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re: wood stove water heater
16 oct 1998
georges wrote:
>>>the text i've seen treats water content as a loss mechanism.
>
>>perhaps not, .....
>
>here we risk straying too far into the realm of the theoretical:)
to make it more real, this might be a "condensing air-air heat
exchanger" as explained on page 3-4 of the 1993 ashrae handbook
of fundamentals...
suppose 20 cfm of hot flue gas with lots of water vapor enters the
fluepipe at thi = 600 f, and 100 cfm of 70 f ceiling air enters the
space between the two pipes at the top, and the 6" fluepipe is rough
and 8' long, inside an 8" pipe, with 1" folded sheet metal between
the two pipes on a 1" pitch.
the area of the fluepipe is 1.6 ft^2/ft, the 8" pipe adds 2.1 ft^2/ft,
and the folded metal adds 4.3 ft^2/ft, for a total of 8 ft^2/ft or
64 ft^2 for 8' of pipe. the area of the ring between the pipes is
pi(4^2-3^2)/144 = 0.153 ft^2, so the airspeed is 100/0.153 = 655
feet per minute or 7.5 mph, and the airfilm conductance is 2+7.5/2
= 5.7 btu/h-f-ft^2, a lot less than the steel and condensing water
conductances, so the number of exchanger heat transfer units is
approximately
ntu = au/cmin = 64 ft^2 x 5.7/100 = 3.6,
e = 1 - exp(-ntu ) = 0.97 = (thi-tho)/(thi-tci),
so the outgoing flue gas temperature
tho = 600 - 0.97(600-70) = 84 f.
the fluepipe joints would run "downhill," lapped so the water runs
back into a small drain near the woodstove, or a bucket collecting
about a pint or two of water per 10 pounds of wood.
this could also work as an outside air-air heat exchanger. we could
turn on the blower when the fluepipe gets hot, with a thermostat,
or with a humidistat, in an airtight house.
>it is far far better, to dry out the wood inside the house than to
>attempt recovery of energy in the steam emitted by the wet wood.
well, it seems to me that wood takes up valuable interior floorspace
and adds undesirable moisture and bugs to a house, and it takes time
to dry wood, and if we want to recover heat from a chimney to increase
the woodstove efficiency, more water vapor is better.
nick
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