Bad experience with evacuated tubes.

[Chipper5783]

Recharging the heat pipes

This is certainly a very helpful thread. My location is western Canada (cold country) I have a 120 tube system – the first 60 about 5 years ago, the second set I picked up used and am in the process of cleaning up and installing. I discovered the issue of split tubes as I was cleaning up this used set – finding 47 split tubes! I started looking at the existing set and stopped after checking two tubes – both were split. I’m still getting a bit of heat out of the original set of 60 tubes, but not like it used to be.

Per the guidelines in this thread, I am starting to refurbish split tubes. I have a few additional thoughts and learnings. I agree that acetone is likely a good choice for the working fluid. I do not doubt that there are other liquids that would work, but for ease of obtaining, ease of handling and properties like the boiling / freezing point and vapor density – I think acetone is better than methanol, propanol, propane or toluene. A freeze protected water would also probably be pretty good. I went with acetone.

Getting the air out (displaced by acetone vapors) is critical for the function of the heat pipe and to minimize the risk while soldering. The approach I took was to:
• cut the very end off the tube
• deburred and cleaned the tube ID (must be bright copper for the flux to work and the solder to stick)
• flushed the entire tube with methanol
• blew the tube out with compressed air (ran a small air line all the way to the top end)
• I then tipped the tube up & squirted 10.5 mils of acetone in (the intent being there will be ~8 mils of acetone when done)
• Heated the bulb with a hot air gun, with a latex glove tight over the top end of the tube like a balloon (it takes a good minute to drive off enough acetone). I let it flow pretty steady
• Pinched the tube (difficult to get a real tight squeeze). Not quite at the end, so there is room for the silver solder to form a nice little plug. I agree with the recommendation of others that soft solder is probably not a good idea.
• Keep the heat on the bulb until there is liquid solder over the end, then pull the hot air gun away (I think the acetone must condense quickly and the vacuum will suck in air very easily)
• Use a torch that is hot and focused (I used oxygen/propane and a brazing tip – basically an oxy/acetylene unit but I’ve been using propane for fuel). I think oxy/acetylene with a fine tip would work fine.

I found that I ended up using about 2.5 ml of acetone for the boil off (displacing out the air). I learned this because on the first several I recharged, most did not function. I cut them apart again, and measured the acetone that came out.

Once the heat pipe is recharged, test it by standing it up at an angle (the same orientation it would be in the panel) with just the bottom 6 inches in a pot of boiling hot water. The bulb at the top end should very quickly get hot. I tried this with a tube that was not split - the bulb was hot to the touch in about 20 seconds.

It probably goes without saying, but: Think it through, be careful – flame soldering in the presence boiling acetone could get exciting.

[Chipper5783]

Recharging the copper heat pipes - still learning

Maybe it is just me? I'm finding it a bit tricky to get a decent re-charge on the heat pipe. The first attempt I charged 7 pipes, only one passed the test and heated the bulb up top. I improved my pinch/clamp arrangement and the second attempt I charged 6 pipes, and had two that would pass the boiling water test I described earlier. When the heat pipe is in the hot water, you can hear the acetone boiling like everything, you can also feel the heat progressing up the tube, the good ones will heat right up to the bulb, the not good ones, the heat never gets to the top. I assume the explaination is that there is some sort of non-condensable drawn into the tube (most likely air, perhaps combustion products off the torch).

I made up a better clamp arrangement, in the third attempt, I succeeded in 3 out of 3. Attempted to attache photos. Next step is a larger sample size. The better clamp reduces the excitement level. I was previously trying to manage the hot air gun that was heating the bulb (my "boil off stage") with a foot controlled switch, since the clamps/vise grip did not completely shut off the acetone vapors. With this latest attempt I get the clamp set up, start the boil off, the boil off is easy to monitor as the heat comes up the pipe - and start pinching off the pipe. The approach I have taken, once the pipe is filled with acetone vapors - and mostly pinched off, is to light off the acetone (then I'm not breathing it as much), which makes it easy to confirm the pipe is filled and validate the pinch is good (pinch it off hard, until the flame goes out). Next step is to remove the heat at the bulb and silver solder the open end (I've chosen to do the actual pinch about 3/8" from the end).

I would be glad to upload pictures. The image upload feature seems to be very limited! The clamp is nothing special: two pieces of bar stock with a couple clamping bolts to squeeze. One piece of the bar stock is long enough to hold in the vise (Obviously the clamp needs to be able to tolerate getting pretty hot during the soldering step).

In my efforts to date, the original heat pipes still heat faster and to a higher temperature, compared to the ones recharged with acetone - when the lower end is in a pot of boiling water.

