rocket stove size for making maple syrup/sorghum molasses

1) I'm looking for some direction regarding the size of rocket stove needed for a pan size of ....18x24 to...maybe...24x36.
2) Is there a good design for a diffuser for the top of the riser to spread the heat over the pan instead of blasting it at one spot.
3) I have yet to find any info on what the proportions should be between the feed tube, burn tunnel and riser lengths.
4) This might be a dumb question, but, I'm new to rocket stoves so I'll ask. With an eye toward making this portable/storable, is it
necessary to insulate it?
I have some 4" dia, 3/16 wall tube that I was going to use as a starting piont to see just what it would do but with all the knowhow
on this site, maybe I can make something closer to what I need the first time.

Thanks
Dave

David,

You can get a good secondary burn without insulating the heat riser. I have several pictures in a thread somewhere on page one in this section, I am running a prototype 6 inch J stove in my backyard.

As to dimensions, I strongly encourage you to just buy the pdf, there's a linky in my signature, and I have heard it is on sale from the already low price of $15, an extended black Friday sale.

I would estimate if you want to fiddle with a diffuser you should probably space the diffuser above the outlet of the j stove just like you were putting a drum over, or maybe a little higher.

How many gallons of sap in the pan?

David Kehr wrote:
2) Is there a good design for a diffuser for the top of the riser to spread the heat over the pan instead of blasting it at one spot.

Use a water bath. A larger shallow pan that your syrup pan sits in, so that it has boiling water under it instead of the direct heat of the rocket stove. Most syrup operations use steam or boiling water to heat the syrup because if the syrup gets too much above the boiling point of water, it can carmelize.

Michael,

Using 7.48 gal. per cubic foot an 18x24 pan with 8 inch sides filled to a depth of 6 inches would hold 11.22 gallons.
A 24x30 would hold 18.7 gal.

I am willing to predict an insulated heat riser will end up putting more heat out the top of the riser. The insulation will have to be warmed up at the beginning of your burn, but once it is warm you should get better throughput at the stack outlet for the rest of the time the stove is running.

Insulated burn chambers definitely burn hotter and cleaner - which tends to increase your heat output per wood.

I like the water bath idea. Another option we've seen (less controlled) is putting some bricks between the hottest part of the flame and the center of the pan. Might be able to combine the two.

My impression is you need quite a bit of heat. Dry firewood only contains so many BTU per pound, so if you build a small burner you are limited on how fast you can feed wood into it. If you are used to burning wheelbarrows full of wood, you will need a lot less with this method because of less waste, but you don't want to get too small.

I'd say a 6" stove would be a pretty good size to start. With serious insulation, you can get enough heat to boil 4 gallons of water in under 20 minutes. With moderate insulation, you might be looking at longer times and less fuel efficiency.
You could also do an 8" stove; this size tends to fit well in a 55-gal drum, which gives you 23" diameter of support for your larger pans. and it will burn more wood at a time, and may be able to do longer steady burns because you can load a larger piece of firewood. When built with brick, the firebox stays hot enough that it can often burn a log on its own coals; but 2 or 3 chunks is a little more reliable for a long, steady, clean burn.

I tend to build my stoves out of portable bricks, rather than trying to make the whole stove portable as a unit. A brick stove can also be left on site from year to year, for example in a little 'sugaring shack'.

For a very portable version, you might look at something like our Poor Man's Forge / Cooking / Canning stove. We did this in the October 2012 Missoula workshops; it's on the "Hot Rockets" DVD that Paul Wheaton released last summer. It's built into a small barrel, with high-grade refractory insulation and perlite/brick. It was a prototype and could use some better workmanship to make it more durable, but the performance was spectacular.

Yours,
Erica W

"Use a water bath. A larger shallow pan that your syrup pan sits in, so that it has boiling water under it instead of the direct heat of the rocket stove. Most syrup operations use steam or boiling water to heat the syrup because if the syrup gets too much above the boiling point of water, it can carmelize."

This will work if you simply want to heat syrup but not effective for boiling sap. Even good sap requires a 40 to one ratio of sap to syrup so you need to boil off 40 gallons of water vapor to get your gallon of syrup.
I have also seen suggestions to set up some type of evaporator like a food dehydrator but the bacteria would eat all of the sugar out of the sap long before you would get to syrup at that low temperature.

My current set up is a 6" square steel tube as the stove. Insulating it was the only way to get enough heat out the top of the riser to boil large quantities of sap. My pan for this set up is a rectangle 12" by 24" and 8" deep. With this pan the most you can expect is 2 gallons of evaporation per hour. That is 10 hours for a gallon of sugar maple syrup. I plan to use this for finishing rather than quantity evaporation. My simple block arch will evaporate 10 gallons per hour but requires allot of fuel. My hope is develop a rocket fuel source for that some day.
The pan also needs to have a shroud around it directing the heat from the riser to the sides of the pan. If you do not do this the sides of the pan act as a heat sync and dissipates a substantial amount of heat, especially in colder weather.

Hope this helps a little bit so you can save some time on your project.