Custom heat pipes

[twilyth]

This is probably covered somewhere so my apologies in advance.

If you wanted to build a heat pipe from copper tubing and you didn't have the equipment or skill to work with the pipe under a vacuum,

1. what would be the best heat transfer medium.

2. how would you seal the pipe?

I was thinking about using ether since it boils at about 36C.

For closing the system, I figure I'm not so much of a clutz that I can't solder at least one end shut (crimp and solder). But I'm not heating a tube with ether, so I figure I can either use a die and create a threaded end or use some kind of compression fitting.

The other question is, whatever medium i use, how would you calculate the best amount as a percentage of the system's volume?

Thanks.

[Oberon]

You have to remember that heat pipes aren't just hollow tubes with a working fluid inside, there is also a wick that needs to be there if you want them to work in any orientation other than vertical.

[twilyth]

Originally Posted by Oberon

You have to remember that heat pipes aren't just hollow tubes with a working fluid inside, there is also a wick that needs to be there if you want them to work in any orientation other than vertical.

Actually I've been reading about that. As long as there is a positive slope to the pipe compared to the base, you can use a regular pipe with no wicking. It works just like an old fashioned steam heat system. Steam goes through the pipe to the radiators. There it condenses and travels back along the same pipe back to the boiler.

If you ever look at a steam heat installation carefully you'll see the slope of the pipes.

However it's more energy efficient to use hot water rather than steam so the the additional cost of installing a return loop is justified by the savings. Also, today, piping is probably cheaper and I don't even think it has to be metal. I guess it depends on what temperature a hot water system works on though.

[Serra]

Agree w/ Oberon

Your next issue is going to be that Ether (or most anything, TBH) just isn't as effective as good 'ol H20. The issue is the latent heat of vaporization. Water takes about 2257KJ/Kg to vaporize, whereas ether only takes 337KJ/Kg. Basically put, water vapor carries a lot more energy up the pipe to the cold end than ether does, so for ether to carry the same amount it would have to make about 7-8 times as many trips! Obviously there are other liquids you could use, but water has a significantly higher latent heat than any easily obtainable liquid I can think of (see here for some examples).

The next question is - what are you planning? Sounds like it must be something fun

[twilyth]

Originally Posted by Serra

Agree w/ Oberon

Your next issue is going to be that Ether (or most anything, TBH) just isn't as effective as good 'ol H20. The issue is the latent heat of vaporization. Water takes about 2257KJ/Kg to vaporize, whereas ether only takes 337KJ/Kg. Basically put, water vapor carries a lot more energy up the pipe to the cold end than ether does, so for ether to carry the same amount it would have to make about 7-8 times as many trips! Obviously there are other liquids you could use, but water has a significantly higher latent heat than any easily obtainable liquid I can think of (see here for some examples).

The next question is - what are you planning? Sounds like it must be something fun

Thanks for the information man!

I had already been there looking at something else. It seems like a great reference site. The problem is that in many cases I'm not sure what the technical terms are so I can't do an efficient search. Letting me know things like 'latent heat evaporation' gets me a lot closer to getting the information I need.

Anyway, this is just a proof of concept. I'm not completely sure where this might go. I just had some ideas that might be worth exploring. If I do anything at all, I'll start small. For example, take an aluminum heat sink, drill it out for one pipe and run it with and without the pipe.

So I don't want to invest a lot of time and expense in something that might just be a pipe dream. (edit - re-reading and just saw that - ouch. That was not intended)

The problem with water is that I don't see how I could easily reduce the pressure in the pipe. Unless you can do that, water isn't going to work very well. The only thing that I can think of that might work is to pack something flammable in one end of the pipe, seal it and then heat the material past it's flash point. that would consume all of the oxygen, but I don't think that would reduce the pressure enough.

As for the wick, I've read in a couple places that it's not necessary and if you think about it, you can see why. The wick just compensates for non-horizontal placements.

[twilyth]

OK, I found this combined gas law calculator. My HS chemistry is coming back to me. Hehehe.

Anyway, I input starting volume and pressure of 1 and temp of 25C. Then I put in an ending pressure of 1 and a temp of 200C. I figured this would correspond to heating an empty tube, sealing it and letting it cool.

I got 8 as the result. Does that mean that once the system cools back down to 25C it would be at 1/8th the pressure? How much would that reduce the water's boiling point by?

It probably wouldn't be 1/8th though since there would have to be some liquid water in the pipe. Although I could heat it and infuse it with steam at the same time. That would give me a T2 of 100 instead of 200 so the reduction in volume would be 3/4ths - which still be enough. However I don't think that would leave me with enough liquid water when the pipe was in use. IDK

Opinions?

[demonkevy666]

neon is not on that list it should be, it has helium on it...

[Serra]

Originally Posted by twilyth

OK, I found this combined gas law calculator. My HS chemistry is coming back to me. Hehehe.

Anyway, I input starting volume and pressure of 1 and temp of 25C. Then I put in an ending pressure of 1 and a temp of 200C. I figured this would correspond to heating an empty tube, sealing it and letting it cool.

I got 8 as the result. Does that mean that once the system cools back down to 25C it would be at 1/8th the pressure? How much would that reduce the water's boiling point by?

It probably wouldn't be 1/8th though since there would have to be some liquid water in the pipe. Although I could heat it and infuse it with steam at the same time. That would give me a T2 of 100 instead of 200 so the reduction in volume would be 3/4ths - which still be enough. However I don't think that would leave me with enough liquid water when the pipe was in use. IDK

Opinions?

Ehhh... I think there would be some question as to how you intend to heat the contents of an open pipe to 200 degrees while keeping an exacting control over how much steam remained in it (because it would all be steam at 200 degrees) and then seal it. It could work, but it just seems like it's probably going to be more work than setting up a vacuum apparatus and likely not work as well.

What you might want to do is talk to some of the guys in the phase change section. They deal with vacuum pumps and phase change points all the time, they might have some ideas on the best way to get this going.

[twilyth]

Originally Posted by Serra

Ehhh... I think there would be some question as to how you intend to heat the contents of an open pipe to 200 degrees while keeping an exacting control over how much steam remained in it (because it would all be steam at 200 degrees) and then seal it. It could work, but it just seems like it's probably going to be more work than setting up a vacuum apparatus and likely not work as well.

What you might want to do is talk to some of the guys in the phase change section. They deal with vacuum pumps and phase change points all the time, they might have some ideas on the best way to get this going.

Yeah, but they speak some other incomprehensible language - delta this and co-efficient that. I'd start bleeding from my ears.

Anyway, my idea was to coil it up with fittings on both ends. I'd leave them open and put the whole thing in a pressure cooker. After maybe 15 or 20 minutes I'd pull it out, put a couple drops of water in from the pot and seal it up. I know it will cool quickly but if I can get it sealed at something close to 100C, that will give me around 1/4 atmospheres at room temp. I looked it up and water will boil around 50C at that pressure - IIRC - I'll have to go look it up again.

On the other hand, I do have a vacuum pump - industrial model that I got from some reseller that deals in pull-outs and such. I guess putting a one-way valve on one end would make a lot more sense huh?

There's a store near me that has a complete stock of HVAC parts. I've been there a few times for plumbing supplies. They're pretty friendly and will probably spend a few minutes and get me the parts I need. I'll just have to check the stats on the pump. It looks heavy duty, but I don't really know.

I have to think about this some more. It's not really a priority right now - to put it mildly. And it might be one of those ideas where a couple days or weeks from now I think 'damn, you musta been high dude.'