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Does air conditioning remove more heat from humid air?

  1. Jul 14, 2016 #1
    Hello,

    assume I have an air condition unit which is not very strong and thus always runs at full power.

    If I let water evaporate in the room without adding heat (e.g. by hanging lots of wet towels), will this improve the heat removal by the aircon unit and thus lower the room temperature, or will there be no difference?

    Evaporating water would add a new heat transport mechanism - it would take away the heat of vaporization from the air and deposit that heat in the aircon unit where the air is cooled down and the vapor condenses back to water. So from this point of view, it should improve the cooling effect. But from the other side, the aircon unit would receive slightly cooler and more humid air - would this increase or decrease its efficiency?
     
  2. jcsd
  3. Jul 14, 2016 #2

    DaveC426913

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    1] Humid air transfers heat better. A warm humid room will feel hotter than a warm dry room.
    2] One of the big things that AC does to help a space feel cooler is to remove a significant amount of moisture from the air.

    So, adding moisture will counteract your attempts to feel cooler.
     
  4. Jul 14, 2016 #3

    russ_watters

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    I'm not sure #1 is what you meant to say. A humid room feels warmer (and clammy) because higher humidity inhibits the evaporation of sweat. #2 is correct and thus the conclusion is correct: adding humidity to the room makes the AC work harder to accomplish its task.
     
  5. Aug 4, 2016 #4
    the question asks will the temperature be higher in a humid room or a dry room...not what will it 'feel like'
     
  6. Aug 4, 2016 #5

    DaveC426913

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    Good point. I made an assumption, based on the knowledge that A/C is generally used to make people feel more comfortable, that this was about subjective temperature.

    If the OP returns, perhaps s/he can clarify the intent.
     
  7. Aug 6, 2016 #6

    russ_watters

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    If you add humidity to the room it will end up warmer because you are making the AC work harder.
     
  8. Aug 6, 2016 #7
    I dont understand what you mean by the phrase 'you are making the AC work harder'
    An AC unit removes heat energy from a room and transfers it to the outside.
    The AC does not know if the air in the room is humid.
    Removing heat energy will lower the temperature in the room.
    Cooler air will cause moisture to condense so the humidity will decrease.
    Condensing vapour will tend to raise the temperature.
    the AC continues removing heat !!!
     
  9. Aug 6, 2016 #8

    russ_watters

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    The AC unit has to remove a certain amount of heat from the room to match the heat flow into the room through the walls, windows, etc. If you add moisture to the room, you increase the amount of heat the air conditioner has to remove from the room because in addition to making the air colder, it also has to remove more heat to squeeze out the moisture you added.
     
  10. Aug 7, 2016 #9
    'squeeze out the moisture'..?
    I think that you are confusing the role of air conditioner with that of dehumidifier. Air conditioner is designed to lower temperature, dehumidifier uses the same principle of lowering temperature with the aim of making water condense.
     
  11. Aug 7, 2016 #10
    Either way, a dehumidifier creates a reduced temp zone in a room and puts the excess heat back into the room,thus no apparent change in temp. An ac unit just puts the heat outside. Water has a specific heat required to vaporize isothermally. It take same amount of heat flow into the water to create vapor as it does out of the vapor to cause condensation into liquid. As the AC unit moves heat somewhere else it will also cause some condensation which is isothermal. That portion of the energy is expended, however, caused no reduction in temp.
     
  12. Aug 7, 2016 #11

    Fervent Freyja

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    Incorrect. Please know what you are talking about before posting. Most modern air conditioners also remove humidity from the air : "This process occurs when hot, moist air in your home comes in contact with the cold evaporator coil. The liquid is then condensed out of the air, making your home less humid. The moisture collected by the evaporator coil goes to a drain and then it is sent outside, away from your home or office building."

    So, like russ posted, adding humidity will force the air conditioner to work harder, thereby keeping temperatures higher than it would be without.

    I personally perceive your responses on this thread to be attacking (nit-picking) other members. I advise you not to do that again.
     
  13. Aug 7, 2016 #12

    OmCheeto

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    This reminds me a bit of the "Instant Vacuum Dryer" thread.
    After thinking about this, I think that wet towels may actually improve the efficiency of the A/C unit.

