At some point we all need compressed air. That means buying an air compressor. When you’re shopping for a compressor keep in mind that more horsepower doesn’t actually mean you get more compressed air. The Cubic Feet per Minute (CFM) is the best indicator of compressor performance.
There are a lot of games being played with horsepower numbers. Peak horsepower may actually be five to seven times the running horsepower. Most companies will give you a horsepower rating but they never say if it’s peak or running horsepower. I recently looked at three different 5-horsepower compressors in Lowe’s. The CFM ranged from 15.3 to 18.1 at 90 psi. That’s a significant difference.
The Tank
The type of usage should determine the size of your air compressor tank. You need this tank because it gives the compressor time to rest and cool off. A big tank takes a long time to fill while smaller tanks are always asking to be refilled.
If you only need air in short quick concentrated bursts, such as filling a tire, then a small tank size is fine. If the unit is to sustain long periods of usage, such as sanding or media blasting a larger tank size will be necessary.
Noise
Some air compressors use oil for lubrication. However there are oil-free compressors where the cylinder is pre-lubricated (mostly with Teflon) for permanent lubrication. There’s a huge difference between the noise levels of these two compressors. Oil compressors are quieter than oil-free ones. This is an important item to consider if you are going to be using the compressor in a closed space. Listen to an oil free compressor before you buy one.
Single Stage vs. Dual Stage
Single stage air compressors work by drawing air in and compressing the air in a single piston stroke. Single stage air compressors can reach pressures of up to 150 PSI. Two stage air compressors compress the air in two steps. During the first step or stage, air is drawn in and compressed to an intermediate pressure. Then the air is piped through an intercooler where it’s allowed to cool and then compressed in the second stage. Two stage compressors are normally good for pressures up to 200psi. Two stage compressors are more efficient at higher pressures because the air is cooled between the stages.
The decision regarding single or dual stage is determined by the maximum operating pressure you need. If you need over 150 psi on a regular basis you need to think about a dual-stage compressor.Size Matters
The single biggest compressor killer is the media blasting cabinet because of the recovery rate. If you have a blasting cabinet you not only need a compressor with a lot of CFM but also a pretty good size air tank. You should be thinking in terms of a 60 to 80 gallon air tank if you do much media blasting.A smaller compressor will work but you have to work in small bursts until the compressor rebuilds pressure. You’ll be doing a lot of starting and stopping while you wait for the compressor to fill the air tank. You want your compressor to cycle on less often and run longer each cycle.
You’ll need at least a five-horse motor for media blasting but you would do well to consider with a 7.5 hp 2-stage compressor. This is the type of compressor that is usually vertical and weighs a couple of hundred pounds. Once you get this compressor home in your garage it’s going to stay there. It’s also going to cost around $1,500.
If you’re just doing a few odd jobs around the garage then a four to five horse motor with a 5-gallon tank might be what you need. You want at least 10 CFM and you’re going to spend less than $1,000 for this size. The good thing is you can actually move a compressor of this size around the shop. They’re great units but don't expect amazing results if you’re using a DA sander or a blasting cabinet. Even a small dual action sander needs 5 cfm at 90 psi.The third category of compressor might be good for putting air in the tires and blowing dust off parts but don’t expect it to do much else. These small units have around 1.5 hp and about might reach 100 psi if you have a 5-gallon tank. These are usually less than $200 and are the models you’re going to find on sale at Home Depot and Lowe’s.
If an air compressor tank feels hot (warm is normal) and the compressor is running continually stop operating the blasting cabinet or air tool and allow the compressor to cool.
The compressor should recycle on and off and normally run no more than two thirds of the time you’re actually using it. No air compressor is designed for continual operation.
You should be aware of the amount of air that’s required for your air tools. You should then allow a 50% overload factor for best results. As an example if your tool requires 10 CFM air then you need 15 CFM of air. The biggest mistake people make is getting a compressor that’s too small. No one has a problem with a compressor that’s too large but one that’s too small will make you crazy.Maintenance
The receiver tank on your air compressor will collect moisture from the air that it is compressing. Drain the tank every time you finish using the compressor. Your air compressor will also loosen its screws, nuts and bolts on a regular basis. Be sure to check these periodically and tighten them up if you find any that have jiggled loose.
A dirty air filter allows dirty air from the outside into the compressor and it also forces it to work harder. Check the filters regularly and change them if you notice a build up of dust and dirt. Change them every six months or so if you use it infrequently. You might also consider changing the oil in the compressor every six months.
Air Tool Description Average CFM @ 90 PSI
Angle Disc Grinder 5-8
Chisel/Hammer 3-11
Cut-Off Tool 4-10
Dual Sander 11-13
Grease Gun 4
Impact Wrench - 3/8" 2.5-3.5
Impact Wrench - 1/2" 4-5
Ratchet - 1/4" 2.5-3.5
Ratchet - 3/8" 4.5-5
Mini Die Grinder 4-6
Orbital Sander 6-9
