[Stoves] cross-section-geometry of stove pipes?

[Crispin Pemberton-Pigott]

Dear Martin

I am not quite following you when you say, "I am aware that other than
circle geometry will have a bigger surface in comparison with the
cross-section."

If you mean a circular pipe, like an ordinary round pipe, then it has the
least surface area per unit flow area. And it has the shape most likely to
maintain a vortex.  Sometimes you want a vortex, sometimes you don't.  It
depends on whether you want the heat to come out or stay in.

For example if you want to heat a room you usually don't want any smooth
flow so the surface gets hot.

By being a bit clever you can get higher or lower losses from the same round
pipe.  Radiation losses are also related to the colour/finish of the pipe so
you can play with that a bit too.

For example you could paint the part of the pipe closest to a stove silver
and a part further away black.  That would provide more heat in the part of
the room that had black pipe.

>Is there a real difference and possibly a rule of thumb, to compare
>different shapes?

I wonder if we have much choice when it comes to pipe shapes!  When I was
very young I lived in a house in Canada heated by 3 coal stoves that had
many metres of round metal chimneys going from room to room and then to 3
chimneys in different places on the roof.  A lot of the piping was
horizontal and I am sure there was very little in the way for vortexes to be
maintained. 

I think if you could make a star-shaped pipe it would be a far more
efficient heat exchanger - perhaps one section in each room where you wanted
the heat most.

Regards
Crispin