Making your own evac tube solar heaters

[willib]

ok had another idea

Take two glass tubes say a 1.5 inch dia. and insert a 1 inch dia tube.

melt them together , pull all the air out , and there ya go!

Not having worked with any melted glass before except while camping as a kid, it seems that all ya need to do is pre form the inner tube so the diameter meets the dia of the outer tube sort of like this--> ][ except at one end only.

or is there a better way?

i'm positive it can be done and i recall from somewhere how to melt a thin nozzle into glass to pull the air out. and then pinch off the nozzle with a hot pincher thingy ?

I'm not sure what would work best to coat the outside of the inner tube with to absorb the sun rays ? and i am surely not thinking of using anything fancy like the store bought ones use.

Just something to get the job done.

a lot of preforming needs to be done to both tubes i'm sure before they can be joined  but is there a right way and a wrong way?

[willib]

http://upload.wikimedia.org/wikipedia/commons/4/47/Evacuated_tube_collector.gif

i dont like the heat pipe idea but this pic is a good description of what i mean.

i prefer a looped copper tube, with a fluid inside, going down into the evac tube.

http://www.appropedia.org/images/9/96/Evacuated_tube_diagram.jpg

[Capt Slog]

I have worked with glass, quite a lot,  I used to repair and make stuff for lab use.  Even with hours of practice I was never very good at it but some of the stuff I made was OK.

What I can tell you is that it is a lot more difficult than you think, a LOT more.  Even with small tubing (1/4") it is difficult to apply heat evenly whilst maintaining the structure, large tubing gets very hard to handle, and this is why the professionals use 'glass blowing lathes'.

From what I've seen of evacuated tubes, it appears to look like one 'test tube' inside another, and then these are sealed together at the rims.  To make something like this you would first make the inner tube, sealing one end and blowing it to make nice round end.  The outer tube would be blown the same way and then a small tube joined to the round end.  I'm not sure how to join the tubes at their rims, it's been a long while since I did any of this, but I can guarantee it would be a sod to do; keeping the two tubes parallel being the problem. I think this used to be done by wrapping the inner tube in card which was burnt away later with acid. Next you'd have to anneal the joint to remove the stresses.

Lastly the structure would be evacuated through the small tube which is pulled closed and fired off.

You'd then have to hope and pray that all your hard work wasn't just going to implode.

Personally, I wouldn't bother.

.  

[joe4324]

I have been around enough evac tubes to know that I personally would never try to make one,  I live in upstate NY and I think I could do 4 season batch heaters up here much more reliably and faster/safer than i could figure out how to do a real evac tube from scratch.   Not that I want to dissuade you in anyway!  I just know my own limitations and patients.

[RP]

Willib,

I learned a lot on how to blow glass by reading a book on neon sign making and buying several sizes of glass tube from a local neon sign company.  It's plain old soda-lime glass and you can get started with a propane torch.

If there's a place around you that makes custom neon, maybe you could talk to the guy/gal that does it for them.

Maybe he/she would be willing to make the collectors for you.

[BigBreaker]

You can't DIY anything approaching a commercial evac unit.  Besides the difficulty of blowing the glass you have:

Sourcing, working with low E glass  (IE not that easy to melt soda lime glass)

Coatings, including the interior reflecting surface on the back half of the tube

Specialized glass metal interfaces.  Typically a special alloy is used with a coefficient of thermal expansion equal to that of glass.  That metal interface is then joined to the copper piping.

If it's a cost thing, make more non-evac units to make up for the heat loss or make non-evac units, sell them and use the money to buy evacs.

[tanner0441]

Hi

An interesting point... I you put a small glass tube inside a big glass tube, draw out all the air and coat the outer glass tube with an absorbant coating.  How will the heat permiate the vacuum to warm the inner glass tube. You can't get convection currents in a vacuum.

Brian

[willib]

Hi Tanner

the dark absorbing coating goes on the outside of the inner tube.

[strider3700]

ok what if we used a copper pipe for the inside pipe,paint it with whatever, slip it inside of a glass tube then seal the ends with silicon?    it saves needing to learn how to melt glass and with a thick enough silicone plug I would think you could still draw a vacuum if you wanted.   

I doubt it's as good as a commercial unit but it's well with in the capabilities of someone @ home.    Having said all that,  I'm collecting parts for a flat plate, drainback system ala builditsolar, simpler and proven.