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I Calculate boiling rate of water

  1. Apr 11, 2016 #1
    How would I calculate the rate that water would boil off? I've done a lot of looking into and found an equation but it doesn't seem quite right. What I found states that the KJ/h delivered to the water divided by the latent heat energy gives you the amount of water that will boil off. I tried this but I got a very low amount (.05 kg/h).
    I'm boiling 4kg of water in an area that's ambient temperature is 60C under reduced pressure (about 30 torr) in a closed system. The boiling point is 30C and the latent heat energy is about 2438 KJ/kg at this pressure. Is the boiling rate just very slow or should I be using a different equation? Or will I just have to test the system and find out for myself?
     
    Last edited: Apr 11, 2016
  2. jcsd
  3. Apr 11, 2016 #2

    jbriggs444

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    That should be 2438 kiloJoules per kilogram.
     
  4. Apr 11, 2016 #3
    Thanks ^-^
     
  5. Apr 11, 2016 #4
    How are you reaching 60° C if the water boils @ 30° C?
     
  6. Apr 11, 2016 #5
    The water itself isn't 60C but the thing boiling the water is
     
  7. Apr 11, 2016 #6
    OK.

    To calculate the boiling rate we need to know the heat transfer rate. Have you calculated or estimated this? It sounds like you are reasoning correctly.
     
  8. Apr 11, 2016 #7
    The thermal conductivity of water at 30C is about .62 W/MK. Since the whole area around the water is 60C the area would be 452.39 inches or 0.0074 Meters (it's in a cylinder with a radius of 6 inches filled up to the 5 inch mark). the temperature difference is 30C and I'm not sure what to say about thickness since the whole area around the water would be at 60C but from what I know the equation would be 0.62 * .0074 * 30/Thickness correct?
     
  9. Apr 11, 2016 #8

    russ_watters

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    What do you mean by "whole area around the water"? What, exactly, is at 60C? Is it a metal cylinder that is directly heated? Air outside of the cylinder?

    Frankly, though, it is probably easier to measure the heat transfer either with a direct measurement of the input or by weighing the water than it is to try to calculate it from scratch.
     
  10. Apr 12, 2016 #9
    The water is in a closed container and that container is being heated from all sides so yeah It could be the air around it and the issue is I can't measure it by the input because this heat isn't being supplied by and electrical force or anything else I can measure like imagine it's in a super hot desert or something, I'm not sure how to measure that input other than the fact that it's 60C :/
     
  11. Apr 12, 2016 #10

    russ_watters

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    This is all very vague. Are you actually building/testing this? Why don't you have access to your own device/experiment?

    Convection is very difficult to calculate accurately and you haven't really even attempted it. The thermal conductivity of the water is not a relevant issue here.
     
  12. Apr 12, 2016 #11
    I actually am going to construct and test this which is why I wanted to have some idea what would have before-hand
     
  13. Apr 12, 2016 #12
    And I don't see how it's vague, the whole container is going to be 60C as if it were for example submerged in warm water or wrapped up in a heating blanket
     
  14. Apr 12, 2016 #13

    russ_watters

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    As if it is wrapped in a heating blanket? Submerged in warm water? In a super hot desert? These are all very different things. Which is it? That's what's not specific (vague) about it. You also haven't been answering all of the questions: Again, why can't you measure the input heat? If it is an electric heating blanket, you can measure the electricity supplied to the heating blanket. Or, can you test the evaporation rate itself by putting it on a scale? And what is the purpose of this device? What are you trying to do/what problem are you trying to solve? Elaboration/specificity would be a big help. We don't know what you are thinking/trying to do -- we can't help you unless you tell us, exactly/specifically/in detail.

    Anyway, here's a good link on convective heat transfer: http://www.engineeringtoolbox.com/convective-heat-transfer-d_430.html
     
  15. Apr 12, 2016 #14
    Ok then specifically I'm putting the container into a compost pile which at it's highest temperature will reach about 60C
     
  16. Apr 12, 2016 #15

    russ_watters

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    Interesting. So, the limiting factor will be the heat generation rate of the compost and getting that heat into your water container. If the compost container is insulated, according to this, it generates about 1,000 BTU/hr (300W) per ton of compost:
    http://smallfarms.cornell.edu/2012/10/01/compost-power/
     
  17. Apr 12, 2016 #16
    I can't log into my account for some reason but it's still me. My compost pile would likely be about a metre cubed which would be 700lbs so the energy would be more like 100 watts so how do I calculate the boiling rate from there?
     
  18. Apr 12, 2016 #17

    russ_watters

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    You had the method right in your first post: heat input divided by specific heat of vaporization.
     
  19. Apr 12, 2016 #18
    Wold the equation to find the heat input be K *A *change in T all divided by D?
     
  20. Apr 12, 2016 #19

    russ_watters

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    No, 100 watts is the heat input (assuming a well insulated compost pile).
     
  21. Apr 12, 2016 #20
    Well the container would only be 12" by 10" and the pile would be a 1m^3 so it wouldn't be able to absorb all the heat would it?
     
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