Evacuated Tube Copper Pipe Seized in Collector

Zxcv1984

Good day,

I have a roof mounted evacuated tube water heating system. I need to temporarily disassemble the system (roofing problems that need to be fixed). Many copper pipes (the pipes inside the evacuated tubes) are stuck in the collector and I can't pull them out. Anybody has experienced this before? Any tips or trick to disassemble them? Any help is appreciated.

Here's what I've tried up to now:
1)WD-40: sometimes helped, but won't free the hard cases, which are many;
2)Working at sundawn, with the system completely cold: does help, but again won't free the hard cases;
3)Working with hot tubes: doesn't help at all AND very inconvenient (very high copper tube temperature) AND maybe safety issues;
4)pushing the pipe further in, then pulling out: does help;
5) Brute force: I have applied limited force in order not to damage the collector. As it's still mounted on my roof and I haven't removed the covers on the collector to confirm its construction, I'm playing it safe and have only applied the force one could apply without the help of tooling (otherwise said: hand force).

I'm thinking of trying penetrating oil, but I'm a bit help back by the potential fire hazard.
I'm thinking of finding something I could use to dissolve the old thermal paste, however this IMHO will be a last resort, as I doubt it would work and the product that COULD work is hard stuff for many plastics (e.g. acetone...)

Larry Weingarten

Hi, I have run across this and it was caused by the header freezing and pinching the top of the tube. If you could wiggle the tube to try and loosen it, that might help.
Yours, Larry

hot_rod

what about removing the entire array without pulling the tubes from the header?

Or try ice around the condenser tube to shrink it from the header

The hotter the tubes the more they expand

Zxcv1984

Larry Weingarten said:

Hi, I have run across this and it was caused by the header freezing and pinching the top of the tube. If you could wiggle the tube to try and loosen it, that might help.

Yours, Larry

"Freezing" like freezing the water in the collector from cold during winter time?

Zxcv1984

hot_rod said:

what about removing the entire array without pulling the tubes from the header?



Or try ice around the condenser tube to shrink it from the header



The hotter the tubes the more they expand

That's a good idea. The thing is, the array structure hasn't been designed to be lifted whilst completely assembled. I looked at it myself and asked the opinion of a few people around, and although everything's possible, this solution would require some work to support / strenghten the array structure so it's not damaged when I bring it down. Additionnal detail: just because of where the building is situated, we doubt we can access it with a telehandler, we'd be looking at a small crane to lift it from the top. I might end up using this solution, however I'm looking for alternatives for now.

hot_rod

What brand, if I may ask? A heat pipe of flow thru design?

Larry Weingarten

Hi @Zxcv1984, I've dealt with freezing water in the insulated header at the top of the tubes. This picture doesn't get it well, but you might see the copper bent in some.
Yours, Larry

Zxcv1984

@hot_rod : Sunmaxx collectors. Heat pipe design.

Zxcv1984

@Larry Weingarten Thanks Larry, there's nothing like a picture. The heat transfer medium in my system is water. Not glycol, just straight demineralized water. We do get very low temperature (way below freezing point) at my latitude. My system is designed to drain completely whenever the temperature sender reads a temperature below a certain setpoint. I need to investigate on what would be the consequences of a broken temperature sender or an electric power failure (does the system flushes or not? Both scenario could help or do harm, depending on the season.) That could explain some things.
I just moved in this house last January and am still in the process of learning this system, which is very effective, but all new to me.

hot_rod

Is its a drainback system, the collector drains down when temperature is reached?

I have not see a good design for completely draining the header? So even a small amount of water in the bottom of the header could freeze and cause damage.

Zxcv1984

When the set temperature is reached (high enough to pump heat from the collector and transfer to the tank), the pump pushes the transfer medium from the tank to the roof. When the set temperature is hit again (e.g. after the sun sets, clouds, etc.) the system drains by gravity. I haven't checked closely, however as the bulb ports (the part where you slide the copper bulb in) is thin walled, I understand how this could happen. I'll take a closer look in the light of what we just discussed. If all this proves to be what really happened and my bulb ports are wrapped, I'm bound for a good job of tearing off stuck copper pipes the hard way, re-shaping the bulb port (not sure how I'd proceed, just thinking out loud), then fitting in new copper pipes.

