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Depth of Water and Energy Required to Boil

  1. Mar 6, 2012 #1
    I have been out of college for more than 20 years and my basic physics knowledge has become quite rusty. Someone made a statement today related to making maple syrup. They stated the energy required to boil a pan of sap 2' wide x 3' long x 6" deep was the same as the energy required to boil 1" of sap in the same pan. Am I correct in thinking the energy required to boil the shallower sap would be less? Wouldn't it take more energy for the air bubbles to pass through a deeper column of sap? Thanks for the input! -John
  2. jcsd
  3. Mar 7, 2012 #2
    I suppose your friends meant the amount of energy per volume of sap, and they did not mean it would take the same energy to boil off six times as much sap.

    The bulk of the energy needed, is to evaporate the water in the sap, and that will be the same for a shallow pan and for a deep pan, however:
    A deep pan will leak more heat through the environment through the sides.

    The energy lost by the bubbles isn't a factor, because any friction would be converted to heat, but when the syrup becomes thick, overheating the bottom of a deep pan
    becomes a problem, because heat loss by convection becomes harder, and it also becomes harder for the steam bubbles to get out of the area.
    To prevent this you need to reduce the heat, and the heat available for evaportation wil then become less, while the heat lost to the air around the pan will remain the same, because the pan is still at boiling temperature.
  4. Mar 7, 2012 #3
    It's true that, boiling water in a higher pressure requires more energy. But, to my knowledge, the heat in the bottom causes convection currents of water and brings heated water to the surface. I haven't seen it, but i think the water bubbles in the surface finally not at the button, unless the heating rate is so much that the convection currents can't transfer it effectively. Industrial boilers are made of an upper thank connected to a lower tank via two bunches of tubes one on each sides. They form loops of water current. The flame heats the pipes on one side more than the other side, this makes the water to go up via tubes on the more heated side and return back to the lower tank via the less heated side. The circulation is continuous and the evaporation takes place in the upper thank because the pressure is lower there.
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