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
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RE: SCFM QUESTION
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RE: SCFM QUESTION
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
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
68 scfm at 90 psig is a mighty small orifice.
RE: SCFM QUESTION
Katmar Software - Engineering & Risk Analysis Software
http://katmarsoftware.com
"An undefined problem has an infinite number of solutions"
RE: SCFM QUESTION
RE: SCFM QUESTION
If it were me, I hunt down that "someone" and ask, "What the heck did you really mean by this?
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Chinese prisoner wins Nobel Peace Prize
RE: SCFM QUESTION
RE: SCFM QUESTION
RE: SCFM QUESTION
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
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
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
Katmar Software - Engineering & Risk Analysis Software
http://katmarsoftware.com
"An undefined problem has an infinite number of solutions"
RE: SCFM QUESTION
Only put off until tomorrow what you are willing to die having left undone. - Pablo Picasso
RE: SCFM QUESTION
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:
David
RE: SCFM QUESTION
Only put off until tomorrow what you are willing to die having left undone. - Pablo Picasso
RE: SCFM QUESTION
RE: SCFM QUESTION
RE: SCFM QUESTION
My Sears cheap air compressor will deliver about 50 cfm at 120 psi...
RE: SCFM QUESTION
http://www.cagi.org/toolbox/formulas.htm
Ted
RE: SCFM QUESTION
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
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
RE: SCFM QUESTION
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
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