Bad experience with evacuated tubes.

[biosolar]

Some photos here: http://biosolar.hu/stuff/uploads/DSC00760_resize.JPG

[russ]

NASA is also a bit pissed about the satellite you shot down

[LucMan]

This is Houston to tandrews, prepare for rentry.
You have been reported to NASA for practicing rocketry without with proper clearance.

[tandrews]

Let me be clear that I attribute these failures to planning over equipment malfunction or failed engineering.
While I'll never know if there was any liquid form acetone left in my 3/8" pressure vessel, that point is rather moot at the instant the cap comes off anyway.
I do think the inner glass tube bottom has cracked, as the silvered bottom now has a bluish discoloration. I have spares and an inner tubing crack is less bothersome as it vents inside the building. Provided the heat absorbent coating does not oxidize immediately it will continue to serve.

As I noted, I had wrapped my heat pipes with a spiral of copper scrub pad material. In the tubes that failed I had used more then others.
This suggests the copper wrap does indeed work as intended and more is better.
It also raises the recorded by failure temperature evac tubes can manifest (450C).
I just wish I had a manifold installed sooner to avoid losing a tube to unintentional rocketeering.

It was not exciting. I heard what I thought was snow leaving the eves and discovered the result later.
Less exciting still to repair something not yet in service.

[mkel]

wow ... I'm surprised ... experimentation can be exciting !

I haven't had any of my repaired tubes fail... I guess I'll put one out in the yard without a header and leave it for a few weeks to see what happens.

I would guess all your liquid turned to vapor and with no way to get rid of the heat it wasn't condensing back to the bottom of the tube... that would explain the pressure.

Did it break your glass tube when it blew?
Glad nobody got hurt... good thing you weren't standing by it.
mike

I must add to this since I have enlightened myself since my last post.
Truth be told, solder can be overcome by the heat generated in a evac tube setup.
I have had a couple of my soldered heat pipes desolder themselves and turn into rockets.

This in itself is not a huge surprise, but the failure mode is rather surprising.
In both cases, the tubes desoldered themselves ON THE BOTTOM.
They effectively became bottle rockets at that point, no splits, nice clean desolder on the bottom end.
One tried to punch through the roof of my thermal shed, then mired itself in the plastic foil bubble wrap I've encased the room in.

I should note this occurred prior to installing the heat removal manifold on top, and likely would not have occurred with that heat removed.
Those tubes that failed had an excess of copper scrub pad wrapped around them and I suspect it insulated and removed airflow up the evac tube. In the absence of a heat draining manifold, heat built up to over the 460 melting temp of my chosen solder.

So, while unlikely and no reason to avoid using solder for DIY, one must make sure they are pulling the heat off the pipe toot sweet or risk bottle rocket heat tubes.
There, I said it.

[russ]

I have had a couple of my soldered heat pipes desolder themselves and turn into rockets.

Maybe we should nickname you Mr Excitement? Sounds like too much fun!

[tandrews]

I must add to this since I have enlightened myself since my last post.
Truth be told, solder can be overcome by the heat generated in a evac tube setup.
I have had a couple of my soldered heat pipes desolder themselves and turn into rockets.

This in itself is not a huge surprise, but the failure mode is rather surprising.
In both cases, the tubes desoldered themselves ON THE BOTTOM.
They effectively became bottle rockets at that point, no splits, nice clean desolder on the bottom end.
One tried to punch through the roof of my thermal shed, then mired itself in the plastic foil bubble wrap I've encased the room in.

I should note this occurred prior to installing the heat removal manifold on top, and likely would not have occurred with that heat removed.
Those tubes that failed had an excess of copper scrub pad wrapped around them and I suspect it insulated and removed airflow up the evac tube. In the absence of a heat draining manifold, heat built up to over the 460 melting temp of my chosen solder.

So, while unlikely and no reason to avoid using solder for DIY, one must make sure they are pulling the heat off the pipe toot sweet or risk bottle rocket heat tubes.
There, I said it.

[biosolar]

Me too

Do you have any photos of the defects?

[roadcracks]

bad news

Me too

[biosolar]

I was informed that our local Apricus distributor announced to stop selling this brand due to heat-pipe faliures.

"It's a serious problem - but rather a shame - that Apricus was't able to supply quality heat-pipes. The heat-pipe replacement under warranty caused extra cost that we couldn't accept."

[biosolar]

I was informed that our local Apricus distributor announced to stop selling this brand due to heat-pipe faliures.

[mkel]

reply

Hi Art,
In my case cost was a huge factor... refigeration valves run close to 3 dollars each and the ones I used were factory seconds (?) at 70 cents.. But the tube on the valves turned out to be very thin wall. I was worried about that at first but they work fine and being as thin as they are if I have a future failure I would expect it to be at that thin wall valve. I don't think I could effectively braze them.

I'm not very worried about the solder melting ... styrofoam cup theory ... as long as there is vapor inside it is unlikely that they will fail. Like soldering a pipe with some steam inside...

As far as how a tube loses it's vacuum ... I would always suspect the brazed points... wouldn't think it could diffuse thru the copper or nobody could have an air conditioner that would continue to work.

I also don't know if all the water would get out ??? as it heats up it would make pressure and you would think some would escape but to do so it would have to be steam vapor and at some point there might not be enought steam vapor to provide enogh pressure to escape...
who knows ... guess it really doesn't matter ... heat pipe no longer works.
mike

Great description of a DIY heat pipe repair.

I would just add that a higher temperature repair is quite easy to make using copper-phosphorus brazing alloy. (It's sometime incorrectly called silver solder or silver brazing) The joint can handle over 800F. MAPP gas is hot enough with a good quality torch. It's self-fluxing and easier to work than any of the tin solders normally used in plumbing.

