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Question about refrigeration

  1. Nov 1, 2009 #1
    Hello, I am a first year engineering student and I have a short question about refrigeration systems. Just want to clarify that this is not a homework question. Yes I do think about things like this in my spare time its just personal interest. I'm taking engineering for a reason :tongue:. Anyways i was wondering if placing a cold object on the condenser/receiver side of a refrigeration system would increase or decrease the efficiency of the system as a whole. I suspect that it would increase the efficiency because if the refrigerant is colder in the condenser (because the cold object would cool the refrigerant down) then my logic says it should condense faster (which, from what I have gathered, is the point of the condenser) and be colder when it is condensed so when it expands in the low pressure portion of the system it should get colder than it would have otherwise. Overall result is less energy input required to run the system. This makes sense to me but I just wanted a second opinion because like I said i am a first year engineering student so I accept that it is very possible for me to be completely wrong about this sort of thing. Just FYI i am not suggesting we all cool things down in our refrigerators then place the cold objects on the warm parts at the back of our refrigerators because in a best case scenario you would be wasting your time because the overall energy use of the system would be the exact same but in a real life scenario you would actually be wasting energy. the purpose of the question is to see if I understand this relationship of the system correctly.
     
  2. jcsd
  3. Nov 2, 2009 #2
    We dont generally talk in terms of efficiency for refrigeration system, there is another term called "Coefficient of perfromance" or CoP for short(thats just the convention).

    Anyways coming to your observation, you are right, colder the space around the condenser, colder is the refrigerant at the condenser outlet & hence at the evaporator inlet, and the specific refrigeration effect per unit work is increased, or in other words, work required per unit referigeration effect is decreased, or in still other words, CoP is increased.

    But how cold can you make the condenser side space?
     
  4. Nov 2, 2009 #3
    You are correct in your understanding. In real-life applications, there are two very common ways that this is put into practice. The first is called a 'precooler', and it is essentially an evaporative cooling pad with associated pump and reservior placed some short distance from an outdoor, air cooled condenser coil on a typical A/C system. The air that passes through the precooler is cooled through evaporation, and that in turn cools the condenser and lowers the high-side refrigerant pressure. Hence, the compressor has less work.
    Another common method is the use of a water-cooled condenser. Cool water is piped through a heat exchanger; the other side of the exchanger holds the refrigerant to be condensed. The water, after absorbing heat from the refrigerant, is pumped to a cooling tower where it is cooled through evaporation and returned to the condenser.
     
  5. Nov 2, 2009 #4
    Soon you'll learn of the second law of thermodynamics. This will enable you to investigate things like these in a very quick and elegant way, especially closed cycles.
     
  6. Nov 2, 2009 #5
    Thanks you guys are awesome :D! I totally appreciate your help.
     
  7. Nov 2, 2009 #6
    I have noticed that all my window A/C units now use the water off of the evaporator to aid in cooling the condensor. They don't have drains like the old ones did, so I assume (oh, OH...) that the additional cooling is how the energy efficiency or CoP( ...yes, I learned something) is increased.

    but, Zack, you didn't answer, ank_gl, how cold...

    dr
     
  8. Nov 2, 2009 #7
    There are some interesting new approaches for air conditioning and refrigeration.

    My company the Dais Analytic Corporation is developing one that uses a specialized membrane and water vapor. An explanation can be found on the daisanalytic.com website.
     
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