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Cooling capacity of an aircon compressor

  1. Mar 13, 2005 #1
    how does the cooling capacity of an aircon compressor exceed that of the power supplied to it?
    from what i understand, energy cannot be created or destroyed. so when we put in 500W of electrical energy into a compressor, why do we expect the cooling capacity to be 1500W.

    does that mean that airconditioners are very effiicient devices

    sorry for the lousy phrasing, i can't think of a better way to put it.
    thanks.
     
  2. jcsd
  3. Mar 13, 2005 #2

    brewnog

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    This is how I understand it. I might change my mind when Russ tells me I'm talking rubbish:

    The cooling capacity (in this case 1500W) relates to the amount of heat that the air conditioner can transfer from inside to outside. It's not really heating or cooling anything, it's just moving heat from inside to outside.

    The power (in this case 500W) is the amount the machine could be expected to draw from the mains, in order to power the compressor, pumps, control system etc.

    I think a good analogy would be thinking of the amount of energy you'd use to carry a pan of hot water from your kitchen to your garden. You're just moving heat from one place to another.
     
  4. Mar 13, 2005 #3
    that is right. The Cooling capacity is the amount of heat that is going to be moved. So if we have 1500W, (usually this is in Btu) then the condensing unit can take out 1500W of heat energy out of the inside air via passing it through the condensing unit and cooling it through the cooling coil. Remember, heat and mechanical energy are not the same. There is a constant which seperates the called Joules constant.

    Regards,

    Nenad
     
  5. Mar 13, 2005 #4

    brewnog

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    Cheers Nenad. Be a darling and tootle on over to the Engineering forum, and de-bug my recent explanation of evaporative coolers in the Portable Air Conditioner thread!
     
  6. Mar 13, 2005 #5
    i see thanks nenad. is there a theoretical maximum amount of heat that the compressor can remove based on the joules constant? sorry, i did not get to study about it in school.
    i found out that joule constant is 4.186J/cal and that 4.186Joules=1calorie.
    if that's the case, the joule constant is simply a conversion of the joule unit to cal unit does therefore suggests that conversion between heat energy and mechanical energy does not result in any loss of either of the two forms.

    sorry to make these assumptions. but if so, does that mean that to remove 1500W of heat from the room, it can take any amount of electrical power to do so, depending on the compressor efficiency.



     
    Last edited: Mar 13, 2005
  7. Mar 13, 2005 #6
    thanks brewnog, u gave a good analogy. are you said that 500W is the amount of energy needed transfer the 1500W of heat out of the room.

    my understanding of aircon cooling is that the compressor mechanically compressess and almost ideal gas to increase its internal energy. this will mean that the amount of mechanical energy put in will not exceed the rise in internal energy of the gas since the gas is not ideal.

    then by allowing the gas to expand, the internal energy falls and temperature decreases. again since the gas is not ideal, temperature fall(cooling power) should not be as great as it should be in an ideal gas.

    Hence, due to the inefficiencies of these 2 steps, cooling power could not exceed the mechanical power put in to the compressor.

    pls correct me. thanks.






     
  8. Mar 16, 2005 #7
    First law of thermodynamics says otherwise :P

    Seriously though, I see what you mean in this context. Don't get saying that often, though.
     
  9. Mar 18, 2005 #8
    You are all correct.

    That's what makes a heat pump so interesting in winter. You get MORE heat into the
    house than you would if you just converted the same energy directly into heat inside
    the house.
     
  10. Mar 19, 2005 #9
    yes, but heat pumps get very inneficient at temperatures below about 0C. The rejection of cool air is hard once the temperature outside is very cold.

    Regards,

    Nenad
     
  11. Mar 20, 2005 #10
    i think i know for how the aircon extracts more heat energy than the amount of electrical energy input and at the same time obeying the laws of thermodynamics.

    the answer lies in the efficient condensor that cools the hot compressed gas before it is allowed to be expanded in the evaporator.
    in other words, the condenser lowers the internal energy of the gas with surrounding air blowing through. when the gas goes into the evaporator, its internal energy is further lowered, drawing more heat energy than the actual amount of electrical energy put into the compressor.
     
  12. Mar 20, 2005 #11

    brewnog

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    It's nothing to do with efficiency weehian.

    You could pick up a 1 kg lump of cold metal, and carry it over to the other side of the room.

    You could then heat up the lump of metal to 500 degrees, and (wearing gloves) carry it over to the other side of the room.

    Just think of it as moving heat from one place to the other.
     
