Davis Tyler

yes the blower will have to be variable speed so I can adjust the burn rate

I plan to have K-type thermocouples at the top of the firebox/riser, and at the exhaust outlet to help me understand what fuel-air ratio gives me a reasonably clean burn at the max output rate

I found this (old) post from Peter estimate 18 kW (61 kBTU/hr) for a 6" batch box; that's not going to be enough for my application.  

https://permies.com/t/59252/Estimated-calculated-heating-power-batch

In my case, the heavy stationary design of the batch box is undesirable; I need to roll it away after use.

I understand those off-grid are loathe to add forced air intake, but in my case it's cheap to buy a blower and easy to connect to an extension cord.  And it will really crank up the BTU output rate compared to natural draft (either J-tube or batch box)

yes, the price is a bit silly, but I'm planning on building one, not buying it.

I heard back from greenlife about my query.

They are stating 75k BTU/hr max output for the 8" J-tube.  That is not going to be enough to meet my application need for 100k BTU/hr, so I am going to need to inject some form of forced secondary air to accelerate the burn rate.

any feedback on this design?

found some references to it several years old, but little feedback on whether the design has been superseded, or is this the "state of the art"?

Is this the same design, or someone else's work?

www.scribd.com/document/342025997/Dragon-Heaters-vs-Rocket-Heaters-pdf

greenlifehomestead.com/8-rocket-heater-core/

I am building a maple sap evaporator based around an 8" L-tube core.  Low thermal mass/no heat storage so the concerns about secondary air intake stalling the draft don't apply in my application

I can't tell where the secondary air is coming in - is that a separate channel in the metal feed tube?

Have the vortex-inducing wedges been tried and tested?  They would be fairly easy to implement in my core.

Glenn Herbert wrote:My current 6 square foot evaporator.

question about the chimney - are you using 6" or 8" diameter ducting?  Hoping 6" ducting is sufficient for an 8" L-tube rocket.  

Are using using regular galvanized zinc HVAC ducting instead of the black single-wall "stove pipe"?  The galvanized is cheaper but I was concerned about high temperatures causing the zinc coating to burn off with toxic fumes.

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?

wow that's one way to do it!  

Five rocket stoves at a time!

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.

oh wow, that's WAY bigger than what I'm looking for!

be a shame to scrap it; have you tried posting it on http://mapletrader.com/community/  ?  A lot of larger-scale commercial sugar makers are on there; should not take long to sell at the right price

I'm in New Hampshire and might be interested in buying your arch and evaporator.  What size/type is it?