Converting Flowrates for Nitrogen Gases 5

[JulesRules]

Good Afternoon,

Would it be possible for someone to point me in the right direction with converting Pure Nitrogen Gas Flow rate of 3,400 lb/Hr to an SCFM value?

Thank you for your time
Kris McPherson
marlboroughman@shaw.ca

 

[iainuts]

Sure, conversion is 0.072465 lbm/standard cubic foot.

3400 lbm/hr = 782 SCFM

 

[Montemayor]

JulesRules:

I don't know with what degree of accuracy you need the conversion, but the actual conversion is done as follows:

(3,400lb N2/hr)(lbmol/28.013 lb)(379.49 Scf/lbmol)(hr/60 min) = 767.66 Scfm (@ 14.696 psia & 60 oF)

Engineering answers are not worth much if they are not documented and "standard" conditions should always be specified. Everyone has a right to their own standard conditions. I take mine from the GPSA.

Art Montemayor
Spring, TX

 

[KernOily]

As John Campbell says - "Knowledge of the basic fundamentals is the required foundation for successful professional practice".

Is this a homework problem?

From Thermo 101:

mass flow rate = density * volumetric flowrate

Sorry if I sound cranky; it's been a long hot day out here in the Land of Fruits and Nuts (California) oil patch. As a mentor to junior engineers I can't for the life of me see why they can't rememeber the very first principles. Like this one.



Thanks!
Pete

 

[Montemayor]

Pete:

I think this is the real thing. As the years go by, I find more people not knowing or remembering what a mol is and the definition of a gram-molecular volume.

That's why you will notice that I show the entire so-called "equation" used to solve this problem. It is nothing more than the basic fact found in Chemistry 101 (not thermo - it's even more elementary) about the fact that one gram-molecular weight of a gas occupies 22.4 liters of volume at the standard conditions of 760 mmHg & 0 oC.

Density has nothing to do with the solution. It is based on the volume of a gram-molecular weight of gas. It's been a long hot day out here in Texas also.

Art Montemayor
Spring, TX

 

[pmover]

initially in my career, i used density until i encountered process engr's whom used technique provided by art. i immediately recognized the value and elementary principle involved and practice it today. i concur with art's example and std used to determine results.

btw art, congrats on the nomination!
-pmover

 

[Montemayor]

Thanks pmover for hitting on what I have tried to emphasize. An engineer has a tough time trying to remember or tabulate "gas densities" for all P&T combinations - when this is not necessary at all. This is why I can identify Kris McPherson's dilemma.

The density of a gas can be basically derived from the simple and established natural fact that a mol (gram-molecular weight) of gas occupies 22.4 liters at 0 oC & 760 mmHg. This is the same figure the GPSA converts to the good ole U.S. units of 379.49 Scf (@ 14.696 psia & 60 oF). I know this is redundant and may be elementary to many; however it may help others to know that all one has to know is one number (@ its reference) and not a whole database to solve problems of this nature. It's not a matter of teaching others how to multiply or convert units; it's an attempt to point to a simple, available and very practical engineering way to resolve a problem. I may have failed to point this out but your post certainly cleared it up.

Please note that iainuts response is essentially correct. Density can be used to arrive at a result. Accuracy or basis for calculation is the only difference. As engineers we are not in "love" with the numbers; we are enamoured with the logic and practicality of resolving a problem.


Art Montemayor
Spring, TX

 

[katmar]

It is important (I believe) that anyone working with gas flows, understands the principles explained by Art above. But I'm a lazy kind of a guy, so I wrote a little program called Uconeer that converts between mass and volume flows in just about any units you can imagine. Much easier and no arithmetical errors.

It also does lots of other engineering units conversions. Once you understand the principles, use the program for the calcs. Grab a free copy at

http://www.katmarsoftware.com

enjoy
Harvey

 

[KernOily]

Art - Excellent point. You are completely right in that I should have taken a step back to a more fundamental method, than the one I suggested especially since his problem was posed in standard units.

The reason I suggested he use mdot = rho * qdot is because I personally NEVER get the luxury of working in standard conditions, so by habit I just whupped the first tool out of the toolbox. Every single problem I am faced with in my daily practice requires actual gas properties, and I am used to dealing with gases that are highly non-ideal due to gas polarity or operation at conditions way far away from STP. Thus I am completely spoiled by HYSYS, Pipephase, and SuperFlo. I am so spoiled that I have gotten to the point where I can get an actual gas density (and whatever else) out of HYSYS in about 10 sec. Plus, I am a dumb ME so I forgot everything I learned in CHEM 1135 ;-). I am so used to mdot=rho * qdot that that was the first thing that came to mind.

Good points, all. Pete

By the way since you're from Texas, you will note that I used the proper spelling of 'whupped'. ;-)

Thanks!
Pete

 

[Montemayor]

Pete:

As I said, I really think this was a real-life problem and unfortunately Uconeer and HYSIS were not around to resolve it on the spot. I also firmly believe in applying practical, down-to-earth techniques to resolve an engineering problem - and the employment of Uconeer and HYSIS is the engineering way to go today. It was different back in '61 when I had to take out my SlipStick and mechanical pencil and work it the way I showed above. That was then; today, you have to accept the practical thing that allows you to compete and stay current in technology within the time alloted to you. I mention this because there may be a young grad out there that mistakenly may get the impression that we don't have to keep up the progressive thinking and improving on how accurately and fast we can resolve problems - because we really have to. And, as Katmar and you obviously agree, we've got to do it with a firm and dominant understanding of the basic principles. It's not enough to apply an algorithm; it's got to be the correct algorithm.
(By the way, is there another way to spell whupped - 'cause that's how I feel right now, after mowing the front lawn just before I enjoy my first Margarita for the day.)
Regards


Art Montemayor
Spring, TX

 

[25362]

One small point. Art Montemayor rightly asks JulesRules on the degree of accuracy requested. If the question referred to an accuracy of two significant digits as in 3.4x10² lb/h, or to four digits as in 3,400 +/- 1 lb/h, the answer would have to be in line.

Numbers generated by electronic calculators cannot be more accurate than the information that led to it. Calculators (PCs included) have no judgment, so you must exercise yours.

 

[ww2]

take a look at

http://www.convert-me.com/en/;

http://www.ex.ac.uk/cimt/dictunit/dictunit.htm