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Using plastic pipes as heat exchanger...?

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Posted by NickW on May 27, 2004, 9:16 am
 
I am going to build a collector on a south facing garage roof. It'll
consist of black painted chipboard, an air gap of a few inches and a
layer of transparent polycarbonate. This area is around 7 x 3 meters.

I am going to collect the heat by running black 25mm MDPE pipe through
the sun space. The incoming water temperature will be about 20-25C.
Pumped at about 15 liters/minute. The wall thickness of the pipe is
around 2.5mm

The question is, how much pipe do I need to effectively collect most
of the heat that builds up in there?

I would assume that laying the pipes out with zero gaps between them
would achieve the best heat exchange efficiency. However, I'd be
willing to sacrifice say 10% if it mean I could use half the amount of
pipe.

Can anyone give me a way to calculate this?

TIA

Nick Walczak.

Posted by NickW on June 4, 2004, 9:06 am
 
OK, forget the maths. Does your gut tell you that a coil of black
pipes 25mm wide with an empty 50mm gap in between each pipe would
collect most of the heat available given a brisk flow of cold water?

Nick

Posted by Anthony Matonak on June 4, 2004, 10:16 am
 NickW wrote:

The materials you are discussing are fairly inexpensive so an
experimental method could give you a good idea of real world
performance. Make a series of small panels using the same length
of pipe with different gaps between, run them in parallel and
measure the temperature of the water exiting from each.

One issue with plastic pipe is that it can melt. This won't
happen under normal operation but if the pump is not working
heat will build up in the panel. Pool heating panels are
typically plastic and don't suffer from this problem because
they are unglazed and lose heat quickly to the environment.
If you add glazing, such as your planned panels, the stagnant
temperature can get up to 250F-300F which can cause damage.
Once again, try it, see what happens. You might do better
not to operate them as a drain down or otherwise unpressurized
system.

Anthony


Posted by NickW on June 4, 2004, 3:10 pm
 
Good point.

Would MDPE or polycarbonate melt or discolour at those kinds of
temperatures. And can wood ignite at these temperatures?

I hadn't planned for it to retain water when switched off, is that
what you mean by a drain down system?

Nick

Posted by Anthony Matonak on June 4, 2004, 4:06 pm
 NickW wrote:

Oops, make that "do better to operate as a drain down..." :)


You can find data sheets on the web and they will indicate the
melting points. I believe wood and paper self ignite around 451F.


Anthony


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