volume-pressure question

[mielke]

If I have a tube with gas being pumped through it, and it is experiencing a large temperature drop (ie the gas flowing in the tube is being cooled) does the pressure or the volume of the gas change or both? (from the ideal gas law PV=nRT)

I guesse I am asking if this is a isobaric process or an isochoric process.

 

[DRWeig]

Mielke,

I don't think the volume can change unless your tube changes its shape...

Based on this and your other post, perhaps it would be best if you gave us a really good description of your project. It sounds like you may be in over your head just a bit...

Best to you,

Goober Dave

 

[ginsoakedboy]

mielke,

Based on your description, I would expect both pressure and volume (density) to change as gas flows through the pipe. I am not sure if the process can be easily categorized as isobaric or isochoric.

Check out

http://www.pipeflowcalculations.com/airflow/index.htm

 

[mielke]

DRWeig,

Can the volume of the gas not change axially with respect to the length of the tube?

 

[DRWeig]

Hi Mielke,

Volume is a fixed amount. The density (kg/cubic meter) will change as you cool the gas, but the volume of your container pipe will not.

Imagine if your tube has caps on each end. The volume of what's inside cannot change unless the pipe changes its shape.

Again, please give us details about your project. This is basic physics we're dicussing here. If we have details, we can share some experience.

Good on ya,

Goober Dave

 

[mielke]

The application is a heat exchanger. We're taking hot compressed gases and cooling them down.

I know compressor industries use heat exchangers. They compress a gas which decreases the volume and increases the pressure, as well as increasing the temperature of the gas (thanks to ideal gas law PV=nRT) after they do this they cool it down with a heat exchanger. If you cool down a gas either the pressure or volume or both go down. It just seems conter-productive if the heat exchanger would lower the pressure by lowering the temperature of the compressed gases and I know that they design heat exchangers for about a 1psi pressure drop (which is just calculated from the friction).

 

[ginsoakedboy]

DRWeig,

I think mielke means specific volume (units: m^3/kg) which is the inverse of density. So yes, the volume will change.

mielke,

I do not claim familiarity with the subject but I would look at compressible flow in pipes as a starting point. Take a look at these links:

http://www.cheresources.com/compressible_flow.shtml

http://www.potto.org/gasDynamics/gasDynamics.php

From what I understand of your process, the pressure and temperature will decrease. The specific volume may either decrease or increase depending upon flow velocity, rate of heat exchange and the gas properties.