost immediately and the air sander would slow down to the point of being useless.
To make matters worse, I hired three people as sanders, so I would have to keep three machines running at full speed all day. I did some math and found that I would need a ten horsepower air compressor with a large tank to do this. I was lucky to find a used one for cheap, but it required 3 phase power and lots of it. More money was spent to have an electrician hook it up to the building’s 208 volt 3 phase power. The big air compressor was so loud it could be heard throughout the building and across the block, but it powered those three sanders from dawn to dusk. The good news is that it paid for itself in sanding time saved very quickly.
Air sanders are aggressive and efficient. They are lightweight compared to their lesser electric cousins. My sanders took to them right away and production took off. I was as happy as they were. Soon there was another machine besides the air compressor that required large amounts of air in the shop: an Onsrud reverse pin milling machine. It was also great to be able to blow the sawdust off the benches and the machine while cleaning the shop at the end of the day. The compressor was also used to spray finishes on the finished furniture.
Years later, I built a smaller woodshop in my house that only required one air sander running at a time. For that store, I bought an air compressor half the size and isolated it in a soundproof room in a corner of the store. I ran ¾” galvanized pipe under the shop floor to three regulators in three different convenient locations. The machine I purchased for that shop was an Ingersoll Rand 5 HP model with an 80 gallon tank. At the 80 PSI required by my dynabrade sander the compressor would produce enough air all day I must say that compressor was very well built all I had to do was keep an eye on the oil level in the sight glass at night I would close the master air valve on the machine side, leaving the electricity on, to silence the compressor at night.
I am assuming that after reading this far you have some interest in using an air compressor to power air tools in your shop. A 2-stage reciprocating air compressor will most likely meet the needs of a small to medium sized shop. As a general rule of thumb, a 5 HP air compressor will power one air sander, a 7.5 HP machine will power two, and a 10 HP machine will be needed for three sanders.
The size of the compressor’s air tank is an important consideration: the smaller the tank, the more often the compressor will have to cycle on and off. This is hard on both the motor and the compressor pump over time and consumes more electricity. I wouldn’t even consider an air compressor used to power an air sander with a tank smaller than 60 gallons and would be much more comfortable with an 80 gallon tank.
The type of electrical power required by an air compressor is another consideration. If you have three-phase power available at your location, good. Three-phase motors tend to use electricity a bit more efficiently than single-phase motors. All large air compressors will require 3 phase power, but the 5 HP models come either way. If you don’t have 3 phase power available, you can make it with a rotary or electronic phase converter like I did in my smaller shop. Whether you use single or three phase power, you will need 230VAC power for single phase motors and 208 or 220VAC for the three phase variety. Be sure to check the voltage and amperage requirements of any air compressor before purchasing. Electricians can be expensive.
A two-stage compressor pump is a must for a machine of this size. Two-stage machines have two cylinders, one larger than the other. Air is first drawn into the large cylinder where it is partially compressed and sent to the smaller cylinder for final compression in the tank. As air is compressed, heat is produced, so a good machine will always have a finned intercooler built in.
Compression not only produces heat, but it squeezes water out of the air that ends up in the tank. Tanks can rust internally over time and if this is not kept under control, the rusted air tank can eventually burst causing tremendous damage and even death. That is why it is extremely important to empty the water tank every day. Most machines are equipped with a drain valve at the lowest point of the tank. If you don’t want to spray water all over the floor under the compressor, you may consider piping it from the valve to another location, such as under the floor or into a drain. Piped water will flow uphill to a sink because compressed air pushes it out of the tank.
You will need at least one regulator and one inline water trap first. These are not expensive. A regulator allows you to set the correct air pressure for the tool you will be using (for example, 80 PSI) instead of tank pressure (for example, 175 PSI).
The air output of a compressor pump is expressed in standard cubic feet per minute (SCFM) or simply cubic feet per minute (CFM). Not all 5 HP compressors generate the same volume of air per minute. This is a function of not only engine power, but also the efficiency of the compressor pump that powers the engine. The higher the CFM, the less the compressor will have to cycle on and off to keep up with the demands it places on it. A small compressor pump in a huge tank will not produce more air than in a small tank. The only difference will be in the number of times the compressor cycles on and off each hour and the time it takes to re-compress the tank each cycle. In the end, you should pay attention to SCFM (or CFM) more than engine horsepower or tank size. Airflow is the end product of any compressor and the CFM should be sufficient for the job at hand.
All reciprocating air compressors spew oil with the air they compress. When the tank reaches its designed maximum pounds per square inch, a pressure switch will shut off power to the motor. Simultaneously, a certain amount of oily air will be released into the workshop environment. Over time, you may see oil buildup on the wall behind the compressor and also on the pump and compressor. This is not cause for alarm, but periodic cleaning may be necessary.
Reciprocating (piston type) air compressors are noisy and this is something to be aware of for yourself, your workers and others around your location. If quietness is an important criteria, you may want to consider spending the extra money on a screw air compressor. Screw compressors do not have pistons or cylinders. The air is compressed into a turbine by a large metal screw that rotates at a very high speed. These compressors just purr compared to the reciprocating type, but they are very expensive. They sound more like a quiet jet engine than a loud truck engine.
I hope this article has been useful to you. Purchasing an air compressor for your woodshop can be quite an expensive investment when you consider plumbing, regulators, hoses, water traps, wiring, and electricians. You will want to buy a machine that is equal to the jobs it will be doing, but no more than that. Buying the wrong air compressor can be a very costly mistake. My intent in writing this has been to provide you with the knowledge you will need to select the right one.
