4 replies to this topic

[tomc]

Hello Members,

Thank you for allowing me to post my question.

 This is a question of gas flows used in welding, but since the flow ultimately exits through an open tube to atmosphere, I think experts on this Venting forum may be able to assist.

 

I am using a mixed gas comprised of 75% Argon + 25% Helium which is purchased (premixed) in a standard shop cylinder. The gas is specified to have dewpoint of  -105 degrees F, which is quite dry.   My standard flow rate from the cylinder is set at 40 cubic feet per hour (CFH) which is measured and adjusted by means of an accurate rotameter type flow meter. Now I must add to this flow, an auxiliary gas supplied from a separate cylinder which is 100% Argon at a flow rate of 25 CFH and also has a -105 degree F dewpoint.    The mixed gas and the straight auxiliary gas exit through separate tubes into a common mixing chamber which is open to atmosphere so neither affects the flow of the other.      

 

Sounds simple so far, but my question is how to calculate the total flow (in terms of CFH) of Argon and total flow (in terms of CFH) of Helium ? 

 

For the 75/25 mix at 40 CFH, is it simply { (75% x 40 CFH) + 25 CFH } for the Argon  and  {25% x 40 CFH} for the Helium?   Something tells me my calculation must account for the weight or mass of the individual Ar and He. 

 

One last detail that might be important, the gas pressures are dropped down from cylinder pressure through a 2-stage regulator to 125psig before it exits the regulator and enters the flowmeter.

 

Please advise if you can help or if perhaps I'm in the wrong forum.

 

Thanks,

Tom

 

 

 

 

 

[Art Montemayor]

Tom

 

I moved this thread here because it has no venting topic related to it.  What it does have is a simple mole balance operation.

 

You say you have a pre-mixed gas comprised of 75% Argon + 25% Helium - but you fail to state the basis of the percentage - a gross engineering error or oversight.  Having spent a lot of years in the compressed gas business, I have to guess that you mean volume percentage.  Correct?  And when you state “CFH” you fail to state at what conditions.  Another major oversight!  Do you mean cubic feet per hour at 60 ^oF and 14.696 psia?  If not, then state your basis.

 

If you identify the standard conditions you have and they are all the same, then the gas volumes you are mixing are in the same proportion as the mole volumes of each gas.  In that case you don’t need to account for the weight or mass of the individual Ar and He - unless you want to.  I usually always did my mass balances in moles instead of volumes in order to stay clear of varying “standard” conditions.  Once I knew the total moles of each gas, then I would multiply the quantity of moles by 379.49 Scf/mole to find the volume of gas at the standard conditions of 60 ^oF and 14.696 psia.  If your standard conditions are different, then use the appropriate gas volume.

 

Your failing in identifying the type of percentage and standard gas conditions is the main reason why I always worked in moles and not volumes.  World-wide there are as many so-called “standard” conditions as there are opinions.  I hope you can understand why I - and a vast majority of experienced chemical engineers - are so critical of a failure to recognize the importance of always citing what standard conditions are when dealing with a gas flow or volume.

[tomc]

Thank you, Art for helping me ask the correct question. Mole balance is not part of my daily skil set which is why I came to visit.  Your assumptions are correct, my per centages were intended to be volume per cent and my flow conditions are very close to the standard that you cited, 60 ^oF and 14.696 psia.

 

But since you have metioned that the gas volumes being mixed are in the same proportion as the mole volumes of each gas then I think the answer to my initial question is indeed, for the total flow of Argon =  { (75% x 40 CFH) + 25 CFH }  =  55 CFH     and the total flow of Helium =  {25% x 40 CFH}   =  10 CFH.  

 

Do you agree with this conclusion ?

 

From this information I determine that my actual gas mixture ratio is 85%Ar and 15%He and this 85/15 ratio becomes an essential variable in my qualified weld procedures.    

 

I'm open and grateful for any commentary or criticism you care to offer. 

Thank you, again.

Tom

Edited by tomc, 25 October 2018 - 07:00 PM.

[Art Montemayor]

Tom:

 

And thanks to you for responding to our comments.   All my comments are based on giving experienced help and never aimed at giving criticism or complaints.  Because the nature of our Forums is directly aimed at giving aid and assistance to other engineers and technical people who are in need of confirming potential solutions or in need of experienced learnings, our comments sometimes come across as criticism or negative comments when - in fact - they are meant to avoid critical mistakes or safety issues.  I am grateful and encouraged by your attitude in recognizing our goals and recommendations.

 

Yes, I agree with your conclusion.  We old process engineers know that mole percentage for gases is equal to volume percentage for the same gas.  

 

Our experienced Forum members are a formidable help and consulting force that I am proud to see helping other members in resolving or eliminating engineering problems.   I am often reminded by my wife of 55 years - who I married while in Peru - who states an old Spanish saying: "The devil is not a clever and a formidable foe because he is a devil; he is that because he is old".

[tomc]

I am grateful for you and this unselfish community of seasoned professionals who are so willing to share their knowledge with people you don't even know.

Imagine, in the entire library of your life's work you have shared a small morsel of information with me. It was an important morsel that clearly answered a question which I studied and struggled with for several days. Now i can further build my own knowledge on that little bit of information and share with others in my sphere of influence.  The seed you have sown will continue to grow on and on in the lives and work of many to follow. This forum is truly wonderful. Thank you Art.