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re: a cheaper outdoor woodstove?
17 dec 2003
steve young wrote:
>there is a lot to be gained here, as you are proving.
with forced draft, the chimney air can be cooled a lot. do any outdoor
woodstoves do this? a water jacket around the firebox would be less
efficient, because it would also cool the combustion process.
>> how can we "connect the pipes, thermally-speaking"? if we have 24 ga steel,
>> 0.024 inches thick, with conductivity k = 24 btu/h-ft-f and slotted necks
>> every few inches, with alternate slots around the circumference bent in and
>> screwed to the inner pipe and bent out to allow airflow, how many vertical
>> inches are needed between necks?
there may be simpler ways to do this, like wrapping the inner pipe
with a few layers of chicken wire and spray-painting it black for
better radiant heat transfer or screwing on lots of drywall l-bead.
page 29 of the 1998 schaum's outline on heat transfer says "fin efficiency"
= (actual heat transfer)/(heat transfer if entire fin were at the base temp)
= tanh(nl)/(nl), where n = sqrt(2h/(kt)) = sqrt(2x1.5/(24x0.024/12) = 7.9
for 24 ga steel. for 90% fin efficiency, 7.9l = 0.58, so l = 0.073' or 0.88".
a 6"x8' pipe has 12.6 ft^2 of surface. it seems we need about 24 ft^2 total
for condensation, so we might screw (24-12.6)/0.9 = 12.7 ft^2 of 0.88" 24 ga
fins to the 6" pipe, ie 12.7/0.073 = 174' of fins. hmm...
or put an 6"x8' pipe with end caps inside an 8"x8' pipe, off-center, and
screw the walls together in a 4"x8' sandwich with some stove cement inside,
then cut a 6' slot through the strip, lengthwise, and bend the inner wall
inward and outer wall outward so room air can get inside the inner pipe,
and attach an 8" to 6" reducer to each end of the 8" pipe to make smoke
travel between the pipes. the 8" pipe has 16.8 ft^2 of surface, making
the total 29.4 ft^2.
nick
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