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Boil water at higher pressure vs. energy consumption

  1. Jan 22, 2008 #1
    (I know that it's easier to get water to boil att lower pressure...)

    The goal is to get the water to an temperature of 100 degress celsius in the most effective way.
    Then If you raise the pressure(with a pump), is it possible to get the water to the right temperature with less energy?!?
    AND you disregard the energy to raise the pressure!

    /rikard
     
  2. jcsd
  3. Jan 22, 2008 #2
    The heat required for temperature change in a system is given as:

    [tex]
    Q = mC\Delta T
    [/tex]

    As such,

    [tex]
    Q \propto \Delta T
    [/tex] (C shows very small variation with T)

    Therefore, a certain amount of heat MUST be provided if the temperature is to be changed by a certain value. Hence, no matter what method you use, the energy required will be exactly the same.

    This is the case in theory. In practice, conservation of energy still applies, but it'd be more efficient to heat the liquid using a fuel burner than to increase the pressure, because for pressure systems, energy losses [here 'energy' means 'usable energy', or the useful energy that can be used to do work in the desired way] are comparatively high as compared to an efficient burner using a good fuel.
     
    Last edited: Jan 22, 2008
  4. Jan 22, 2008 #3
    Okey, yes heat would be added anyway, not only pressure to achieve the right temperature.
    Would higher pressure result in that less heat can be provided to the water and still get it to the right temperature?

    Would it go faster to heat water in pressure?

    (We could say that the pump is driven by an human, so it wouldnt use any of the energy that could/would be use to get the water to the right temperature)

    /rikard
     
  5. Jan 23, 2008 #4
    At normal pressures, i.e., the kind you might achieve with a pump, the specific heat of water may be considered a constant. Thus, you would use almost the same amount of heat and require almost the same amount of time whether you are heating with a loose fitting lid or a pressure cooker lid.

    You would have to apply enough pressure to noticeably compress the water before you would also see differences in the specific heat.

    You may be thinking of the fact that you can cook foods faster in a pressure cooker than in atmospheric pressure, but that is due to the elevated boiling temperature in the elevated pressure.
     
  6. Jan 23, 2008 #5

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    To amplify just a bit, the elevated boiling point not only keeps the water liquid, but makes the reaction rate of cooking much faster due to the higher temp, thus saving energy. It doesn't loose heat quickly, and continues to cook for quite some time even after the heating source has been switched off. That's why you get the best results if you don't open the lid too quickly.
     
  7. Jan 24, 2008 #6
    And, from experience, you get quite a reaction if you open the lid quickly!
     
  8. Jan 24, 2008 #7

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    That is right.

    Actually, you just cannot open them due to friction until the pressure has fallen below a dangerous limit. Of course, I haven't tried to force it open with a wrench or something. But I've read about this point in ads. And whenever I've tried it a bit too early out of impatience, oh yes, I found out about the latent heat of condensation. Should NOT be tried. :eek:
     
  9. Jan 24, 2008 #8
    There is very little that is both stupid and dangerous that I haven't tried at least once.
     
  10. Jan 24, 2008 #9

    RonL

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    As was stated, the pressure has to reduce, but you can still put your potatoes "on the ceiling" :eek:
    Talking while absentmindedly fiddeling with the handles, there is a time when the lid will slide on the gasket, even though there is pressure in the pot.
     
  11. Jan 24, 2008 #10

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    You two are people after my own heart. :tongue2:
     
  12. Jan 25, 2008 #11
    Umm... I think your missing something fundamental here. The whole reason water is easier to boil at a lower atmospheric pressure is that it boils at a lower temperature. once the vapour pressure of a liquid becomes equal or higher than the atmospheric pressure, it will boil. I've done work on fuel pumps where this happens at room temp due to the suction pressure sometimes involved. Its the root of all cavitation problems... but thats another story.
     
  13. Jan 25, 2008 #12

    RonL

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    I had wondered why boat props underwater had this problem under certain conditions, never gave it much thought, i always assumed air was being drawn in somehow:rolleyes:
    Low pressure near the hub would develop a gas pocket, would this be correct ?
     
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