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Microwave heating

  1. Feb 9, 2005 #1
    I'm currently sitting in a very cold room, it's cold because heating costs too much darn money (we haven't got central heating:(). Well, this got me thinking over cheap ways to heat a room. The idea of heating air with microwaves struck me being really cheap and effiecient. But it's so obvious, I thought, that it should be common place. Why isn't it? Is there a reason why the technology of microwave ovens can't be turned to heating nitrogen in air, or water in central heating pipes, immersion heaters, or a kettle even? (and yes, I know about the superheating problem, but this seems easy enough to overcome)

    The only applications I've seen on the net are industrial, but how easy would it be to scale these processes down? I'm left thinking that it's possible, but maybe copyrights are getting in the way of it being used.
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  3. Feb 9, 2005 #2


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    So you feel a blast of hot air coming out of the microwave every time you open it? No?? Microwaves are tuned to heat up water, not air.
  4. Feb 9, 2005 #3
    Yes, but as I understand it, it is the frequency of the microwave that is tuned to h2o in ovens. Industrial microwave heaters can be set up to heat other materials by changing the frequency to it's resonant freq. [afaik:)]

    Why not tune to nitrogen or oxygen or co2 if that's the case?

    Also, even if it can only do water, then why not water in central heating or powershowers? Why can't I take the prehistoric 3kw electric element out of the immersion heater and replace it with an 0.5kw immersable microwave generator that's twice as fast?
    Last edited: Feb 9, 2005
  5. Feb 9, 2005 #4
    the effiecency of a heater is 100% already...... what is the point of microwave heating air????
  6. Feb 9, 2005 #5


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    Micro waves heat water by inducing molecular level vibrations, this energy is transfered to Kinetic energy during collisions with other water molecules. So one thought would be that in a gas this type of mechanism would not be as efficient due to the lower frequency of molecule to molecule collisions.

    You need to know the bond length of O2 and N2 to find the frequency needed to induce intra molecular vibrations. Is this frequency in the microwave range?

    I don't know the answers off the top of my head. If you can answer those questions you may be able to discover if your concept is viable.
  7. Feb 9, 2005 #6
    Not entirely true I'd say, heaters lose energy to light in some cases. Those that are fairly efficient (electric oil filled radiators) spend a good deal of energy heating the oil first, microwaves are direct. Probably only IR heaters approach 100% efficient, and they are expensive (and overkill for domestic use).

    Central heating though, is at best (ie condensing boilers) only 85% efficient, surely direct microwave heating would be better in this case?
    Last edited: Feb 9, 2005
  8. Feb 9, 2005 #7
    This would not be a good idea. Exposure to microwaves tend to cause sterility and cataracts. Probably not something you want to be pervasive in your home.
  9. Feb 10, 2005 #8
  10. Feb 10, 2005 #9


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    I'm getting images of those old ladies who put their pet cat into their microwave oven to warm them up...
  11. Feb 10, 2005 #10


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    Light is just high frequency heat. And as long as it stays in the room, the heat stays there as well (the light gets absorbed by the walls, turning it to heat). Yes, all forms of electric heat are about (at least - heat pumps are more) 100% efficient.
    Well, boilers use gas or oil - they have hot exhaust, which is hot air being blown out the top. That's where the 85% efficiency comes from. 100% efficiency applies to electric only. The reason they are cheaper is simply a matter of the economics of the fuel.

    Now infrared and microwave heating would have an advantage over other forms: they do not require heating the air. This is why at some restaraunts and driving ranges, they use infrared (also, its actually illegal to use conventional heat outside) - you can heat people directly, reasonably inexpensively. But I wouldn't want to risk using microwaves.
  12. Feb 10, 2005 #11
    Ok, well, some of you got the impression that I meant to open the door of a microwave, break the safety latch and switch on... That's not quite what I meant. Obviously the microwave generator would have to be inside a metal box, with diffraction gratings top and bottom to allow air to convect up and out (drawing more air in as it does).

    However, looking into it more, I found that what Integral said was true, that microwaves only heat water, or hydrogen bonded molecules(fats etc) from what I can tell.
    So thanks for the info guys, I'll put this idea to bed.

    However, heating the water in central heating pipes or immersion heaters still seems feasible. Industrial units that do similar jobs can raise the temperate of liquids from 30c to 200+c in a high flow rate system, with only a few feet of exposure, ie in a matter of seconds. These are 100Kw systems tho, but I would still think that, when scaled down, this method would be more effecient that electric heater coils which require direct contact with water and can only heat the water they touch. It requires convection to take the heat away from the coil, where as microwave can heat it a)quicker and b) evenly.
    Last edited: Feb 10, 2005
  13. Feb 10, 2005 #12


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    Actually he might be on to something.

    The only problem is, as Integral stated, you will need to know the frequency of O2-N2 oscillation, and plus this is extremely inefficient for gases. Your best bet is to heat water in a large pipe which lays on the bottom of a floor, with microwaving equipment at 2 ends, shielded and outside of the domestic area preferrably.

    Another thing is - will it be efficient at all?? Your idea seems viable but the engineering aspect of such an enclosed container with all the microwaves propagating at water under pressure is hard to accomplish safely.

    Edit: you edited it! :grumpy:
  14. Feb 10, 2005 #13
    (the edit was only for typos:))

    Safety is quite easy to accomplish. The industrial system uses a teflon pipe, which is transparent to MW, and pumps the water through a focused microwave beam.
    It's a powerful system (and heavily copyright by the look) and it says somewhere on that site it is slightly less efficent that trandition heating, but benefits from smooth heating gradients. But that system is constantly on, ie its constantly heating cold liquid. Whereas a domestic system is sealed (more-or-less), and the benefit would be that microwave heating would be faster, so cost less in way of kwh that traditional heating, maybe:confused:

    Edit: I said the site says it's less effecient, but that not quite the case.
    I understand tho, that it's a corporate site, so is purposely vague and defence about mw heating.
    Last edited: Feb 10, 2005
  15. Feb 10, 2005 #14


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    A reasonable question to ask is how efficient is the process of creating Microwaves? This is a fair amount of heat generated behind a working microwave, this is all wasted energy. You may want to reseach the effeciancy of a microwave magnetron.
  16. Feb 10, 2005 #15
    Haha, yet again, spot on with your advice, thanks Integral. This from wikipedia.
    That incredibly ineficiant if you ask me, they should design a better one. :devil:
  17. Jul 27, 2005 #16
    Hi, I would be very interested in using microwaves to heat the water for my central heating system. I wou think that the energy consumption should be less because the heating time would be reduced. Even if a part transmitted directly as heat and not as microwave energy would not be a big problem as we could also use this heat to haet up the water for my centrel heating system. I popped a question for information at the webadress I found above. waiting for the answer. Will keep you posted.

  18. Jul 27, 2005 #17
    Hmm, heating up the air seems terribly ineffecient... I think I'll design a microwave heater tuned to h2o frequency that skips the middle man and heats up the people in the room directly. :biggrin: :biggrin:
  19. Jul 27, 2005 #18


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    Microwaves are bad for our bodies. They are so bad, they are being developed as weapons by the US military. It was a secret for a long time and soon to go into the field for the first time.
  20. Jul 27, 2005 #19
    As far as I know, microwaves don't ionize human tissue, only heat it up, so the most damage you can sustain is being cooked.
  21. Jul 28, 2005 #20


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    They're also so bad for our bodies that they're being used for mobile telecommunications all around the world. :tongue:
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