can I get 100,000 BTU per hour from an 8" J-tube?

interested in building a rocket stove for boiling maple sap.  I've seen Aprovecho designs for boiling in a 2-3 gallon pot, but my evaporator pan will be 2'x4', so I need to get ~20 gallons of sap up to a boil and drive off ~10 gallons of water per hour to make it comparable to commercial wood-fired evaporators.  

Will this thing eat wood fast enough for me to boil off 10 gallons of sap per hour?

mapleresearch.org/wp-content/uploads/wilmot_energy.pdf

It takes ~ 10,000 BTU to convert 1 gallon of cold sap to steam

To evaporate 10 gallons per hour would require 100,000 BTU/hr (or about 30 kW)

Firewood at 20% moisture content contains ~6700 BTU/lb of energy

www.spikevm.com/calculators/firewood/btus-pound-wood.php

Ignoring stove inefficiency and heat transfer losses, that would require burning ~15 lbs of wood per hour. 20 lbs per hour to account for 25% efficiency losses

With natural draft only, will an 8" J-tube rocket consume 15-20 lbs of wood per hour?

Will it deliver 100k BTU (30 kW)?

For comparison, the woodstove I use to heat my two story house is rated at 15k-40k BTU/hr. Wen it's running on high it's putting out a LOT of heat, and I will need more than double that for boiling sap.

Most rocket mass heaters are sized to burn 1-2 loads per 24 hour cycle.  But I'm trying to get a continuous max output.  I don't mind standing there feeding it wood; most commercial evaporators have a 10-20 minute reload cycle.

I'm not a rocket scientist, so I am not able to provide the math to it all but I have found a thread you might find interesting.

https://permies.com/t/26083/Rocket-Evaporator

The part that I found some value in was the video in the thread.

wow that's one way to do it!  

Five rocket stoves at a time!

I have built two rocket-powered evaporators, one 8 square feet and one 6 square feet. Both were powered by L-tube cores 8" wide x 9" high x 32" long, with about 30" risers. I used 8" diameter x 10' long chimneys. This was good for 6 but not quite for 8 sf pans.

I typically used about 1/4 cord of wood (mostly mixed oak and fir deadwood, well dried) to get around 3 gallons of syrup. I cut wood to 30" long, and only split if needed to get 4-5" maximum width. The bigger the logs, the better they hold their shape before collapsing to coals, and the more complete the burn. When ideally sized, there is no more than an inch or two of ash buildup in a 12-hour burn and no raking needed.

I will be rebuilding the 6 sf evaporator for the 2025 season in a new location, with a few improvements, and will post about that when done.

Glenn Herbert wrote:I have built two rocket-powered evaporators, one 8 square feet and one 6 square feet. Both were powered by L-tube cores 8" wide x 9" high x 32" long, with about 30" risers. I used 8" diameter x 10' long chimneys. This was good for 6 but not quite for 8 sf pans.

I typically used about 1/4 cord of wood (mostly mixed oak and fir deadwood, well dried) to get around 3 gallons of syrup. I cut wood to 30" long, and only split if needed to get 4-5" maximum width. The bigger the logs, the better they hold their shape before collapsing to coals, and the more complete the burn. When ideally sized, there is no more than an inch or two of ash buildup in a 12-hour burn and no raking needed.

I will be rebuilding the 6 sf evaporator for the 2025 season in a new location, with a few improvements, and will post about that when done.

8"x9" is an odd-shaped core - is that based on a standard firebrick size?

32" long burn tunnel is longer than typical, no?  Is this to accommodate long firewood, or intentional design choice?

any estimate how many gallons per hour you were able to boil off?  

I see chimney exhaust is at the top - any consideration to exhausting from the bottom like a bell design, to take advantage of gas stratification?

Standard firebricks are 9" long, so 8" w x 9" h is the easiest shape to make without cutting or special fitting. A little bigger would be good, but I will have to decide on a setup that will require cutting and fitting, or more bricks.

31 1/2" is an exact number of firebricks lined up in soldier orientation, and is long enough to combine with the riser to get full combustion before reaching the pan. Cutting long firewood saves labor and helps with making a spaced-out fire bed so air can get around logs for good combustion.

I understand the theoretical maximum boil rate is about 1 gallon per square foot of pan per hour, and I can get close to that with dry wood.

For a sap pan, you need to keep the air moving to heat the pan at the maximum rate. Stratification to extract lower-energy heat is not really a thing; it might be useful if you added a preheat pan or barrel.

project update - I got everything put together and tested it with some boiling water.
I was able to get the hot spot under the riser boiling, but the far ends of the pan only reach about 170F and lightly give off steam instead of a vigorous boil

You can see I moved the outlet to one corner and put in some steel baffles to make the hot gases linger under the pan but that didn't seem to work.

I'm measuring 1800F at the outlet of the riser, and the smoke from the chimney is clear, so I'm getting fairly complete combustion.

Any ideas to improve this design, or should I give up on the passive rocket stove idea, and put a forced air blower into the firebox to get the heat flux rate higher?


http://photos.app.goo.gl/51dVnXdy4bso2Ats7

http://photos.app.goo.gl/rUAea1V2uALWKH4e8

http://photos.app.goo.gl/Sq8SD9vf5SHVaAQ68

http://photos.app.goo.gl/y1F6H72b8fPw26w66

http://photos.app.goo.gl/P6HrKfdN9uWB9YMt9

Davis Tyler wrote:

if someone can tell me how to embed images, I will do so

putting the URL in between [img] tags doesn't seem to work

Hi Davis,
you will have to either upload the photos to a file host or attach them to the post.
Google photos does not work unfortunately.

I recently explained it here: https://permies.com/t/269517/Heating-big-space-efficiently#2860766

Or follow the link in my signature

images attached

 That looks like a good start.  My notes on an 8" J tube says 10 to 12 # of wood with a 45min burn time, with a 30min feed interval, would put you right @ 100K btu.  I sounds like that is very close to what you have just short of the 125Kbtu you need for a good boil on the full 8sqft.  Lookin at your pictures I like the layout, out and back on top of an insulated chanel.  Are you sure your cross section at the top of the riser is large enough to make a unristricted 90° turn? Also did you try running it without those metal baffles? It sounds like you only need a little bit more burn rate to get the heat you need.

Tuning experiments:
1:  Lift the cover over the riser about an inch
1.5:  And/or notch the top of the riser to add flow left and right
2: Increase the opening down into the lower return. 90° turn @ least 10"x8" opening.
3: Add 4' to the top of the chimney, more pull.
4: Bevel the intrance to the chimney, that wall penetration should be at least 9" in diameter with 45° bevel.  Easy way to do this is a 10" x 8" chimney adapter. Or just shape the insulation into a funnel.
5: Raise the chimney penetration off the floor at least 2" (1/4 diameter) will increase flow.

Note: airflow into a hole in a plate comes in from the side of the plate not straight into the pipe.

Fyi: A round hole in a plate only flows about 62% of what a tube of the same diameter flows.  That's why i think you need more cross section at those transitions.  Flares in and out of pipes help flow alot.  Tom