[Gasification] Condensing heat exchanger

[Steve Carroll]

Does anyone have any ideas on building a condensing
heat exchanger?  I figure most people on this list
with condensor experience are just trying to cool 
their gas down before mixing it with secondary air.  I
am trying to cool the combusted gasses and recover the
energy from the heat of vaporization (heating
application, not for ICEs).

I have a loose idea of how to put one together but at
this point it is all guesses.  What I envision is a
piece of rectangular duct mounted vertically in the
open air.  The duct is about 5 feet tall and 9" x 18".
 It probably needs to be stainless.  Hot exhaust
gasses enter at the side of the duct very near the
bottom and flow upward.  Somewhere along this duct are
tubes soldered or welded through the duct so that room
temperature air can pass from left to right through
the duct.  Above that I will probably add a copper
coil that can circulate water through the exhaust
gasses.

As of yet, I haven't done any actual calculations to
know what kind of exhaust temperature I would have.  I
don't know if I would have too much surface area or
not enough or even what the temperature of the cross
flow air or water is going to be.

I assume that if does get the exhaust temperature
below 212°F that there will be quite a bit of
condensate running to the bottom of the duct where it
would somehow need to drain.  This might be a problem
for me because this is a draw through design and I
can't afford air leaks.  I suppose a P-trap could fix
that?

Some concerns I have:
Condensate will be acidic.  Do I have to use
stainless?
Will silver solder work to attach the heat exchanger
tubing?  If not, what about welding?  Will that be
hard to do on thin metal?

There seems to be very little information on how
condensing heaters works.  Is the condensing part as
simple as it seems or are there some things that I
should know about?  I don't have any experience with
this so any help would be appreciated.

Thanks,
Steve

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