compressor SCFM ratings

[Gary Hoemann]

I need information on air compressors for spraying finishes. If the compressor is rated at 5.5 SCFM @ 90psi, how does that translate at different pressures.
Does that equate to 11 @45 or 2.75 @ 45 or what?

[Curt Harms]

I need information on air compressors for spraying finishes. If the compressor is rated at 5.5 SCFM @ 90psi, how does that translate at different pressures.
Does that equate to 11 @45 or 2.75 @ 45 or what?

Oftentimes there are 2 numbers, X CFM @ 40 psi, Y CFM at 90 psi. I think the rating at 40 would apply to spraying, 90 psi for tools. Nailers, impact wrenches and such often use around 90 psi air.

HTH

Curt

[Mike Cutler]

I need information on air compressors for spraying finishes. If the compressor is rated at 5.5 SCFM @ 90psi, how does that translate at different pressures.
Does that equate to 11 @45 or 2.75 @ 45 or what?

No. Flow is equal to the square root of the delta pressure across a system or an orifice. It's not a linear equation.
There should be a curve available from the manufacturer that represents the flow output for a given pressure.
At too low of a pressure the curve will drop off due to dynamic losses and the mechanical design of the final orifice restriction device.
Additionally, some compressor designs have a constant pressure, and or flow designed into them that envelopes a wider application envelope. This design would have an impact on the curve.

[Maury Saggus]

Gary,

The flow of a recip compressor will not vary a lot through the range of the output capability. It will make a little more air at the 40psi due to a decrease in loss across seals, rings, etc. Conversely it will make a little less air at 125 psi due to an increase in losses. The use pressure does not affect the cfm rating, only the pressure that the compressor is putting in the tank.
Usage cfm is affected by pressure (more pressure, more use - less pressure, less use) and will cause the compressor to cycle more/less. So set the regulator as low as possible to do the work required, and the compressor pressure switch as low as possible to supply the air, and you will maximize the capability of the system.


Use the 90 psi rating for a good average...

Your spray gun will publish a number which is probably the average, not the max. It is calculated by the max flow * % usage. Typical usage can be 50%, which makes the max capability double the published number!

[David Weaver]

No. Flow is equal to the square root of the delta pressure across a system or an orifice. It's not a linear equation.
There should be a curve available from the manufacturer that represents the flow output for a given pressure.
At too low of a pressure the curve will drop off due to dynamic losses and the mechanical design of the final orifice restriction device.
Additionally, some compressor designs have a constant pressure, and or flow designed into them that envelopes a wider application envelope. This design would have an impact on the curve.

this is slightly off subject, but given that you'll find that lots of compressors are nowhere near close to each other on ratio of scfm at 40 psi vs. 90 psi, if you had a compressor that was designed to be optimal at 90 psi, couldn't you use a regulator to do what the original poster was suggesting? Not that the compressor pump itself could deliver constant amount, but that X scfm at 90 PSI will always be Y scfm at 40 psi if you get to ignore the design of the compressor and use a regulator instead?

I'm asking this because my compressor develops something like 6 or 7 scfm at 40 psi, but 5.0 at 90. If I let it run at 90 or 115, and use an HVLP conversion gun with constantly at 35 input to the gun (after regulator), the compressor still keeps up with the gun. If I ran 40 directly to it, I think it would lay an egg.

[David Epperson]

I need information on air compressors for spraying finishes. If the compressor is rated at 5.5 SCFM @ 90psi, how does that translate at different pressures.
Does that equate to 11 @45 or 2.75 @ 45 or what?

Sounds like yours is of a similar size to David Weavers.

I'm asking this because my compressor develops something like 6 or 7 scfm at 40 psi, but 5.0 at 90.

I would expect similar numbers. On the upper end a compressor may be limited by how much it can push, but at the lower end it's also limited by how much it can suck in. It becomes a vacuum pump with a leak (OK a big leak )

[Maury Saggus]

this is slightly off subject, but given that you'll find that lots of compressors are nowhere near close to each other on ratio of scfm at 40 psi vs. 90 psi, if you had a compressor that was designed to be optimal at 90 psi, couldn't you use a regulator to do what the original poster was suggesting? Not that the compressor pump itself could deliver constant amount, but that X scfm at 90 PSI will always be Y scfm at 40 psi if you get to ignore the design of the compressor and use a regulator instead?

I'm asking this because my compressor develops something like 6 or 7 scfm at 40 psi, but 5.0 at 90. If I let it run at 90 or 115, and use an HVLP conversion gun with constantly at 35 input to the gun (after regulator), the compressor still keeps up with the gun. If I ran 40 directly to it, I think it would lay an egg.

