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SCFM QUESTION

SCFM QUESTION

SCFM QUESTION

(OP)
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

RE: SCFM QUESTION

(OP)
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

RE: SCFM QUESTION

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

RE: SCFM QUESTION

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.

RE: SCFM QUESTION

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"

RE: SCFM QUESTION

(OP)
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

RE: SCFM QUESTION

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

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RE: SCFM QUESTION

(OP)
yeah I meant the original someone hehe...  

RE: SCFM QUESTION

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

RE: SCFM QUESTION

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

RE: SCFM QUESTION

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"

RE: SCFM QUESTION

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

RE: SCFM QUESTION

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"

RE: SCFM QUESTION

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

RE: SCFM QUESTION

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:

Quote (mouse over to see answer):

d.  Whatever two people agree it is

David

RE: SCFM QUESTION

d

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

RE: SCFM QUESTION

That's one.

RE: SCFM QUESTION

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

RE: SCFM QUESTION

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...

RE: SCFM QUESTION

Hydtools,
That is an interesting variant.  Never seen partial pressure called ppm, but that's no big deal.  I'm sure that the ppm value is really "theoretical partial pressure of moisture at 100% RH" even though it doesn't say so (if not then you would be applying the RH factor twice).

David

RE: SCFM QUESTION

I will actually disagree with all of you smile

SCF and Sm3 and Nm3 are all MOLAR flow. Yes, when you are talking specifically about air - its also mass flow, but for a given gas (be it methane or hydrogene) a SFC is 1.19804 moles (0.0026412 pound moles - no matter if its air, methane or any other gas.

Best regrads

Morten

RE: SCFM QUESTION

That is a very interesting point Morten and you are correct. Saying scfm is analogous to mass flow assumes one is referring to one gas. But , I believe that most equations that use scfm also factor in density, and the two together are analogous to mass flow. But, I also haven't given it a whole lot of thought.

RE: SCFM QUESTION

For those who haven't seen Morten's magic number before - remember that this applies only if your standard conditions are 60 F and 14.73 psia.

It is like the magic number of 22.41 litres of gas per gram mole that those of us who grew up under the old IUPAC rules of standard conditions being 101.3 kPaa and 0 C came to know and rely on.  Unfortunately IUPAC revised their standard pressure to 100 kPaa and the magic number became 22.71 litres per mole.

Katmar Software - Engineering & Risk Analysis Software
http://katmarsoftware.com

"An undefined problem has an infinite number of solutions"

RE: SCFM QUESTION

Katmar

It true that IUPAC changed _their_ refernce condition - but that why you shold always state your definition somewhere - even when referring to SCF. In Denmark even the Danish Authorities uses 1 atm as the reference pressure for fiscal metering - not 1 bara.

But the most common mistake is to confuse nm3 (at 0 deg C) and Sm3 (at 15 deg C). Here its more than 5%.

Best regrads

Morten

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