[Chipper5783]

Heat pipe clamp - trying again with the pictures

DSCF4168 - smaller.jpg DSCF4166 - smaller.jpg DSCF4167 - smaller.jpg

[inetdog]

Maybe it is just me? I'm finding it a bit tricky to get a decent re-charge on the heat pipe. The first attempt I charged 7 pipes, only one passed the test and heated the bulb up top. I improved my pinch/clamp arrangement and the second attempt I charged 6 pipes, and had two that would pass the boiling water test I described earlier. When the heat pipe is in the hot water, you can hear the acetone boiling like everything, you can also feel the heat progressing up the tube, the good ones will heat right up to the bulb, the not good ones, the heat never gets to the top. I assume the explaination is that there is some sort of non-condensable drawn into the tube (most likely air, perhaps combustion products off the torch).

Yup, that is likely the explanation. Whatever it is must have a lower molecular weight than acetone, otherwise the hot vapor would rise through the contaminant to the top and the tube would work, just not as well.

[LucMan]

You could try installing 1/4 refrigeration access stubs on your tubes, you could then evacuate the tubes with a vacuem pump to remove the air and then suck the acetone into the tubes.
With the access fittings it will be easier to adjust the amount of fluid, or change fluid to alcohol & water to determine which works better.

[Chipper5783]

Recharging heat pipes

You could try installing 1/4 refrigeration access stubs on your tubes, you could then evacuate the tubes with a vacuem pump to remove the air and then suck the acetone into the tubes.
With the access fittings it will be easier to adjust the amount of fluid, or change fluid to alcohol & water to determine which works better.

Hi LucMan,

Thank you for the suggestion on charging the tubes. I don't know about these "access stubs". I saw mkel's experience with "Refrigerant Access Valves" (Dec. 2011), so went straight to crimping and S-solder. I notice that you replied to him, so I'm guessing these "stubs" must be something else? I do not have a vacuum pump (I do have a hot air gun and can simply add a couple cc's extra acetone and let it boil off), but if I did get a vacuum pump, I don't know how I would monitor if it was pulling air or acetone vapours. There isn't very much liquid acetone to work with, it would be easy to flash it all off.

Using alcohol (methanol?) - I didn't try it. Others recommended acetone, which seemed to make sense. At room temperature and pressure, methanol vapor is lighter than air, so with the procedure I'm using to get the air out, I wasn't so sure of being successful. Acetone vapors are nice and heavy, I think that would make it safer (I'm pretty comfortable knowing that the oxygen is out of the tube).

I think it is a pretty suret that water would be better in nearly every aspect - except for the freezing temperature. Certainly the original water filled tubes (even ones that have been in service for several years) heat faster and to a higher temperature than the acetone charged ones when subjected to the "pot of boiling water test" I described earlier. I thought that since I'm only trying to pull about 30 watts of equivalent energy out of a given tube, the reduced transport capability of condensing acetone compared to condensing water will be made up for with a little higher temperature in the evacuated tube (and likely a bit longer initial start time in the morning).

Always glad to hear other ideas.

Regards, David

[inetdog]

Using a vacuum pump you suck on the empty tube and then let the vacuum suck your measured amount of alcohol in. No need to pump on the acetone-filled tube.

[Chipper5783]

Recharging heat pipes

I suppose there is more than one way to get'er done. I'm pretty low tech and try to work things out with what I've got on hand. The actual cash to complete the recharge is modest (fuel gas, S-solder, some power, 5 cents for acetone). The all in time to clean all the parts, recharge the heat pipe and reassemble the tube is about 1/2 hour - the time certainly adds up (that doesn't count the time to get it wrong, work out the steps, making fixtures/tools). I'd certainly like to hear of techniques that others have used.

I'm landing a bit better than 90% success rate on recharging the tube using the boil off method (using the "pot of boiling water test as the measure of success"). How long the S-solder plug lasts (or if the pinched tube now cracks) remains to be seen. I've been tweaking the procedure a bit with every batch of tubes that I work through.

[lark]

Purchasing help

Running out of what little part time steam I had when first got excited about these evacuated tubes. Seems like only 2 sellers for passive systems in states are Dudadiesel and Solarbank. The latter says goes to negative 35F cause uses copper dust/water in ht but have sold very few and hard to judge their rep. Lot pricier then Duda. Maybe worth it if quality superb but big if. No warranty on tubes. Will the vacuum leak? both use unknown china manufacturers.

Someone else also, can't remember but they cleverly hid fact didn't use heat rods just water in the tube.

Mention here of Apricus, kingman, thermomax but don't see any passive system at least with those tubes

Lots of problems and not sure I trust either of above. Asking people here to please chip in and tell me how, from whom to get a dependable machine won't leak or freeze. Otherwise maybe go flat panel and drain in winter. At least with FP won't have vacuum leaks. Haven't looked too much on those yet but seem pretty scarce also. Who there?