Zxcv1984

As for the setpoint, I know it's dependent on the accumulation tank temperature, however I'd have to check if it's dependent on other parameters. But I digress... Kind of like to learn about how this setup works.

hot_rod

Basically two types of solar thermal being installed currently. A pressurized system, typically with glycol antifreeze protection. In some climates plain water is used, on the equator for example

A drainback would have a separate small tank that allows the fluid from the collector and piping, usually a 6 gallon size tank.

I much prefer drain back as it eliminates over-heating of the glycol which can break it down in a few years time.

One issue with evacuation tube drain back is the condenser bulb in that well with no fluid flowing across it can reach 300°C, reflector type up to 350C, according to German lab tests.

Look into the Dow high temperature silicone grease. The grey zinc oxide or silver based paste tends to harden and glue components together. Happens with any copper sensor in a well after time.

The very best thing is keep a constant load on evacuation tubes, when the stagnate things get hot in a hurry.

Here is an Oventrop tube making tea water on a cold winters day.

Also a journal we produced on solar thermal basics.

https://www.caleffi.com/sites/default/files/coll_attach_file/idronics_3_0.pdf

Zxcv1984

@Hotrod Thank you! Is the DOWSIL™ SC 102 Compound the product you refer to when you speak of DOW thermal paste?

hot_rod

Zxcv1984 said:

@Hotrod Thank you! Is the DOWSIL™ SC 102 Compound the product you refer to when you speak of DOW thermal paste?

I'll check with our lab on Monday. I remember it being clear, almost like a squeeze tube of silicone.


You might also contact DOW directly at the link they show at the bottom, explain the application. Often they send samples.

Or small tubes are also found on Amazon, once you get the correct part #.

https://www.ellsworth.com/globalassets/literature-library/manufacturer/dow-corning/dow-corning-brochure-enhancing-the-reliability-of-your-electronics-designs.pdf

Zxcv1984

I gave a call to DOW early this week, they recommend DOWSIL™ SC 102 Compound for the application. I'll give it a try. Thanks again!

Zxcv1984

Ok, I'm almost through the roof repair project (it's ongoing as I write), I'll give a more lengthy report as soon as I can free a bit of time, but now I need to check: what angle would you consider minimal for a gravity drain system? Literature has different numbers, I've read from 1 deg up to "at least 3 deg" up to now. I'm inclined to go with 3 and work from there. Anyone had experiences with this?

Thanks for the support!

hot_rod

Do you mean the pitch in the piping? I use 1/4" per foot, like you would slope drainage piping. No low spots of course.

Are you trying to do an evac tube drainback? I have had limited success with that. The heat pipe type evac tube type work best.
Mount the array on it's side. I think you have heat pipe style if the sensor bulb was stuck in the header?

If it is a U tube style where the fluid travels in the evac tube, I think you would want the header at the bottom and pitched to one side.

If questionable drain back ability you could put a glycol mix in a drainback for extra insurance.

Larry Weingarten

Hi, Just looked it up and 1/4" per foot is 1.2 degrees. That's fine as long as there is no sagging, (as HR said) and that can be difficult to insure over time. Of course, you would want the headers sloped also if they could freeze. Then there are details like not putting any reducing bells on horizontal runs...

Yours, Larry

Zxcv1984

Much obliged, I take your advice into account.
I've made more check since my last post, the original design look like its around 1,5 or 2 deg.

@hot_rod not sure how you'd call the setup I have, with I think is pretty usual, gravity drain aside. I'm adding a picture so you can see for yourself.

@Larry Weingarten thanks for pointing out about the piping. Many small details to keep the thing hassle free.

Zxcv1984

@hot_rod whilst looking for answers about the recommended slope for gravity drain someone brought the idea that there would be such a thing as too much incline. Too much slope would mean faster flowrate, which in turn would be detrimental to optimal heat transfer. You got an opinion on that?

Just on top of my hat I have my doubts, I'm under the impression that the system fills the pipe completely. There's event a cycle called "filling", if I understand right.

Zxcv1984

So, long form report, for the benefit of the community.

For reasons not related with the solar thermal collector system, the roof shingles on my house needed replacement.

The roof features a shallow angle of 15 degrees.

