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Conservation of Mass Principle applied to Refrigerators

  1. Aug 14, 2013 #1
    Dear all,

    Can the conservation of mass principle be applied to the mass flow rate of a refrigerant in an ideal refrigerator system? And what about for an actual refrigeration system?

    I'm reading the principle here but can't work it out. Says that it applies for steady flow, but I assume for a refrigerator the flow would not be steady because of the evaporation/condensation processes, but maybe it still applies. If so, how exactly? Can anyone help?

    I want to eventually find out if the refrigerant mass flow rate is equal in all parts of the refrigeration process, for ideal and actual systems.

    Thank you very much
     
  2. jcsd
  3. Aug 14, 2013 #2
    Also, if you have any useful links on this please send them, thanks.
     
  4. Aug 14, 2013 #3

    SteamKing

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    Most refrigeration systems work on a closed cycle, so yes, conservation of mass of the refrigerant applies, unless there is a leak in the system.
     
  5. Aug 14, 2013 #4
    The term "steady flow" (steady state operation) means that nothing is changing with time at each specific spatial location in the system. As long as the refrigerator is running continuously, nothing is changing with time at each specific spatial location, even though the parcels of refrigerant passing through the system are experiencing changes along their paths. But, if the system is switching on and off intermittently as needed to maintain a certain temperature in the refrigerator, then the system is not operating at steady state, and things will be changing at each location with time. However, eventually, if the system is allowed to stay on long enough, it will again approach steady state operation.
     
  6. Aug 14, 2013 #5
    Excellent, thank you both very much.

    I will discuss about both in my report: possible losses in system leaks and also about intermittent operation causing steady state condition to change.

    Related Question

    I remember doing a venturi lab test when the mass flow is conserved, but i think there was some difference in the mass-flow rate of one side of the pipe to the other, even though the cross-sections were equal. Is this meant to be possible and what can cause this, can friction losses upset the steady state conditions in this case?
     
  7. Aug 14, 2013 #6
    If the upstream and downstream pressures were maintained constant, and the mass flow rate upstream was maintained constant, then you should have achieved steady state conditions. Possibly you are referring to the possibility that the downstream pressure was lower, so the downstream density was lower. Under these conditions, the downstream velocity would be higher than the upstream velocity. But mass would still be conserved, and the system would still be at steady state. Friction losses do not affect whether the system is at steady state.

    chet
     
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