Bad experience with evacuated tubes.

[Art VanDelay]

Photo? Copper Powder?

Mkel,

Did you find any copper powder in the failed tubes? Did you cut open an undamaged tube to see what the factory fluid is?

Another American solar engineer has postulated that the copper powder provides "nucleation sites" for crystallization to begin.

If you provide a good closeup photo of the header near the outlet, I might be able to identify the manufacturer if it's one of the "big 3".

[mkel]

reply to Art

copied from my previous post about fluid found in (27) unbroken tubes.
Since my tubes are on the roof I don't plan on going up there anytime soon ... but next time I'm up there I'll take a pic and see if you know who made mine. Really doesn't matter to me anymore as I modified them.

>>> 5. In this process I cut all the tubes that had not burst. I found that every tube contained a different amount of fluid with a variance of as much as 8cc. Some tubes had almost nothing.. maybe 2cc. The fluid appeared to be dirty water and I suspect the copper dust is not a product of engineering but residue from poor manufacturing of deburing or pipe prep. The amounts of residue went from a trace to quite a bit and some had none. <<<

Mkel,

Did you find any copper powder in the failed tubes? Did you cut open an undamaged tube to see what the factory fluid is?

Another American solar engineer has postulated that the copper powder provides "nucleation sites" for crystallization to begin.

If you provide a good closeup photo of the header near the outlet, I might be able to identify the manufacturer if it's one of the "big 3".

[Art VanDelay]

Evacuated tube technology originated in the US in the 70's

This thread contains a juicy irony:

I can't find a US engineer selling heat pipe evacuated tubes who knows for sure how they are freeze protected. I've gotten a few folks to speculate, and I don't mind speculating either. Pointed questions to the Chinese manufacturers result in the "trade secret" answer.

A few things are pretty clear, however:

1. Quality problems or design errors have resulted in a high historic heat pipe failure rate.

2. Since some heat pipes work and don't freeze break, it is possible to make the technology work for North America.

3. All the large Chinese manufacturers use water in the heat pipe as the working fluid.

4. Overfilling of the heat pipes with water can cause freeze breaks.

5. Not all of the heat pipe failures are from freezing. A high percentage of pre-2006 Apricus tubes stop functioning without any external evidence.

So, be sure you get an enforceable warranty from your supplier. Better yet, get the supplier to explain how their freeze protection works and how they maintain the quality to ensure they are manufactured correctly.

For you "Evacuated Tube Geeks", here's a quote from a forthcoming NREL report on the topic of Chinese collectors:

"Production and use of evacuated tube collectors is growing worldwide; the nature of glass tube production favors high degrees of automation. In the late 1970

[LucMan]

I have seen no vintage 70's version of evacuated tube systems . I have seen several 70's flat plate systems, among those only a few drain back systems were still operational.
All of the glycol systems were dead.
Has anyone run into a silicon fluid based system that still is operational?

[mkel]

speculation

That's some great history you posted !

here is some speculation ... on #5.

If the solution in the heat tube is water and the tube has lost it's vacuum I doubt it would be capable of vaporizing and transmitting heat to the bulb. Allthough it may turn to steam at some point, it's effectiveness would be greatly reduced...... purely my own thoughts ...
mike

5. Not all of the heat pipe failures are from freezing. A high percentage of pre-2006 Apricus tubes stop functioning without any external evidence.

[Designer_Mike]

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.

[tandrews]

Heat tube vacuum

I will suggest that if your heat tubes have burst, you should not replace them with the same type.
Rather, drain them and refill with acetone, then solder them shut.
I say this because if you put hot coffee in a thermos bottle and leave it outside (sealed) in -20 it WILL freeze. Evacuated tubes (refer to the glass, not the heat tube) are fabulous insulators, but they are not perfect insulators. An exposed heat tube neck will accelerate the process, but an unexposed one is still in -20. Replace water filled tubes that failed with water filled tubes and expect the same results - eventually.

This talk of vacuum in a heat tube/pipe is directly relative to the temperature applied.
That is, when I made mine, the application of vacuum was impossible, until I had the second end cap soldered and the tube cooled.
See link earlier in this thread for the method I used. (End cap soldered, 8cc acetone added, heat bottom to push 02 out, then solder second cap as tube cools and goes through zero ambient pressure)

If an evacuated (glass) tube fails, the inner tube will very quickly lose it's coating to oxidation, or at very least become distinct in appearance from it's sealed neighboring tubes.
If a heat tube fails, efficient heat transfer drops off - it does not cease.

If you have failed HEAT tubes, try this:

thaw and drain fluid from tube
Heat tube with a heat gun or torch to drive all water out of tube.
Use a syringe to inject 8cc(ish) acetone into split.
Rotate tube to put acetone in bulb/manifold end at the bottom end.
Work split closed with a vice, pliers, or whatever method is easiest for you - no need to flatten pipe.
Prep split area for soldering with sandpaper and flux
have a helper heat bottom of tube while a finger seals the split (neoprene, latex gloves will keep greasy human oils out of the mix)
Allow the pressure from boiling acetone to push the air out past your finger at least a couple times to ensure most 02 is removed
Apply heat to split and seal with solder - this is the artsy bit as it needs to happen when acetone is neither pressurizing nor pulling air into split
Expect failure of that last step more then once, but don't worry about some solder in the bottom of your tube. Skill is required, but there is no huge learning curve.

Wa la. repaired heat tube that will never freeze.

Nay sayers will comment that solder will never hold.
They should consider this is the very bottom of the temp gradient in the heat tube/pipe, and the cost to do this repair is mostly time.

Boiling acetone will not burn without oxygen.
If you get a flame at split when torching to solder split, this means you have removed oxygen from tube - good for you - now seal that sucker before it starts sucking 02 in.

If you fear repairing a split, get some HVAC 3/8" end caps and cut the split off the heat tube.
While one end is open, rinse with acetone to get any other crud out.
If heat pipe length is critical to ensure manifold retention, replace the lost length with copper scrub pads under the heat pipe at the bottom of the evacuated glass tube.

Step last - pass it on if it saved you X dollars.
Happy Holidays.

[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.

[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.

[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.

[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.

[biosolar]

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

[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."

[roadcracks]

bad news

Me too

[biosolar]

Me too

Do you have any photos of the defects?