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dbc

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Posts: 252
Reply with quote  #1 
I put together a prototype screen frame, made from 1 5/8 metal stud + track.  I was looking for an alternative to the wood or aluminum window screen frames used in most screen collectors.  I have used both, but hey, why pass up a chance to do something the hard way!  This design is sort of a mash-up of some work done by others here on Simply Solar with some new elements.

The standard materials each have some limitations:

Wood - Easy to work with, but it can be heavy, prone to warp, twist or split (tried to buy a cheap, straight, stable 1x2 lately?).  Wood uses up 3/4 inch worth of depth for each frame, which can be a problem with shallow collectors.

Aluminum screen frames - Super light and take up very little space, but kind of flimsy, and it's hard to get the screen tight without the frame distorting.  Vinyl spline and plastic corners may be vulnerable to heat.  I used sisal rope on my 2x16 ZP, which worked OK, but didn't hold the screen as tight or smooth as normal spline.  (Note: More on the spline / heat topic in my 4x8 turnaround ZP in Projects section).

So, I made a 20 x 24 frame out of metal stud, like this:

Frame top 01, 041718.jpg 

Frame side stud, 041718.jpg 

The above 2 photos were taken at different time of day, but in the first, the 20 inch 'top' is on the left.  You can see that the stud is cut the same length as the track.  The stud is inserted into the track facing the opposite way, so the whole assembly forms a rectangular tube.

In the second picture, the top of the frame is at the top of the photo.  The inner stud pieces for the 24 inch sides are cut shorter than the side tracks, so they fit between the upper and lower studs.  The picture shows the stud just before being pushed into the track.

Once everything was pushed together and squared up, I riveted all the corners, and riveted the side pieces once in the center.  For a real frame, I would use more rivets; this was only a test.

I arbitrarily designated one surface as the bottom.  Here I added some triangular gussets at the corners:

Gusset, 041718.jpg 

The frame was pretty stiff, considering I only used enough rivets to hold it together.  In fact I almost ran out of rivets, and switched over to self-tapping screws for the gussets and the screen (below).

I flipped the frame over, and attached a piece of fiberglass screen to the top side.  The screen was sandwiched between the frame and some pieces of drywall corner bead:

Screen on frame, 041718.jpg 

Once again, I used only the bare minimum of screws.  You could get the screen tighter by clamping everything beforehand and using more screws or rivets, maybe one every 2 inches or so.  Unfortunately, that adds up to a lot of rivets.  I think I would also run a thin bead of silicone to the face of the frame before fastening the screen.

Overall, I would say this represents a usable method, although it does require a lot of material and hardware, not to mention time and effort.  There is a lot of cutting and drilling.  Another potential problem is that the screen frame eats up 1 5/8 inch of depth in the collector, way more than an flat 1x2 frame.  Depth would equivalent to a 1x2 frame with the boards set upright; you can ask, "why not just 1x2's and be done with it?"  With 2 frames, such as in a Zero-Pass, you end up with a pretty deep collector, which raises internal shading issues.  Also a deep collector may not be acceptable aesthetically for some.  That said, the stud frame is probably lighter, straighter, and more stable than an equivalent wood frame.

gbwillson

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Reply with quote  #2 
Interesting idea Don-

It gives me an idea for a new(er) collector frame. Why not build the collector box itself with the inner and outer stud track? The biggest issue with using stud track framing has been the lack of attachment points inside the collector box. This would also greatly stiffens the frame. Yes, you would double the cost of framing materials for the box, but it's still not a huge investment. I would either cut strips of polyiso to fit tightly between the inner and outer stud track, or fill the void with a can of Great Stuff foam. I'd also spray the entire frame with a rust preventative paint. The condensation inside the collector box has rusted the galvanized stud track in places on my current collector box. 

Greg in MN
dbc

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Reply with quote  #3 
Yeah, I think I like your idea better.  Another advantage is that it opens up a new way to build longer frames.  A 16 ft. long frame, for example, could have 10 + 6 for the track with 6 + 10 for the inner studs, giving you a 4 ft overlap in the middle.  The resulting assembly should be reasonably stiff.  You'd have 2 ea. 4 ft. scraps of stud and track left over from the 16 ft. sides, so you should be able to build a 16 x 4 frame out of 4 ea. 10 ft. tracks + 4 ea, 10 ft. studs, and end up with zero waste.  At $6 apiece (Home Depot price for 10 ft x 3 5/8 inch studs or track), that's $48 for a 16 x 4 frame.  I could live with that.

Strips of Polyiso (or canned foam) inside the tubes should stiffen it even further.  What I'm not sure about is if you'd still need additional insulation inside the finished frame to overcome the thermal bridge created by the 1 1/4 inch walls of the metal tubes.  Not sure how to predict that - my 'empirical' test has been to feel the outside of the frame when the fan is running; if it's not (too) warm, that's good.  It would sure be easier to attach things to the inside if you could screw directly to the inner studs.

