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Poppy   Steam Forum  

Cooling hot water til Freezing vs Cooling Cold Water til freezing

Wednesday, January 16, 2008   By: Ross [390] 5 Stars
Hi all

Thanks for the help on my question - Before.

I work with 2 operators that argue with me about what freezes first. I say cold water freezes first and they keep on and on giving good points about how quick hot water looses heat and cold water is slow. I check the internet and no one seem to have it all.

I just dont have the smarts to defend myslf but I know it does not make any sense that hot water will freeze before cold water. I am hoping you can set them straight.

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Thursday, January 17, 2008   By: Joe [371] 1 Stars
Hot water will cool down faster, but once it reaches the temperature of the colder water both will freeze at the same rate.
Cold water freezes first.

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Friday, January 18, 2008   By: Ross [390] 5 Stars
Joe

Thanks, this is what I have been saying. I was hoping for some science. Much appreciated.

Ross

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Saturday, January 19, 2008   By: Murphy [21] 4 Stars
I think this is easy to solve.

Consider 1-pound of water at 60F and cool it to 32F in a very-very large freezer, but keep a close eye on the cooling rate. There are approximately 28-BTU per pound of water to cool down to 32F. This is a well known fact throughout the world.

The cooling rate is in one-hour 28-BTU will be lost or transfered from the one-pound of water to the freezer.


Now, consider a 200F container of water that is in the same freezer and will be cooled to 32F. It is a well known fact throughout the world that there are approximately 180-BTU per pound of water to cool down to 32F.

We are going to solve for the unknown value of pounds of water per hour for the 200F container. We will now compare:

1-pounds/hr (28-BTU/pound) = (x-pounds/hr) 180-BTU/pound

Lets transpose...

1-pounds/hr (28-BTU/pound)
------------------------------------- = x-pounds/hr
180-BTU/pound

BTUs/pound cancel out!

Lets take a look

0.15-pounds/hr = x-pounds/hr

This means, in order for the 200F container to reach 32F at the same time as the 60F container, which has one-pound of water, the 200F container can only have 0.15-pounds of water. Furthermore, if the 200F container had an equal amount of water (one-pound) the 200F container will take 6.6-hours to reach 32F from 200F.

I hope this is sufficient!

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Monday, January 28, 2008   By: C Hall [27] 0 Stars
The question posed is not the same as what my answer will address. However, I believe my discussion is what causes some people to believe that "hot" water freezes before "cold" water.

The reality is, hot water lines sometimes freeze before cold water lines. This happens when the hot water line has condensation on it. The latent heat of vaporization of the condensation causes the cooling rate of the line to exceed that of a "cold" water line, under the same low ambient temperature (and identical material constants).

This happened to yours truly at my house. I had the hardest time coming to grips with a "hot" water line that froze, instead of the cold water line; but I'm here to tell you it did happen.

I have to admit that my heat transfer knowledge is rusty, but I do believe this argument has merit. In this situation, heat transfer influences on the hot water are not the same as on the cold water line. Heat transfer constants are the same (for stationary water in identical copper lines), but the hot water line has an additional heat transfer mechanism: condensation on the outside of the line.


What do you think?

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