  13. Mar 20, 2005 #12

    Clausius2

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    Maybe this helps a bit:

    Take a control volume in such a way that all the aircon machine is rounded. You'll have different inputs and outputs of energy, but the fact is that for steady state operation and considering there are no losses of heat through pipes and valves, then:

    [tex] Q_{condenser}=W_{compressor}+Q_{evaporator}[/tex]

    It is only a question of energy conservation. The compressor adds enthalpy to the flow of refrigerator each time that it passes through the compressor stage. This enthalpy is added to enforce the fluid movement from lower pressures to higher pressures. In addition to the heat absorbed at the evaporator by means of a state change, the fluid must carry on an additional energy to enhance the jump of pressure. Eventually all this enery is released at the condensator.
     
  14. Mar 20, 2005 #13

    russ_watters

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    "Heat pump" is an unusually descriptive word - thats precisely what it is. Really, an "air conditioner" is also a heat pump - you just pump the heat in the opposite direction. In a heat pump (as opposed to a cooling-only air conditioner), there is a little extra piping and a valve that reverses the direction of flow of the refrigerant and not much else to it.
     
  15. Mar 21, 2005 #14
    ThaNKS Clausus2, the equation solves the puzzle. and thanks for helping me understand this topic. From many airconditioners, seems like the Qcondenser forms a bigger part of the equation than the Wcompressor. Eg. 500W(compressor) of power into the compressor can cool down 1500W(evaporator). therefore, by subtraction, the condenser would help to remove 1000W(condensor) of heat assuming all things ideal.



     
  16. Mar 21, 2005 #15

    thanks russ, how nice if we can make a huge a evaporator to sit in the north and a mega condenser and compressor to sit and the south and connect all of these together with a refrigerant. during summer in the north, ppl will be comfortably cooled and ppl in the south who are having winter will be warmed by the heat from the compressor and the evaporator. heat pump in the south and aircon in the north :smile: only unless there is no heat loss.
     
  17. Mar 21, 2005 #16

    Clausius2

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    Such heat pump would have a poor efficiency. I mean, the more different are both temperatures, the more power you have to supply to the compressor for each unit of heat released.

    I think russ is going to agree with the fact that surprisingly, any refrigerator machine works the best when both temperatures are similar, just when nobody wants to refrigerate nothing. Refrigerators seems to be something that go counter-nature, and so the work of engineers like russ is to fight against nature to cool the home of some capricious guy. :rofl:
     
    Last edited: Mar 21, 2005
  18. Mar 21, 2005 #17

    russ_watters

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    True, but there is an application where just that scenario (sometimes) exists: in an apartment building. Consider springtime - if its 55 degrees outside, the people on the sunny-side of the building are going to get hot and the people on the shady-side, cool. If the building shares a central water loop, people who turn on their heat pumps in air conditioning mode will be putting heat into the system and and people operating their heat pumps in heat pump mode will be pulling it out. They quite literally pass heat back and forth between apartments.

    Even better, we did a job for a college that was building classrooms in the first floor and basement of an apartment building. The floors above are arranged like fins, so every apartment has outside wall exposure, but the 1st floor and basement are perfect squares. Except for the absolute perimeter, all the classrooms need air conditioning all year-round. Utilizing a water-source air conditioner, the college was actually pumping heat into the building-wide water loop, which was then utilized by the residents to heat their apartments. This resulted in substantial savings for the landlord (not sure how they charged their tenants), but the college declined our offer to calculate how much, but its thousands of dollars - perhaps as much as $10,000/month for 6 months a year.
     
    Last edited: Mar 21, 2005
  19. Mar 21, 2005 #18
    Actually, the "idealized" heat pump is always better for heating regardless
    of it's efficiency.

    ["Idealiszed" means my heat pump's compressor is *in the house*
    and only the heat exchanger coil is outside. Real heat pumps rarely
    do this.]


    If I have a 1000 Watt resistance heater, I heat the house at the rate of
    1000 Watts. If I have a heat pump to which I deliver 1000 W, even at very low
    efficiency I will get *more* than 1000 W of heating into the house (assuming as
    I did that the compressor is *in the house*.)

    This is beasue the ineffieicncy of the system translates into heat released
    into the house by (practically speaking) the compressor and motor.

    In the extreme case of it being "absoulte zero" degrees outside, I would
    recover the original 1000 W of heat from a zero efficieny heat pump.
     
    Last edited: Mar 21, 2005
  20. Mar 21, 2005 #19

    Clausius2

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    It seems a good idea to take advantages of transferring heat one zone to another into the same building. Right. :approve: I hadn't never thought of it.

    I have another little question for you, russ.

    Why don't aircons (or refrigerators in general) use water as refrgerant liquid? The other day I didn't know how to answer to Astronuc who questioned me about the practicity of pressurizing the cooling circuit to avoid the freezing of water. I mean, I would pump the water into the circuit, I'd seal it at some pressure above the atmospheric one, and so It could reach temperatures below 0ºC at the evaporator without freezing. By the way there won't be any contamination via CFC's. What is the main problem with this? I could imagine it is only a question of money. :confused:
     
    Last edited: Mar 21, 2005
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