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re: a cheaper outdoor woodstove?
16 dec 2003
gary coffman wrote:
>>your air to air heat exchanger as described will be very inefficient...
how much is "very"?
>we could use 6", 8" and 10" pipes, or connect the pipes, thermally-speaking,
>and put them inside one chamber of an 8'x2'x1' foamboard box with 2 1'x1'x8'
>flues, for more airflow and less heat loss. would anyone care to estimate
>the efficiency of that heat exchanger?
no? ok, i'll try. assuming condensation inside, the heat transfer surface
is pi(6+2x8+2x10)/12 = 11 ft^2 per foot of chimney, or 77 ft^2 with 7' outer
pipes. if 10 cfm of 1200 f smoke flows into the center pipe, ntu = au/cmin
= 77x1.5/10 = 11.6. with 550 cfm of room airflow, the capacity rate ratio
z = 10/550 = 0.02 and e = (1-e^-0.98ntu)/(1-0.02e^-0.98ntu) = 0.99999. is
99.999% "very inefficient"? :-)
to assure condensation from 100 f smoke, we only need e = (1200-100)/(100-70)
= 0.973, which makes ntu = 3.67 and a = 24.4 ft^2. with no 10" pipe, a = 40
ft^2. there's a lot to gain here, compared to a non-condensing woodstove that
sends about (1200-70)10 = 11k btu/h up the chimney.
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?
nick
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