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Gas work and regulators

Gas work and regulators

Gas work and regulators

(OP)
I'm old and rusty and have been struggling with a thought experiment.  Can someone set me straight please?

Lets say you had a piston designed to expand some gas from 2200 psig to 10psig.  And lets say its happening slowly with plenty of heat transfer so that its effectively isothermal.

Its fairly easy for me to imagine and calculate the work done by the piston by just integrating the force on the piston over the length of the stroke.

So lets assume that you feed the same piston, from the same 2200 psig source, but through a 10psi regulator.  Again lets assume its nice and slow so its isothermal.  So if I was to calculate the piston work in the same way as above, the answer would be much much lower.

Where did that energy go?  

(The reason for my inquiry:  A potential client is using 220SCF gas bottles to maintain pressure in a remote application - the bottles require weekly replacement.  I've been asked to comment on the feasibility of extending the life of the bottles by using the stored pressure to run a pump to pressurize the application with ambient air rather than pressurizing the application directly)

RE: Gas work and regulators

Can they tolerate the imprecision of using a non-bleeding regulator?

 

Mike Halloran
Pembroke Pines, FL, USA

RE: Gas work and regulators

(OP)
They can tolerate just about anything - the application is allowed to vary from 6 to 10 psi.

The issue is conserving the gas, however.  It doesn't make much difference if the pressurant is N2 from the tank or air from the environment.  The idea here is to utilize the potential energy in the tank in addition to the gas itself with some type of pump.

If that makes any sense.

I don't really know what a nonbleeding regulator is, but for the concept to have merit we need a ~10x improvement in the life of the tank.  So based on a few minutes of Googling I'm not sure a regulator change is the ticket.  :)

RE: Gas work and regulators

Regulators work better when there's a little flow through them, so much so that some of them actually have intentional leaks built in on the regulated side.

Study the specs for the regulator in use, or throw a soap film over one in operation.

 

Mike Halloran
Pembroke Pines, FL, USA

RE: Gas work and regulators

The question is an interesting one about thermodynamics rather than the details of how a regulator works. I don't have the answers and am therefore also curious.The same issue is encountered with steam engines. You can fill the cylinder with steam and then let the steam pressure drop as it expands as the piston moves, or you keep the cylinder at full steam pressure though the stroke of the piston.
In the second case the full cylinder volume at full steam pressure has to be dumped at the end of the stroke.

One simple solution to your problem may be to use the high pressure N2 to power a ejector (venturi) to compress air to 10 psi. It could also be done with pistons but that would be more mechanically complicated. The fact that your N2 supply pressure is no constant is another complication.

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