Types / categories of Compressor

For "rougher stuff" like running air tools there are certain types of air compressor you may be offered which you need to avoid. You need to avoid "oil-less" compressors and ones with no air tank.

The only ones with no air tank are teeny little ones for use with paint-sprayers. You press the trigger and the compressor starts up and runs until you release the trigger. They have no other use.

An air tank smoothes out demand. A lot of air use is very intermittent, so it avoids the compressor perpetually starting and stopping. It also allows you to use air tools which draw more air than the compressor can continuously supply. And with a lot of tools, you will do a little fettle here and a little fettle there... So you can use air tools drawing half as much again as the compressor can supply continuously without ever running out of air. A bigger air tank is usually a good thing - but takes up space. A 3HP machine could have a 50litre or a 100litre air tank. The choice is yours.

The categorisations applying to small workshop compressors are:

Oil-lubricated vs "oil-less"

I believe "oil-less" compressors are only found as small single-cylinder direct-drive machines and that these are only useful for small artistic paint-spraying guns, like graphic artists and model-makers use. Those air-guns little larger than a fountain-pen, held in the same delicate way. With no oil contaminating the air, it keeps things nice and simple. "Oil-less" compressors became possible when PTFE ("Teflon" - the stuff used for non-stick pans) became available.

However, oil in compressor not only lubricates. It also cools the mechanism and carries away heat to be dissipated elsewhere. Decent lubrication and heat dissipation are needed for hard work. For happy hours of craftspersonship with air tools, you want an oil-lubricated compressor. I have seen single-cylinder direct drive oil-less compressors offered very cheaply at a supermarket (about a third of the price in a tool dealers) - with a tack-gun for upholstery - and I gather from conversation with air-tool users that is about the limit of their reasonable usage.

Direct-drive vs belt-drive

With direct drive the electric motor connects directly to the compressor crankshaft. Therefore the compressor spins at the motor's speed. What you often see in tool stores are single-cylinder direct-drive machines with a little receiver tank, perhaps 25litre, which the pump and motor sit up. Oil-lubricated, of course, otherwise it wouldn't be in a serious workperson's tool store. Such a machine would be useful to a carpenter who has to walk into houses carry his/her tools. And they use a pneumatic nail-gun and similar tools with very intermittent short uses of air. That's their legitimate niche.

Belt-drive compressors have a V-belt running from a smaller pulley wheel on the motor to a larger pulley wheel on the pump. therefore the pump spins slower than the motor. This is the traditional arrangement for a long-lasting capable compressor. The majority of compressors you should be looking at are of this type. My compressor shown previously is of this type, if you need to picture what such a machine looks like.

An additional advantage is that if anything were to go wrong you can replace the individual parts. Motors and pumps are commercial items. So if you buy a good one it should work well for a long time and even if after a long time it were to start showing problems you could soon have it back on top form.

A very reputable manufacturer has started offering direct-drive V-twin compressors. Certainly the revolutions of an induction motor running off the mains is low compared to the rate at which car engines spin these days and there is no reason to thing that a well designed and made machine would not be a real workhorse.

Portable vs fixed

There's some assumptions which go with portable compressors vs fixed compressors which you need to know to realistically know which compressor you want. Portables are portable, obviously. That is a very big advantage for a craftsperson. On the other hand, fixed compressors are more heavily specified and pump more air. A popular compressor is the "3HP compressor". This relates to single-phase motors which can run off a domestic supply. The largest electric motor you can run off a domestic supply is 3HP. You can do higher if you have special starting arrangements, but in general, 3HP is the max. So you will tend to go for this. Now in reality "3HP" portables have a motor which is more like 2~3/4HP. Whereas fixed 3HP compressors have a true 3HP motor. This is one part of why for "3HP compressor" at 90 p.s.i. a portable gives about 7 c.f.m. FAD while a fixed workshop compressor will give about 10.5 c.f.m. FAD.

A 3HP portable is very useful in that it can be taken to jobs and it is very useful in that it can do serious work. Whereas if you have a workshop and lots of heavy-duty work there you might want a machine which supplies more air.

Single-stage vs two-stage

This is something a bit fundamental. In a single-stage compressor, in a single action you send the air you are pumping from atmospheric pressure to delivery pressure. Now the problem with this is that as you are pressurising one volume of air, the work you are doing on it heats it up - which means that its pressure rises and it opposes the pumping force. So you have to drive the piston harder, which means that you are putting more heat into the air... The air coming out of the pump into the tank cools and lowers in pressure, so you don't get advantage from this extra pressure. This reality of "adiabatic compression", to use the terminology of the science of Physics, is the reason why you get a poor correlation between the power you put in at the compressor and the power you get out of our air-tools.

And by the way, it is a fact that the discharge pipe from the pump to the air tank gets very hot...

From the above, you would deduce correctly that if you could throw out the heat as you pump, you would be better off. One way which doesn't work would be to pump slowly enough that the heat can escape. That would be too slow, by thousands of times, to make a practical workshop compressor. The workable way is to make a compressor with a succession of stages, routing the air through a cooler between each stage. For a workshop compressor a two-stage compressor is manageable. You can get about 25% higher air delivery for the same motor power from a two-stage compressor.

But a two-stage compressor costs more to make, so often it is cheaper to get a more powerful single-stage compressor...

While using less energy is a good thing, compared to the energy used in heating and by the chemicals industry, the energy used by a compressed air system is not big. So their are single-stage and two-stage compressors out there. Most small workshop compressors are single-stage. You might be offered a two-stage compressor to get the highest FAD given that the largest electric motor you can run off a European domestic supply is 3HP. That would be a stationary compressor. A portable would not be if it had to have an intercooler as well!

(Richard Smith, June 2005)