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Q & A: making steam?

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Most recent answer: 04/28/2014
Q:
Is it possible to determine the speed of water that has been vaporized? Using gas laws, I am aware as to how I can determine the relationship between a change in volume to a change in temperature. So lets say a piston changes the volume of a cylinder, through compression, and an independent hollow sphere is traveling through that heating chamber. If water was within that sphere, it would be correct to assume that it would heat up, due to thermal condution (assuming the pipe is a good thermal conductor). However now that the water has been heated. how long will it take to heat the water to turn to steam and what would the speed of the steam. Considering that the use would be to drive a flywheel. Please help. i have searched everywhere and can't seem to get a definitive answer. I ask the question because it is an essential part of a research paper I am currently writing.
- Mike (age 17)
Edmonds, WA
A:

There are a couple of parts to this. You ask "what would the speed of the steam?" I'm not sure if you mean how fast the water molecules in it move, or something else. The typical speed of the water molecules is easy to calculate using equpartition: the rms speed is sqrt(3kT/m) where m is the mass of a water molecule, k is Boltzmann's constant, and T is the absolute temperature. You can look them up and do that calculation.

What about how fast the steam is formed? You're putting energy in by doing work on the piston. You can calculate the power your putting in, we don't have the info needed.  Before it boils it takes a little while to get the water to the boiling point, but the total amount of heat needed to get some water from room temperature to boiling is much less than the amount needed to boil it, described by the latent heat of vaporization. So most of the time is spent in the boiling, not in heating things up first. While the steam is forming, most of that energy should be going into the latent heat, which you can look up. Dividing the power by the latent heat per mole will tell you how fast the steam is forming, say in moles/second. 

Mike W.


(published on 04/28/2014)

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