advice on rocket maple sap evaporator

[twd000]

I need to boil about 200 gallons of maple sap to make syrup this spring

Had the idea of buying a 2'x4' pan and making a homemade rocket-powered evaporator for the base.

There is no thermal mass in my plan; I want all the heat going into the sap pan, and I don't want residual heat when I run out of sap to boil

See attached photo of my sketch idea

The core is based on Matt Walker's "SuperHot J Eight Inch Core"

walkerstoves.com/product/superhot-j-eight-inch-core-cut-plan/


Do you think that a J-tube feed is better or worse than a batch box feed for this application?

The rocket feed tube would be integrated into the face of an insulated sheet metal box.  The heat from the riser heats the maple sap pan from below, then stratifies gases and heats the remainder of the 4' long pan, before exiting at the lower rear of the box.

The structure of the box would be a steel 2'x4' shelving system.  Something like this cut down to 36" tall or so: www.shelvingdirect.com/metal-point-plus-steel-shelving-unit-with-no-decking

I would attach sheet metal roofing panels to the shelving frame, then insulate all the interior walls with 2" thick ceramic wool blanket.  The sap pan sits on the open top of the heater.

Chimney exits at the low point of the rear wall so only the coolest gases can escape.

Any guess as to how many BTU's / kW I can generate feeding an 8" rocket with dry wood?  What would you expect the feed rate to be?  I don't mind a near-continuous burn/feed as I never leave the boiling pan unattended


Open to any suggestions on how to improve this design.

[martyn]

Well I think the J tube is good for giving you control over the heat, if you use a more powerful design like a batch box, then you would need a method of regulating the heat but I dont know how without giving it a lot of thought.
However an 8” J tube will have a 4’ tall riser so you will need to address that issue! Possibly sinking the fire box or building a platform around the stove.
Personally I would not build your stove from ceramic fibre for heath reasons.
Basically you want the pot to sit above the riser and the heat to travel around the sides, so you need to build an insulated container to hold the pot with a 1-2” gap all around so the heat will warm the sides as well as the base.

[twd000]

Oct 1, 2024 23:10:13 GMT -8 martyn said:

Well I think the J tube is good for giving you control over the heat, if you use a more powerful design like a batch box, then you would need a method of regulating the heat but I dont know how without giving it a lot of thought.
However an 8” J tube will have a 4’ tall riser so you will need to address that issue! Possibly sinking the fire box or building a platform around the stove.
Personally I would not build your stove from ceramic fibre for heath reasons.
Basically you want the pot to sit above the riser and the heat to travel around the sides, so you need to build an insulated container to hold the pot with a 1-2” gap all around so the heat will warm the sides as well as the base.

thanks for the reply

good to know I'm not wildly off-track

I don't require control over the heat; essentially I want to run full-throttle to boil off sap as fast as possible. When I'm running out of sap, I'll stop feeding wood.

tall riser: where are you seeing 4' tall riser?  I'm looking at this design, which calls for a 24" riser on top of a 12" base, which is 36" (standard kitchen counter height): walkerstoves.com/product/superhot-j-eight-inch-core-cut-plan/

you're concerned about silicosis from the ceramic fiber?  It will all be contained and sealed inside the interior of the heater; it won't be disturbed or exposed when in-use. 

heating the sides of the pot: on the contrary, I want all the heat on the bottom of the pan.  You can easily scorch your syrup if there is heat applied to the walls of the pan

"more powerful design like a batch box" - this sounds promising.  Is there a design guide you can point me to?  I just need to build the core/riser, without any surrounding mass.

[martyn]

Ok fair enough, you seem to know what you want!

[twd000]

question about sizing- I see ISA as an important metric. If I count the interior walls of my proposed chamber, that is 52 square feet, or 4.8 square meters. If that appropriate for drawing a draft with an 8" square J-tube riser?

[twd000]

is this the source of the "4 feet tall" estimate? donkey32.proboards.com/thread/4007/minimum-height-bell-ceiling-riser

Do I really need 12" gap between the top of the riser and the "ceiling" (in my case the sap pan)?

Was planning to leave a minimal gap there - only 2" or so of clearance so the flames shoot right up into the pan.

Will this small gap restrict the gases? I'm assuming they can spread sideways since the width will be 2 feet.

[martyn]

There is a standard, tried and tested, formula to build an efficient J tube.
It is well documented but if you want to experiment and go over old ground that is fine, it might work!
The formula is 1-2-4 depth of fire box, length of burn tunnel and hight of riser.
Example … one foot deep fire box, two feet long burn tunnel, four feet riser.
You can also try to get away with 1-2-3 as without any mass it might work?
The gap above the riser on an 8” J will be a minimum of 5” but better at 6”
There should not be any flames coming out of the riser, just very hot gasses.
There are also tried and tested pot boilers versions as I described in my first post!
If you are not bothered about regulating the heat, then look up ‘Batch Box EU ‘ it is a web site with all the dimensions but I think a 8” batch box will have a 50+ inch riser.
However a 6” batch will offer similar performance to a 8 inch J tube.

Rocket stoves are great, one of the main benefits come there ability to counter flow the hot gasses into a bench or bell, in your case you dont need to do that but still bear in mind, that a natural gas oven flame, runs around 2000c (thousand) directly under a pan, a 8” J will be more like 6-700 (hundred) above the riser. 
That is why other people have built an insulated box around there pot and allow the gasses to escape around the sides of the pot.

[twd000]

thanks for the design rules of thumb

now I'm wondering about the heat capacity. 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, will an 8" J-tube rocket consume 15-20 lbs of wood per hour?

Will it deliver 100k BTU (30 kW)?

Most of the rocket cookers I've seen are focused on a 2-3 gallon kettle of water. I'm trying to bring 20 gallons to a boil all at once, and drive off 10 gallons of steam per hour.

[martyn]

Peter will know the BTUs but I very much doubt if a single 8” J tube will continuously boil such a large tray.
Like I have said a batch box is more powerful, they burn for an hour or so before going to coals but with such a large tray you might be better off with two!
Maybe two 8” Js would do it but the cost to build two and the fact you you need to feed them regularly might not make that very practical.

[twd000]

is a batch box more powerful than a J, or just accommodates a larger fuel load? I assume it's limited by riser diameter

I don't mind feeding them continuously; the pan requires constant vigilance/fiddling so I wouldn't leave it unattended even if I could

the commercial evaporators increase the BTU output for forced air fans under the fire, but the natural draft of a rocket stove is inherently self-limiting.

[martyn]

Yes basically a batch box is more powerful because it burns far more wood at a given time but it also has a pre heated secondary air supply and a very efficient mixing method that happens at the base of the riser.
There are other factors that can effect any form of rocket stove like quality of build and materials used, abeyant temperature, wind, size and hight of chimney, ease of flow through the system, size and type of fuel, moisture of fuel and probably many more factors …..