Also, the common diameter of a heat pipe tube is .312 which I think is 3/8" refrigeration, not carried by Home Depot.

And I have to wonder, why don't the factories use acetone?

Reply to Mkel:

If the heat pipe has a physical leak, air comes in and as you say, ruins the vacuum and raises the boiling point of the working fluid.

After a few days, the water vapor would all escape through the same physical leak, so the tube would be empty.

Another theory is that the vapor diffuses out through the copper wall, or air diffuses in. I should have checked my failed Apricus tubes for water before I recycled them. If there was water but no physical leak then the air diffusing in theory could be right.

But if air can diffuse through copper, any of these copper heat pipe style collectors will eventually fail.

[russ]

Another theory is that the vapor diffuses out through the copper wall, or air diffuses in. I should have checked my failed Apricus tubes for water before I recycled them. If there was water but no physical leak then the air diffusing in theory could be right.

But if air can diffuse through copper, any of these copper heat pipe style collectors will eventually fail.

The liquid inside the tube refluxes (evaporates/condenses) in the tube - if there is a hole in the tube all will escape and no reflux action - end of game

Air diffuse through copper? Not gonna happen under these conditions - that 'theory' hasn't checked in with science or practical knowledge - if air does diffuse through copper does then cancel all refrigeration units -heat pumps etc.

[Art VanDelay]

Reply to tandrews

Great description of a DIY heat pipe repair.

I would just add that a higher temperature repair is quite easy to make using copper-phosphorus brazing alloy. (It's sometime incorrectly called silver solder or silver brazing) The joint can handle over 800F. MAPP gas is hot enough with a good quality torch. It's self-fluxing and easier to work than any of the tin solders normally used in plumbing.

And I have to wonder, why don't the factories use acetone?

Reply to Mkel:

If the heat pipe has a physical leak, air comes in and as you say, ruins the vacuum and raises the boiling point of the working fluid.

After a few days, the water vapor would all escape through the same physical leak, so the tube would be empty.

Another theory is that the vapor diffuses out through the copper wall, or air diffuses in. I should have checked my failed Apricus tubes for water before I recycled them. If there was water but no physical leak then the air diffusing in theory could be right.

But if air can diffuse through copper, any of these copper heat pipe style collectors will eventually fail.

[mkel]

some clarification

I've added some schematics roughly showing my system.
As reference to your comments quoted...
I've only had one glass tube fail ... the glass gets warmer to the touch and I'm sure it loses heat but I still use it to experiment with. Inside still gets very hot .. unless you had a bunch of glass tubes fail I'm not sure if you would even notice a difference in your system temps.

As far as my repaired tubes and vacuum ... I currently have a few outside testing with a hard vacuum on them, however since the internal space is so small I haven't been able to figure out what I have them pulled to.. it is super easy to suck the acetone out so these have been slightly overcharged with acetone and then a vacuum pulled on them for a second or 2. the weather hasn't been clear enough to determine if there is any difference beween these and the tubes evacuated via the torch method.
I'm considering this to be a long term experiment.
here is the link to my post about repairs I did....

http://www.solarpaneltalk.com/showthread.php?4096-Bad-experience-with-evacuated-tubes&p=37594&viewfull=1#post37594

My system consists of a solar loop and solar storage tank...
domestic water goes from the pre-heat tank thru heat exchangers and back to pre-heat tank
at the top of the pre-heat tank there is a mixing chamber 2"x8" .
right above this chamber our hot water recirculator returns hot water from the furthest fixture in the house. This maintains a flowing loop thru the homes electric water heater. As the pre-heat tank gets hot the heat rises into the mixing chamber and the recirculating pump circulates it thru the hot water system. If it's cloudy and the pre-heat tank is cooler than the domestic water than the cooler water in the pre-heat tank doesn't affect the mixing chamber.
If hot water is used in the house our cold supply enters the pre-heat tank which warms it. In the winter our well runs at around 40 degrees so even on the worst cloudy weeks the pre-heat tank runs at around 75 to 80 and our electric water heater doesn't run much. Tempering the well water this way really makes a difference even when we use a lot of water. Most of the year our electric water heater remains unplugged .... with exception of a few times in the winter.
mike

primary-schematic.jpg floor-schematic.jpg

Can I get some clarification about your set up please. And you could also confirm my understanding of them below.

My understanding of evacuated tubes is that there are two tubes under vacuum:
The (external) glass tubes are under nearly full vacuum. A tube inside a tube sealed at both ends and coated in high heat absorbing material. This provides an isolation from the exterior temperature and wind extremes that kill productivity.

The internal (heat tube) which is what has failed, I was told is under a partial vacuum to allow the water inside to "boil" at a lower temperature and transfer the energy to the water header above.

Obviously if the heat tube splits, it is game over. If the glass tube fails and the heat tube is still in tact, I would think it would still function but at a greatly diminished efficiency.

When you did your experimenting with different fluids and water, did you apply any type of vacuum to the heat tube? Sorry if you mentioned it in one of the posts, but I didn't have time to completely read the entire thread.

Acetone boiling point is about 135 deg F which is roughly equivalent to the boiling point of water in 25in of vacuum.
Of course the minute you get any water boiling, the vacuum will be reduced and as the temperature increases you will get increasing steam pressure which is part of how the whole thing works.

It would be very interesting to take some of the Acetone tubes and put them under a modest amount of vacuum and test production changes. Since I believe you still have a valve mounted to the end of the tube, it should be fairly easy to use a small hand vacuum pump and evacuate the heat tubes and simply close the valve.

The vacuum does not affect the freezing temperature of the water.

Does anyone have any idea how much vacuum the heat tubes are. It would be very interesting to have a pressure gauge on the tubes as they function.