1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

What would be the limitations on an AC unit that worked like this?

  1. Aug 8, 2013 #1
    The unit consists of a single, long rectangular chamber. On the inside is a moving wall. On one side of the wall is a vacuum, and the other side is room-temperature air. The moving wall starts much closer to the side with the air and moves slowly in the vacuum direction. As this happens, air particles that make contact with the moving wall will lose kinetic energy, and thus the temperature inside will be lowered.

    I just realized as I'm writing this that I don't have a good way to bring the machine back to its starting state, but maybe one of you can come up with a method. Anyway, is this entire line of thinking silly? Would such a device cool the air significantly? I'm only a first year university student so go easy on me.
  2. jcsd
  3. Aug 8, 2013 #2


    User Avatar
    Staff Emeritus
    Science Advisor

    It would not be effective. Do you know how a standard AC unit works?
  4. Aug 8, 2013 #3
    I looked up the way a commercial AC unit works just before posting, and I know this method would not even come close to the current efficiency standard. I am more interested in the principle. Maybe I should have simply asked, would an expanding chamber cool the air inside it?

    A similar question I had when thinking about this was why doesn't a fan warm the room, as it is adding kinetic energy to the system?
  5. Aug 8, 2013 #4


    User Avatar
    Science Advisor

    Yes, such a design could be made to work. One possible design for such an air conditioner would be to start with a cylinder half-full of outside air and withdraw a piston, leaving the cylinder full of [roughly] half-pressure, cooler air.

    Then use some sort of heat exchanger to let the room air lose heat into the cylinder air.

    Let the piston fall back and now you have a cylinder [roughly] half full of warmer-than ambient air. Exhaust that to the outside and refill with outside air, taking the opportunity to move the piston the rest of the way to the original halfway mark.

    Repeat as neccessary.

    You are using energy since the piston stroke takes more force and covers more distance on the cool-air pull than it gets back on the warm-air push. You are putting heat into the environment because you're taking in ambient temperature air and exhausting higher-than-ambient temperature air.

    It does.
    Last edited: Aug 8, 2013
  6. Aug 8, 2013 #5


    User Avatar

    Staff: Mentor

    What you are describing is a normal reciprocating air compressor (you're just kinda describing it backwards). They do indeed work as functional air conditioners (heat pumps), just not very efficiently. However, this is actually similar to the technique used for cooling gas so far it liquefies.

    When the air gets compressed, it gets hot. Water or air is circulated around the cylinders or air/tubing to cool the compressed air back down. Then the air gets expanded when you use it and it cools below ambient.
    Last edited: Aug 8, 2013
  7. Aug 9, 2013 #6
    Thanks for the answers jbriggs444 and russ_watters!

    One more theoretical question for you guys: would it be possible to design an AC unit that doesn't need to discharge warm air? My gut says the answer is no because such a system would cause a reduction in entropy, but I want to know what you guys think.
  8. Aug 9, 2013 #7


    User Avatar

    Staff: Mentor

    You are correct. Another way to think about it is that you are moving heat, so you are moving it from the cold side to the hot side of the AC unit.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook