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Why don't air conditioners have regenerative heat exchangers?

  1. Jun 24, 2009 #1
    I've been pondering this question for a while and I think I must be missing something. In a regular refrigeration cycle you have a high pressure liquid at nearly ambient temperature entering a metering device and then an evaporator coil. At which time it is a low pressure and low temperature gas. This gas exits the evaporator while still fairly cold. Why not put a regenerative heat exchanger to cool the liquid as it's entering and heat the gas as it's leaving? The gas is just going to the compressor anyway and the compressor doesn't care what it's inlet temperature is. The resulting high temperature gas from the compressor outlet would be hotter, but this would allow better heat transfer to the ambient air in the condenser. Seems like a surefire way to increase efficiency to me, but I don't see any real world applications. What am I missing?
     
  2. jcsd
  3. Jun 24, 2009 #2

    Danger

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    Gold Member

    This is not an area of knowledge for me, but I would suspect that it's a matter of manufacturing costs.
     
  4. Jun 24, 2009 #3

    russ_watters

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    That was my first thought. I can't offhand think of a technical reason why it wouldn't work, but I'll sleep on it...
     
  5. Jun 25, 2009 #4

    turbo

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    It would work, but it wouldn't be energy-neutral. The boot-strapping involved would violate a basic physical law. Can we figure out why it won't work without additional energy input from outside the system?
     
  6. Jun 25, 2009 #5
    Regenerative heat exchanger Or Suction line-liquid line heat exchanger(as they are called) do not always result in an increase in COP, it depends on the refrigerant in use. True that the enthalpy of refrigerant going into the evaporator coil decreases, enthalpy of refrigerant going into the compressor also increases, hence the work input also increases. Whether the COP will increase or not, then is the question of which refrigerant is used.
    Secondly, it might be the cost factor
     
  7. Jun 25, 2009 #6
    I ran through some calculations, apparently the only refrigerant showing a decline in COP with a regenerative heat exchanger is R718(water), mainly because the colder fluid experiences a lesser temperature change compared to the hotter fluid(colder liquid has higher specific heat), hence the enthalpy drop for hotter fluid(which has to enter the evaporator) is much greater than the enthalpy lift for the colder fluid, & enthalpy drop outweighs the increase in compression work.

    :zzz:I will put my $$ on the cost factor
     
  8. Jun 25, 2009 #7
    While what you've said is true, you are forgetting that it is a heat pump. Having a higher enthalpy going into the compressor does not indicate more work done by the compressor, but instead more heat rejected to the environment. At least that's how I see it.
     
    Last edited: Jun 25, 2009
  9. Jun 25, 2009 #8
    In a properly designed heat pump, the working fluid is in the liquid-vapor or liquid state when entering the evaporator.

    Anyway, to answer your question, this is done and is common practice. You may not find it in your mini-fridge but I believe you will see it with things like HVAC systems. See pic.

    http://www.betterbricks.com/graphics/assets/images/Building_Ops/BOpEqSysChillers_6w.png [Broken]

    Notice the heat exchanger between the evaporator and condenser reservoir exits.
     
    Last edited by a moderator: May 4, 2017
  10. Jun 25, 2009 #9
    Heat pump??:confused:, how does that change the situation?

    Do you mean heat loss to environment in the suction line? Again, how does that change things?
    Higher enthalpy at compressor inlet means higher work input to the compressor(assuming same pressure ratio to be maintained). Remember that isentropes diverge, higher the enthalpy, flatter is the isentrope in P-h diagram, higher is the enthalpy difference =>higher work input.
     

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  11. Jun 25, 2009 #10

    crk

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    Cooling the low temperature liquid before it leaves the metering device would improve the cooling but would be redundant, because that is what the condenser coil is designed for. Transferring heat to the exiting gas would increase its' temperature which would increase the work the compressor has to do to compress the gas and because compressing the gas increases its' temperature even further, you run the risk of overheating the compressor and the motor. Although the resulting higher temperature liquid would be able to release heat more quickly at any given ambient air temperature, it would still be counterproductive because the condenser would first have to remove the extra heat and once the liquid temperature drops to what it would have been if the additional heat had not been added the liquid would still have to be cooled as it had been before. So it would just add to the cooling load the condenser has to deal with.

    But it's a good question. Keeping thinking outside the box. There is still much to be learned.
     
  12. Jun 26, 2009 #11
    Condenser design isn't as critical as the evaporator's. So what if the condenser inlet temperature is high? Usually all the condensers are over sized, so that's not much of an issue. Yes, the compressor temperature also increases, & usually the refrigerant itself is used to cool the compressor, so that will increase the cooling load.

    Anyways, calculations show that COP does increase by introducing suction line-discharge line heat exchanger in a simple refrigeration cycle. Hence work done per unit cooling load decreases.

    Its a trade off in design constraints.
     
  13. Jun 26, 2009 #12
    The heat pump comment was because it's easy to think of an AC unit as a heat engine and get your head messed up. The goal is to move heat not to make the heat do useful work so the more heat you can reject to the environment with the least added energy the better.

    Guess it all comes down to price vs benefit. Since most of the heat is removed through the latent heat of vaporization of the refrigerant the amount of superheat on the vapor line is trivial. Probably something that is beneficial for larger units but just cumbersome for smaller application.

    I think that pretty much answers my question. Thanks guys!
     
  14. Jun 26, 2009 #13

    crk

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    I have worked in the A/C industry for several years and evaporators and condensers are always matched to give the best performance. Condensers are slightly oversized to accommodate heavier loads at peak demand, the very time when the increased load will become a significant factor.

    What works on paper doesn't always work in the real world because most such calculations do not take into account a dynamic environment and are limited to specific scenarios.
     
  15. Jun 27, 2009 #14
    I am not question your experience(I am sorry if it seemed so). I graduated just a month ago, so i dont have much of experience, only theoretical reasoning.
    As far as I can reason, evaporators are to be closely matched to the cooling load, because if it is oversized, compressor work increases, & if it is undersized, we aren't extracting the maximum. While in case of condenser, an undersized one will increase the enthalpy of refrigerant at the cooling coil inlet, while an oversized wont do anything, because the lowest temperature that can be achieved is atmospheric temperature, oversizing won't cool it down anyways, although it would accommodate any fluctuations in the unit.

    There are a lot more techniques of improving the CoP of the system, example, multi pressure systems, ejector compression etc. But you wont find anyone in a home unit. Like I said, its a trade off in design constraints.
     
  16. Jun 27, 2009 #15

    crk

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    Have no fear I was not being defensive, I was merely stating what has been my experience.

    You are right that evaporators are matched to the load but in practise, so are condensers matched to the evaporator. This becomes doubly important if the system is used as a heat pump and the jobs of the two coils are reversed to provide heating and cooling.

    That said, I am beginning to see wisdom of the original premise and find that I must rescind all previous objections.

    Thanks for the enlightening.
     
  17. Jun 27, 2009 #16
    Ah yes:blushing:. Just forgot that one completely:redface:
     
  18. Jul 9, 2009 #17
    A similar system to what you are describing, only working on the air side, is a wrap-around-heat-pipe. It pre-cools the entering air by a phase change in a refrigerant coil placed in front of the cooling coil, the cooler air hits the cooling coil at a lower temperature improving dehumidifying, then leaves the cooling coil even colder, where it hits the wrap around coil that recools the refrigerant to a liquid phase.

    The energy hit is in increased fan external static pressure, but the system does dehumidify well.

    http://www.heatpipe.com/heatpipes.htm" [Broken]
     
    Last edited by a moderator: May 4, 2017
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