.
Basketeng:

Volumes stated in Nm3/hr are usually volumes measured at standard conditions of 1 atmosphere pressure and 273.15 deg Kelvin (usually but not always ... there are many definitions of standard conditions).

You can convert Nm3/hr to actual m3/hr at any other pressure and/or temperature by using:

P₁V₁/T₁ = P₂V₂/T₂

where:
P = absolute pressure (i.e., gauge pressure + atmospheric pressure)
V = volume
T = absolute temperature (i.e., deg Kelvin or deg Rankine)

Make sure that you ascertain the standard conditions that were used to define the Nm3/hr (i.e., V₁) and use them for P₁ and T₁.

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

mbeychok

I disagree. Almost any gas mixture will need a calculation of the Z factor in order to be correct when the P/T differs significantly from the "normal"/"standard" conditions.

Furthermore: N is (as you stated): 0 deg C 1 atm and never anything else. There is also "standard conditions". This differs some.

For standard cubic feets (SCF) - its 60 Deg F, 14.5 psia
For standard cubic meters (Sm3) its 15 deg C, 1 atm

(note that 60 def F is close to 16 deg C).

If the question is how to convert from Nm3 to Sm3 then your observations are correct.

Best regards

Morten

To avoid lengthy discussions kindly visit Thread798-106556, where you may find more references of previous threads.

Morton:

Thanks for your comments on my response to the original question.

If you will read some of the many previous threads on essentially this same question, you will find some responses stating that Normal conditions are not always defined as 0 deg C and 1 atmosphere ... and that is why I said "usually but not always".

As for SCF, in the 40+ years that I worked in the U.S. petroleum industry, SCF was defined as being at 60 deg F and 14.696 psia (i.e., 1 atmosphere) rather than 14.5 psia.

Again, reading some of the many previous threads on this question, there are literally more than half a dozen definitions of SCF, Sm3 and Nm3. Just as one example, our U.S. EPA defines standard conditions as 1 atmosphere and 68 deg F (i.e., 20 deg C). As another example, since 1982, the International Union of Pure and Applied Chemistry (IUPAC) has defined standard conditions as being 0 deg C and 100 kPa (which is 1 bar) whereas 1 atmosphere is 101.325 kPa. Finally, the American Gas Association uses both 60 deg F and 14.696 psia as well as 60 deg F and 14.73 psia.

The lesson to be learned is that we should always state what standard conditions are being used.

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

basketeng,

I have written a simple program (which is a free download) which should help you with the calculation. It is called "Uconeer" and is available from www.katmarsoftware.com

It is basically an engineering oriented units conversion program, but includes a calculator for converting gas flowrates between mass and volumetric units.  It can be used to convert from one set of volumetric units or conditions to another by converting from the first set to a mass flowrate, and then converting from the mass flowrate to the second set of volumetric units and conditions.

But please take note of the items raised by Milton Beychok and by Morten.  They both highlighted very important aspects.

When I wrote Uconeer I was under the same impression as Morten that Normal Conditions were ALWAYS 0 deg C and 1 atm.  Subsequently I learned that IUPAC had changed their definition (See Thread378-97454) to 0 deg C and 1 bar.  I am currently in the process of updating Uconeer, so while you are using the current version (1.e. version 2.3) please just key in the correct temperature and pressure conditions for your situation.

Morten has also pointed out that the gas compressibility (Z factor) must be taken into account. Unfortunately it is not always easy to get the value for Z for your particular conditions.  Probably the best source would be to get it from a Process Simulator, but if you have access to such a program then it would do the conversion for you as well.  Uconeer will estimate the Z value using the Peng-Robinson equation of state and as long as it is not too far from 1.0 it will be good enough.  If the Peng-Robinson EOS estimates Z to be far from 1.0 then you should check your result using a more sophisticated approach.

regards
Katmar

I checked a little more on IUPAC's homepage and came across the following documen that contains a definition of "standard state".

http://www.iupac.org/goldbook/S05925.pdf

I do however think that the statement in the IUPAC definition that "prior to 1982 101325 Pa was usually used" is a little to self confident.

I have worked in the Danish/Norwegian oil industry for 10 years as a process engineer (consultant) and have never come across anybody who used 1 bara - everybody uses 1 atm.

Best regards

Morten

MortenA:

I also worked in the oil and gas industry for a long time (40+ years) and I also never heard of anyone using 1 bara as the standard condition pressure. Never-the-less, this is the statement at http://www.iupac.org/goldbook/S05921.pdf:

Quote:

Standard pressure: .... In 1982 IUPAC recommended
the value 10⁵ Pa, but prior to 1982 the value 101,325 Pa (= 1 atm) was usually used.

That very clearly says that before 1982, the IUPAC's standard pressure was 1 atmosphere ... and in 1982 they recommended 1 bara as the standard pressure.

Like you, I had no knowledge of IUPAC's standard and I continued to use 1 atmosphere as the metric standard pressure ... as many still do. Again, that is why my original response in this thread stated that:

Quote:

Volumes stated in Nm³/hr are usually volumes measured at standard conditions of 1 atmosphere pressure and 273.15 deg Kelvin (usually but not always ... there are many definitions of standard conditions).

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

The use of the term Nm3/hr is quite ambiguous and has been the cause of innumerable errors over the years.  Not only because of the exact definition of pressure and temperature(and often confusion with Standard by the specifier) but because, for example with air, it is necessary to know the exact RH that the specifier used.

Unfortunately it continues in frequent use despite being completely inappropriate for displacement machines such as fans and blowers.  The reality is that Nm3/hr is really an oblique way of expressing mass flow.

The best solution is to find out exactly what density the specifier used when calculating the "Normal" condition.  You must then calculate the density at the "actual" condition at which you require the volumetric flow rate.

Actual m3/hr = Nm3/hr x "Normal" density / actual density.

While we are all confessing our ages, let me admit that I have worked quite a bit longer than Morten but not quite as long as Milton!  And I have also not seen anyone in industry use the 1 bara basis for Nm3.  This was why I was so thoroughly shocked when I first came across the IUPAC statement referred to above.

However, I think it is just that it is such an entrenched convention that none of us questioned it.  After I saw the IUPAC statement I questioned my son who is studying physics at high school and he confirmed that he was taught the 1 bara basis. By the way, we are in South Africa. Can anyone else with academic links in other countries confirm what is being taught at schools and universities?

The bottom line, as has been regularly emphasized here, is that whenever a gas flowrate is communicated, the details of the temperature and pressure must be given.  For safety's sake I try to give the flowrate in mass units as well - I mean real mass units like kg or lb and not a pseudo mass unit like SCFM or Nm3/h.

regards
Katmar

I think that the conclusion (that was also reached previously) is:

Allways state your "stadard" definition somewhere early in your calculation! If for nothing else then for reference.

Best regards

Morten