Bring compressed air form 68psig to 130psig using 2000psig cylinders

[czarSS]

What we are looking for is the amount of 250 cubic feet cylinder at 2000 psig that we need to purchase to make up the difference between the instrument air (68 psi expected) and the test pressure of 130 psi.

The sytems total volume is 170 cu. ft. Medium is air. Temp will not be a factor.

How many cylinders will it take to bring the 68psig to 130psig in a 170cu. ft system?

Thanks for your help.

 

[zdas04]

Convert your 170 ft^3 at 68 psig to a volume at standard conditions. Then convert the 170 ft^3 at 130 psig to standard conditions. Subtract the first number from the second, that is the standard cubic feet of gas you need to add.

Now turn your bottle volme into standard conditions at 2000 psig. Then compute the bottle volume at some number approaching 130 psig (how close you come is a function of how long you are willing to wait, I would use 145 psig myself) to standard conditions. Subtract the 130 psig number from the 2000 psig number and you get the standard cubic feet of usable gas in a bottle.

Divide the second number into the first to get the number of bottles.

David Simpson, PE
MuleShoe Engineering

"Belief" is the acceptance of an hypotheses in the absence of data.
"Prejudice" is having an opinion not supported by the preponderance of the data.
"Knowledge" is only found through the accumulation and analysis of data.

 

[nickelkid]

assuming you are providing gauge pressures you will have 3,480 cu.ft. of air available at test pressure from 1 cylinder.

Boyles Law Pt Vt = Pc Vc

 

[zdas04]

RobinHandy,
Not sure how you calculated that, but it is wrong. The required volume is 716 SCF. If you assume that the bottles contain 250 SCF at 2000 psig (since if they hold a physical 250 ft^3 you would need a 60 inch ID "bottle" 12 ft long) which has a usable volume (at 145 psig terminal pressure) of 230 SCF. That says that you need 3.1 bottles.

David Simpson, PE
MuleShoe Engineering

"Belief" is the acceptance of an hypotheses in the absence of data.
"Prejudice" is having an opinion not supported by the preponderance of the data.
"Knowledge" is only found through the accumulation and analysis of data.

 

[nickelkid]

Yes zdas04 you are absolutely correct 4 bottles are needed. This is my reply post of shame… no excuse for sloppy posting – like anything else an eng-tips deserves a Preview and check to ones work. An unverified cut-n-paste on my part would have cause problems without out your diligence. Thanks.

 

[czarSS]

zdas- when I calculate using your first explanation I get 1 cylinder. I agree with RobinHandy, the req'd volume to bring the 170ft^3 system from 68psig to 130 psig is 716 ft^3. If I convert the 250ft^3 cylinder at 2000psig to standard condition it yields 34,013 ft^3. If I calculate that same 250ft^3 bottle at 145psig it yields 2,466psig. The difference in these is a usable volume of 31,547 ft^3. I only need an additional 716ft^3, so is it safe to assume I only need 1 cylinder? Please confirm...

I appreciate everyone’s help.

 

[czarSS]

*2,466 ft^3 at 145psig

 

[zdas04]

You are confusing ACF with SCF. The bottle has a physical volume around 1.8 ft^3 to provide 250 SCF at 2000 psig. In my first post, the third step is "250 SCF" since turning SCF into SCF is multiply times 1.0.

RobinHandy,
That is the beauty of eng-tips.com, when you make a mistake someone in the world will call you on it. I really love that feature of this site because I make mistakes all the time.

David Simpson, PE
MuleShoe Engineering

"Belief" is the acceptance of an hypotheses in the absence of data.
"Prejudice" is having an opinion not supported by the preponderance of the data.
"Knowledge" is only found through the accumulation and analysis of data.

 

[czarSS]

Thanks, really appreciate it.