AUF Question

[Paddymountain]

According to the Vermont Maple research paper you need 12 3/8 holes per square foot in the floor
of your firebox. Also, there isn't supposed to be any holes 8 inches from the doors.

So how do you figure it? Square foot total and no holes in front of the doors?
Or, just the square foot of the floor minus the 8 inches from the front?

[Hunt4sap]

I'm in the middle of building a new oil tank evaporator and I'm glad you asked this b/ c I didn't know not to put holes within 8" of door

Are 3/8" holes correct size for any application?

[Russell Lampron]

This is how W.F. Mason set up my AUF. He made 3/4" holes, 2 per brick and the bricks start at the door. My arch also has air blowing into the fire from the front which makes a big difference. I was using my original wood fired arch with AUF without an airtight front before. I saw my evap rate increase more than 50% and my production increase from 5 gals. of syrup per hour to 8 gals. per hour boiling 14% concentrate.


1871818719

[maple flats]

You put the 12 holes/sq ft, 3/8" diameter in all except the 8" from the door.

[Hunt4sap]

Is the sq ft just the footprint of the firebox size or the sq ft of the footprint of entire pan/ fire traveling path area?

[Hunt4sap]

I just noticed Patty Mountain showed floor of the Firebox, I'm guessing the reason for not putting holes with an 8in of the door is to eliminate fire blowing out of door when opened. I'm wondering if you turn fan off or way down before opening door you can get away with putting holes closer to the door to maximize air injected Into the Fire?

[Hunt4sap]

I'm not trying to hijack the original post either, but all questions are good I feel as long as you're getting quality answers back. Another question I have is I have a piece of 2 in electrical metallic tubing that I just had laying around here that I plan on using for the air Under Fire tube to feed it into my fire box but I'm wondering is will it get too hot and damage it?

my original thought was to weld it into the evaporator in the very bottom but the more that I'm thinking about it I think I'm going to leave it fit so that I can remove it if it gets damaged or I want to alter it after burning and see how it works out is my latest thought, I will be doing this part of the project within a week and I'm ordering a custom-built 27" by 48" flat pan ( I batch boil and it works out for so far)

[Paddymountain]

no need to worry about hijacking this thread!
Dave and Russell: I just figured it out and I'm 5 holes shy . It calculates out to 2.75 square foot. That should
be 33 holes, right now I'm at 28. I'll get them drilled out, and also, I am using dust collection gate valves on my air delivery
pipe. I want to put in two 4" ball valves, the others leak a ton of air. I need to put in a stack temp gauge too. Currently using
a laser temp sensor. I'm getting about 325 degrees on the outside of the stack above my steam hood, but what does that mean?
The stack probe will be way more accurate. I'm hoping to pick up 3/4 gph with a little tuning, right now I've just let my air where it is, because
I feel without knowing the actual stack temp I'm just guessing anyway.

[Russell Lampron]

I need to put in a stack temp gauge too. Currently using a laser temp sensor. I'm getting about 325 degrees on the outside of the stack above my steam hood, but what does that mean? The stack probe will be way more accurate.

I have a laser thermometer too and don't know where to set the emittance to get a correct reading. I can't find an emittance chart that gives a setting for patina'd stainless steel. I'm going to get a stack probe too so that I can fine tune my blower.

[maple flats]

While it does not stick to SS I use a magnetic probe, it has about a 6" probe into the stack. With AOF/AUF I get temps of about 650-800F Occasionally I see temps of 850 but that is unusual. With just AUF you will likely see temps of 900-1100F if I recall.
I fuel by a timer set at 9 minutes when I have mostly good solid hardwood and 8 minutes if I have some punky wood or softwood. My wood is all split fine (wrist size or smaller) and it is well seasoned. I do not have the floor with holes in it, I just have the original grates, then under the grates I have a heavy wall stove pipe (20 ga.) 4" diameter which extends from the back to about 6" from the front. In that I have 3 rows of air holes that are 5/16" spaced every 2-2.5", one row top center, and one each facing towards the outer edge of the grates. My AUF also uses the high pressure blower for the AOF so those may not work if you have a squirrel cage blower.

[Paddymountain]

I do have air over fire, and it works well. I used to get just a little streak of scorching along the pan wall of whichever side I was drawing off.
Since AOF that has disappeared. I still think I need to push a little more fire back on the flue pan. I really should just get the ambition to build a
new arch. Mine is a really old Soule, I had rebuilt when I bought it back in 2009. I've gone though about 3 upgrades since then. But I'm sure
if I built one from the ground up I would get a little more efficiency.

[Paddymountain]

Since I started this thread awhile ago, I have done some minor upgrades. I drilled some more holes in the floor of my
firebox. I've also replaced my dust collection gates with 4' ball valves. Also, I've bought a digital temp pid controller with
K-type thermocouple. I have it mounted in a 6" box, it really turned out great. I'm sure it will be a good improvement.
Next question: Has anyone thought of using a radon manometer to monitor their draft? It looks like you can get one for $15.00
or so. It seem to me for that price it would be a great way to monitor your draft!
Any thoughts out there?

[minehart gap]

Possible a stupid questions but why do you need to monitor this and what exactly would you be measuring? I haven't been doing this long enough to understand the reason.

[Paddymountain]

It was in the UVM article improving combustion with AUF/AOF. Maybe I need to go back and reread it. Some of it applies to
oil fired evaporators. Anyway, besides monitoring stack temp you would know if your draft is consistent.

[raptorfan85]

I'm on the same level as minehart... if you have auf and aof wouldn't it be a forced draft at that point? Meaning that there is no natural draft and it's a forced draft because of the blowers?

[Super Sapper]

I'm on the same level as minehart... if you have auf and aof wouldn't it be a forced draft at that point? Meaning that there is no natural draft and it's a forced draft because of the blowers?

It would depend on how air tight your evaporator is. If it is air tight you will force the air through the stack, if it is not it will come out where ever it has an opening once you have more air coming in than the draft can handle. Granted you will be hotter with auf and that will improve the draft some.

[Paddymountain]

You know, you guys are probably right! With a stack temp gauge, I'll be able to monitor my overall boil rate better.
Having that draft gauge to look at too, might be overkill. It's like isn't there enough to keep track of when you boiling
let alone another gadget!!

[Super Sapper]

Ask Chuck what happens when you have too many things going at once.