Don, what you say is true, but it is not so simple, and there is more to it than just CFM.
You have to take into consideration duty cycle, heat, wear and tear, and also air line pressure requirements for tools.
Consider a typical 135psi single stage compressor. With a 25' 3/8' hose, 3/8' or High flow couplers, 135 psi will get you about 90 psi working pressure At high consumption tools. By that I mean 1/2' impacts, drills, sanders, die grinders, etc. But, keep in mind that this is at the compressors kick off pressure. By the time the compressor kicks on, you Have dropped to 105psi. That's going to result in a pretty dramatic loss in pressure at the tool, and be very noticeably down on power. Now, if you are running an HVLP gun, who cares, because most run 20-50psi, but for most common tools, you are going to deal the drop. It's even worse with a 50' hose. It was an incredible pain in the butt to run my 1.3hp 4-1/2' angle grinder consistently off a typical 135psi single stage compressor. Same with air drills using hole saws. Drill half way through and run out of power. Been there, tried that, didn't work out. When trying to get a job done, it's incredibly frustrating to deal with the large fluctuations in power as pressure drops.
With a two stage 175 psi compressor, the kick on pressure is typically 135-145 psi. With this set-up, I can set a regulator to 120-130psi, and run air tools with no fluctuations in power, which is huge when hogging away material with a grinder, or using hole saws on a drill. As long as tool usage is at or below compressor output, the two stage will give you enough head room to maintain working pressure at the tool.
There is a durability factor as well. Compressing air requires considerable force and also results in a lot of heat. Single stage compressors do this compression all in one shot. If the kick off pressure is 135psi, the pressure in the cylinder is at least 135psi as the piston nears tdc. With a two stage, each stage shares the load. The first stage is usually around 70-120 psi, and the final small high pressure cylinder piston takes it to 175. This results in less heat in each cylinder because neither one of them is pumping the air up as much as a typical single stage compressor. Then there is the load side of it as well. For a similar swept displacement, the single stage will see higher forces on the piston skirts, piston pins, rods, rod bearings, etc... as a result of the higher internal pressure. You may say the two stage has higher pressure on the second stage. Yes, but remember the piston is smaller and has far less surface area, so the force on downstream parts is still lower. Less pressure on the bearings, etc results in longer wearing pumps given similar materials and tolerances. add all this up, and it is why most single stage compressors have lower duty cycles than two stage compressors. There are good and bad examples of both, but the average is far easier to deal with heat and wear in two stage designed than in single stage.
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IMHO correct sizing is everything with a screw - any larger industrial compressor, they don't like stop and start. They especially can not tolerate only intermittent run. Its a fundamental nature of the way they lubricate them selves. Equally they have a fair bit that kinda goes wrong. If you only genrally need a little air 4 days a week then a lots one day, you need a system with 2 separate compressors. Trying to do it with just a large one will cost you more in parts and electric down the road unless you put in large air storage.
Me i like hydrovanes, sure there not as efficient as a good screw, but so long as you keep on top of the oil changes and they run long enough cycles to get upto operating temp to keep the oil dry they just go forever. Like screws a key part of that means that if your only using it intermittently, you need it inside the heated space, not outside in a typical compressor enclosure so it can stay at correct running temp. Soon as you start building moisture up in the oil your heading for problems with any compressor.
As to screws taking out shaft seals in less than a hours, sounds like a major issue either alignment or a design failure or a back pressure issue.
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