[MikeSolar]

Guys, there are over 1000 manufacturers of vacuum tubes in China, some in back yard garages and some in modern factories. Most importers here just buy a container of stuff which, in reality, comes without a warranty so most warrant them themselves (or not, haha). I am told by some Chinese solar people over there, that there is no warranty on tubes as they are so cheap. People just spend a $1 and get another one.

all that said, and as fun as it may be to play with brazing, you wouldn't have this issue with a flat collector. Just sayin...

[RJM]

Replacement Solar Heat Pipes

Sadly I am one of the unlucky ones to get defective tubes on my array as well. After reading most of the posts here it is obvious this problem is wide spread which is sad for such great technology. I have come across a company not from China that manufactures replacement heat pipes they say they are good for -36 Degrees Fahrenheit. They also use water and an additive and stated this was the best combination they found after years of research, they too will not say what the additive is but said theirs have survived 10 years without incident in very harsh environments of -23 Degrees and tested them to -36. I mentioned using Acetone but they tried it and they did not hold up over an extended period of time...not sure why.

Since these are coming from overseas I wanted to see how many people might be interested in purchasing them....at this point I am guessing they will cost between $10-12 per pipe with all the shipping and customs etc. Please let me know if you are interested and how many so I can put an order together.

[J.P.M.]

With all the hassle/cost/failures of evec. tube solar thermal collectors vs. the relatively simple, and it seems more reliable closed loop flat plate systems, or more simple yet open loop for non freezing climates, I'd opt and indeed have opted for the flat plate design. 7 yrs. and one expansion joint gasket replaced with no measureable performance deterioration. Solar DWH heating fraction > ~.95.

[Chipper5783]

Sadly I am one of the unlucky ones to get defective tubes on my array as well. After reading most of the posts here it is obvious this problem is wide spread which is sad for such great technology. I have come across a company not from China that manufactures replacement heat pipes they say they are good for -36 Degrees Fahrenheit. They also use water and an additive and stated this was the best combination they found after years of research, they too will not say what the additive is but said theirs have survived 10 years without incident in very harsh environments of -23 Degrees and tested them to -36. I mentioned using Acetone but they tried it and they did not hold up over an extended period of time...not sure why.

Since these are coming from overseas I wanted to see how many people might be interested in purchasing them....at this point I am guessing they will cost between $10-12 per pipe with all the shipping and customs etc. Please let me know if you are interested and how many so I can put an order together.

Hi RJM / JPM,

My supplier has indicated that they will provide replacement copper, no cost to me (I'd still have a fair amount of work - but way less effort than what I've already been through). They claim that the tubes they get now don't have this problem. I don't have the replacement tubes and have not tested them, so right now it is just another of those projects I am hoping to get to in the next few months. I think it should be quite easy to test the bare tubes - measure the lower end, stand them up in a freezer through a few cycles - then remeasure. Obviously, they should not swell at all. On my system, the few tubes I had that were still good - were like new (had not bulged at all). As one would expect, they were either perfect, or split.

Regards, David

[RJM]

Well JPM I have to say with all the problems I have had and heard of I would think you are correct but, when you see the amount of energy you get out of Solar Evacuated Tubes it is impressive. Even with my system compromised I see 100 degree water at 9am. I am quite surprised they have not resolved this problem sooner. In my research I found that the flat plates lose a lot of energy compared to the EV tubes and don't get hot till later in the morning. My system is on my roof with a long run of pipe so heat loss from the panels would just be adding to the loss specially in the Northern climates. Flat Panels do have their place for sure but, I am trying to get heat to heat my house as well as my hot water and my system is doing that already just not as well as it could until the pipes are fixed.

With all the hassle/cost/failures of evec. tube solar thermal collectors vs. the relatively simple, and it seems more reliable closed loop flat plate systems, or more simple yet open loop for non freezing climates, I'd opt and indeed have opted for the flat plate design. 7 yrs. and one expansion joint gasket replaced with no measureable performance deterioration. Solar DWH heating fraction > ~.95.

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[RJM]

David,

Who is your supplier and will they sell heat tubes to others who may need replacements? Since this seems to be such a problem I would think someone would be able to supply them as long as they are better than the originals. Thanks.

Hi RJM / JPM,

My supplier has indicated that they will provide replacement copper, no cost to me (I'd still have a fair amount of work - but way less effort than what I've already been through). They claim that the tubes they get now don't have this problem. I don't have the replacement tubes and have not tested them, so right now it is just another of those projects I am hoping to get to in the next few months. I think it should be quite easy to test the bare tubes - measure the lower end, stand them up in a freezer through a few cycles - then remeasure. Obviously, they should not swell at all. On my system, the few tubes I had that were still good - were like new (had not bulged at all). As one would expect, they were either perfect, or split.

Regards, David