CFM vs SCFM

[AHayden]

I have been running around in circles trying to figure a relation between CFM and SCFM.

I have a perforated bar with 243 holes. The pressure on the inside of the bar to be 2psi. The pressure outside the bar is 1.143psi (there is a web laying on top, which represents the cushion pressure). I have calculated the airflow per hole to be 0.5cfm. I believe that is ACFM (calculated to be 121.5 ACFM), but when I calculate SCFM, its extremely small. I should have about 200-250SCFM in the bar...

Thoughts?

 

[asimpson]

Are those gauge or absolute pressures?

 

[TD2K]

Depending on the equation you are using, it could be cfm or scfm. If the pressure on the inside of the bar is 2 psi (I assume gauge), scfm and acfm should be within 15% of each other unless you have a big temperature correction.

I would go back to the source of the equation and see if they define the flow as scfm or actual cfm. Do they have another formula in say mass units that would allow you to decide whether you have cfm or scfm?

 

[hydtools]

SCFM is ACFM flow corrected to standard pressure and temperature. Standard pressure is 14.7psia. Standard temperature is 69F, I believe. I did not look up the temperature definition. Someone else will provide the correct figure, I am sure.

Ted

 

[hydtools]

Or go here:

http://www.engineeringtoolbox.com/scfm-acfm-icfm-d_1012.html

Ted

 

[dgallup]

Thee is nothing standardized about standard temperature and pressure. Every organization has their own idea about what is standard. It's a mess.

----------------------------------------

The Help for this program was created in Windows Help format, which depends on a feature that isn't included in this version of Windows.

 

[hydtools]

Here is what the CAGI uses:

http://www.cagi.org/working-with-compressed-air/conversion-scfm-acfm.aspx

Ted

 

[zdas04]

dgallup has it exactly right, "standard" is whatever any two people think it is (and no one I know of uses 69F). Some of the notable values I've seen are:
[ul]
[li]ISO uses either 101.325 kPa (14.696 psia) and 0C (32F) or 100 kPa (14.5 psia) and 20C (68F) depending on which ISO document you read ("standard" is not standard within a single organization)[/li]
[li]AGA and ASME use 14.73 psia and 60F for measurement and other values for other things[/li]
[li]EPA uses 14.696 psia and 68F (20C)[/li]
[li]State of Lousiana uses 15.025 psia and 60F[/li]
[li]Finally, I reviewed a contract recently that specified something like 17.5 psia and 100F, not sure why, but once the contract was signed that was the basis for the exchange of money[/li]
[/ul]

Who's "right"? All of them. None of them. The ISO approach of 100 kPa and 20C at least reduces the round-off garbage.

David Simpson, PE
MuleShoe Engineering

"Belief" is the acceptance of an hypotheses in the absence of data.
"Prejudice" is having an opinion not supported by the preponderance of the data.
"Knowledge" is only found through the accumulation and analysis of data.

 

[rakuday]

AHayden,

What are the pressure and temperature you have considered to calculate SCFM? As told by zdas04 & dgallup there is a lot of confusion over what is the standard temp and pr. In your application if there are no big difference between the actual condition and "your defined standard" condition, then there shouldnt be a great difference between ACFM & SCFM.

Also, SCFM and ACFM are predominantly used in the compressor industry to specify the capacity of the compressors and to compare machines of different manufacturers on a common platform.

 

[kingnero]

also regularly used when specifying the volume needed by gas-powered devices (oxy-fuel cutting, free flow of flame arrestors, propane heaters, ...)

 

[zdas04]

Rakuday,
"Predominately" covers a lot of ground. I would say that SCF us predominantly used in custody transfer of gases since there are hundreds of millions of custody transfer meters and barely millions of compressors.

David Simpson, PE
MuleShoe Engineering

"Belief" is the acceptance of an hypotheses in the absence of data.
"Prejudice" is having an opinion not supported by the preponderance of the data.
"Knowledge" is only found through the accumulation and analysis of data.

 

[rakuday]

zdas04,

The point I was trying to make was that these terminologies are used in the larger applications and industries. For areas such as the current problem, there will not be a significant difference between the ACFM and SCFM.

 

[rb1957]

rather than the detail about what pressure is standard, i think the bigger question was asked early on ... gauge pressures ?
if the 2psi is absolute pressure, then the pressure ratio to standard is 2/14.7 (or something like).
if 2psi is gauge, then the pressure ratio is 16.7/14.7

Quando Omni Flunkus Moritati

 

[rmw]

David,

That is because much of the state of Louisiana is below sea level and the pressure is higher down there.

rmw

PS: I couldn't get the smileycon thingy to work, sorry

 

[zdas04]

Yeah, something like that. I could almost buy it (not really), but then New Mexico picked the same number--the whole state is above 3000 ft, and some of it is above 8000 ft. There is no explaining government actions, I'd rather try to explain the actions of a female Labrador Retriever.

David Simpson, PE
MuleShoe Engineering

"Belief" is the acceptance of an hypotheses in the absence of data.
"Prejudice" is having an opinion not supported by the preponderance of the data.
"Knowledge" is only found through the accumulation and analysis of data.