There are six 30 evacuated tube collector units on the roof.
Four of them on a gravity drained open circuit for heating.
Two of them on a separate glycol closed circuit for hot water.

The system is about 10 years old and several evacuated tubes in the collectors are stuck in place, either as result of damage from freezing inside the collector or cooked thermal paste. It's hard to tell the difference once it's stuck, but I have good reasons to believe that at least some of the stuck tubes are the result of freezing damage.

The collectors are otherwise in very good shape, with only two damaged tubes out of 180, which I'll replace once we're done with the roof repair.

The selected mean of getting the collectors out of the way was to manually move the collector units with a team of workers. Since the roof has a very low angle it's rather easy to move around, even when carrying a load.
At a glance the tubes look like they didn't suffer any damage. Some metal fixtures were bent a bit, however nothing that can't be fixed easily as far as I can see.

The contractor responsible for pipework asked to be present before final placement to ensure a good piping job.

At this point there's a single day of work left to do to make final placement for the gravity drain units, the piping, take care of loose ends (mostly waterproofing with caulking and pitch in some spots) and clean up.

Up to now my best advice would be to choose well you subcontractor and work with them so they don't overlook anything. Special projects ask for special care.

I'll let you know how it turns out. The real test will be when I remove the tarp from the collectors and recommission both circuits.

Cheers,

hot_rod

They look like skinny U tube style, but if they have a bulb not copper tubes into the header they must be heat pipe.

Here in my experience with a U tube drainback. I had the array mounted horizontally, it actually worked for a few years. I suspect the copper tubes sagged after time inside the glass, see the split.
Notice also how hot the copper must get inside those tubes when no fluid is moving. I suspect the intensive heat inside the tube anneled the copper and it sagged, trapping water from draining back. The black scaling is like when you burn copper with a torch, had to be 500- 600° or more! Certainly no place for soft solder joints.

Trust me, there is no such thing as water moving too fast through a collector. It is not about the "linger" time inside the collector to maximize the heat output. The longer the fluid stays in the collector, slow flow rate, the more of that heat energy is lost through the glass and enclosure to the ambient air. If the ambient is even 1° cooler than the collector there is heat transfer, hot to cold.
That is the whole concept behind evac tube style collector, minimize that loss to ambient with the vacuum insulation space.
That collector efficiency number is calculable, and it also shows up in the SRCC test data. The trade off for higher flow is of course pumping power. Also keeping the velocity below 5 fps for protection against erosion wear, and of course velocity noise.

I'm using all ECM circulators now on my solar thermal, to lower that energy conversion cost. My system is running right now, I'm pumping 11 gpm, 5- 4X8 collectors, with 43W.

How often do the glycoled collectors stagnate?
That can be a real tough operating condition for glycols. There was some ultra high temperature solar glycol available from a German manufacturer, specifically for evac tube systems. It was never really promoted here, I don't think it could pass the FDA or GRAS testing with all the heavy duty inhibitors it required. I'd use a double walled HX if you do go with any heat transfer fluid that is not potable rated. Same with the old solar "oils" like Bray oil, Slytherm, paraffin, etc.

mattmia2

I'd venture to guess that black oxide is formed with the tubes hitting closer to 1000 F. Temperature controlled soldering irons will sit all day below about 700 F without much oxidization, once you set them around 800 f, they start oxidizing badly.

I don't think that a reducer in a horizontal pipe would be an issue. It will hold a little bit of water which will freeze, but it is a small fraction of the total cross section of the pipe so it will not grow large enough to damage the pipe. A good idea to avoid it, but unlikely to cause damage.

hot_rod

The manufacturer of those collectors should have a suggested flow rate. It is usually the gpm they are tested at and certified. The SRCC website has that info if the collector was listed, most are so they were able to get incentive $$.
In a drainback the “downcomer” pipe is sometimes reduced or throttled to help establish the siphon.
The pump mainly fills the collectors and overcomes the piping resistance. It’s the siphon moving the water.
A control with a variable speed is best for drainback, especially systems with high lift. The pump speed is modulated based on delta T through the collector to maximize solar harvest.

Zxcv1984

About copper tube oxidizing: I've observed some degree of black oxide on some of my copper tubes, however nothing as extreme as what's shown by @hot_rod. My tubes are a good 10 years old. Could it be something that will naturally occur over time (years) with lower, albeit hot temperature? Anybody can enlighten us on the subject?

