CFM and mass flow rate for compressed air 2

[IntrepidLearner]

I apologize if this seems a bit basic to some of you. My 1st question is this:

Say, for example, I have a piece of equipment whose pneumatic valves are consuming 10cfm of compressed air at 100psi. Now, if the pressure is then increased to 120psi, will the actual flow rate increase proportionately with the pressure? That is, will the new flow rate be:

10 * (120/100) = 12cfm ?

(Assume that the temperature does not change.)

Second question:

Does the mass flow rate remain constant?

 

[dcasto]

I do not understand how a valve consums 10CFM continiously? A controller tied to a valve my use 1 cfm, but its 1 cfm at standard conditions, or 10 CFM at standard conditions. So because the load is measured at standard conditions, raising the pressure will not increase the volume required.

 

[katmar]

In order to operate a valve you need to inject (to a first approximation) a fixed quantity of air at a fixed pressure to get the valve to move against its spring or other load. If you increase the pressure of the supply air you will be able to get that amount of air into the valve more quickly and the valve will switch position more quickly.

For an on-off valve if, once the valve has reached its new limiting position, you continue to supply air then the pressure in the cylinder will continue to rise and yes you will use more air in the ratio you gave. On the other hand, if you have a positioner that closes off the air once the valve is in position then you will not use more air.

For a modulating valve, the positioner will take only as much air as it needs to get the diaphragm or piston to the correct position. In this case the air consumption (to a first approximation) will be independent of its supply pressure.

In all valves there are small air leaks and these will increase with pressure. But they should be a minor effect and that is why I said above "to a first approximation".

Katmar Software
Engineering & Risk Analysis Software

http://katmarsoftware.com

 

[IntrepidLearner]

Based on what katmar says above, if I have either:

(1) a modulating valve, or
(2) an on/off valve which shuts off air when in position

the volumetic air consumption is independent of pressure. Therefore, if a manufacturer quotes a piece of equipment to consume X scfm, then I know that at any pressure, the air consumption will still be X cfm (assuming that temp & humidity are the same as standard).

How will the mass consumption be affected? I assume that the air consumption by mass will increase with increasing pressure, correct? So then, if I'm interested in the air consumption of my entire factory, wouldn't the mass consumption be the better means for comparison?

 

[quark]

Still there is some confusion in your understanding. If you just get to the basics, you should have enough force to act upon a specific area to turn a valve or to push it or to do any other equal act. So if your actuator volume is y cf, the pressure required to actuate the valve is z psia and if you operate the valve once in a minute then your consumption will be yx1 acfm (i.e y cfm at z psi pressure).

For convenience sake and to have uniformity of data about the consumption of compressed air by various devices at various pressures, we calculate the consumption in terms of scfm (standard cfm) which in all ways gives you as much flexibility as the mass flow data gives you.

In the above example, the scfm will be yz/14.696.

In your OP, if you raise the pressure to 120 psia from 100 psia then the acfm required is 10*100/120 = 8.33.

the volumetic air consumption is independent of pressure. Therefore, if a manufacturer quotes a piece of equipment to consume X scfm, then I know that at any pressure, the air consumption will still be X cfm (assuming that temp & humidity are the same as standard).

This is not correct. You should get the manufacturer to specify at what pressure the flowrate was defined.

How will the mass consumption be affected? I assume that the air consumption by mass will increase with increasing pressure, correct?

There is no easy rule. The variables are downstream pressure, flow condition and type of equipment etc.

 

[Ashereng]

In almost all I/P, and many positioners, there is a constand bleed of instrument air or instrument gas. This constant bleed is still there even when the valve is not moving (being stroked).

Is this what you are referring to?

"Do not worry about your problems with mathematics, I assure you mine are far greater."
Albert Einstein
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[IntrepidLearner]

quark:

In my mind, there appears to be a contradiction with what you proposed.

I assume that in your 1st paragraph, you mean that air consumption will be yz acfm and not yx acfm. (If not, where did x come from?)

Now suppose (from your example) the pressure to actuate is increased to (z + 10) psia. Air consumption will then be y(z + 10) acfm. Hence, indicating that acfm increases with pressure.

So why, then, would increasing from 100 to 120 psia cause a decrease in the acfm (from 10 to 8.33)?

What is it that I'm still not getting??

 

[quark]

It is neither yx nor yz but simply y acfm (it is actually y multiplied by 1 or y x 1).

By definition ACFM is referred with respect to a certain pressure. SCFM is at 14.696 psia pressure.

Now if we come to your case, there are two options here. One is to provide enough power to actuate the valve. I considered it to be at 100 psia and corresponding 10 cfm flowrate. I further considered that you are increasing the air pressure at the compressor and not to enhance the actuation. So, I stuck to 10 ACFM at 100 psia which is about (100*10/120) acfm at 120 psia.

If you want to enhance the actuation then obviously it will be 120*10/100 = 12 acfm.

In any case, you are exhausting the air to atmosphere and should check, once, the duration of valve closure with respect to pressure.

 

[IntrepidLearner]

OK, I think I've got it now...

If I'm increasing air pressure by way of a regulator, I will effectively be enhancing the actuation, and, therefore, increasing my acfm.

But if I increase the air pressure at the compressor in an attempt to maintain the required force of actuation, I will cause a decrease in my acfm.

 

[quark]

Yes, you are right and it is my personal observation that you electical guys beat us mechanicals in logic.

I will just explain further, to avoid any future confusion. When you are filling a chamber, the more the air pressure the more is the volume inside the chamber. Ultimately, it should be exhausted to atmosphere and thus when you increase the pressure of the chamber, you are using more air (provided you are generating air at this pressure).

The mass flowrate of air through a compressor, at constant ambient, geographical conditions and speed of the compressor, remains same irrespective of discharge pressure (so, you can safely say that scfm remains constant). When you compress the air to higher pressures, the volume reduces and thus you get reduced acfm from the generation source. Still, the acfm through the actuator increases till such a point that all user flowrates equal the generated flowrate. Beyond this point, the compressor pressure drops and it can't generate any higher pressure. This is precisely how a back pressure type fluid handling devices function.

Good luck,