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Cool down below ambient?

  1. Jul 31, 2006 #1
    Hi,

    Can a radiator ie: air to air intercooler in a car (or industrial application) cool the air passing internally through it cooler than the ambient air passing externally through it without the aid of liquids.

    Thanks
    Darren
     
  2. jcsd
  3. Jul 31, 2006 #2

    Danger

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    Wecome to PF, daz. Depending upon how you choose to look at it, the radiator doesn't cool or heat anything. It's the fluid or gas flowing through it that does the work; the metal is just a container for it and conductive medium for the heat transfer. By that token, one that has no circulating contents (or contents that aren't altered in another part of the system), will have no effect upon air that encounters it.
    Whether the air is heated, cooled, or unchanged is dependent upon the temperature of that 'working fluid'.
     
  4. Jul 31, 2006 #3

    rcgldr

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    As the intecooler pressurizes the internal air, it raises that high pressure air's temperature to well above the the temperature of the external air, which allows the internal air to be cooled via radiation via the metal interface between internal and external air. For a real intercooler, the temperature is still probably higher than the external air, but less than if external air were just pressurized without heat transfer to external air stage. If the intercooler reduced the pressure back to the same as the external air's pressure, then the internal air would end up cooler.

    It could be possible to boost the air to a much higher pressure, say 40 psi, then go though the heat exchanger, then lower the pressure to 10psi, and end up with much cooler air, but I'm not aware of intercooler setups that do this. This would require some sort of computer controlled valve mechanisim to control the rate of air flow to maintain the 40 to 10 psi differential while alllowing a variable rate of air flow. It's also possible that pressurizing the air to 40psi cost more power than what is gained by lowering the intake air temperature futher still.
     
  5. Jul 31, 2006 #4

    rbj

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    Jeff, this is just about what refrigeration does. it works for air, but works better for a more compressable gas.

    you don't need a computer controlled valve or computer controlled anything. you need nozzles, piping, and a pump (a.k.a. "compressor").
     
  6. Jul 31, 2006 #5

    Danger

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    Hmmm... sorry, guys. I think that I misunderstood the question. :redface:
     
  7. Jul 31, 2006 #6

    rbj

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    i think the strict answer to this question wihtout making other unstated assumptions (like a compressor, etc.) is no. without doing "work", the 2nd Law of Thermodynamics implies that heat will always naturally flow from hotter objects or substances to colder objects or substances. the "cooling" air on one side of the heat exchanger will get warmer as it draws energy from the air getting cooled until they are virtually the same temperature.
     
  8. Jul 31, 2006 #7
    Thanks for your replies, so if the air inside the intercooler was at ambient temperature rather than the )hot compressed charge from the turbo) could it be cooled to below ambient?

    Thanks
    Darren
     
  9. Jul 31, 2006 #8

    rbj

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    i think that you need to be more specific about what your intercooler is. "heat exchanger" is a common term and fit the description you first made. is that what it is?
     
  10. Jul 31, 2006 #9
    Yes it is a heat exchanger, just an aluminium radiator that sits on the front of the car, the heated charge from the turbo goes through the intercooler then out to the engine.
     
  11. Jul 31, 2006 #10

    Danger

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    I think that in your case, then, you'd be really lucky to get the turbo outflow below a couple of hundred degrees (rough guess there), particularly if the intercooler is situated behind the engine radiator. Of course, that also depends upon what your air intake is like. With ram-air, you'd go a lot cooler than if it draws from under the hood.
     
  12. Jul 31, 2006 #11

    rbj

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    yup what Danger said. i just wanted to say that the (pretty solid) theory governing a heat exchanger is that the hotter fluid gets cooler and the cooler fluid get warmer and the temperatures of both will not cross each other in magnitude. otherwize you would have heat from the colder fluid being transferred to the hotter fluid and, in conduction, heat just doesn't travel that way.
     
  13. Jul 31, 2006 #12
    Thanks for your reply,

    I have been asked this question by a fellow

    Why can water freeze in puddles or buckets, when the air temp is above zero, or how coolant with a freezing point of -30c can freeze in a radiator while you are driving when the air temp is only -15c

    My answer is it cant to both questions but he insists that it did.
     
  14. Jul 31, 2006 #13

    chroot

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    Water in puddles can freeze even when the air temperature is very slightly above ambient. This can happen at night, when the puddle loses more heat via radiation into the emptiness of space than it receives back from space. Of course, the water also needs to be in some kind of a vessel that insulates it from the warmer ground, and it needs to have a very high surface area to volume ratio (i.e. a shallow, wide puddle).

    - Warren
     
  15. Aug 4, 2006 #14
    Jeff,

    I like that idea, all I would need to do is put a restrictor after the intercooler, so the pressure in the intercooler and before it is much higher than after it, the turbo should not need to work any harder as the flow rate has slowed just the pressure has increased.

    What do you guys think of that idea?
     
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