arch mods

[Brien]

A couple years ago I purchased a 2x3 flat pan and welded up a 2x3 firebox to fire it. This has worked ok, except feeding that whole box of wood is a bit of a chore and is probably using more wood than needed. I would like to modify this so the fire box is 20inches deep and then ramps up to a narrow opening before it exits into the 6in chimney.
Does this transition have to be sloped. I was thinking of making it close to 90 degrees, that would help maintain that narrow opening of a foot before it exits the chimney. Or I could make the fire box say 18in and have a 60 degrees angle to slope it to the narrow opening?
What do you think?

The Narrow opening I was thinking of having around the same Area of my stack, My fire box is 24in wide - 2in of brick = 22wide x 1.25 = 27.5, which is close to the area of a 6in pipe.

any input would be appreciated!

[Super Sapper]

You can to straight up, many of the hobby arches are made that way.

[aamyotte]

I am in the process of finishing up my evaporator build. I built mine 90 degrees like the professional ones. I figured it should perform the same as them.

[therealtreehugger]

Your math is good for the area. (pi r^2 = l x w). Mine is straight up, 90 deg, too. The quicker you get the heat close to the underside of the pan, the better. As long as you leave enough room for the length of your wood, 20 inches should be fine.

[30AcreWoods]

I had one of those 2x3 hobby arches that went straight-up and it worked well enough. If you have power, consider adding a blower - it will improve your evaporation rate rather significantly for that size evaporator. And you probably know this, but: only fill the firebox 1/2 full max., and add fuel frequently to optimize your evaporation rate. Or, fill it a little more, slow down a bit, crack a beer and enjoy having some friends over!

[Brien]

thanks for the replies. I will go with 90degrees and see how it works. I should be able to get the mod done before winter and give it a test boil with water. I did try out a blower last year with limited success. But it was free and off who knows what, I don't think it has the cfms I am after.

[Super Sapper]

Without the restricted area in the back, you may have just been blowing the heat under the pan and out the stack.

[30AcreWoods]

That's what I was thinking. You need an obstruction to create turbulence on the way out so that more heat transfer takes place. Without it, you were likely cooling the sap in the pan. Injecting air over the top will ignite the volatile organic gasses and further increase evaporation rate and results in a cleaner burn as well.

[Brien]

The blower was blowing air under the fire through my ash pan, so I think it was going right up the stack.... Which reminds me I would like to buy a stack thermometer to see what's happening with the temps

[Super Sapper]

That's what I was thinking. You need an obstruction to create turbulence on the way out so that more heat transfer takes place. Without it, you were likely cooling the sap in the pan. Injecting air over the top will ignite the volatile organic gasses and further increase evaporation rate and results in a cleaner burn as well.
You want the air added below the grates to increase your temperature. Adding over the top would cool your pan. The ramped up portion forces all of the gasses to come in contact with the pan bottom. Otherwise it can short-circuit around the pan and up the stack.

[30AcreWoods]

Actually, you do both under and over. The air over, as long as it is ingested at the front of the firebox, ignites the gasses and produces more heat. I agree that you wouldn't do over if you didn't have air under, but if you have both, it's much more efficient. That's why stoves have reburner tubes - same theory; more heat, cleaner burn.

[Brien]

Well, I completed my arch mods, and reduced the depth of my firebox to 20ish inches. I did a test boil with water yesterday and everything went really well. The front of the pan started to boil first, then once the fire was ripping the whole pan started to boil. Seems like a success and I definitely used a lot less wood. I ended going straight up 90degrees, and it seems like the right decision. Thanks for the input. I got a pic of when the fire was burning down, you can kind of see the inside.

Never mind, kept saying error when I tried uploading picture

[RC Maple]

What did you do with the area behind your vertical wall? Just wondering since I have a 2X3 barrel stove with a firebrick wall 24" back from the front. I built it up to about 2" from the pan bottom but there is nothing behind it but insulation around the sides. I always thought it worked well that way but maybe filling much of that void to 2" or so of the pan bottom with vermiculite and a fire blanket over it may be even better?

[30AcreWoods]

That area won't be doing much for you. When I had a 2x3 it pretty much served as an ash trap (I had a forced air under with that setup).

[RC Maple]

It does collect a few ashes but mostly looks the same each time I clean it out. I've never had an issue with getting a good boil at the back of the pan. It usually starts under the fire (middle), spreads to the back, and then I have to work at keeping the fire towards the front until that part of the pan gets a good boil going too.

[maple flats]

The way Leader says to do it with their 2x hobby arch, called a Half Pint, is that they say to build the back wall up to about 2" below the pan, full width, then leave the space behind it open. That forces the heat up against the flat pan. Doing it that way you get far more heat to the bottom of the pan, maximizing the boil. With good dry wood, split to wrist size you should be able to get 6 GPH evaporation. Your best boil will come from refueling using a timer. Experiment a little, I suggest you try fueling every 7 minutes. Then try 8, and 9, then 6. Find which works best for your evaporator.
I use a loud timer, in case I'm bringing more wood in or something. When fueling add as quickly as you can without banging up the pan and get the door closed again. Only fill it to about 5-6" below the pan. Repeat as indicated by the timer at the interval you find works best.