Posted by Morris Dovey on March 26, 2009, 11:30 pm
Yes - getting the heat /inside/ the hot head was, and to some extent
still is, a difficulty with the low-temperature engine.
Yup - that's the reason I was willing to spring for copper...
Why didn't /I/ think of that! I went through a ray-tracing exercise a
while back and noticed that more heat would be applied from the sides
than from elsewhere. It seems like the design should be able to take
advantage of this...
DeSoto, Iowa USA
Posted by Bruce Richmond on March 27, 2009, 12:50 am
From a posting in alt.solar.thermal
There was this link for spray paint.
Don't know anything about it but it sounds like what you were looking
for. For all I know bare copper is better.
For increasing surface area of the tube to the working fluid, I seem
to remember seing inserts with a cross section like * that you could
slide inside a pipe. Might even be able to find a source for pipe
with them pre-installed. Would be best to slip a cold insert into a
hot pipe so they would have better contact for heat transfer.
Might be able to make a cheap vacume tube out of a used floresent
While you may get the high temp you are looking for by focusing to a
3/8" line on a 1" pipe, to heat the working fluid inside the pipe you
need as much area as possible hot. So the heat from that 3/8" strip
will flow out to the rest of the pipe, spreading the heat out and
dropping the temp. Might be better off not bringing it to such a
narrow beam to start off with.
Putting a small reflector on the back side of the pipe would not only
reduce losses due to re-emission, if the beam is made a bit wider than
the pipe it could be used to heat the back side of the pipe.
I don't *know* that the ideas above will help but they might be worth
Posted by Morris Dovey on March 27, 2009, 2:11 am
Bruce Richmond wrote:
I should have bookmarked it then. Have it now - thank you.
I'll dig around and see what I can find. I like the idea, but am
beginning to think that I might try this thing out first with an empty
tube just to see how that plays.
One of the trade-off factors here is that the viscosity of air increases
with temperature, and the air in this long narrow hot head will need to
move /fast/. I do want to distribute the heat as efficiently as
possible, but I don't want to waste power overcoming flow resistance.
It'll be an interesting task to find the balance point...
I read that thread, too - but I think when I get to that point I'll just
buy one of the tubes used in solar water heaters...
I've had that same thought - and what seems to make most sense is to set
up the mounts (they're just U-bolts) for the tube so that the tube can
be raised so that the focus is at the bottom of the tube, and lowered to
the point where the focus is coincident with the center of the tube.
I'll file that one for more thought, but the current geometry won't let
me do that (I'd need to raise the focus, which would complicate the
Until I'm sure that it can't be further improved and still be built at
reasonable cost, /everything/ is worth thinking about.
DeSoto, Iowa USA
Posted by sno on March 29, 2009, 6:09 pm
Morris Dovey wrote:
When I was a kid we had a neon tube sign maker where I lived....he had
glass tubes about 8 ft long....
Have often wondered if you could not increase the efficiency of a home
built concentrating collector by using one of these tubes around the
collector....you can draw a vacuum in them....and could coat one side
with a reflective material to bounce back radiation....
Do not know how you would seal the ends...maybe melting the tube around
Do not know if you can still get these glass tubes or how much they
cost....or their diameters ....
May or may not be practical....
Posted by Morris Dovey on March 29, 2009, 6:41 pm
Thanks - I, too, remember a local shop where such magic was worked. As a
kid I'd have been underfoot for hours at a time if I'd been allowed. :)
I think Neon John is the 'local' expert these days - but I think I'll
probably just buy an off-the-shelf tube when I get to that point.
I /like/ your idea of the reflective coating on one half of the tube.
DeSoto, Iowa USA