Use of Nm3/hr Unit 2

[sanchem]

Hello everyone,

Can someone explain the use of the term Nm3/hr for calculating air flow rate?
When to use it and when to not while sizing equipment like scrubbers, fans, blowers etc?
I see it used in many design problems. Plz explain the basics.

 

[MortenA]

Nm3 is a "standard condition reference volume" unit - equivalent to a fixed no. mols of gas. The specific reference conditions is 0ºC and 1.01325 bara. At these conditions equivalent to 0.0446158 kmol. Please note that even though the work "standard" is used this is NOT a standard because UIPAC has some years ago redined the pressure to 100,000 Pa (or 1 bara) but this is not uniformly accepted - and i think that _most_ people uses 1.01325 bara. The standard (or normnal) volume flow is idependant of gas because it relates to mol of gas not mass or actual volume. You can calculate the mass flow and actual volume flow if you know the MW, P & T and composition based on the std. vol. flow.

This illustrates that you should _always_ specify your reference (standard) conditions somewhere.

See also this entry in wikipedia

https://en.wikipedia.org/wiki/Standard_conditions_for_temperature_and_pressure

 

[zdas04]

sanchem,
I would echo MortenA's admonition to always specify your reference conditions, especially in contracts or engineering calculations. I review a lot of gas-sales contracts and the first thing I do is look for the reference conditions. I won't sign off on a contract without them, I don't care what they are, I just want them specified. Regulatory agencies are terrible about picking a pressure/temperature that is not in general use (the US EPA has at least six different "standards" depending on the background of the moron writing the regulation, we often have to convert from what the royalty people at the federal government require to what is required for one monthly EPA report and then convert to a different base for the quarterly report).

If you think about an air compressor at Denver, CO in the U.S. (elevation 1600 m ASL). Suction pressure is 84 kPa(a) at 15 C. Discharge pressure is 1.1 MPa at 143 C. If the compressor is moving 2800 m^3/day at suction conditions then it is moving 300 m^3/day on the discharge. There is nothing in fluid mechanics that says this is wrong (mass flow rate has to be constant, not volume flow rate). To make this make sense, we use a set of reference conditions and pretend that the gas was at those conditions all along. In this case (using 101.325 kPa(a) and 15 C as the reference conditions), it is 2300 SCM/day at both the suction and discharge.

I always use the term SCM for "standard cubic meters" (and "kSCM" for thousand cubic meters, etc) and reserve m^3 for actual conditions. I have seen several authors follow this convention and it significantly reduces the confusion. I have seen people confuse km^3 for cubic km, and thought that that was foolish until I found a regulator in an European country who required data to be reported in cubic km.

The original gas measurement standards in many countries were a straight conversion of the U.S. 14.73 psia and 60F (i.e., 101.325 kPa and 15.7 C) and this was called "standard". Other regulators used 14.73 psia and 32F (101.325 kPa and 0 C) and this was called "standard". A grass roots rejection of this foolishness started at a number of refineries, big engineering companies, and large gas facilities. These guys came up with "normal" conditions to represent "normal" as 1 bar (100 kPa, 14.5 psia) and 15 C (59F). The exact numbers didn't really catch on, but the concept has lingered. Today when I review operations outside the U.S. I have to ask "what are the conditions of 'standard' and 'normal'?" The answers are distressing. Frequently the "standard" number will quote a sales contract and I'll find a second or third contract with a different number and a regulation with a still-different number. It is also common for everyone to "know" what the "normal conditions" are, but for 5 guys to give me 5 different numbers. The concept of "normal" added a level of confusion that was not justified, period. Why does anyone have any invested ownership of an imaginary number? Why the heck does it matter to anyone? I tell people to pick a standard and make any engineering contractor sign off that he will always use those numbers.

David Simpson, PE
MuleShoe Engineering

In questions of science, the authority of a thousand is not worth the humble reasoning of a single individual. Galileo Galilei, Italian Physicist

 

[EdStainless]

David,
Amen, and then try to explain this to a non-technical person......

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, Plymouth Tube

 

[zdas04]

Oh the stories I have. Just getting over the hurdle that the "standard" conditions that they use have no real analog in reality and as long as everyone agrees and uses the same pretend pressure and temperature everything will be fine. This is one of those concepts that "everyone understands" so no one wants to bring it up in polite company and if you use 14.5 psia and I use 15.025 psia then my number will be 4% higher than yours for the same number of molecules passing under the meter. Lawsuits are built on such "understanding".

David Simpson, PE
MuleShoe Engineering

In questions of science, the authority of a thousand is not worth the humble reasoning of a single individual. Galileo Galilei, Italian Physicist

 

[sanchem]

Hey GUys EdStainless, zdas04, David Simpson, MortenA,

Can Someone actually explain how to make use of this concept and what significance it has with some Numerical Examples?

If you have any sums or numericals where this concept is used, please share it.

 

[LittleInch]

With a gas flow, the fluid is compressible. For everyone to communicate technical information, the accepted practice is to use Nm3/hr or SCFH to a set definition of what that is (see above) as a base case number. In its most basic form the ideal gas law is
(P1 x V1) /T1 = (P2 x V2)/ T2. Here P1 is 1 bara, V1 is 1, T1 is 15C (288K). Hence as soon as anything changes in your particular condition, normally Pressure, then the Actual volumetric flow of your gas changes.

Remember this uses absolute values (bara and Kelvin) so if say your blower has a volume flow rating of 100 Nm3/hr, but produces 1 barg (2 bara) pressure but still at 15C (288K) then the ACTUAL volumetric flowrate will be 50 Am3/hr (using 1 bara and 15C as your reference point)

As you get more complex then you introduce compressibility factors (z) and as you get to metering standards, then you introduce even more complex calculations to convert your ACTUAL conditions back to some agreed "Standard" or "Normal" conditions. As noted above, a bit like Humans, there isn't a fixed "standard" or "Normal".

However for many purposes the accuracy isn't that important so 1 bara at 15C will be good enough to get you an initial figure then go and do the detailed calculations and agree with all the parties what "Standard" or "Normal" actually is.

Does that help?

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[MortenA]

I guess for a compressor/blow the point is that your actual volumetric flow changes across the unit but the standard flow remains the same. This means that you wont have to specify exactly where you refer to your nominal capacity.

 

[zdas04]

Mass flow rate is constant if you don't add or remove any mass. Mass flow is

mass Flow = volume flow * density(relative to volume flow)

So since mass flow rate is constant

volume flow (std or normal) = volume flow (actual) * density(actual) / density(std)

David Simpson, PE
MuleShoe Engineering

In questions of science, the authority of a thousand is not worth the humble reasoning of a single individual. Galileo Galilei, Italian Physicist