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Cooling between chambers

  1. Dec 13, 2013 #1
    I have a practical problem that I am trying to solve regarding using air chambers. I hope someone can help me with the problem. I have two air chambers connected by a tubing and a valve. One is pumped to a low pressure, chamber A (~30 kPa), and the other is at near atmospheric pressure, chamber B. When I open the valve I get air rushing into chamber A. Once the valve is closed, the remaining air temperature in chamber A is hotter and the air in chamber B is colder. Both started at room temperature. So far this makes sense and I understand the reasons why this occurs.

    However, I am looking for a solution to minimize the temperature drop in chamber B while still transferring the same total amount of air (30 L) in the same period of time (2-3 seconds). Any solution to this general problem is welcome.

    I also have a specific implementation in mind however that I hope someone can comment if it is logical or illogical. The solution is to have a long tube or relatively large diameter (2 inches) connect between the valve and chamber B. The diameter of the tube will be significantly larger than the opening of the valve (0.5 inches), so there will be very little pressure differential per unit length in the large diameter tube. The idea is that the low temperature will be created at the valve to tube interface. The cold air will be formed at the valve. The long length of the tube will provide the necessary thermal diffusion distance for the air to be heated by the tube walls before the low temperature reaches chamber B. This of coarse only makes sense if the temperature differential is actually forming at the valve itself and not uniformly a long the tube and chamber B.

    I am willing to experimentally do this solution, however if someone sees this as flawed logical it could save me a lot of time.

    Thanks,

    Near
     
  2. jcsd
  3. Dec 13, 2013 #2

    Baluncore

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    The temperature of the bulk gas on both sides that is not transferred will be subjected to a change in pressure. That will result in a temperature change. Unless you can thermally couple the non-exchanged volumes of air you will see the change in temperature.

    The temperature change on the low pressure side will be due to the bulk pressure increase and the mixing of the cooled air released through the valve. The high pressure side will not have heat energy transferred against the airflow.
     
  4. Dec 14, 2013 #3

    CWatters

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    If the walls of B are insulated then as the gas expands it's temperature must fall. Only solution is to add energy to B somehow.

    http://www.etomica.org/app/modules/sites/JouleThomson/Background2.html

    Even if the walls of B are good conductors of heat and there is an outside heat source it takes time for energy to move. So that may not stop a short term fall in temperature before it recovers. The thermal time constant of the system may matter.

    The gas inside B also has thermal resistance - so even if the gas near the walls of B could be kept at a constant temperature, the temperature of the gas in the middle might fall before recovering.

    Scuba divers have the opposite problem. As their tanks are filled they heat up. They put them in water to improve the rate at which heat is lost by a factor of 25 and that helps keep them cool..

    http://www.scubadiversions.com/services.htm

     
  5. Dec 14, 2013 #4

    CWatters

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    Possible solution might be to partly fill B with a constant water spray or copper wool? Something that would add a distributed thermal mass. It wouldn't stop the temperature dropping totally but might reduce the magnitude of the drop.
     
  6. Dec 14, 2013 #5

    Baluncore

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    One chamber could be a large vehicle radiator or heat exchanger radiator, enclosed by a very slightly larger rectangular box chamber. The two chambers would then have both thermal mass and close thermal contact.
     
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