Converting from standard to actual 2

[NPC5]

Please if someone would help with a sanity check. Converting from standard cubic feet per minute to actual cubic feet per minute is simply the ratio of densities right? i.e. scfm/standard density = cfm/actual density. I have been checking some process data and my numbers do not match with what I am seeing. Please and thank you for any replies.

 

[katmar]

The unchanged parameter is of course the mass flow. The definition of density is mass per volume, so to get mass from density and volume you must multiply and not divide i.e.
scfm x standard density = acfm x actual density

Katmar Software
Engineering & Risk Analysis Software

http://katmarsoftware.com

 

[zdas04]

Katmar,
I've been struggling with that for a couple of years. It never makes sense to me that ACF=SCF*stdDensity/actualDensity. Every time I see it (about once a week), I think that the ratio is upside down and go back to my reference material.

Yesterday, I got frustrated and went to mass flow rate and then to ACF and it became clear (for a minute). I wish you had posted your simple explaination last week.

David

 

[NPC5]

That's right...I always have the same problem as zdas04. Thanks a lot Katmar.

 

[rmw]

I have had to sit back and calculate myself out of a brain fart several times lately regarding ACFM vs. SCFM. I like the symetry of Katmar's formula; S*sd = A*ad. Makes it easy for a pea brain like me to remember.

rmw

 

[katmar]

Simple people give the simplest explanations!

Because I have to convert between acfm and scfm (also Nm3 and mass flow) so often I have included a calculator for it in my units conversion program, Uconeer. This is a free download (see my signature below).

In Uconeer click on the fan icon to bring up the gas mass to volumetric flow rate converter. To convert from scfm to acfm you have to do a double conversion via mass, ie set the conversion to be from volumetric to mass, set the conditions as standard and enter the flowrate. This will give the flowrate in mass terms. Click the swap button to convert from mass back to volumetric and set the actual conditions and the answer is there for you!

Katmar Software
Engineering & Risk Analysis Software

http://katmarsoftware.com

 

[rmw]

Katmar,

I have had your software loaded for a long time now and use it regularly as in just this morning. In fact, I have it loaded on every computer I use. I do like the mass to volumeric flow, both liquid and gas. Thanks for making it available.

rmw

 

[mbeychok]

Instead of having to calculate the two densities, it might be easier to just use the combined gas law;

V₂/V₁ = (P₁/P₂) (T₂/T₁) (Z₂/Z₁)

If you think ideal gas behavior fits the situation, then just remove the compressiblity factors (Z₂ and Z₁) ... and all you need is the temperature and pressure at your standard conditions and at your actual conditions.

If you think the situation does not warrant assuming ideal gas behavior, then use Z=1 for your standard conditions and determine the the Z needed for the actual conditions. You would have to do that even if you used the densities (as per Katmar) instead of the combined gas law.

Milton Beychok
(Visit me at www.air-dispersion.com)
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