    Say you have wet towels that hold 1 kg of water.
    As the water evaporates, the the towels will cool off, cooling off the environment. 2,265 kilo-joules of free swamp cooling!
    But, the extra moisture will cause the A/C unit to have to expend that much energy to re-condense the moisture: Zero sum (theoretically....)
    But one thing I noticed about my brand new window A/C unit, is that the condensate is routed to the outside section, and the hot radiator actually sits in the pool of cool condensate.*
    So as that water evaporates outside, it will act as a kind of swamp cooler, and carry that energy away, making the unit consume less energy.

    wet.towels.and.air.conditioners.png





    *I actually thought they had forgotten to put a drain hole in my old A/C unit, and was going to drill a hole in the bottom. Fortunately, it died of heat stroke last year before I got around to it, as when I saw my new unit didn't have one either, I decided that they must be designed that way. Then I scratched my head for a few moments, wondering why, and then said to myself; "Ah ha! Brilliant!"
     
  14. Aug 8, 2016 #13
    advice duly noted
     
  15. Aug 8, 2016 #14
    you response makes physics sense...I am not familiar with the phrase 'squeeze out the moisture' from another response.
     
  16. Aug 9, 2016 #15

    russ_watters

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    Nope, I'm not confusing the two. As you say, they do basically the same thing. So if you add enough moisture to the air to exceed the moisture content of the air conditioner's supply air, you force the air conditioner to remove that moisture.

    Maybe it would be helpful if you picked some conditions and we worked through the energy requirements.
     
  17. Aug 9, 2016 #16

    marcusl

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    There is a quantitative answer to this question, folks. The latent heat of vaporization for water is approximately 2.3 kJ/g (with a slight temperature dependence). You can calculate the amount of water vapor present in, say, a liter of air--it depends on the relative humidity, temperature and pressure--and compare the energy required to condense that vapor into water to that required to cool the air by a certain amount (1K, for instance). The AC unit can remove only a certain amount of energy per unit time, and energy removed from water vapor cannot be removed from (and therefore cannot reduce the temperature of) air. I leave the calculation to those interested, but note that the latent heat quoted above is so large compared to the specific heat of air (1 J/g-K) that it can will significantly reduce the air temperature drop at high humidity.
     
  18. Aug 9, 2016 #17

    OmCheeto

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    Ok. I did one calculation.
    My 736 watt window A/C unit would take 51.3 minutes to remove 1 kg of moisture from the air.
    Assuming it did nothing else, and COP of 1.

    Does this mean that I would have wasted 629 watt hours of energy, as there was no cooling?

    I think one problem with this thread, is that there are (at least) two separate questions:

    1. Does air conditioning remove more heat from humid air? (The title)
    2. ...would this increase or decrease its efficiency?

    I think the answer to #1 is yes.
    And #2? My guess is, that because the A/C unit has to run for nearly an hour, with less than 100% efficiency, just to remove the water, the answer is "decrease".


    ps. I just went outside and measured my A/C's "condensate pool" volume. It should hold almost exactly 1 kg of water.
     
  19. Aug 10, 2016 #18

    russ_watters

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    All else being equal, the answer is yes.
    I think #2 is a "wrong question". To an engineer, "efficiency" is heat out divided by energy in: almost all air conditioners are more efficient when the air is moist because the heat transfer in condensation is very effective.

    But I don't think that's what the OP wants to know. The right question is: does it take more or less energy to cool moist air. It takes more.
     
  20. Aug 10, 2016 #18

    russ_watters

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    Does the coil just sit in the water or is the water dripped or sprayed onto the coil?

    I think such a feature is required in Europe and Japan, but I haven't seen it much in the US.

    I have run tap water to sprayers on the condenser of my split system AC unit, which gives up to about a 20% efficiency boost (half output increase, half input reduction), but it is only economical when it is hot out due to the water cost. Using the condensate is of course free and I may try to do that.
     
  21. Aug 10, 2016 #19
    I tried this on my old A/C, and it actually reduced efficiency! I was perplexed, and then I realized the fins were so close together, that the water had bridged the gaps and reduced the air flow! Very counter-intuitive at first.

    The other concern is that minerals in the tap water might build up and coat the coils/fins and reduce thermal conductivity over time. The condensate should be mineral free, and only contain a bit of dust/dirt from the air, but I would think that would tend to wash away?
     
  22. Aug 10, 2016 #20

    rbelli1

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    Most of the window units I have seen splash water on the condenser using a ring built into the fan.

    BoB
     
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