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Decreasing temperature using pressure

  1. Aug 1, 2009 #1
    Hi everyone,

    Since there is a clear relation between temperature and pressure, I would like to know if it is possible, at the simplicity of our homes, to design a simple experiment where, one can decrease the temperature of an object (liquid, gas or solid) just by varying the pressure levels.

    Do let me know if it is possible....
  2. jcsd
  3. Aug 1, 2009 #2


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    That's exactly how an air conditioner or refrigerator works. A compressor compresses a refrigerant to a high temperature, interfaces it with the much cooler ambient air pulling energy out of it, and then letting the refrigerant expand. When it expands, it will be at a much lower temperature that can be used to cool the air the refrigerator/AC is attempting to cool.
  4. Aug 1, 2009 #3


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    Isn't that what the compressor in your refrigerator and air conditioning units already do?

  5. Aug 1, 2009 #4
    Thanks all for the replies...

    I know that the airconditioners and refridgerator work on this principle, but these gadgets use costly parts and high voltage to achieve this.

    I was looking for something very simple which could be tried at home.

    For example, I saw in one science museum that there was one thin copper pipe kept, over which air was blown fast from one end. In a few seconds, the temperature at one end of the pipe dropped considerably.

    Something on similar lines...
  6. Aug 1, 2009 #5


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    Yes, air compressors do it too. Any time you rapidly compress or decompress any gas, this happens. Are you just curious about this or are you trying to build yourself a cheapr air conditioner?
  7. Aug 1, 2009 #6
    That pipe was a thermoacoustic device (I assume). Thermoacoustics can be pretty simple and home-made, and there's no moving parts in the simplest designs! Its pretty interesting stuff, too.
  8. Aug 1, 2009 #7
    That's the best current technology can do. If you'd like a measure of efficiency of doing this you can use the EER for air conditioners, for example. The number tells you how many BTU's of cooling is achieved for each watt of power consumed. An EER of 12, for example, means for each watt of electricity you pay for, you get 12 BTU's of cooling...pretty good these days.

    If you look at typical window air conditioning units in Home Depot for example, maybe 5,000BTU to 12,000 BTU range (approximately) you'll see EER's range from about 9.8 to around 10.8. Central air conditioning units in homes might be about 13, much, much more efficient.

    Exterior air conditioners can be used for efficiently when kept in the shade, say under a large tree which blocks direct sun, but allows for free airflow.

    I wonder why they don't use the same measure for home refrigerators?? Instead, they utilize an "energy use" measure reflected on the yellow energy tags.

    One way to make home refigs more efficient would be to exhaust heat in the summer and retain that heat inside in winter. But I'm guessing the cost of ducting and automatic shutters and heat loss and gain of the ducting itself means it's a long,long,long,payback.

    "For example, I saw in one science museum that there was one thin copper pipe kept, over which air was blown fast from one end. In a few seconds, the temperature at one end of the pipe dropped considerably."

    That sounds like a heat exchanger....if the pipe was already hot or the air cooled, sure that's what happens, but the heating is what takes most energy, not the movement of air. Pressure changes alone do not produce cooling, it's a means of achieving temperature gradients.

    If you search THERMOACOUSTIC REFRIGERATION you'll find experimental work being conducted at Rutgers and Penn State.
    Last edited: Aug 1, 2009
  9. Aug 1, 2009 #8


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    FYI, the government mandated minimum for central air conditioners is 13 SEER. High end air conditioners get to about 18.
  10. Aug 1, 2009 #9


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    Aerosol cans do this, too. Get a can of computer dust spray and let 'er rip (outdoors!) In short order, the can will become too cold to hold comfortably and may even develop frost on its surface.
  11. Aug 1, 2009 #10
    Thank you all...for your replies!!! Much Appreciated

    It s a pleasure to know there are still people in this world who are genuinely interested in the sciences.

    Wish you all bright success in all your endeavors!
  12. Aug 1, 2009 #11
    Russ...glad you posted that info...I had to look up my home AC...I had thought it was the steady state EER of 13 but the spec sheet says it's 13 SEER...

    and also we should have mentioned some theoretical CARNOT CYCLE heat considerations to the poster...see
    Wikipedia for a discussion of EER/SEER/COP as well as Carnot cycle theory at

    And split systems may offer additional efficiency, from the above wiki:

    (I wonder why a split system is more "efficient"??)
  13. Aug 1, 2009 #12
    Talking about efficiencies, brings my thoughts towards our electricity transmission lines.
    Of all the electricity that is generated by either water or coal, only a tiny percentage actually manages to get transmitted. So somewhere during the conversion, electricity is lost and efficiency decreases.
    Friends, please advise if I must post this in some other relevant section of the forum, as this is change in topic...
  14. Aug 1, 2009 #13


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    I don't see where that is implied. Most residential air conditioning systems are split systems. Packaged units typically have to be located on a roof (the entire unit). Unless they mean to compare them with window units (but I don't see that as being implied), which are limited in efficiency because of their physical size.
  15. Aug 1, 2009 #14


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    I'm not sure what you mean. The loss in the transmission lines themselves is only something like 7%. Transformers may have another 10% loss altogether. So something like 85% of the power produced by a generator gets to usage points.
    You're ok.
  16. Aug 1, 2009 #15
    Russ. you post # 13 is likely correct...I was thinking "two zone" rather than split...which probably refers to separate compressor(outdoor) and evaporator(indoor).
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