Bad experience with evacuated tubes.

[tandrews]

Curious Daves, have you adjusted your output calculations for area?
That is, for a given flow rate through both types are you calculating your output temps (edit: should have been BTU) for the collector area?

You note your choice was based on marketing so without this adjustment it would not be a fair comparison right?
It looks like the evac system is not even half of the flat panel considering the exposed area of the internal evac glass.

[daves]

I'm interested in the heat pipe collectors because I think they would be more reliable in a cold-climate recirculation or thermosiphon freeze protection strategy.

Heat pipes has too many risks. Freezing water in heatpipe, lost vacuum in the glass tubes, heat transfer loss between the head of HP and the manifold. The best choice is flat collector, even if it has poor performance in winter. If you have enough space, you can make many flat collectors, make much bigger aperture size to cover the heat loss and get some heat from it.

See http://cwc-das.com/logger/meters/sol...?genimage=true (the picture is rendered every time with actual temperatures) - In this case I do think flat collector has bigger aperture size and vacuum tubes could be gone (like experiences from this thread). Anyway, good to watch.

U pipe is imho kind of compromise. Vacuum is in the glass like in thermos bottle, not in tube like heatpipe has. So less risk with vacuum loss. Also U pipe is filled with antifreeze fluid used in system, so it can not freeze. Also it is directly connected to the manifold, so the are no loss due to oxidation or some plays like on heatpipe. Finally it has round shaped aperture, so it covers much bigger size than common heatpipe.

I wanted to buy heatpipe vacuum tubes from china, but i give it away. To many risks for too much money. U pipe has much bigger chance to work in long term.

[Art VanDelay]

U pipes are fine with antifreeze

I'm interested in the heat pipe collectors because I think they would be more reliable in a cold-climate recirculation or thermosiphon freeze protection strategy.

For more information on that point, see http://www.heatinghelp.com/forum-thr.../direct-system

[daves]

water in heatpipe

I have email from china manufacturer, where he wrote they use clear water in heatpipes since 2009. For extra 2$ per tube they can add copper powder to make "antifreeze". He agreed they have still problem with freezing.

Then I found this forum, while looking if its true. So it is.

Maybe instead of spending lots of money for diy heapipes, isnt the U pipe better way with less risk ?

[tandrews]

I'm not yet convinced that you had dieseling or autoignition. Did you see flame marks or other evidence? All the other phenomena can be explained by the sudden release of a compressed gas out of the bottom tip. But if you had a flame, this is the answer to a question I posed above, which was why don't the manufacturers use acetone?

I'm not convinced either, nor am I trying to convince anyone. Simple observation and hypothesis.
I don't agree though that simple gas release was the cause. I heard the tube when it failed, the noise travelled through the wall/siding of the house, two layers of R5 silverboard and foil bubble insulation and there were still two distinct bangs. One when it popped and one when it hit the roof I'm guessing. 63" of 3/8" copper hvac, 8' of copper wool and an end cap left the evac tube all at once. I know the tube would act as a rocket launcher, but it wasn't a "foomp", rather a bang and that's a fair bit of mass to move. That, and the tube is not connected to the wrap, rather sits on a bed of it at the bottom of the evac tube. (lurkers please don't drag air testing into this)
I think I suggested it was likely liability that prevents acetone use from manufacturers. Also theory.

I would not expect flame marks from a stoic ignition as there are no flames. Just kablooey and it's out.
I didn't see any, but they would be at the bottom of the tube where it's dark anyway.

A few photos would be awesome.

I understand, pix or STFU...
Not high on the todo list, but may happen.
Sit by.

As for whether the top should be ever hotter than the bottom, the top is always being cooled by ambient air convection. There is a little air convection at the bottom, but that air is much hotter than ambient.

More likely zero air convection in these instances due to the wrap, but the top of the heat tubes sit only a couple inches from the end of the evac tube in the heated air that does escape.
I think I had leaky heat tubes. If I get failures on top when it's all insulated I'll change my theory. I hope to put all that energy in the tank though.

But since we were talking about safety, once you install these babies in the manifold, you've sealed the top of the vacuum tube. If you stop all the hot air from convecting out, then everything has the opportunity to get even hotter. But now the failure mode isn't copper wool being blown out or a copper bottle rocket....... well, we might not be as amused by your next report, and "engineering failure" would be the correct description.

Nice seqway. Someone was amused?
My goal is to transfer ALL the heat through the tubes to the manifold and on to the milk tank. I don't plan to seal the evac tube ends however, rather place manifold and insulate around it to allow air heat to transfer to manifold as well (a loop of copper with manifold tubes soldered on). Hotter is better as long as I collect it.
I can't have an engineering failure if it wasn't engineered. DIY experimental would be more apt. (Easy mods... not a secret and I'm sharing)

Like I said, if this was a simple temp desolder tube fart scenario I would have more than two failures by now.
In order to to ensure tubes had zero O2 in them, one might have to heat inductively in a nitrogen atmosphere and hard solder.
Quality control requirements, liability, expense - all good reasons not to do acetone commercially and water is a better transfer medium.

