evaporator design principles and efficiently capturing heat

[SapTsunami]

Looking for some feedback on my plans to build an evaporator. I assume there's good reason why commercial evaporators are designed and built the way they are, but likely I could benefit by understanding why before I get started.


I'm going to buy a pan because I can't weld stainless (or do any sheet metal work for that matter).


I need a lightweight and weatherproof evaporator that I can store away in the offseason. I don't have a sugar shack, so I will set it up in the driveway for sap season then roll it under the porch in the offseason. I can't roll a several—hundred pound steel and firebrick evaporator around my hilly property safely. And I don't want it rusting in the woods in the off season. So my plan is to build it from a simple metal frame, with sheet aluminum sides and ceramic fiber blanket insulation. Minimal removeable firebricks to make a firebox. I will add a forced air blower to provide AOF.


My first question is related to the aspect ratio, length:width. I'm looking at a 2'x4' pan for hobby-sized operation. I see people measuring 800-1000F flue stack temperature which seems an awful waste of hot flue gases. Is there a reason why you wouldn't want to extend the length of the evaporator base to capture more heat? Wood stoves typically run 400-500F exhaust temperature, so much more of that heat goes into the house instead of up the chimney. You could make an 18"x64" evaporator with the same pan surface area as a 2'x4', with 33% more running length. Wouldn't this capture more of the heat from the fire?



Related to capturing wasted heat from the chimney, has anyone tried one of these "heat reclaimers"? https://www.hy-c.com/magic-heat-product/heat-reclaimers

They're sold for wood stove, to provide a basic air-to-air heat exchanger, they claim captures 30% of the waste heat. Seems it would be easy to set this in your exhaust stack and point the fan at your preheat feed pan.



Second (third?) question is about the position of the duct boot for the exhaust stack. Is there a reason why it's typically set high instead of low on the evaporator base? There is a common design in rocket stoves, to take advantage of temperature stratification by setting the exhaust outlet low, so only the coolest gases are allowed to leave the chamber. The hottest gases always rise to heat the ceiling of the "bell" or the maple pan in our application. See the photo here: https://rocketheatergamera.wordpress.com/principle/


Likely these ideas have already been proposed and discarded but would appreciate any insight before I start fabricating my evaporator.

[littleTapper]

If you're planning on running AUF/AOF, then you likely won't see 800-1000F stack temps; improving efficiency considerably.

Easiest way to be more efficient is to RO the sap and not worry so much about the evaporator.

[Pdiamond]

The way an evaporator runs is totally different than the way a wood stove operates. There are some manufacturers that make evaporators with wheels. I am not so sure how well an arch with aluminum sides would hold up, even with blanket insulation and firebrick.

[littleTapper]

FWIW, my 2x4 is on wheels. Blanket through most with a small amount of brick. Super easy to move, even up the slight ramp into my shop with sweet in the pan. I would not use aluminum on the sides either. Some sheet steel is fine. I put a bead of gasket sealer on, put the sides in place, then tack welded along it. Still is sealed up nicely.

[maple flats]

I'd use Little Tappers ideas but modify. You don't need to weld the steel sides, they can be welded or bolted. For the metal sides I'd suggest either 20 ga SS or 18 ga galvanized, either will work fine, the SS will look good much longer. Fill under the flues with vermiculite, available at large green house suppliers fine or course grade are good. Don't buy it from a big box store or you will pay several time more. In the firebox use half brickes purchased from a masonry supply (lower cost). See if you can get wheels that will retract or remove for in season.
Split your wood wrist size and have it real dry.
For the vermiculite, on my 3x8 raised flue evaporator I calculated how many of Lowes biggest bags I would need to insulate a 3x6 raised flue sap pan area. It came to over $700 back in 2008, I bought 3 hugs bags of it at a greenhouse supply at $23 a bag. When finished I had about 1/3 of a bag left over.
If you can come up with a high pressure blower such as on an old oil burner and blow the air in on top of the fire aimed down about 15 degrees it will greatly improve combustion efficiency. Look up a stickey post titled combustion efficiency in evaporators, it will help lot. More heat to the pan, less heat lost up the stack.