Bad experience with evacuated tubes.

[russ]

Hi mkel - Welcome to Solar Panel Talk!

Good post and a lot of research it seems!

The IR temp measuring device should show a big temp difference as in many degrees comparing bad tube to good ones. The tube is collecting heat and the vacuum doesn't let it escape.

I generally think the IR devices are useless for measuring temperature but they should be great for a go/no go indicator in a situation like this.

They rely totally on the emissivity setting and that is rarely adjusted correctly by the user.

It seems to make sense to use a liquid with a low melting point rather than water -

[mkel]

heat tube repair comments

I've been experimenting with evac tubes for a few years now. Have 80 tubes on the roof and last year I started to have heat tube failures. I want to add some comments about my experiences and maybe they will help someone else.... some of these comments are my opinions with no scientific tests.
Please don't rip me if I miss-spell something --

1. I read where a infra-red heat detecter could be used on the glass tubes to determine if a heat tube had failed inside. I did a reading on all 80 tubes and wrote down each reading. there were a number of tubes that read 4+ degrees higher than others (supposedly indicating the heat tube is bad). When I pulled the tubes I found no correlation based on this method.

2. My ratio of failed heat tubes was about 40%. They had split and I do agree with statements in this forum that this was a product of freezing.

3. Found a number of tubes that had NOT split yet were expanded at the bottom.

4. My supplier offered to replace all my heat tubes however I declined because I have no faith that the new tubes will have anything different in them and would just become a future failure. I have repaired all 80 tubes by soldering a refrigerant access valve on the bottom of the tube. Pulling the schrader valve out I can charge the tube with fluid and then replace the valve. Heating the tube I depress the valve until I feel a bit of moisture from the fluid. At that point I close the valve and replace the valve cap.
These tubes work great and because of the valve I was able to experiment with different mixtures and quantities to find the best ratio for my tubes.

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.

6. SUMMARY -- I believe the tubes that split had much more fluid (water) in them than the ones that didn't split. the pipes that were expanded had more fluid in them than the ones that looked normal.
It appears the manufacturing process is sloppy at best. The random liquid levels in the heat tubes may explain why the infra-red heat reading didn't work. In experimenting with different amounts of fluid in the tubes I found a wide swing of temperatures depending on the amount. I lined up 8 tubes with different amounts of fluid from 2cc to 14cc. This provided interesting results and I found 7cc to be the perfect amount for my pipes. Less than 7cc worked well in direct sun but at 7cc I got some heat even when it was cloudy... only dropping off when clouds were very thick. Solution of choice turned out to be acetone. Nothing else I tried provided as good of results... and it won't freeze. There is ZERO chance of a failed acetone heat tube contaminating the water circulating in a evacuated tube system. Also for those who are paranoid of explosion... I put one of these acetone heat tubes in my burn barrel and cooked the crap out of it with no explosion.. solder did melt and acetone escaped with no event noted.

Hope some of this info helps others. It is a shame the manufacturing of these tubes has left the customers with this problem. Worst part for me was the huge number of trips up and down from the roof removing and replacing the tubes.

EDIT 3-2014 -- INFO UPDATE -- I had repaired tubes with refrigeration valves on the ends... This was a failure after 1 year ... the refrigeration valves could not take the heat inside the tubes for an extended period of time.
Next try was to crimp and solder the bottoms --- worked great for a long time but then we had a power outage on a sunny day and the system started dropping off on temps and wasn't producing heat like it had. I pulled some tubes and found that under stagnation in direct sun the solders had melted. 460 degrees ... my pressure relief did go off but that didn't prevent the heat tubes from failing.
Last try -- same as before -- crimp tubes but used silver solder this time .. only did 20 tubes about a year ago and they are all still working great. I intend to do the rest this spring. I did see that the suppliers of evac tubes are starting to sell replacement heat tubes on ebay.... could you trust them? My guess is they are filled with water just like the ones that failed in the first place ... and they are asking a lot of $ for them if you are replacing a lot.
END EDIT.


bulb-stop.jpg heat-tube-valve.jpg broken-tubes-opt.jpg
mkel

[sunno]

Yes this happened to me also, it is very important I think not to buy those cheap chinese tubes, but make investigation on the web before byuing and building your solar vacuum tube system. Well even if some tubes breaks, it is still very efficient product.

[SolarHome]

Apricus Heat Pipe Failure

I had multiple heat pipe failures like "Windy." I obtained new heat pipes from my Apricus dealer, but now I am going to check my installation with and infrared camera. I live in Northeastern Maine and the cold temps may have been responsible. The bottom of the pipe was expanded and ruptured. If you remove the tube and the bulb on the pipe is not not, but the heat pipe is, especially after being out in the sun for a few minutes after being removed from the manifold, your heat pipe is bad.

[hamish]

hi gonsh, apply a mild acid like spirit vinegar to the area around the heat pipe where it meets the
manifold and leave it for a few hours. It will slowly disolve the buildup between the heat pipe and manifold. You could also use a good penitrating oil or brake fluild if its very stuborn, the main thing is you must leave it on for a long time keep reapplying it over night or a day or two and it will come loose.

[Art VanDelay]

Copper powder

In a six ft. heat pipe, there was about 2 ml of water, and 1ml of powder.

So no, the water level is above the level of the copper powder, by 1-2 cm in the 1/4" tube. The shape of the tube at the bottom end is also probably important to get the water to freeze "correctly".

