Musings About Evaporators
by Wayne Crouch
February 12, 2001
Someone once asked, "Wayne, why does your evaporator look like that?" and I said: An evaporator consists of three parts. The arch which contains the fire and
supports the pan(s), the pan(s), and the flue.
Design considerations Arch
Restricted air flow so just the right amount of air gets to the fire. Too
little air, fire isn't as hot as it could be. Too much air, fire is cooler
than it could be because so much cold outside air is getting in and too much
heat is being sucked up the flue
Our arch is a 275 gal oil storage tank. There is a door to put wood in
through and a round hole with a little flap that we can use to adjust the
amount of air flow. Very inexact, but good enough.
A grate is ideal so air flows up through the fire, but very hard to do
cheaply. We do not have a grate. At the end where the door is, the bottom
of the arch is filled with cement blocks and sand which creates a concrete
surface on which we build the fire. (Not a good design if you want to move
the evaporator. We just leave ours in place.) The air just flows in
through the cracks around the door and through the official little round air
The cement block/sand platform extends about 30 inches into the arch. From
there to the end where the flue is I had a "false floor" welded in which
slopes up to just below the hole in the back of the arch where the flue
attaches. That keeps the heat close to the bottom of the top pan instead of
filling that whole large area at that end of the arch.
Bare metal exposed to the 1000 to 1500 deg F heat that is generated in there
radiates a lot of heat out into the surrounding space, may glow red hot, may
melt, and deteriorates at some unknown rate. We stack fire bricks along all
inside surfaces as much as we can to minimize these problems. That helps a
Surface on which fire is built should be about 12 inches below the bottom of
the pan. If it is too close to the pan there isn't room to build a big
enough fire. If it is too far from the pan not as much heat gets to the
Top of arch should be open to the pan. Heat should go directly to the
bottom of the pan.
We have metal covers we can put on if we want a fire going but no pan in
place or when we have the large pan hoisted with finished syrup being
Design considerations Pan(s)
If you have two, the smaller should be near the end where you put the wood
in (this one is usually called the finishing pan), because when you are
finishing off, you want to be able to reduce the heat and that is easiest
done at that end of the arch. The hottest part of the arch is the end near
Avoid lead solder. Use welded pans only. You probably won't give any kids
enough lead from maple syrup to matter, but lead is now know to be very bad
for kids. We are trying to keep it out of maple syrup.
Stainless steel is best. Galvanized metal is okay. I don't know about
other possibilities if there are any.
You need thick enough metal so pan is rigid enough to handle with sap/syrup
in it and to support the weight of the liquid as you boil. The pan should
be supported by the arch only around its edge so as to expose as much of its
bottom to the fire as possible.
It will not "burn" under normal conditions because liquid conducts heat away
very efficiently. When you let the liquid all boil away (even over just a
portion of the pan's bottom) or when you let the concentrated sap (syrup or
nearly so) boil up until it is all bubbles and no more liquid, the last of
the liquid will be charcoaled onto the pan's bottom. I suppose a thick
bottom of the pan would resist getting melted a little more than a thin one,
but this is not the biggest problem at this point. The charcoaled syrup is
the problem and if the heat is hot enough to melt/destroy the pan bottom it
is probably going to do that anyhow by charcoaling it beyond cleaning.
It your finishing pan is small, you may be able to lift it off the arch when
the syrup is finished. Ours is not small so we have the chains and block
and tackle to lift it.
You must get the finishing pan off the heat before pouring the syrup out
because when almost all the syrup is out, the remainder will burn. A very
Design considerations Flue
The taller the better. Produces a better draft.
The Manual suggests three times the length of the arch, but most anything
will work. The little efficiency lost because of a shorter flue is of no
significance to us amateurs. If fire really won't draw well, too short a
flue might be the cause.
Secure all parts of the flue pipe to one another and to the arch with
screws. Our flue fell off three times before we learned that lesson. Luckily,
we didn't burn the garage down.