SCFM QUESTION

[USAeng]

If someone says they want 68SCFM airflow @ 90psi what they really want is 68cfm at 90psi right? scfm would be 68cfm at 14.7psi correct? thanks

 

[IRstuff]

Isn't there also a temperature associated with SCFM?

TTFN

FAQ731-376
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[USAeng]

right... maybe they just put it in terms that the shop guys would understand or maybe they just didnt know the difference... temp is 68F from Wiki

I did some internet search and I think they should have stated it like 68CFM @ 90psig so they should have left out the standard part

 

[BigInch]

Yes, so it would usually be at 60[°]F, the standard-standard reference temperature, although it is not unknown to have a different standard reference temperature.

One might assume they mean 68 SCFM (14.73 psiA & 60[°]F) compressed to 90 psiG and 60[°]F, although that 60F is a guess.

Note the difference between psiA and psiG
If it was psiG and the temperature was different, say 75[°]F, then you would convert to actual CFM at 90 psig & 75[°]F conditions by,
68 * 14.73/(14.73+90) * (459+75)/(459+60) = 9.84 ACFM
A == Actual


Only put off until tomorrow what you are willing to die having left undone. - Pablo Picasso

 

[Duwe6]

This is a typical requirement for PRV's. Flow capacity is measured in scfm, at a specified pressure forcing the flow through an orifice..

68 scfm at 90 psig is a mighty small orifice.

 

[katmar]

Do not think of SCFM as a volumetric flow. Because it is associated with a specific temperature and pressure it should rather be thought of as a mass flow rate. This way you will not confuse it with the actual CFM (written ACFM) flowing at 90 psi, which will be a lot less than 68.

Katmar Software - Engineering & Risk Analysis Software

http://katmarsoftware.com

"An undefined problem has an infinite number of solutions"

 

[USAeng]

So I have to figure out what they meant... I see what you are saying... 68scfm if the standard is 90psi is a lot different than 68scfm if the standard is 14.7psi... I am thinking maybe they meant supply 68scfm and compress to 90psig which will give less flow as mentioned... I will have to ask more people to try and figure out what they meant

 

[IRstuff]

Well, I've generally found that asking anyone but the "someone" that wants "68SCFM airflow @ 90psi" is usually unproductive. No one you ask can possibly delve into the mind of that "someone."

If it were me, I hunt down that "someone" and ask, "What the heck did you really mean by this?

TTFN

FAQ731-376
Chinese prisoner wins Nobel Peace Prize

 

[USAeng]

yeah I meant the original someone hehe...

 

[USAeng]

at least I hope I can find the original people that made the statement... might not be possible though... Ill find out tomorrow... thanks

 

[zdas04]

Keatmar,
SCF is NOT mass flow, it is an excellent surrogate for mass flow, but it is still a volume (even though it is at an imaginary temperature and pressure).

60 SCFM at 90 psig (and some unspecified temperature) only has meaning if "standard" is defined. If the application is not custody transfer (where the standard condutions MUST be specified in a contract) or regulated (where the standard conditions MUST be defined by the regulator) then use the standards common in your industry. In Oil & Gas, we use 14.73 psia at 60F. In most industrial applications people uses 14.696 psia at 60F. The EPA uses 14.7 at 60F (actually they say 20C because they are truly un-American).

So if you work in Oil & Gas and your standard is 14.73 psia at 60F and your atmospheric pressure (which is a function of local elevation) is 14.5 psia (sea level) then 68 SCFm at 60F and 90 psig is 68*(14.73)/(90+14.5) or 9.59 ACFm because you can treat air at 90 psig and 60F as an ideal fluid (compressibility is assumed to always be around 1.0), and your delivery temp is the same as standard (60F=520R).

For most equations that are used to estimate compression hp, you need ACF/time. To calculate velocity you need ACF/time. To aggregate fluid flow at different pressures and temperatures you either need mass flow rate of SCF/time.

