A poster a couple days ago posted a message (the son of the guy who started and run Eaton Compressor Manufacturing) and what he said totally threw out just about everything I thought I knew about single stage compressors vs two stage compressors. I now have a whole new respect for single stage units. I was always under the impression that two stage units were more efficient and pumped more air --- definately not the case.
I can't believe I didn't figure the whole thing out a long time ago.
Most single stage pumps are two pistons and cylinders of the same diameter. Both cylinders pump the same amount of air independently, that goes directly into the air receiver tank. On the other hand a two stage pump has two cylinders but one cylinder is larger than the other. Air is sucked into the first cylinder (the larger one) and the air is compressed, that compressed air is then pumped into the second (smaller cylinder) where it is compressed even more. From the second cylinder the air is then pumped directly into the receiver tank. So logically if you set a single stage unit next to a two stage unit both having basically the same piston displacement and pump rpm the single stage unit is going to pump more air and logically is a more efficient air pump. While it is true that the two stage pumps are designed to pump air to a higher cut off pressure than the single stage units. Most two stage units are designed to cut the pump off at 175 psi while the single stage units are designed to shut off around 125 psi.
I'm now questioning if there is any REAL advantage to a two stage unit compared to a single stage unit of comparable quality and piston displacement. While the two stage unit will pump to a higher pressure which means more air reserve in the receiver tank the single stage pumps more air to make up for the less reserve capacity. It should also be kept in mind that the two stage is going to take considerably longer to pump up to 175 compared to a single stage unit pumping more air to only 125 psi. The single stage will cycle more often but the two stage pump will run quite a bit longer per cycle compared to the single stage unit. So to me the advantages of a two stage is cancelled out in favor of a good quality commerical single stage unit.
NONE of the air tools in a body shop require air of more than 90 psi. So air coming out of an air tank at 175 psi is just going to have to be regulated down to 90 psi anyway. The extra air pressure in a two stage unit is wasted in the type of work we do in an autobody/paint shop.
I think where the single stage units got/and get a bad rap is that most home owner compressors are not real good quality single stage units that have electric motors and pumps running at twice the rpm that a quality made commerical single stage unit would be running at. For those little single stage home owner units doubling the rpm means almost doubling the air output for that inexpensive small unit. What they are doing in reality is sacrificing quality and longevity for an inexpensive unit that to the unitiated appears to be an excellent buy with a good cfm output. The down side is those home owner units will self destruct in a fraction of the time it would take a quality commerical unit to wear out and reach the end of its useful life (or just get rebuilt, quality units warrant a rebuild when needed. Not just throw it away and buy another one).