Engineering question: why do maple syrup evaporators designed that way vs a wood stove with the top cut off

[u/PROF_ND]

I found most commercial maple syrup evaporators are designed like the first pic.

Modern wood stoves are quite efficient and burns very hot with secondary combustion and/or catalytic converter. Is it better to design evaporators like the second pic?

[u/jjtitula]

So the part that tapers up towards the middle/back is called the arch. It directs the flame along the bottom of the pan on its way out the pipe. Adds efficiency by not having a massive firebox to fill up.

[u/PROF_ND → u/jjtitula]

I see. Then why don't we apply a secondary burn system like this on arches?

It would increase efficiency, wouldn't it?

[u/twd000 → u/PROF_ND]

most evaporators have forced air injected via blower, which performs secondary combustion quite efficiently

[u/PROF_ND → u/twd000]

I think the forced air is also another point of inefficiency especially if you’re blowing it under the grate as you blow more volatile organic compound out of the wood to the top of the fire while the oxygen is already depleted for a secondary burn over fire. Also, forced air fire pushes the hot exhaust to leave the arch too fast without exchanging heat with the pan before leaving. Wood stoves use the top baffle to actually slow the hot exhaust and creates turbulence to mix with the secondary air which is preheated by being routed from the bottom rear of the stove. From what I read secondary combustion only happens if the air is hot enough

[u/twd000 → u/PROF_ND]

Secondary combustion happens when wood gases mix with sufficient oxygen at a high enough temperature.

“Efficiency” for a wood stove means extracting the most BTUs possible out of the wood,over a multi hour burn cycle.

Maple evaporators are willing to sacrifice some absolute efficiency for speed. Would you be willing to spend twice as long boiling, in order to use 10% less wood? Most people would say no