Ice melting

Discussion in 'OT (OFF Topic) Forums' started by Andy W, Sep 2, 2004.

  1. Andy W

    Andy W Hardcore MB Enthusiast

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    Does anyone know a formula or the rate of melting ice for the following.
    One block of ice 0.5 metre melting at an ambient temperature of approx 23 oC. I estimate approx 72 hours, what do you think. This is not a crank question it is something that I could do with knowing.
     
  2. andy_k

    andy_k MB Club Veteran

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  3. se97mlm

    se97mlm Hardcore MB Enthusiast

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    Ooo an Engineering problem; i like those!
    Well i can get half way..... and say how much energy is needed to melt the block and consequently how much power would be needed for the two times suggested above if you were to use a heater or something. Don't know how much heat energy in watts is transmitted to air of temp 23° though yet.Suspect it would have to be a giant room not to cool down with a block that size!! Well here goes...

    0.5m x 0.5m x 0.5m gives volume equals 0.125m^3 (which is big!!)

    Density of ice (less than water) is 917kg per m^3

    So mass of cube is 917x0.125 = 114.625kg (bl**dy heavy)

    Energy needed = mass x specific heat of material x temp. rise.

    Assuming simply that the block goes from -5° to 0°C, and specific heat of water = 4186J/kg/°C

    Then

    Energy needed = 114.625 x 4186 x 5 = 2399101.25 Joules (or 2.4 MJ)

    Power = energy / time

    so for 5 hours, the atmosphere would need to take 2399101/5x60x60 = 133W of heat, which is a little high i think, judging by the heat you get off a 100W lightbulb

    for 72 hours, you would need the atmosphere to be able to dissipate heat at a power of 2399101/72x60x60 = 9W which must be closer to the money but i guess still a bit much.

    Have to put my thinking cap on in the morning. Is probably really not very difficult, but my brain stopped working when i knocked off at 8 tonight!!

    ps. if the block is in sunlight or a breeze, the melting rate will be orders of magnitude higher than in a sealed room with static air. The sun provides a max of about 700W per m^2 i think if it is shining directly downwards. a breeze is no different to air cooling used on some engines, so you can see that can be very effective indeed.
     
    Last edited: Sep 3, 2004
  4. OP
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    Andy W

    Andy W Hardcore MB Enthusiast

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    Bloody hell Se97mlm ! The ice is on display in a public shopping centre so an average room temp of about 23oC is about right, there is no assisted defrosted i.e. no water trickling no hot air dryers just naturally defrosting.
     
  5. Will

    Will MB Club Veteran

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    Surely the time taken for the ice to melt will also be relative to what temperature the ice has been frozen to? ;)

    Ie, if it has been frozen to -1°C, surely it will melt quicker than if it has been frozen to, say, -18°C? :confused:

    And what about the fact of the size of the ice block becoming smaller as it melts, would this have an effect on the rate of melting?

    Also, would the 23°C ambient temperature not become cooler in the vicinity of the ice block as it is exposed to such cold temperature, thus affecting any calculations?

    Does anybody know the answer to this?

    Cheers,

    Will
     
  6. Benzowner

    Benzowner Hardcore MB Enthusiast

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    Went on a cruise recently, and someone asked what they do with the ice sculptures when they melt :D :D
     
  7. andy_k

    andy_k MB Club Veteran

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    we based our 5 hours on the time it took to almost melt an ice sculpture at a party our son went to at the weekend - he was there for 4 hours with the sculpture (about 3' tall) in a marquee in a garden (fairly warm but not boiling hot day) and there wasn't a great deal of it left :)

    would the air around it get colder - I would have thought that air in a shopping centre would be constantly on the move with the effects of the air conditioning and people disturbing it. This movement would accelerate the melting process rather than slowing it as the air on the surface would always be warmer than the ice block

    Would the temperature it had been frozen at have any effect? I'm no physicist but I would have thought that the effect would be minimal as the ice melts from the outside and it's surface temperature would soon raise.

    just to throw in another pointer to melting times - anyone remember this

    http://www.germancarfans.com/news.cfm/newsid/2040517.004/volkswagen/1.html

    where it was estimated 8.5 tonnes of ice would have taken 36-48 hours to melt.

    Now my head hurts :)

    Andy
     
  8. pluggers

    pluggers Hardcore MB Enthusiast

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    Might take longer to defrost with that lovely DIAMOND inside :cool:
     
  9. OP
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    Andy W

    Andy W Hardcore MB Enthusiast

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    Oh damn, pluggers has sussed me out, blame the wife not me !
     
  10. pluggers

    pluggers Hardcore MB Enthusiast

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    What will you do if you win it?

    Dont forget,The ice only has to melt enough to free the diamond!
     
  11. OP
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    Andy W

    Andy W Hardcore MB Enthusiast

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    The wife wants it, as if she hasn't got enough diamonds already as she already works in a jewellers in the Potteries Shopping Centre so gets discount off jewellery anyway.
     
  12. NormanB

    NormanB Hardcore MB Enthusiast

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    Good so far but you forgot the energy needed to change state from solid to liquid (latent heat).
     
  13. Will

    Will MB Club Veteran

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    I must also had that one of the most important factors has been neglected!

    What is the ice sitting on??? Plastic, glass, metal??? :rolleyes:

    Air is such a good insulator compared to most solid materials that I would imagine the material of whatever the ice is sitting on to be almost as crucial in any calcualtions. You would have a 0.5m x 0.5m square sitting on this material, along with 5x 0.5m x 0.5m squares exposed. As the ice melts, it may also fall over - how could you calculate this! :eek:

    Any ideas?

    Will
     
  14. mark.t

    mark.t Banned

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    ice

    is it cold ice or very cold ice, that is ice frozen to differant temps will thaw at differant rate + was it tap water or bottled water as the molicules will act in differant ways,if cl2 is presant it can create air bubbles which will slow down the melting rate :D
     
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