[Art VanDelay]

I'm not yet convinced that you had dieseling or autoignition. Did you see flame marks or other evidence? All the other phenomena can be explained by the sudden release of a compressed gas out of the bottom tip. But if you had a flame, this is the answer to a question I posed above, which was why don't the manufacturers use acetone?

A few photos would be awesome.

As for whether the top should be ever hotter than the bottom, the top is always being cooled by ambient air convection. There is a little air convection at the bottom, but that air is much hotter than ambient.

But since we were talking about safety, once you install these babies in the manifold, you've sealed the top of the vacuum tube. If you stop all the hot air from convecting out, then everything has the opportunity to get even hotter. But now the failure mode isn't copper wool being blown out or a copper bottle rocket....... well, we might not be as amused by your next report, and "engineering failure" would be the correct description.

[tandrews]

Russ's suggestion prompts another possible cause for my failures.
If I had a poor solder seal, however small the leak, the cycling of pressures night to day would vent acetone and suck air until stoic conditions were met.
Since they didn't fail immediately upon installation this may be an equally likely scenario.

I'm torn on the hottest part of the heat tube.
On one hand, I thought the amount of copper wrap around the heat tube may have prevented cooling air from circulating, creating a more then others sealed oven at the bottom of the evac tube. Maybe.
On the other hand, the acetone should boil off from the bottom transferring heat to the top. (Where manifold would take it away if installed, air since it wasn't)
On the third hand, acetone will only boil until heat tube pressure tells it to stop, then the fluid will rise in temp. No air, no autoignition.
On the fourth hand, I may have had a poor solder seal - leak, suck, stoic, flashover igniting tube gas leakage.
On the tail, there was ignition, so regardless of failure mode, evac heat tubes can reach acetone autoignition temps.

One tube did not launch but blew out copper wrap. So - autoignition of the gas mix in the evac tube.
Since the other launched with relatively high energy, it likely dieseled once it reached stoic (inside heat tube) and sufficient pressure due to temp.

Any way you view it, 2 out of 35 is not a bad failure rate for a newby solderer/acetone air evacuator.

I did think about how to test these heat tubes after build, but couldn't come up with a practical test to ensure they were sealed.
8ccs of acetone might be difficult to differentiate in weight from extra solder, or variations in tube length regarding a weight test.

[russ]

All joints leak is a generalization but not far off. Over the years I worked with the direct reduction process (of iron ore) we tried to minimize hydrogen leaks to an acceptable point. Trying to stop all leaks was a fools game - a lot of work with no success.

Of course the plants were outside so if anything leaked it went up-up and away. In a confined space H2 can cause exciting moments. You have to consider that static electricity is more than enough to ignite hydrogen - not every time but often.

[Art VanDelay]

"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."

"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."

Well, now we know Apricus has had an unacceptable number of dead heat pipes. If I were forced to reach a conclusion, I'd blame leakage, not diffusion. According to Refrigeraton Technologies, "all joints leak":

http://www.refrigtech.com/Product/Manuals/LeakDetectionManual.pdf

[Art VanDelay]

The top is cooler in your case

If you hadn't yet assembled the header, I would think that the top is the coolest spot on the heat pipe. It's losing heat to the air while the bottom tip has more insulation around it.

[tandrews]

Valid point Art, but I did make a point of getting a roll that had 460C written on it.
I shall locate said roll to confirm as I too have shared your thought and an unplanned explosion of any kind can make one second guess themselves...

Still, wouldn't you expect a failure at the top of the tube in any case since the bottom should have any fluid last?
If I did choose waxy solder, I'd expect this to happen:

http://www.youtube.com/watch?v=5m5BqXR4YJE

Rather then just two.

[Art VanDelay]

I'm confused by your temperatures

"It also raises the recorded by failure temperature evac tubes can manifest (450C)." - Tandrews

Most of the readily available plumbing-type tin soft solders will melt at under 240C (464F). (95/5 tin/antimony)

So I'm thinking that you may have mixed up F & C in the above observation, and that your joint melted at 240C. Are you sure about that solder melting point?

[tandrews]

I agree Art that brazing would have been preferred.
I simply didn't have facility to do so, nor funds to farm it out and all the digging I did suggested 315

[biosolar]

fearful type

I am wondering how dangerous the acetone and air mix is. I mean a fire on the roof that could ignite the series of rockets. What do you think?

[Art VanDelay]

Awesome Learnings!!

Let me be clear that I attribute these failures to planning over equipment malfunction or failed engineering.

Nope, Murphy's Law ALWAYS rules. Even if you had the header on, the pump or controller can fail, or the fluid might leak out of the whole system, or if you go on vacation, your controller might shut off the pump on tank high limit, etc. Or some other failure mode we can't even think of right now.
(phos-cop brazing is actually easier than soldering because it's self fluxing and you don't need a cap. Just pinch the end.)