Air Compressor Question for the Guru's...

[Jeff Duncan]

I have a question in regards to how motor hp relates to cfm's generated by any given compressor. In general bigger compressors in hp provide more cfm output. But does the hp directly effect the cfm's generated?
Or maybe a better way to ask is this, I run a 7 hp (yup, way inflated #, actually 15 amps @ 220v) on a 60 gal tank. If I were to replace the motor and upgrade to a 7-1/2 hp 3 phase motor, would it actually increase the cfm's??? My guess is it wouldn't, I would think that the pump and cylinder design will still produce the same cfm, but may just make life easier on the bigger motor. But I'm curious enough to see if I'm correct or if the motor could increase the output?
thanks,
JeffD

[Bob Feeser]

Think of it this way. If you were in a car in 4th gear going up a hill at about 25 mph, would an 8 cylinder engine give you more torque than a 4 cylinder? Yes but to overcome the radical difference between 1st gear with such a little bit of resistance, and 4th gear with a radical amount of resistance would take a lot of horsepower. The measurement of a compressors ability is how many SCFM does it put out at how many pounds of resistance. So when you get to the top of the compression cycle when the tank is getting over 100 PSI and the pump is still running, it is shoving against 100 PSI worth of pressure. That is where the additional horsepower comes in. At 5 pounds of pressure in the tank, it doesn't take as much horsepower to give you some air. When you get into a 2 stage compressor, such as a 5 horsepower with an 80 gallon tank that runs 175 PSI, you are dealing with a lot of resistance at the upper end of the compression cycle. I had 2 compressors like that running on 3 phase with a large shop. We hooked the piping together and always had more air than we needed, even with 5 to 10 guys working at once.
The motors that ran those compressors weighed about 100 pound apiece. They are not the same thing as a Home Depot compressor rated at 6 hp.
I guess I am rambling here. I currently have a Campbell Hausfield, that I bought at Home Depot, and kept a 2 stage pump head, and acquired a 5 hp single phase, that I have yet to hook up. I just keep using the 6 hp CH with a 60 gallon tank and it seems to be all that I need. It was about a 400 dollar unit.
How does this relate to your question? Putting a larger motor that runs at lets say 1725 RPM would push the same at the lower PSI range, but when it got to higher PSI ranges of resistance, it would produce more SCFM. BUT real pressure gains are made when the entire design of a compressor is balanced. With a large motor mated up to a larger compressor head, would be better able to take advantage of the additional power. Then you have the max pressure capacity of the tank to take into consideration. With a higher PSI range, you have the advantage of more capacity. It is hard to keep up with a blow gun for example, or a high output spray gun. When you are dealing with limited capacity, you invariably have to build up more air than you need, and then as you need more air than the compressor can provide, you eat that up. Finally you wind up with a lower pressure than you need, whereas the compressor can't keep up. If you are spraying some finish, that results in less than stellar results.
My advice is to get a bigger compressor, or get a second compressor that runs in the same PSI range (120 max pressure for example) and hook the outgoing pipes together. Make sure the tanks and pressure relief valve are rated at the same PSI.

[Tom Veatch]

I have a question in regards to how motor hp relates to cfm's generated by any given compressor....I run a 7 hp on a 60 gal tank. If I were to replace the motor and upgrade to a 7-1/2 hp 3 phase motor, would it actually increase the cfm's???...

The cubic feet per minute output is a direct fuction of the pump displacement and the pump RPM. If the current motor runs at or near it's rated RPM, you can put as big a motor on it as you want, and unless you increase the pump's displacement or RPM you're not going to get a single CFM more out of the compressor than you did with the smaller motor. The only effect is that the big motor will loaf along under a light load.

To increase the CFM, get a bigger or faster turning pump and use the bigger motor to drive that bigger, faster pump.

One thing you could do. If its a 1725 RPM motor, you could replace it with a 3450 RPM motor. That would give you more CFM. At least for however long the pump lasts ;).

[Jeff Duncan]

Thanks guys, that's pretty much what I thought. Which begs the question...why do compressor manufacturers go so wild in their hp ratings? If my compressor puts out 11.8 scfm @ 40 psi, what difference does it make whether it's a 3 hp or a 7 hp motor driving it? As long as it's providing the air volume they claim, the hp of the motor seems irrelevant?

As far as my compressor goes it provides more than enough air for my current needs. I am thinking of changing my spraying from a turbine to a compressed air system which may require me to seek more air depending on which gun I choose. But this was really more of a curiosity question as I wouldn't bother fooling around with swapping motors anyway.
JeffD

[Lee Schierer]

...why do compressor manufacturers go so wild in their hp ratings?

It's the Tim The Toolman effect more power is better and mine is beigger than yours mentality! The sales people know this and cater to it. That's why the Hp rating of motors doesn't always reflect what the motor actually delivers. Did they figure the Hp with the starting current, full load current or something else?

Why do I now need 15 Hp to cut the same lawn with a 42" deck that I cut with 10 Hp for 20 years? It isn't cut any better or faster, all I do is use more gas.

[Jason Beam]

That's because people think HP is the end-all be-all measure of a tool. And marketers figured that out a long time ago. Which is why you see 6hp shop vacs.

[Greg Cole]

"ILS" HP ratings these days... IF LIGHTNING STRIKES a shop vac might be 6HP......;)

Greg