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Re: Water Boil Water



At 21:14 3/15/98 -0500, Chuck Britton wrote:
[Ludwik]
it does boil, experimentally. Here are some details.

Outer container --> sauce pan diam=17.5 cm, depth 11 cm
Inner container --> soup can daim=7.5 cm, depth 10.5 cm

[Me]
Ludwik found a way to defeat the expected outcome....
He chose a rimmed can which can hold a little superheated steam from the
surface in contact with the heater.

I doubt that this inadvertant trickery is required.

Chuck Britton

Here's how I rationalize my experimental findings:

For a glass bottle of height 25 cm, half full of water
and 2/3 submerged, I estimate the base of the bottle is in contact
with outer water at an over pressure of about 5 cm water gage.
The excess temperature to boil water at this overpressure is about
150 millidegrees C
The thermal conductivity of glass is about 1 watt per meter kelvin
and so the flux through the base is about 1/4 watt and the walls
conduct perhaps 1 or 2 watts more. (For a bottle of base thickness
1 cm and diameter 5 cm)

For a bottle which may contain 200 grams of water, it is difficult
to sustain a rolling boil on this power input.
If I have worked these numbers correctly ( and I hope Chuck will
tell me if not) I expect a net loss of a milligram of water per
second from the bottle to justify an observation of minimal bubbling
after the water has deaerated.

Substituting tinplate for glass to improve thermal conductivity,
and providing a base rim to catch a little steam to increase the
delta T indeed improves matters - but that is not the experiment
which we were asked to judge.

Sincerely

Whatcott Altus OK