Understanding AOF

[11-Nick]

I have no fan and would like to increase the gph for next year. AUF is logical. I do not understand AOF. I would think I'd want the air under the fire where it can increase combustion and heat from the wood. I don't grasp the concept of AOF, though. On the surface, it doesn't seem like it would impact the boil as well as AUF would. Can you explain it, please?
I read that a fan can double my boil rate. Can you break that down and somehow quantify the how the rate is increased with just AUF vs AUF and AOF both? Almost definitely adding a very simple AUF system of some sort. Trying to understand AOF so I can decide if I want to go through the hassle of adding it.

Side note: I don't like the search engine on the site. I remember reading some time ago that it only recognizes words that are larger than three letters. I'm shot in the butt when I search for AOF, AUF, or fan. I don't recall what it was, but I've searched for at least one other short word in the past, but couldn't get the job done with this search tool.

Sorry if my AOF fan question is elementary. I tried to look it up on my own before asking!

geez. my life is so friggin empty now that I can't boil.

[maple flats]

The air under fire (AUF) is easy to grasp. I'll try to explain air over fire (AOF). When wood burns, you are really not burning the wood, but rather the wood gases given off when the wood is heated high enough. To see this, light a wooden match and look at the flame, it hovers over the match, and it does not touch the match. It rises, mixes with oxygen and then burns. In AOF you are adding high pressure air, blowing it down at an angle and from all sides. This mixes the air (containing the needed oxygen) with the wood gas. This gives you a much hotter fire and generates the heat sooner, under the pans. Without this AOF the wood gases consume the oxygen and then the rest of the wood gases go up thru the stack unburned. Finally they exit the stack, get a fresh supply of oxygen and burn there, just above the top of the stack, much like the wooden match burned. Adding this AOF gives you far higher efficiency (more complete combustion of the gases) and generates a faster boil without burning more wood.
When you use AUF, you get more heat too, but there it burns more wood to get that heat.
As an example, I added AOF and AUF from the same high pressure (HP) blower a few years ago. I ran a 4" PVC pipe from the blower (outside, under my head tank platform, with a sound shielding box around it, HP blower are loud. Under the evaporator, my 4" y's into 2 pipes, each 3", then I have a valve to regulate the air flow balance. My AUF then enters under the grates in a 4" heavy galv. stack type pipe. Under the grates I then have holes drilled in that pipe, 3 rows of 5/16" holes, one top center, and one facing each outer edge of the grates, spaced about 3-4" in each row. The pipe ends and is capped about 6" from the front of the arch. For my AOF, that goes thru another 3" valve (mine are actually brass ball valves because I got them used and free, but any full flow valve type will work) and then runs into a manifold I had made that enters the arch thru the ramp. Once thru the ramp area, it goes across the back of the firebox area at 6" below the pans, continues along each side and then stops at the firing doors of the arch. Every 6" I have a 1/4" pipe nipple (nozzle) welded to the manifold, angled down at the prescribed angle (refer to the AOF link http://www.uvm.edu/~pmrc/Combustion.pdf) and facing towards the opposite side or end. The first year I had this I experimented with the valve openings to get the best performance. Before the end of that first season I settled on about 25% open on the AUF valve and maybe 70% open on the AOF valve. This gave the best burn, and those valves have not been changed since.
My previous fueling schedule was every 7 minutes, now I fuel every 9 minutes, my boil increased about 30% and my wood use dropped by about 1/3. Thus, with the AUF/AOF combination I boil faster, and use less wood. I used to get a ball of fire out the top of my stack, easily seen at night, I now get no flame at the top, my pans run hotter and my stack temperature actually runs about 300-400 degrees cooler (because I use the heat under the pans now, rather than send so much up the stack).
While I ran both off one larger HP blower, most run it separately, using the proper size squirrel cage blower for AUF and the right size HP blower for AOF. The AUF does not require high pressure but the AOF does or you will get little if any mixing of the gases and little if any gain in efficiency. I'll try looking up the link mentioned above and add it when I get a chance.

