2 stage -- 2 cyl compressors

[Tom Godley]

I am a little confused

I have been using a Sears unit that I bought 20+ years ago (3-hp 20-gal Hor w/wheels) It has worked well for me over the years.

What exactly is a 2 stage compressor? -- are they also two cylinder?

Do they have single cylinder two stage?

I see that IR compressors come in both twin cylinder then for a little more money "two stage" - but the numbers do not look as good?

I need a compressor education

[bob hertle]

Hi Tom, it wouldn't be practical/possible to give you a crash course in compressor-ology here, but I'll take a shot at the difference between 2-cyl, and 2 stage.

In a two cylinder single stage, the air is raised from atmospheric to set tank pressure in one shot. Both cylinders in one compression stroke take the air from 14.7psi to lets say 120 psi. The cylinders are exactly the same bore and stroke. The result is heated and potentially wetter air due to the heat of compression and the ability of the warmer air to hold more water vapor. Works fine for home shop and even the corner gas station!

A two-stage compressor is also at least 2 cylinders, but they are different sizes. (bores are different, strokes the same since they run off the same crank). The first stage has a large piston which raises the pressure from 14.7 psi to some intermediate pressure say 60 psi. This air is sent thru a heat exchanger (called the intercooler) where a good portion of the heat of compression is removed, then it "supercharges" the small cylinder in which it is raised to final pressure of say 120 psi. The air entering the tank will be cooler, and drier than the single stage can do. And, due to a trick of thermodynamics, less horsepower is required to produce the same volume of air. I.E. the 2-stage is more efficient. The 2-stage is usually capable of higher pressures as well. Typically they can run 175psi all day. Nearly all recip compressors used in industry are 2-stage.

Hope I didn't bore the heck out of ya!

Bob