AOF "passive" installation ideas

[Pete S]

I have been working things out in my head for the creation of a new evaporator for 2015.

Very serious about installing an AOF (air over fire) system.

Lurking about here I didn't stumble on to anything that was a "passive" installtion as in NO BLOWER.

We have a 10+ year old Jutol wood stove in our home which has AOF as part of it's design. The burning gasses from the wood actually give a ghost like presence and this thing creates a TON of heat with proportionately very little wood. It has a 3" inlet opening, but the stove is small.

I am interested in the AOF as I think it would really help creat more of a complete burn as it's the gasses from the material that are actually burning and NOT the material itself.

My issue with a pressurized AOF is that I just don't want to deal with the noise and the necessity to rely on power (although readily available) to make our syrup. Not that using electrical power to make syrup is bad, just want to aviod the reliance on it.

Thoughts, comment, and designs are greatly appreciated.

I am anticipating that my tube size would need to be a bit larger, and have multiple holes rather than nozzels. (?)

[Paul VT]

I don't know a lot about the AOF. But I know some have the blower outside there sugarhouse and run PVC piping into it. This way you don't hear the blower. Next year I may do some experimenting with AOF.

[maple flats]

Since AOF won't even work with a blower that does not deliver high pressure, I see no way to do it passively. You need to create a turbulence in the firebox, above the fire to make the gases burn much more completely. If you can achieve that passively you have achieved a miracle.

[CharlieVT]

.....Very serious about installing an AOF (air over fire) system......
My issue with a pressurized AOF is that I just don't want to deal with the noise and the necessity to rely on power (although readily available) to make our syrup. Not that using electrical power to make syrup is bad, just want to aviod the reliance on it....

I suppose some type of increased efficiency could be achieved with a passive AOF but you have a lot of considerations when trying to adapt what your Jotul stove has. I have a couple of woodstoves that are designed basically like this:

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This design has tubes across the top of the fire box which have rows of holes. The natural draft pulls air up through the manifold and out the tubes thus mixing air with the gases above the fire. An important part in this design is a baffle between the top of the firebox and the top of the stove. In a maple arch, this baffle would be separating the fire from the bottom of the pans. It seems to me that this would significantly reduce the heat to the pans.

These "high efficency" wood stoves are designed to improve the burn and decrease the amount of combustables going up the stack. But the stoves are also designed to radiate heat from the stove into the surrounding room. In the maple arch, we are trying to keep the heat in the arch, under the pans.

No doubt you could make a more efficient evaporator using some type of passive AOF, but I don't think you can just copy the design of the these EPA approved high efficiency wood stoves. So you'd have some serious home brew engineering and experimentation to do.

Maybe someone has heard of someone doing something like this, I haven't.

There are lots of folks who have built AOF using information available on this site and in this publication: http://www.uvm.edu/~pmrc/Combustion.pdf

[motowbrowne]

I've thought about this too. My woodstove is a Morso, similar to your Jotul. The reason I don't think it would work in an evaporator is that in the woodstove, the firebox is basically airtight, when the primary intake is partially or all the way closed, the negative pressure pulls air through the baffled secondary intake. With an evaporator, you'd never close your primary intake creating negative pressure. If you designed something with passive AOF, you might be able to create a higher efficiency rig, but you'd have to make your box airtight, and you'd end up with a slower evaporation rate than running it all out with the primary intake fully open. I hope that helps you in your figuring.

[nymapleguy607]

Ae evaporator fire box can be made air tight. Nearly all commercially built gassifier/ air over fire arches are air tight. The reason evaporators use blowers is the will make a ton more BTU's than using a natural draft system can. Look at the burn rate of your woodstove vs the rate on your evaporator, you refill the evaporator every 7-10 minutes, and the wood stove every 2-3 hours. If noise is the problem then locate the blower outside of the sap house and pipe the air in. Once you boil with a blower you won't want to go back, the evaporator boils harder and evens out faster after firing. Good luck

[Robert K]

Its kinda like drag racing, once you have driven a blower car you don't want to go back to naturally aspirated!!

[Robert K]

I own a Pacific Summit woodstove and have thought about this for awhile, the idea could work if you were willing to do trial tests and figure out how to get it to draft at an optimum rate. To bad we could'nt rig a turbo to the stack exhaust to bump up air charge pressure coming in. Now that might require some thought??? Hmmmm

[warners point]

check out this guys gasification system. it seems like you could tinker with this and see if it works on an evaporator.

https://www.youtube.com/watch?v=AaZn...e_gdata_player.

Browse through his YouTube page he even goes so far as to use gasification to make a generator run off this thing.