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re: heating water with a woodstove
11 may 2000
georges wrote:
>>>if you are a tinkerer, it's not that hard to wrap a coil of copper
>>>around the stack, and plumb it into the hw via a shell and tube heat
>>>exchanger.
oh wait, you are georges, not george ghio... maybe this will work...
>>that's fairly expensive and useless, because of poor thermal contact
>>between the copper pipe and the flue gas and the extra heat exchanger.
>there are better ways to increase heat transfer. i note that you left
>out the part where i said to insulate the copper pipe from contact
>with the air with kaowool. this does increase the heat transfer rate.
seems like the rate would still be small... 600 f slow-moving flue gas
heating 8 ft^2 of 200 f pipe; (600-200)8(1.5) = 4800 btu/h, about a kw,
with perfect stovepipe to tubing conductance, vs a 5 kw water heater?
the insulation doesn't increase the heat transfer much if the thermal
conductance between the flue gas and the pipe is many times larger
than the thermal conductance between the pipe and the room air...
and the woodstove would operate more efficiently with a lower flue temp,
using more flue pipe or a gas-air heat exchanger.
>it's not useless because it works.
well, maybe it's just expensive...
>during the middle of january, when the stove is on
>nearly all the time, it keeps a 50 gallon tank at 150f.
not hard to do, with minimal water usage and good insulation...
what i had in mind was thermal storage in external water drums
in an insulated closet for more controllable space heating with
less frequent fires, more than just water heating.
>...the primary function of my wood stove is to heat the
>house, hot water generation is a scavenging operation.
i see your point...
>if you make the heat transfer too good... you have to develop
>a system to dump the excess heat...
how about boiling, in an unpressurized system?
and lots of thermal storage...
>...if you are suggesting that the coil be in line with the primary hw,
>then you have a potentially messy failure mode...
maybe two copper coils, one in the stovetop and one in the near-boiling
unpressurized tank...
>>better to cast a flat top for the stove with some concrete and wire,
>>with a flat soft copper coil in the middle. this can also make a nice
>>temp-controlled cooking surface.
>are you sure it's better?
the solid-to-liquid heat transfer conductance might be 60 btu/h-f-ft^2
or even 20 k btu/h-f-ft^2 when boiling occurs, vs 1.5 for gas-liquid...
>the average temperature of the concrete is somewhere between the
>temperature of the wood stove, and the temperature of the surrounding air.
it might be a bit more than 212 f most of the time,
good for simmering stews and teapots.
>if your coil is in the middle, you cut the heat transfer significantly.
>what's the thermal resistance between the stove and the concrete?
concrete's about r0.2 per inch. make the coil near the bottom,
right on top of some chicken wire that conforms to the stovetop.
use bending springs to make 50' of 3/4" soft copper tubing into a
pancake spiral. say the stovetop is 800 f and the 10ft^2 of tubing
is 200 f, with a 1" average concrete thickness between them. (can
we make it thinner, or add steel to the concrete to decrease its
thermal resistance?) that's about (800-200)4ft^2/r0.2 = 12k btu/h
for a 2'x2' top, right?
>are you casting the concrete directly onto the stove top.
sure. with some plastic film over a vertical wood picture frame
to contain the ferrocement and provide a mold-release.
>if so, then you have a differential temperature expansion problem.
it would just rest on the top, in close contact. it might rest
on a thin bed of high temp stove cement...
>consider the failure modes. a problem with a cast solution is
>spalling due to differential thermal stresses. did you try this
>design, or are you recommending it without thinking it through?
i'm recommending it after thinking it through, without trying it.
use lots of chicken wire.
>are you certain that the thermal coefficient of expansion of concrete,
>cast iron and copper are identical?
i'm sure they are different.
>what are the other possible failure modes?
earthquake, tornado, deus ex machina. some giant foot
comes out of the sky and smites flat the owner...
>does the copper pipe ever break inside?
i hope not.
>if it does break what method do you have to insure you do not
>cause steam pockets and the resultant concrete shrapnel?
an unpressurized system, with 212 f concrete?
>finally, you're assuming, of course, that the stove has a flat top.
no. i was thinking of doing this with my horizontal 55 gallon drum stove.
>are you suggesting that one purchase a stove to implement your
>speculation?
no. just cast the top to match the stove top, and make the upper
surface flat.
>i'd be interested to hear from people who have actually tried this method.
me too.
nick
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