Join Date: Dec 2001
Location: Belleville, Illinois
Thanked 22 Times in 17 Posts
Re: Shop Compressor? Any thoughts?
In a two-stage compressor there is a big piston that pumps air into a small, second-stage cylinder, which pumps it into the tank. There are mechanical limitations, I don't know what exactly, that cause a single stage compressor to become very inefficient at higher pressures. Usually they have their pressure switch set at 125 PSI.
Two-stage shop compressors typically are set at 175 PSI, and run well at those pressures. High-pressure pumps, like scuba compressors, have 5 to seven stages, and pump to 3,500 PSI or more.
And as I said earlier, there are two-cylinder, single stage compressors. There are big ones with 5 HP motors and 50 gallon tanks that look pretty impressive, but the fine print will say 125PSI. That may sound OK to run a tool that calls for 90 PSI, but with pressure drop in the dryer, oiler, fittings and hose, you need almost 125 in the tank to have a full 90 at the tool. And once you start using the tool, the tank pressure will drop, and it takse a single-stage pump a long time to get it back to 125.
A two-stage compressor set will cost quite a bit more than a single stage, twin cylinder unit of similar size. I suspect that the reason is that a two-stage unit is likely to live in a commercial setting and run more than it's shut off, so they have to be built better.
I bought a used Ingersoll Rand for about as much as a similar size single stage at Sam's, and am glad I did.