David

 

[katmar]

David, I accept that any unit that has "cubic foot" in it has to be a volume, but let's just say SCFM behaves more like a mass than a volume. If you know the molecular weight of the gas and you have agreed on a standard then SCFM tells you 100% how much mass you have - which is of course why it is used for custody transfer. But you need significantly more information (viz. pressure, temperature and compressibility factor) before you can convert it to a meaningful volume.

Also, in any flow situation (which is where we would be using SCFM) the flow in mass terms or in SCFM terms remains constant along the pipe but the volumetric flow rate is constantly changing as the pressure and temperature change along the pipe.

So I think for someone who is being exposed to SCFM terminology for the first time, it is much more helpful for them to think of it in terms of mass than volume. I suppose that is exactly what you were saying by calling it a surrogate for mass flow. Is there an English word for something that is more than a surrogate, but just less than the real thing? Something like an "acting" president. That would be the word we need to describe SCFM.

Katmar Software - Engineering & Risk Analysis Software

http://katmarsoftware.com

"An undefined problem has an infinite number of solutions"

 

[BigInch]

ACFM changes, but SCFM doesn't change along the pipe length, hence no problem. Actually I think I used MMSCFD for a couple of years before I knew anybody ever worried about mass flow.

And tools don't ask for lbs/minute, because that doesn't give any indication of capacity or power. Mass flow isn't the best unit to use in all circumstances.



Only put off until tomorrow what you are willing to die having left undone. - Pablo Picasso

 

[katmar]

BigInch, the default units vary from industry to industry. It is probably very rare in the petroleum gas industry to ever talk in mass flow units. But nobody would ever rate a steam line in SCFM - it would always be in mass units. And then the brewers and distillers rate their liquid lines in hectoliters per day. Even in a "hard" science like engineering you can't always bank on people being logical - consistency is the best we can hope for.

Katmar Software - Engineering & Risk Analysis Software

http://katmarsoftware.com

"An undefined problem has an infinite number of solutions"

 

[BigInch]

That's exactly what I'm saying, in this example machine tools don't care about mass flow, or SCFM alone either, they always ask for flow and pressure; never pounds/min. I'm not sure this is even important now. The answer to the OP question in both SCFM and ACFM has been posted since yesterday. Nobody was ever interested in mass flow here.

Only put off until tomorrow what you are willing to die having left undone. - Pablo Picasso

 

[zdas04]

Katmar,
I think that "surrogate" is the right word. As in "A surrogate stands in for the principal". That is what flow rate at imaginary (standard) conditions does. I has the important characteristics of not changing down the pipeline like mass flow rate (continuity) and being able to be aggregated like mass flow.

In fact, I wish that standard conditions had never been though of and that we would have had to build our industry-specific language on mass flow rate. I would spend a lot less time saying things like "you can't use SCF to calculate velocity or Reynolds Number" and "just because your air compressor suction is about atmospheric pressure, it is not SCF and the suction flow rate is different than the discharge flow rate--continuity does not apply". But someone did think of it and we're stuck with it. I think the only way to be successful with it is precision in language.

For my recent course in Johannesburg I sent out a pre-course questionaire with the question:

Standard (Normal) Temperature and Pressure is:
a. 14.73 psia (101.56 kPaa) and 60F (15.6C)
b. 14.69 psia (101.30 kPaa), 32°F (0C)
c. 14.69 psia (101.30 kPaa), 68°F (20.0C)
d. Whatever two people agree it is

20 people have taken the test and not a single person has figures out that the right answer is:

d. Whatever two people agree it is

David

 

[BigInch]

d

Only put off until tomorrow what you are willing to die having left undone. - Pablo Picasso

 

[zdas04]

That's one.

 

[Compositepro]

The use of scfm is "analogous to" mass flow rate.

 

[MiketheEngineer]

I have a simple box fan that I an sure pushed 68 cfm - but at no real change in pressure. Need to know what pressure they want it at.

My Sears cheap air compressor will deliver about 50 cfm at 120 psi...