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Post by Donkey on Oct 15, 2014 20:54:12 GMT -8
I was listening to the podcasts over at Permies.com about the Rocket Stove Innovators gathering at Paul Wheaton's place recently. One of the things that got mentioned was Heat Pipes. For a good explanation of what Heat Pipes are, you can read this article at WikipediaThinking that it might be a good way to move heat around without moving parts, I decided to build one out of some bits that I have laying about and see what there is to see. I took a piece of 3/4 inch copper pipe 5 feet long, soldered on a permanent cap to one side and a threaded end to the other that can be closed with a brass cap. My first experiment used water as the transfer medium. I poured in about 1/4 cup of water, heated the bottom of the pipe with a torch until it steamed actively and the pipe was too hot to touch all the way at the top, then I screwed on the cap tightly. When cooled, this created a partial vacuum in the pipe, lowering the boiling point of the water inside. I then made myself some tea. In the water left over inside the teapot, I stuck in one end of my heat pipe. Instantly, I could hear the water boiling inside the pipe as heat began climbing upwards. It seems that the condensation point moves slowly up the pipe, progressively heating it as it goes. After a few minuets, the pipe was quite hot around 30 inches up (more or less). After some time, it seemed that heat would not go any further up the pipe than that. It did seem very evenly heated all along that length, touching it with my hand I could not detect any variance except for a few inches towards the top of the heated area. I have poured the water out and replaced it with 99% alcohol, used for stove fuel. I copied the steps that I made with the water, heating as before and capped it off. Then I had to come inside, so the heat pipe with alcohol (as transfer medium) awaits my return. Tomorrow I will make tea with it again and let y'all know how it goes.  |
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Post by matthewwalker on Oct 15, 2014 21:05:18 GMT -8
Nice dude, we had fun playing with them in Montana for sure. Acetone gave pretty good results at the temps we were playing with. Brainstorming applications was probably my favorite part of playing with 'em. Endless possibility.
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Post by Donkey on Oct 15, 2014 21:09:01 GMT -8
How far up a pipe would acetone heat, what temperature(s) were you playing at, how much acetone did you use and how big were the pipes you were using?
Yeah, I've ALWAYS loved brain-storming sessions with others..
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Post by Deleted on Oct 16, 2014 3:00:57 GMT -8
1/4 cup of water is far to much.
It should be not much more than needed to cover
the complete inner surface with a thin film.
For a 3/4 inch copper pipe 5 feet long that is less than a foxglove.
A well done heatpipe heats up the oposite end within seconds.
The temperature difference at both ends of a well done heatpipe
should be not more than 1°C.
I was the first who has published how to do it
at a German computer forum about ten years ago.
Acetone is far inferior to water.
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Post by photoman290 on Oct 16, 2014 3:42:01 GMT -8
i had a look at those not long ago. there is a table somewhere with the figures. acetone and methanol are the best. as methanol is pretty nasty stuff i would use acetone. you can buy acetone from glassfibre suppliers. plenty of youtube vids on how to do it. think the evac tube solar panels with 2 inch stubs in the heat exchanger produce around 60 watts each in the commercial ones.
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Post by Deleted on Oct 16, 2014 4:31:21 GMT -8
Water is by far the best media,
as it can move more than double the amount of energy.
It is not just the much higher phase change energy,
but also the much higher change in volume which make water far superior.
One mol gas accupies about 22.4 liter at 0°C,
which translates to a volume change for water of bout 1200 times at 0°C
and about 1600 times at 100°C.
Calculate it for the other stuff yourself.
I have created 8mm pipes from copper water tubes capable to move about 150 watt,
to keep a CPU within the safe range.
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Post by photoman290 on Oct 16, 2014 6:01:23 GMT -8
thanks for confirming that karl.I wonder why people go to the trouble and expense of using acetone then? maybe water is too simple?
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Post by Deleted on Oct 16, 2014 6:19:38 GMT -8
Just a thinking error.
Acetone or alcohol have lower boiling points at standard conditions,
but in vacuum acetone or alcohol do not longer enjoy that advantage.
