"Normal" is a Euro unit and a favorite in the "Mother Continent". It is never used in the USA, nor have I seen it employed in Latin America - with the exception of some chemistry or physics profs in academia referring to it in books. It simply is another variation of "Standard" for defining gas conditions at a datum level of pressure and temperature. Like all such attempts to "standardize", it is subject to the whims, likes, preferences, and needs of the user and, as such, should always be followed by an identification of the base temperature and pressure to which the originator ties it to. Otherwise, like all such attempts, the title itself is useless because it is subject to any one's idea of standard (or "Normal") conditions.
Hoping not to confuse, but SCFM and its other "Standard" or "Normal" relatives is actually a unit of MASS flow, not volume flow. SCFM magnitude won't change when flowing temperature and pressures change.
In the UK we use NTP and STP for normal temp and pressure and standard temp and pressure. The only difference is that normal is usually 15 degrees celsius and standard is 0 degrees.
Pressure is taken as atmospheric at 1.01325 Bar 101.325 kPa.
I use N for Newtons, but have never come across Sm3/sec or Nm3/sec??
This might be peculiar to some of the countries on the main continent of Europe.
OldSohioEngr i disagree - its a unit of molar flow not mass. Any SCF will have the same number of mol gas (1.195) be that argon, methane, air.... But it will not have the same mass.
Its the same for Nm3 and Sm3. Just to confuse those on the "new continent" S in Sm3 is not defined at the same temperature as S in SCF allthough the pressure is the same
I recently had this "discussion" with another individual who has, admittedly, many more years experience than myself. However, once I started questioning his viewpoint, he refused to defend his position (probably shrugging me off as a younger engineer not worth his time). Perhaps you can pick up the torch for him.
I do agree that since SCFM relates to a given pressure and temperature condition that you are indeed referencing a given mass flow (that must still be calculated), however I disagree that SCFM should be refered to as a mass flow, simply on the basis of units and basic fluids equations (i.e. m(dot) = rho * q).
My argument was a simple one: 1 SCFM = what mass flow? If SCFM were indeed a mass flow, the value should not be different. I feel it's important to make the distinction.
In classes I teach, I refer to "SCF" as a "surrogate for mass flow rate". It's not really a unit of molar flow because the units are volume per unit time.
It is a good surrogate because once you know the density of that particular mix of gases at STP, then you can calculate mass-flow-rate at any temperature and pressure from SCF/unit time.
The error I see most often in going from volume-flow-rate units to mass-flow-rate units is the transition from SCF to ACF. Next hardest thing seems to be calculating density at flowing conditions. You can avoid both of these pitfalls by simply multiplying the densitiy of air at STP times the mixture specific-gravity and multiplying that times SCF/time to get mass flow rate (which can't change within a flowing stream unless you add or subtract mass according to the continuity equation).
Neither pipe diameter, pressure, nor temperature is required for the translation which makes it a lot easier to get right.
How can it be mass flow when you need the molecular weight of the gas to convert it from molar flow to mass flow? Take a SCF of gas - any gas - and you will allways have the same number of moles but not the same number of pounds. Its even so neat that you will have the same number og lb-moles and kg-moles.
HYSYS has MMSCFD and Nm3/hr in the molar flow category. Therefore normal and standard conditions are a molar flow. I have no reason to doubt it. The SG of a gas is related to the molecular weight ratio with air.
The question over N and S conditions has been raised previously on ENG TIPS and there are lots of different S and N's depending on your persuasion (engineer, scientist etc) and country of origin
MortenA,
SCF isn't mass, but it can be used to get to mass. The specific gravity times the density of air gets to mass/unit volume, that times SCF is mass. The molecular weight is contained in the SG.
"SCF isn't mass, but it can be used to get to mass."
That is the crux of my side to that argument. If mass flow needs to be calculated from SCFM (or what have you) then why would one want to confuse the issue and say that it is already a mass flow? I have no doubt in my mind that it references a mass flow because the conditions are fully defined. However calculations are still required to obtain the actual mass flow.
Again I ask...1 SCFM (at whatever you define S at)is what mass flow?
It's a matter of convention. SCFM is NOT a unit of volumetric flow. We use AFT ARROW for compressible flow calculations; they include SCFM, SM3/M, NM3/M and other such designations in their units for mass flowrate. They don't have units for molar flowrate. Chem E's work with molar stuff all the time, most Mech E's don't. The conversion to real mass flow comes from either mols/scf or lb/scf.
Fred,
I guess I just don't get your point. Gas is bought, sold, transported, etc. based on standard cubic feet. Compressors, separators, dehy's, etc. are usually sized in some multiple of SCF per unit of time.
Compressors don't compress SCF. SCF is meaningless for calculating velocities, but it is often the only information available. If you know the gas specific gravity you can calculate ACF, velocity, or mass flow rate from SCF. It is a useful shorthand notation for some very complex engineering and commercial activities.
SCF isn't mass, it also isn't a volume or a velocity. But it is a useful common denominator for doing a lot of stuff. So, what is the disagreement here?
David,
I guess my viewpoint is nit-picky. I have issue with using the term "mass flow" with a quantity that is volumetric flow. That's all. When I size up a compressor, say at 1600 SCFM, I know that the mass flow is about 120 Lbm/min, not 1600.
When I ask "1 SCFM = ?? in mass flow?" I am trying to say what you said earlier:
"...multiplying the densitiy of air at STP times the mixture specific-gravity and multiplying that times SCF/time to get mass flow rate."
1 scfm of air is not equal to 1 lbm/minute. It is approx .076 Lbm/min. A calculation has to be performed to arrive at the mass flow.
There's a disconnect here between what the CE's use as mass flow and what myself as an ME uses. I have a hard time coming to grips that a cubic foot of anything is a direct measure of mass.
That's all. I just am trying to understand where the convention started.
I checked with my trusty "Campbell" and they _tend_ to side with you - it used to be mass because it was for air - thats where relative density comes in i recon. But being a chemist I still think its more mol than mass :-9 (they also write "there has been a lot of trivial controversy in former English unit areas about what comparable SI units to use"...