David,

The different rating for pressure on a compressor have nothing to do with the pressure at which the air is used. By that I mean it is strictly limited to how fast the compressor can pump air into a tank. The ratings are for those specific pressures of production of air, not use. When you start from 0 psi in the tank, the compressor is most efficient because it is pumping against very little back pressure. As the pressure increases, back pressure causes more losses and more current draw for the compressor (more HP req'd). Most people will leave the pressure setpoints alone on a compressor and only adjust the regulator. In that case, the CFM rating for the compressor is based on the pressure switch (most will be 90-120psi).

Use rate of air is greatly affected by regulated pressure. Overpressuring an application increases use of air and this overuse is known as artificial demand. The use rate of a known psi can be readjusted for a different pressure by the following formula:
(New psi + ATM)/(known psi + ATM) * (known cfm) = new cfm
ATM is atmospheric pressure (14.7 psia at sea level, 14.5 here in GA)

Example:
You have a sprayer which requires 8 cfm @ 40 psi and you adjust the regulator to give 90 psi instead. The gun will now use:

(90psi + 14.5psi)/(40psi + 14.5psi) * 8cfm = 15.3 cfm

You also will lower the cfm by lowering the pressure, but the gun may not work correctly.

Hope this helps!

[David Weaver]

Right, but I'm running the regulator in the opposite direction - input into the regulator is 115 psi, say, and coming out is 40 psi.

And right on the equation - it's the old PV=nRT (pervnert I think we called it), right?

I guess what I'm not grasping is what the pump on the compressor actually provides. If it's advertised at 5.0CFM at 90 psi, does that mean it will produce 5 cubic feet of air existing at 90 psi, or does it mean that it will take in five cubic feet at some prescribed pressure (atmospheric) and push it into the tank when the tank is pressurized at 90 cfm?

[Maury Saggus]

Right, but I'm running the regulator in the opposite direction - input into the regulator is 115 psi, say, and coming out is 40 psi.

And right on the equation - it's the old PV=nRT (pervnert I think we called it), right?

I guess what I'm not grasping is what the pump on the compressor actually provides. If it's advertised at 5.0CFM at 90 psi, does that mean it will produce 5 cubic feet of air existing at 90 psi, or does it mean that it will take in five cubic feet at some prescribed pressure (atmospheric) and push it into the tank when the tank is pressurized at 90 cfm?

David,
If you are running the regulator at 40 psi, it only affects the use rate of the gun. If the gun is rated at 8 cfm @ 40psi, then the use rate is 8cfm. If the gun is rated at 8cfm @115psi, and you set the regulator to 40 psi, then the actual use rate is:
(40 +14.5)/(90+14.5) * 8 = 4.2 cfm

As far as the compressor is concerned, all compressors are rated as SCFM which is calculated as output of air based on inlet conditions (sounds crazy).
Your 5 cfm is intake air (atmospheric), so if it is compressed to 100psi the actual volume is (14.5/100) * 5 = 0.725 cf or 5.42 gallons of tank space.

Also, the regulated pressure may not be the actual use pressure. Loss through the regulator, hose, and fittings may reduce the pressure way below the regulator setpoint. And the gauge on the regulator will not reflect the pressure drop!

[David Weaver]

Thanks Marty, that clears it up. The gun uses 10 cfm at 40 psi, so that's probably roughly what it's getting after the regulator.

There is definitely leakage along the way between the compressor and the gun, no doubt about it. The regulator's not too bad for loss, but it's not airtight.

[David Epperson]

Right, but I'm running the regulator in the opposite direction - input into the regulator is 115 psi, say, and coming out is 40 psi.

And right on the equation - it's the old PV=nRT (pervnert I think we called it), right?

I guess what I'm not grasping is what the pump on the compressor actually provides. If it's advertised at 5.0CFM at 90 psi, does that mean it will produce 5 cubic feet of air existing at 90 psi, or does it mean that it will take in five cubic feet at some prescribed pressure (atmospheric) and push it into the tank when the tank is pressurized at 90 cfm?

SCFM stands for Standard Cubic Feet per Minute - measured at standard pressure and temp - 14.7 psi not at 90 psi.

[Maury Saggus]

Thanks Marty, that clears it up. The gun uses 10 cfm at 40 psi, so that's probably roughly what it's getting after the regulator.

There is definitely leakage along the way between the compressor and the gun, no doubt about it. The regulator's not too bad for loss, but it's not airtight.

David,

The loss I am referring to here is pressure loss, not air leakage. The flow of air through the restrictions(regulator, hose fittings, etc) will cause a pressure loss through them. Think of pressure as voltage, and cfm as current. The hose, regulator, and fitting are resistors, and will result in the pressure(voltage) being lower into the gun than at the regulator.

Leaks will also reduce the pressure at the gun, but I was referring to resistive pressure loss only.