cfm compared to psi chart?

[logical]

Pressure and Volume are indirectly proportional. As Pressure increases, Volume decreases and vise versa. If a compressor states ACFM(Actual CFM) 10.6 at 90 and 12.4 at 40. It's real simple. It means it will deliver 12.4 ACFM at 40 psi continuous and 10.6 ACFM at 90 psi continuous. If you have any doubt whatsoever, put a flow meter on the line and measure it. These raw equasions being put forth here are lacking as they do not account for your local Atmospheric pressure and humidity factors to start with. Those equasions are designed for laboratory use. Some of you guys have garages so clean they could be labs, but that's another story.

Kinda sorta half right. Pressure and volume are indirectly proportional but that is a relationship that assumes a constand mass of air that you are increasing and decreasing the pressure by changing the size of the container it is in (volume). Once you are talking about cfm you are talking about a machine's ability to supply a quantity of air at a given pressure. Think of "cfm at 90 psi" as "how much air can I consume per minute at 90 psi and not ever run out of supply as long as the motor is running".

I can't believe (well, yes I can actually) how many people know all these formulas but have no clue when they apply or how to apply them.

 

[Bob C]

Kinda sorta half right. Pressure and volume are indirectly proportional but that is a relationship that assumes a constand mass of air that you are increasing and decreasing the pressure by changing the size of the container it is in (volume). Once you are talking about cfm you are talking about a machine's ability to supply a quantity of air at a given pressure. Think of "cfm at 90 psi" as "how much air can I consume per minute at 90 psi and not ever run out of supply as long as the motor is running".

I can't believe (well, yes I can actually) how many people know all these formulas but have no clue when they apply or how to apply them.

I thought I said that.

 

[DekeT]

Pressure and Volume are indirectly proportional.

I think you meant to say inversely proportional.

 

[Bob C]

I think you meant to say inversely proportional.

Oh my! I sure did! Thanks for the correction.

 

[Mister Moose]

I have a degree in Physics, and I don't even want to tackle this empirically.

I doubt there is an easy equation, likely the correct version would leave you frustrated anyway in it's accounting for real world factors and the variable nature of the problem at hand, ie it might not be linear.

It seems to me that the compressor is able to deliver less air on each stroke as the pressure in the tank increases. In other words, if the pump displaces 1L, at start up the compressor pumps 1L air at (atmospheric pressure) and places it in the tank. On the second stroke, the tank is at some increased pressure, and less then a full liter of air is able to be pressed into the tank. The cylinder retains more air due to the increased back pressure.

Consequently, there is a real reduction of flow as pressure in the tank increases, as the spec states.

I would think you would need to know and include a dozen variables such as pump displacement, tank size, cycles per second of the pump, rate of cooling of the tank from the heat of compression, etc, etc.

In the real world what matters is how long you have to wait for the compressor to recover if it can't keep up. Many tools (Like a DA sander) use a variable amount of air anyway as speed is throttled up and down. Field experience would trump theoretical predictions, in my view.

 

[RPH]

Mister Moose hit the nail head. It's what you get in the real world is all that matters.

 

[cheechi]

The reason that many compressors provide you the SCFM @ 40 & @ 90 PSI is so that you can interpolate the information you need. Granted, your compressor tells the flow rate at such a high pressure as a selling feature but if they provided you one other set of values you can use linear interpolation to estimate the flow at any pressure you want.

This is a system with several components. The CFM coming from the compressor, the pressure of the tank as air is used as a flow, the pressure of the tank being replenished by the compressor as another flow, and a set of related rates. the % of water condensing out of your air has to be as a ratio of either volume or time.

Regardless of the measurements you can take, the constants you use, the assumptions you make, you solve this with differential equations. Algebra isn't enough here.

 

[fomocoforrester]

I think this whole discussion is being protracted more than neccessary by a certain lack of appreciation of the diferences between .....

ACFM - Actual Cubic Feet pr Minute
SCFM - Standard Cubic Feet per Minute
CFM FAD - Cubic Feet per Minute - Free Air Delivered
ICFM - Inlet Cubic Feet per Minute.

There's a nice little pdf here that explains it quite well, particularly the importance of geographical location....

http://www.ohiomedical.com/pdfs/11-icfm.pdf

I was suprised to see that on a normal day in Denver you would need a compressor 30% bigger than you would need in Los Angeles and on a hot humid day you would need one 40% bigger to do the same job....

 

[cheechi]

Why surprised? air there has less density at higher altitudes, meaning there is less mass of air. So basically, there is 'less air in the air' meaning your compressor needs more time to fill the same amount of mass at higher elevations.

The real issue isn't just bigger, but it will wear out faster since it must cycle on more frequently & run longer than compressors at lower elevations.

 

[DekeT]

To address the original question, empirical observations are what is needed to chart a general formula for cfm delivered at various psi. From what I have seen of many compressor ratings, use about 1.5 to 2 cfm higher for each halving of psi for any typical model. Despite all the other interesting theoretical ramblings in this thread this is my take on the original question.

This thread did compel me to peruse a number of old college textbooks on thermodynamics and mechanical engineering. I found them as dusty as my brain.

 

[gt1040b]

the defining formula is related to the physics of gases, PV=nRT.

P = pressure, V = Volume, and the physics of the compressor doesn't change, the actual quantity of air and the gas constant (R) are equal, and the Temperature (T) is approximately the same.

so you can relate P1 (psi) *V1 (cu.ft.) = P2 (psi)*V2 (cu.ft.)

that is why the 1/2 estimation was a reasonable approximation from 175 psi to 90 psi, whoever told you that isn't misleading you.

this is only true if you are actually regulating the pressure of the system down, or have a regulator at your tool though. if your tool can accept 90-120 psi, for instance, it will be operating at the higher end of the range if you are feeding it the higher pressure, and thus you'll be consuming more air.