[BreezyHill]

Nick AOF is simply put as a high pressure jet of oxygen to totally burn the unburned gas and carbon that is seen when opening the fire box. Think of a torch...it burns blue when the oxygen is turned to the right amount and when off the torch gives off black smoke. AOF is turning up the high pressure oxygen.

Result: less smoke=happy on lookers
better combustion= less wood...happy wood processor
faster boil rates= happy fire tender

I plan to attach my fan this summer to my unit. Relatively easy to do but I am setting it up so it is preheated and using the exhausted from my vac pumps so there is no oil vapor loss at all after the reclaimer unit.
Also adding a stack temp pid to tell me when best to fuel the chamber. Did well on wood this season with avg of 40+G/ cord but still don't like or have the time to be getting lots of wood.
Don make the design any harder than it has to be...I am running the tubing from the rear of the rig to the front in place of the top section of fire brick and C blanket behind. Last season I had the pans of and designed it out and figure it should take about 2-3 hours with the torch and welder to install. Jet holes will be predrilled and add the jet tubes when installed in the rig. Test fire with water in the rig and be ready for 2016. My sugar house in below ground mostly on three walls so it is a great place to work in the summer heat.

I Currently have air under and have a switch on my PID for my auto draw setup. When I shut down the AUF fan my temp of syrup doesn't rise much while drawing off. If I don't I have had rises of as much as 3 degrees and have and near disasters.

Send me a pm and when I do the build I will send you a text and pics of the progress.

Ben

[11-Nick]

Tell me about the high pressure fan and jets.
What do I need to look for when shopping for a fan? What is the typical use for these fans in the "real world" so I might be able to search for one used/cheap vs new/expensive?
I am guessing the jets come in various sizes. Is there a typical size that is used, or am I taking a shot in the dark?
It sounds like the AOF does not run the full length of the pan? Just from the door to the ramp?

[Maple Hill]

Yes the AOF is just in the fire box,6in below the pan. For the jets all you need is black pipe,for mine I used 3/8 id every 6in.For the fan look under HIGH PRESSURE FAN.

[11-Nick]

Is there a reason it has to be high pressure? simply piping the oxygen into the box/above the fire isn't enough?
I know a guy who ran an electric leaf blower in as AUF (Hi Patrick!). I don't know how that worked out for him for AUF, but it seems that there would be a fair amount of pressure coming out of that to pipe it in under the pan rail and port it out into the firebox for AOF??

[CharlieVT]

http://www.uvm.edu/~pmrc/Combustion.pdf)

I made much use of the document that Maple Flats linked above.
Lots of the questions posted in this thread are addressed in it.

[maple flats]

Without the high pressure you can't get the mixing action needed in the gases above the fire. Poor mixing means the efficiency gain will be drastically reduced. However, I suspect you would still get a portion of the gain possible. If a HP blower just isn't in the cards now, it may still help to set it up on a squirrel cage blower now and then save your pennies until you can switch to the HP blower.
Since I have never tried the squirrel cage for AOF I can't even guess at the percentage of possible gain you might realize, but if you try that approach I suggest you follow the design set forth in the link I gave you and then plan to get the HP blower asap.
You should realize that a HP blower is still not very much pressure, but when sized correctly, it IS enough to create the turbulence and mixing needed to get the combustion you need.
There are several newer designes in new arches that utilize this principle to maximize the combustion and get closer to 100% (likely no design will ever really get that 100% burn, but they can get much closer with proper design)
Anytime you can get the gases to burn more completely, rather than having to cut, split, dry, and add more to the fire each and every time you fuel, it is a win-win-win-win. Remember, you likely set a timer to fuel and the wood you add each time went thru multiple steps before it got to the point that you loaded it into the arch, and likely you or someone close to you were intimately involved in those time consuming steps. The HP blower is not cheap, but along with an RO it is one of my best improvements I ever made, it pays dividends every 9 minutes for me.

[WI Sugarpop]

We built our new arch for the 2014 season with AOF and AUF based on Dave's design that I saw on a different thread. I have to say we are so glad we did that. This year we boiled down 2380 gallons of sap to 70 gallons of syrup using only 2 cords of wood. No RO either. No smoke and not much ash. It works great. Oh, and we doubled our gph from our previous arch that did not have forced air draft.