Cob: rocket stove question

[Shannon C. Dealy]

On Wed, 28 Nov 2001, Royce Hays wrote:

> We'd like to run the flue of a rocket/J stove under an outdoor courtyard
> area but am not sure how long of a flue/pipe that I can get away
> with.  I know that the length will be determined by how hot the fire
> burns (the hotter, the longer the flue, I believe).

In the rocket stove/heated cob bench design such as is used in a number of
cob cottages (particularly those built by Cob Cottage Company and their
students), I don't believe that anyone has ever established any maximum
length, and it is possible there may not be one, though you will
definitely reach a point where there is no longer any heat worth
extracting.  The pipe running through the bench should be viewed as an
exhaust pipe/heat exchanger only, and (unlike conventional stoves) not a
part of the combustion system.  Everything is driven by the draft
created by the chimney in the barrel next to the "firebox".  We just built
a new one in Cob Cottage Company's site near Coquille, Oregon which I
think had 37 feet of pipe in the bench (I should remember better, since 
I was one of the people who laid out the pipe) -- photos are going to be
added to the web site one of these days (I have them, just haven't had
time to do the update)

> A releated question to this is how long should I run the pipe
> under/through floor/benches before allowing it to vent up and out so as
> to not have a glowing hot pipe sticking out of the ground?

It doesn't take much to get the temperature into a relatively safe to 
touch (but not keep your hand on) range, twelve feet will probably do
it, though this will of course depend on the quality of construction (if
the pipe isn't well bedded in the cob bench, you can end up with air gaps
and lose alot of your heat exchange.  How your stove is sized and the
length of time you run it will also make a difference.

> What materials have folks tried that work well for the combustion
> chamber and nearby
> piping such that the chamber won't burn through due to the high heat in
> a few years?

For the chimney/combustion chamber, stove pipe will work, but will
burn through, for the one in Coquille, fire brick was used for these
areas, then a large (55 gallon) drum was placed upside-down over the top
of the chimney (which was wrapped in insulation), with sufficient gap to
allow the hot gases to flow down the sides of the barrel, and then out
through a hole in the side that was attached to the "exhaust pipe" in the
bench.  The 55 gallon drums seem to hold up pretty well over the years,
and by the time the hot gases reach the exhaust pipe, they have already
cooled sufficiently to significantly reduce the possibility of damage to
the pipe in the bench, though I would tend to recommend using a heavy
guage piece of stove pipe for the very first section in the bench.

> Also curious about how much curvature/bending I can get away with.

Probably as much as you are likely to want in your piping, the bench in
coquille had a couple of 90 degree bends and the tightest 180 degree bend
we could manage without any problems.  As the gases cool, their volume and
velocity through the pipe are both reduced, so the effect of bends should
also be reduced the further you are from the fire.

> We're primarily using this to heat the earthen floor and earthen
> benches in an outdoor/covered courtyard area.  Ideally, I'd like to run
> the pipe down the length floor on one side and back on the other and
> provide smaller branch sections of the pipe that could run up into
> benches and reconnect with the main pipe.  Don't know if/how that'll
> work but that's why I'm asking.  Rough area is 20' x 10' area with
> bench areas mostly on the perimeters.

Don't see any reason why this couldn't work, but I personally wouldn't
setup branch sections, just bend the pipe to run another loop back through
the area.  My concern is that the gases will just take the path of least
resistance and not really properly flow through the branches.

One thing to keep in mind, the further the pipe is from the fire, the less
heat it will be able to deliver to the bench, so you might want to
consider doing a few things if you are going to try and get somewhat
uniform heating throughout:

   1 - Put the rocket stove in the area that is likely to be coldest
       (perhaps near the door), and with the most thermal mass (such as a
       wider or more massive part of the bench.

   2 - At the far end of the bench from the fire, you could double the
       pipe back when it reaches the end, so that more heat can be
       transferred into this area of the bench which is getting heated by
       only the coolest exhaust gases (this is why we had the 180 in the
       Coquille bench)

   3 - Run the pipe closer to the surface of the bench, the farther away
       it is from the fire.

All of these have tradeoffs, but they should probably at least be
considered.

For anyone who is interested in designing/building their own wood burning
stoves, I highly recommend the Cob Cottage Company's pyromania workshops.



Shannon C. Dealy      |               DeaTech Research Inc.
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