Another question is how to attach and seal the back polyiso.  If you had a rigid back (plywood or something), you could screw that directly to the back of the frame and drop the polyiso in from the front.  If you didn't have a 'structural' (heavy) back, you could screw some L-trim to the inside walls and insert the polyiso from either the front or back, depending on where attached the L-trim and which way you turned it.  Either way, you should be able to seal the polyiso to the frame OK with silicone, although, without a solid back, you may also want some cross-straps to keep the frame from spreading apart and breaking the seal.  If you wanted a plywood back, but didn't want the plywood edges exposed, you could use L-trim to fasten both the plywood and polyiso to the inside walls of the frame, maybe glue the plywood and polyiso together first - sort of a 1-sided SIP.

A square tube gives you multiple options for attaching the glazing, either to the side or front.  Going into the front may be stronger, since you would be fastening through two layers of metal.

I don't think I have a spot for a 4x16 collector, at least on the wall, but a lot of people do.  For me a 1-piece, 16 x anything would also have to live outside during construction, but I can figure something out - tarps or such, as long as it's light enough to move around.  I still want to try another 2x16, but built as one piece.  That would be a lot easier to handle than 4x16, and you could still see where you were going!  Wheels may be a good idea, although my ground is pretty rough. 

Maybe it's time to look at a ground mount.  I bet you could use metal framing to build that too, especially if you had some internal wood blocks at the attachment points.  Too many ideas...

By the way, I haven't seen any rust (yet) on my stud-track frames, although we're in a much drier climate here.  The older one is painted, but the 2x16 ZP is not, mostly because I was in a hurry to get it running.  I should probably paint it too some day.  This may be a dumb question, but did you by some chance use any pressure-treated wood in your collector?  That can cause corrosion even to some galvanized metal, more so than plain untreated wood.  Don't think I'd knowingly use PT in a collector - one more chemical to worry about breathing.

Bert

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Reply with quote  #4 
Don.

That's a very clever idea and seems to be close to the optional spacing.


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Bert K.
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Bert

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Reply with quote  #5 
Whoops!

I didn't read your first post careful enough.

I thought you were using one frame for both screens. You would have to open up the ends though and that would make it too weak.
What if you used a flat bars for the ends? That would be strong and let air through. You could put both screens on the same frame on the inside.
That would save space and be less material. 

How does the corner beads work out for holding the screens? Do you just stretch the screens out and screw them down?
You could put them on the inside but maybe the stud would work sandwiching the screen in between the track and stud.

The inside spacing may be very close to 1-3/8".





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Bert K.
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dbc

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Reply with quote  #6 
Bert,  I think you have identified the biggest flaws in my prototype: (1) you need two frames, (2) the frame is too deep, and (3) it uses too much material.  As it is, I doubt I would ever build it.  I agree, it would be way better if you could have one frame for both screen layers.  Solar Dan made a frame that way couple years ago out of 1 5/8 stud.  The screen was wrapped around the outside of the stud, so the spacing was 1 5/8 (width of the stud).  He used stud with a slot cut in it for the ends.  He had to leave a little bit around the slot for strength, so the screen gap was less there.  It seemed like a good idea that just needed a little more work.  I thought about using some threaded rod for the ends, but that doesn't leave much to attach the screen to, and it would still partially block the air channel.  Maybe flat bar would be better, if it was stiff enough.

I thought about sandwiching the screen between the track and the stud.  I didn't see how to hold everything together tight, so I eventually just attached the screen afterwards as a separate step.  The corner bead seemed to hold the screen pretty well, even though I just pulled on the screen with one hand and drilled in the self taping screw with the other.  if I was making something permanent, I would clamp everything better, and maybe attach the free end of the screen to a piece of wood with a couple wraps to keep the tension more even while I attached it to the frame.  I would also use more screws (or rivets) and add a bead of silicone.

The hold-down piece doesn't really need to be angled unless you were attaching on the side instead of in front.  The corner bead was cheap and available, main reason I used it.  You'd still need something different on the ends.

I'm still thinking about how to make a single frame for both screens.  I ran out of enthusiasm a little bit while working on the prototype, because I ran another 'bake test' with some aluminum screen frame with a piece of screen held in with spline and covered with foil tape (more details in the 4x8 turnaround ZP thread).  It held together better than expected, which left me thinking the pre-fab screen frames were probably a better solution.  Biggest downside is probably the cost.  I heated up the test sample in a solar cooker, which got me thinking...how's your oven project going?  Last I recall, you were fine tuning the gasket design and researching the reflectors.
Bert

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Reply with quote  #7 
Don, If I make another collector I may try a few things based on your ideas. It would be neat if it panned out. Steel flat bars should be strong enough. Don't think the dissimilar metals would be much of a problem.

I haven't worked on the solar oven since my last post there. With the nice weather it won't be long. Just have to do a few other projects around the house first. You know, stuff the wife wants done. 😉  Also two squirrels made it into my chimney and flopped out when I opened the damper. Dirty with soot they dumped it all over the house on their way out.  Once my bang up knee gets better I will be on the roof to fix that.

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Bert K.
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