@hot_rod : about flowrate: the setup you're describing is mostly how I observe my system works. The controller modulates the pump speed depending on the circumstances.
I'm attaching pictures of the innards of my evacuated tubes, I hope it clarifies as to whether it's a u-tube or heat pipe design.

Zxcv1984

hot_rod said:

How often do the glycoled collectors stagnate?
That can be a real tough operating condition for glycols. There was some ultra high temperature solar glycol available from a German manufacturer, specifically for evac tube systems. It was never really promoted here, I don't think it could pass the FDA or GRAS testing with all the heavy duty inhibitors it required. I'd use a double walled HX if you do go with any heat transfer fluid that is not potable rated. Same with the old solar "oils" like Bray oil, Slytherm, paraffin, etc.

@hot_rod I just realized that I missed this question.
The glycoled circuit has its glycol streaming non-stop if I'm not mistaken. It dumps the collected heat in the DHW tank. There's a temperature sender on the tank.
When the upper temperature limit of the DHW tank is reached a solenoid valve diverts the glycol toward the heating tank (about 1,500 L of water).

I have no metric or observation about the frequency of stagnation condition.
My impression is that if I make good management of the period running from May to August the system seems to regulate itself.

I've checked the Glycol condition about three months ago before de-commissioning the system for the roofing project.

Density was exactly where I expected (50%).
pH showed that the inhibitors were depleted (somewhere between 7 and 8 if I remember right).

Previous check has been done about 8 months ago, same results were observed then.

I guess I'll just carry on with checking pH and density regularly. There's just too much at stake with glycol going bad anyway. Better be safe than sorry.

I'll try to remember to let you know how the new brand I'm about to charge the system with fares.
My old glycol was branded as "Solar II", and was apparently designed for solar. It's discontinued, but a reliable vendor offered me this alternate, equivalent product: "Recofreeze PG".

hot_rod

Those are heat pipe type collectors. The system fluid does not flow through them. They are sealed with a methanol mix, pulled under a vacuum so the boil and steam inside that bulb, around 80- 100°

Those condensor bulbs are not in an air tight connection so I would expect some oxidization being exposed to atmosphere. Probably a removal every 5 or so years to clean and re dope them with heat transfer compound. It's a tough life on the roof like that.

The main thing is to prevent stagnation, over heating conditions under low or no loads conditions. Over heating the metal may contribute to some scaling also.

Some controllers have over-heat function built into the logic. It may turn the pump back on when the collectors reach an adjustable high limit, maybe around 250F. Some controllers have an evac tube setting. In some cases a dump zone is required to shed away that excess btu harvest, if this is a daily occurrence in summer months perhaps.

Apricus has a small outdoor radiator that is used to dump from their evac tube systems. It can mount on the side of the building or right at the array. When the collector array reaches that high limit, a 3 way zone valve diverts flow to the dump radiator.

Zxcv1984

One year later: Recofreeze glycol check:
Density: 45% pH: over 8, less than 9 (litmus strip test).

Stagnation is common occurrence on cloudless day from June to mid- July. Otherwise said: starting with a DHW tank at 30 deg C at sunrise and a cloudless day, the collector will reach stagnation T by 9h30 - 10h00 AM. I haven't lived through July yet, I just assume from my June experience that working from the longest day on June 21st, it gives me about three weeks on each side of the longest day. Weather permits, I'll carry on with the checks in July to confirm this.

Question for you guys: my system is designed to provide heat all year long. This this frequency of stagnation to be expected?

@hot_rod this Apricus device makes a lot of sense, for the right price.

Zxcv1984

15 months later: Recofreeze glycol check:
Density: 44,5% pH: about 8 (litmus strip test).

So Recofreeze will have lasted about two years, since it's having its hardest season in summer. Planning on recharging the glycol circuit with fresh glycol before autumn kicks in. I'd rather play it safe, and it's not winter time that will break it down but summer, as far as I know.

Stagnation is common occurence during summer. Didn't gather data on it but a few sunny days in a row and it's stagnation everyday. I'm venting excess energy from the thermal mass through the floor of my garage, which is free and better than nothing, but insufficient.