Closing a tube filled with steam expanded 1600 times
results in a very strong vacuum once the phase changes,
causing the water to steam even below 0°C.
A vacuum created by phase change of acetone or alcohol
will not be nearly as strong
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Post by Donkey on Oct 16, 2014 6:45:21 GMT -8
1/4 cup of water is far to much. It should be not much more than needed to cover the complete inner surface with a thin film. For a 3/4 inch copper pipe 5 feet long that is less than a foxglove. A well done heatpipe heats up the oposite end within seconds. The temperature difference at both ends of a well done heatpipe should be not more than 1°C. I was the first who has published how to do it at a German computer forum about ten years ago. Acetone is far inferior to water. OK. then perhaps I did something wrong. As to how much water, I used 1/4 cup (rough guess) because I read (some place) that they work best when there is always a little liquid in the bottom. When ALL of the liquid us in it's gaseous phase, the pipe stops working. I also wanted to be sure that there would be enough water in it after (steam) loss from creating the partial vacuum. I will try again (after the alcohol experiment) today with less water. |
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Post by Deleted on Oct 16, 2014 7:08:57 GMT -8
One gram of water will fill 1.6 liter (1.2 at 0°C).
For a 3/4 inch copper pipe 5 feet long one gram can fill
the whole pipe, cover the complete surface with a thin film
and let a small amount at the bottom.
For creating a pipe one needs slightly more to compensate for the loss
before the pipe can be closed.
People have opened commercial pipes and wondered why there was no
detectable amount of water inside.
BTW the water does not run back in droplets, as people like to think,
that would be far to slow.
The water runs back by impulse which is lightning fast.
Water molecules changing phase at the surface are kicking other
molecules out of the way, which creates an impulse chain
moving a chain of molecules at once exactly one molecule wide
into the direction of the next empty space.
Just like the famous toy with a row of balls hanging on chains.
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Post by Donkey on Oct 16, 2014 7:40:09 GMT -8
Fantastic.. Thanks Karl, you're amazing!
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Post by Donkey on Oct 16, 2014 7:50:42 GMT -8
I just looked through some Youtube videos on the subject.. None of the ones I found (that apply) are all that great..
Anyway, I noticed that people are building copper heat pipes with a bulge or two at the ends. You see 1/2 inch bulge on the end(s) with 1/4 inch length of pipe between, or 3/4 with 1/2.
I am assuming that it takes advantage of surface area differences to improve and/or limit heat transfer selectively. ??
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Post by matthewwalker on Oct 16, 2014 7:56:24 GMT -8
Nice one Karl, great info as always. Donkey, we had a very small amount of liquid in the pipes, and they transferred the whole distance almost immediately. We just had them stuck in the riser flame at the top, we never really got past just playing around with them.
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Post by Deleted on Oct 16, 2014 8:11:37 GMT -8
The people with the bulges are just clueless
about the physical laws at work in heatpipes.
A bulging reservoir is not needed.
More water than necessary makes the heatpipes only sluggish.
The larger surface of the bulge may offer a slight advantage for transfer into the pipe,
but hardly enough to justify the added costs.
Bulged pipes are limited by the smaller middle tube.
there is no way to overcome this limit.
Implementing two thinner pipes instead one with bulge is simpler and cheaper,
while offering significantly higher capacity.
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Post by Donkey on Oct 16, 2014 9:38:50 GMT -8
I tried the alcohol with roughly the same outcome as the original water experiment.. Using about the same amount of alcohol (as I did earlier of water), the heat pipe performed about the same. Heat moved in the pipe just a little bit faster and travelled up the pipe just a little bit further (45 inches instead of around 30).
Then, after that attempt, I emptied the pipe of alcohol and rinsed it WELL in water. Assuming that the pipe was now wet relatively evenly, I added 1 more CC of water, heated with a torch till the entire pipe was hot and steaming actively, then closed the lid tight.
After the pipe was cool, I put the bottom end in my hot (just boiled) tea-pot, stirring slightly. What a difference! In just a few seconds,the pipe was evenly hot from end to end.
It is clear that less is better here. For my next experiment, I will dry the interior of the pipe COMPLETELY and use 1 CC of water total.
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