Temperature drop during emptying of pressurized tank

[thermorookie]

Dear Forum,

i have a system with a pressurized air tank connected via a filter regulator (automatically adjusting throttling valve) to an air motor. Air is dry.

The concept is that when I open the valve, air passes from the air tank through the filter regulator which adjusts the outlet pressure to a certain level. The air motor will then be able to operate at constant pressure as long as the pressure in the tank is higher than the set point of the filter regulator.

Now my question is; what will happen to the temperature inside the tank, assuming that the heat capacity of the tank is negligible and the tank is isolated.

I know that over the filter regulator no external work is done , so w=o, and assuming adiabatic conditions -> h_in = h_out, and there will be a small temperature drop due to the Joules Thompson effect. Over the air motor, assuming an completely isentropic process the temperature will drop acc. to laws of isentropic expansion:

T2/T1 = (p2/p1)^(1-1/gamma),
gamma = cp/cv = 1.4 for air

However I am unsure how this affects the temperature of the remaining air in the tank. During the open valve period the tank pressure drops from p1 to p2, but what thermodynamic conditions govern?


Thanks

 

[willard3]

Is this an ongoing process or do you stop adding air to the tank?

 

[zdas04]

Joule-Thomson effect has very little ability to transmit temperature changes upstream. The air cools in the regulator and the temperature change is carried downstream. Some amount of the temperature change will transfer to the pipe walls, and upstream piping will cool some for a very short distance (inches, not meters). I've seen flow rates out of an air receiver that resulted in frost on the downstream piping but did not make a measurable change in the receiver temperature.

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.
The plural of anecdote is not "data"

 

[IRstuff]

I would think that it would depend on the delta pressure change in the tank, i.e., if the change is insignificant relative to the overall pressure, the temperature change would not be noticeable, but as the delta pressure becomes bigger relative to the remaining pressure, there would be a noticeable temperature drop.

TTFN
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[zdas04]

With natural gas, I've seen drops from over 2000 psig to 45 psig, with no measurable upstream cooling (significant downstream cooling). Say the upstream gas is at 80°F and the JT cooling is 70°F, the downstream gas is well below freezing, the pipe wall gets very cold and ice forms on the outside of the pipe from humidity condensing. Temperature of the pipe at the upstream threads on the regulator cannot be seen to change from the bulk source. I've checked it many times, because it just doesn't seem possible, but what little temperature change that moves upstream is sucked out by the gas in the pipe. I've uploaded an example of a particularly interesting example.

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.
The plural of anecdote is not "data"

 

[IRstuff]

Is that just specific to NG? I've used Freon and CO2 canisters that frost over when releasing gas.

TTFN
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[zdas04]

I think it is a function of the masses involved. A can of air gets too cold to touch, but when we're talking about industrial volumes and the mass of a pressure vessel the outflow of gas is (in my experience) enough to keep the vessel at about the same temp.

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.
The plural of anecdote is not "data"

 

[chicopee]

If your tank is large enough in a relatively warm workshop and with the compressor pumping hot pressurized air into the tank, you should not have freeze up situations. You indicated dry air, how is that accomplished or is this a school thermo assignment?

 

[thermorookie]

Hi,

Thank you all for the input, but I am looking for a thermodynamical answer rather than practical.

Please notice that the case I am talking about the tank is:
[ul]
[li] isolated, so adiabatic[/li]
[li] the tank itself has no heat capacity[/li]
[/ul]

There tank does not receive air from a high pressure source during the operation.
The starting pressure in the tank is 7 barg
The pressure after the filter regulator is 3 barg
The pressure after the air motor is 0 barg (1 atmosphere absolute)

 

[thermorookie]

regarding the answer from zdas04

That is an interesting picture. My explanation for it is that the pressure on the inlet side does note change significantly fast (if at all) compared to the heat transfer with its surroundings.

In my case the tank size is sized in such a way that the pressure would drop from 7 barg to 3.53 barg during 10 minutes of operation (open valve).

Also I consider my system as adiabatic and the piping and tank material has no heat capacity --- so purely a speculative setup.

 

[zdas04]

Whenever people start talking about "thermodynamical instead of practical", I hear "student post" and stop wasting my time on a thread that will likely be deleted.

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.
The plural of anecdote is not "data"

 

[thermorookie]

zdas04. If you read my initial post you would know the conditions of the system I was asking about.

I don't know which kind of engineer you are, but if you are like me I want to know about the physics involved when designing a system, and not only rely on practical experience.



For the record, this is not a study related question, so I will qoute your signature:

"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.
The plural of anecdote is not "data"

 

[dvd]

Draw a control volume around your tank, forget about the regulator. You are converting the internal energy of the air in the tank into enthalpy of the gas flowing out of the control volume. It is a problem presented in practically every thermodynamics text. Conduct a search using terms like enthalpy + "internal energy" + "tank emptying".

 

[25362]

You may find some satisfying reading in the following:

thread124-202829
thread391-78680
thread798-242268