[Art VanDelay]

Interlocking Extrusions

Once you remove those end caps, I think you'll find two interlocking extrusions. Maybe you can tap the top one off with a hammer. If urethane foam has them all glued together, it will be really difficult.

Hey, keep me posted, I'm gathering a summary of the failure modes of this type of collector.

[tandrews]

Query for Art:
How much copper was included in the mixture?
That is, would it be sufficient in quantity to allow the volume of water to be below the powder in the tube?
Curious now...

[Gonsh]

Removing cover on header

anybody ever remove the cover on the manifold? It has two plastic end caps that are riveted on. Is this even possible with the tubes attached? Dont trust the built in freeze protection on the controller. Trust glycol! Thanks Art.The manifold will work fine in the summer at 15 psi so i hope the crack isnt too bad. gonna start poppin rivets.

[Art VanDelay]

Your Manifold Cracked Because it Froze

GRRRR, My manifold has cracked somewhere and to fix it i attempted to remove my tubes from the header assembly. The glass slips out just fine but the heat pipe is stuck in the manifold!! I have already broken one heat pipe trying to remove it. Please can anyone help me. It does look green up in there and i cant even remove the bulb nwith much force from the one i already broke!! Help me please. I have it covered so it will cool off and i hope it will disengage when it does.

When the manifold (header) freezes, it clamps down hard on the heat pipes. I know this because it happened to me. But luckily, my header didn't split, so the collector still works fine.
So, don't worry about getting the heat pipes out. Remove the header insulation and maybe you can find and braze the split closed. Solder won't hold.

[Art VanDelay]

freezing and water heat pipes

Heat pipes use very little fluid. About 2-4 ml of water, acetone or whatever.

The mechanics of pipe damage due to water freezing are as follows:

1. Water in the bottom of a mostly empty tube tends to freeze from the top of the water down. That's because the coldest water becomes buoyant at 39F and rises up to freeze (that's why we can skate on partially frozen lakes)

2. The freeze plug that forms on the top surface of the water sticks hard to the sides of the pipe.

3. As the freeze plug expands downward, it pushes on the incompressible liquid water, which splits the pipe.

So, if you can coax the water into freezing at the bottom tip of the tube first, and expanding upward, there is no split.

The Chinese solution is: a small amount of water mixed with a coarse copper powder.

As I say, I haven't seen any documentation on this. I just tore apart a few heat pipes to see what was inside. I also put them in a freezer to see if the tip diameter increased when frozen. It didn't.

Don't get me wrong, I'm not against acetone, I'm just reporting what is in millions of Chinese heat pipes (SunRain)

[tandrews]

Water is safer, cheaper and transports the most energy per gram of just about any fluid.
Freezing is an issue that is being solved by using very little water, and they add a coarse copper powder to the water. I haven't really found the official reason for the powder. But it apparently causes the to water to freeze in the tip before making an ice plug. If it freezes bottom first and expands upward, no damage will occur.

Safer for installation, transport and install agreed. For troublefree use in Northern climates, not so much as is evident by known failures.
Perhaps the copper powder is intended to manage crystal formation and diffuse lateral pressures at the bottom of a heat tube? As long as the amount of water covers the bottom of the capped end, there remains risk of a split in the event of freezing. While I agree that water would be preferred for heat transfer, it's properties alone also add risk to freeze failure. I'd love to read some reasoning for copper powder additive if someone can speak to it directly. For now I'm sticking with Acetone, but reserve the right to flip my opinion if sufficiently informed otherwise.

I'm not sure how copper powder would cause freezing to occur in the tip first (If by tip, you mean upper manifold end?).
If it freezes bottom first, damage may very likely occur. Why do you say it wouldn't?

In a failed evac tube freezing might occur anywhere in the water based heat pipe, but in a good tube exposure to temps is limited to those areas exposed to weather or poorly insulated from it (grommets, flashings etc). They're not perfect insulators, but they're not perfect vacuum either. The chances of Acetone freezing are well past -78C directly exposed. We see -35C on occasion here, so water = bad in my part of the world. Your mileage will vary.

[Gonsh]

heat pipe siezing in header

GRRRR, My manifold has cracked somewhere and to fix it i attempted to remove my tubes from the header assembly. The glass slips out just fine but the heat pipe is stuck in the manifold!! I have already broken one heat pipe trying to remove it. Please can anyone help me. It does look green up in there and i cant even remove the bulb nwith much force from the one i already broke!! Help me please. I have it covered so it will cool off and i hope it will disengage when it does.

[Art VanDelay]

Chinese heat pipes use water and Cu powder

This is how I made my heat tubes:

https://www.youtube.com/watch?v=LU4eynU6R-8

Why a manufacturer would use water eludes me. Regulations perhaps.

Water is safer, cheaper and transports the most energy per gram of just about any fluid.

Freezing is an issue that is being solved by using very little water, and they add a coarse copper powder to the water. I haven't really found the official reason for the powder. But it apparently causes the to water to freeze in the tip before making an ice plug. If it freezes bottom first and expands upward, no damage will occur.

[tandrews]

The fellow in the video is merely my inspiration for using acetone as transfer medium.
I am a humble self educating newb working toward some less processed energy for heating and perhaps TEG generation next summer.
Like many here, it's something I must do.