Difference between 1 and 2 stage compressors producing same PSI?

[metlmunchr]

A standard cubic foot of air is one cubic foot at 68°F and 14.7 psia, or 0 psig.

A cubic foot of air at 90 psig and 68°F contains roughly 7 cubic feet of standard air. Only 6 cu ft is deliverable as pressurized air, while 1 cu ft would remain in the tank at 0 psig.

One deceptive claim you still see from time to time, usually on cheap compressors, is advertising piston displacement rather than air delivery, because displacement is a larger number.

A lot of what I've seen recently is advertising max psi and tank gallons, neither of which have any bearing on the actual ability to deliver compressed air. In fact, many posters here on GJ seem to refer to compressors by tank capacity alone. i.e. I wanna buy a 60 gallon compressor. Gives no more useful information than saying I wanna buy a truck with a 60 gallon tank. Would that be a F150 with an auxiliary tank, or are you shopping for a tri-axle Peterbilt?

In general, the faster the compressor runs, the greater the pumping losses. That explains why a pump driven by a 5hp motor at 1400 rpm may deliver 15cfm while another pump driven at 900 rpm by the same 5hp motor will deliver 18 CFM.

 

[pcmeiners]

I think it's also about volumetric efficiency. if you have a single piston compressing to 175 PSI, then the air in the cylinder at TDC can never be less than 175 PSI. on the downstroke that'll fill the piston and you'll be able to **** in less fresh air. the higher the pressure, the more air "remains" in the cylinder.

With a two-stage you can do less compression on the first stage, which improves first stage efficiency.

You left out the importance of relative humidly of the air intake, and the operating temperature. Since water expand 1700 times with heat, the higher the compressor output temperature the more the moisture and air expands. So a single stage compressor expand the moisture and air more than a lower temperature 2 stage compressor, as single stage machines run at a higher temperature by it's nature. The difference between single stage output temperature air and moisture expansion and a two stage compressor's air/moisture expansion is all wasted energy ( as to pneumatic needs), as the air cools in the tank, air lines and from cooling as the air is used going from a higher pressure to lower pressure within pneumatic engines and tools.

 

[u2slow]

For cfm, aren't we letting the tank volume and pressure do the work?

I mean, there's no way I can sustain my gun on pump output alone. Just ain't happening. My single-stage is rated to cut out at 135psi. It actually presses up to 140psi. Since I know my exact motor/comp setup is often sold with a bigger tank, I'm just going to double up on 20gal tanks.

 

[PoorUB]

For cfm, aren't we letting the tank volume and pressure do the work?

I mean, there's no way I can sustain my gun on pump output alone. Just ain't happening. My single-stage is rated to cut out at 135psi. It actually presses up to 140psi. Since I know my exact motor/comp setup is often sold with a bigger tank, I'm just going to double up on 20gal tanks.

I had a 60 gallon single stage that while running a DA sander it would finally get down to around 60 PSI and maintain. Luckily the sander would function adequately. I replaced it with a real 5 HP 80 gallon. It will cycle on and off while sanding, so in that case the pump is really doing the work.

You can put a 1/4 HP compressor on a 1,000 gallon air tank and have all the air you might need for a day or two, but then it takes a week to refill so if you need more air tomorrow the large tank does you no good.

For years I ran a three man shop from a 3/4 HP compressor. We used impact wrenches so it was a quick blast from time to time. If that is all you need then a small compressor with a larger tank will work for you, but long run times with a larger CFM tool and you will need some horse power.

 

[2ndGearRubber]

For cfm, aren't we letting the tank volume and pressure do the work?

I mean, there's no way I can sustain my gun on pump output alone. Just ain't happening. My single-stage is rated to cut out at 135psi. It actually presses up to 140psi. Since I know my exact motor/comp setup is often sold with a bigger tank, I'm just going to double up on 20gal tanks.

CFM is output of volume. You can have the biggest tank and motor in the world, and a 1/8 diameter output line ain't moving CFM.

Ideally, your fittings and piping are not a limiting factor, despite the fact they often are. The impact drops pressure across the air motor, using CFM, and the hope is the volume of the tank plus the motor refill rate can outlast your demand.

Theres a reason lots of shops run 120-160 static. 90psi dynamic is a taller order than people think. Unless the tool needs a limited pressure input, wide open, no regulation is the way to go IMO. You can always throttle CFM back. Hell add an extra coupler between you and the compressor and that's going to drop pressure. Once you're 100ft from the compressor, through 60yo cast iron pipe, spliced into a gas line, jumped into a reel with a coupler, the 50ft of hose, then another coupler, you need big numbers static to fight the pressure drop.