10/29/09
christina writes:
I would really suggest you get the (about $6) "Heating Water with Wood"
manual from Lehmans.com (Amish general store) and get it into the hands
of your designers if they haven't heated water with wood before.
Temperatures in the firebox easily exceed 500 degF - they can be double
that, which is why that manual advises Stainless Steel piping in the
firebox and for a few feet beyond the firebox because copper piping
could melt. You get the most heat from the wood with a heat exchange
pipe that is within the stove firebox, but this is what makes it
difficult to have antifreeze in the pipes because of the high
temperatures. The Amish don't use antifreeze, they just never let the
room the wood-fired waterheater is in ever drop below freezing in
winter, thus they don't put this system in buildings that are only
occasionally heated and they don't leave home for long in winter unless
they get a housesitter. The manual covers the numerous safety features
such a system should have when you are dealing with temperatures higher
than 212 degF so the water can become steam unexpectedly, with
potentially explosive force.
I'm not sure where 2" pipe diameters came into the stove to tank loop,
since the Amish have done successful thermosiphon systems with much
smaller (and cheaper) diameter pipes. Also, I don't know what to make of
"corn" glycol, other than corn is the source material, it doesn't tell
me what the end product is chemically. I'm only aware of two types of
antifreeze: propylene glycol (food grade, used in solar DHW systems) and
ethylene glycol (poisonous, used in cars). There may be other chemical
formulations of antifreeze than those, but I don't know anything about
its properties. If you think you might want to heat potable water with
the same system as you are using for heating the building, code would
require you to use a food-grade antifreeze. If it is not going to be
food-grade, you should take precautions to ensure that ignorant folks
like children can't get into contact with it at the tank (warnings,
locks, etc).
There is the possibility of putting your heat exchanger piping on the
outside of the woodstove box, which causes a dramatic loss of efficiency
of getting the energy contained into the wood into the watertank. But if
you absolutely need to have antifreeze in the system, this may be your
only option. There are off the shelf external heat exchangers for
woodstoves (I believe Lehmans sells some), which typically have flat
metal panels with piping welded or brazed onto it. These are mounted
about a 1/4 inch from the surface of the woodstove, and can only capture
some of the radiant heat coming off of the surface of the woodstove,
which is a lot less than the amount of heat available in the firebox.
That is why these external heat exchangers typically have the pipes
doubling back and forth over most of the external surface area of the
stove, which also means that the thermosiphon loop is much longer, and
therefore the resistance to flowing in the pipe is greater in the
external heat exchanger. These off the shelf units are flat panels, so
maybe one could replace the heatsheild at the back of your stove, but
they may not work well (or aesthetically) at the sides your existing
woodstove, which is slightly curved, isn't it? I still think that many
of the safety features in "Heating Water with Wood" might be wise to
incorporate into this system, too.
These form incompatibilities may cause you to decide that you want to
"make your own" heat exchanger, embedding it in cob like your intern
suggests, but I can't predict how this will work. Even if you were to
replicate the length and configuration of the tubing in the
off-the-shelf model that has been time tested for successful flow & temp
range, you could still have unpleasantly different results. A heat
exchanger & thermosiphon loop should always have the path of the heated
water be slightly uphill, thus roughly horizontal pipe runs between the
switch-backs, so there are no places where an air bubble could form and
be trapped and block the flow (as could happen if the pipes run up and
down w/ u-returns - if flow stops, possibility of steam flash
explosion). The store bought heat exchangers don't have a conductive
material between the stove and the heat exchanger; this air gap may keep
the pipes cool enough that the antifreeze can't get cooked or the slowly
moving water flash to steam. Also, the pipes will expand and contract a
lot over the operational temperature swing of the stove, and this may
cause the cob to crack and break up. Finally you should definitely test
to see how your particular cob mix reacts with both the metal of the
water pipes and the shell of the stove at high temperatures over time
before you cover large portions of the stove or pipes with the cob. If
the materials in the cob aren't PH neutral (unlikely) or are the metals'
polar opposite on the galvanic spectrum, there could be a lot of
corrosion to either or both. But it may be worth experimenting, since
rocket stoves work and they involve putting cob over a metal firebox &
flue, though its likely different metals.
While running solar system pumps with solar panels is a workable idea
(at least theoretically), since you shouldn't have much solar heat to
move if you don't also have the solar electricity, too, the same logic
is not applicable to heating water with wood. The wood heat is much more
intense and is more likely to be in effect when the sun is not shining
than when you have sun to make electricity. Thus you'd need to have a
big battery bank to store solar electricity for the pump for your
woodstove loop. Even with a battery bank, it should still be backed up
with a grid connection, because if the pump loses electricity while the
woodstove is still running, the water stops moving because it can't get
past the pump and the results can be catastrophic. That is why a
well-designed, time-tested thermosiphon loop system is safer than a
pumped loop, since the water is moved by the heat itself, and the hotter
the stove gets, the faster it moves, and the flow won't stop until the
stove is cold again. The Amish system uses a closed loop thermosiphon
but at atmospheric pressure with substantial air space at the top of the
tank that permits expansion of the fluid. This arrangement could
probably work for the external heat exchanger you likely need just the
same as the Amish internal heat exchanger. I wouldn't recommend a sealed
pressurized closed loop typically seen in a pumped system, because even
if the pump is fully functioning, the high temps of a wood stove could
cause pressure in the system to rise beyond the capacity of the
expansion tank to compensate for.
I know all this technical stuff is intimidating to you, so feel free to
forward my remarks to the Harlows directly, and to have them call me on
my cell phone (734-355-2528). In short synopsis of the above, w/out all
the explanations, for a system with antifreeze in it, I think the
off-the-shelf external woodstove heat exchanger replacing the heat
shield on the back of your stove (and possible side mounted exchangers
too?) might be your best bet, paired with a closed loop unpressurized
thermosiphon design with no pump, similar to what is described in
"Heating Water with Wood" from Lehmans.com. It will mean that you have
to burn lots more wood to get the tank topped off with heat, but the
anti-freeze in the system will allow you to walk away from the building
for weeks or months in the winter with no worse consequence than the
temperature dropping to close to the outside temperature.
Best of luck with your project!
Christina
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