AOF/AUF Opinions

[DocsMapleSyrup]

Earlier today, I spent quite a while talking with a maple syrup equipment manufacturer about the value of an AOF/AUF combination for smaller arches. Specifically the 2x8 I am planning on building. The manufacturer felt that forced AUF was needed on smaller arches but that on arches less than 3' wide, there was not enough area to burn the exhaust gas prior to getting into the flues and, therefore, caused a cooling effect rather than improving combustion heat. I have read the UVM extension report and it makes sense to me to use both AOF/AUF, however; I have no experience in the area. I am wondering if those of you who have 2 - 2.5' wide arches that you have converted to AOF/AUF would comment as to what your real life experience has been as far as improvement(s) in efficiency or BTU output that you may or may not have noticed. Much appreciated.

[jmayerl]

I built a arch that is 2x7' and very similar to a intensofire. I get 55-60gph and fire every 45 mins when adjusted to "gasification mode" or every 25 mins when on full force blow. I have arch temps that are 2000+F and 500F in the chimney, zero smoke. Just because someone doesn't manufacture it doesn't mean it is a great idea.

[CharlieVT]

...The manufacturer felt that forced AUF was needed on smaller arches but that on arches less than 3' wide, there was not enough area to burn the exhaust gas prior to getting into the flues and, therefore, caused a cooling effect rather than improving combustion heat...

The blower has to be sized and adjusted properly for the firebox. Location and angel of air injection ports may have a signification effect too.
But I don't see why a smaller firebox can't benefit from AOF.
Any size arch can suffer from too much AOF air. In my home brew 4x10 AUF/AOF if I put too much air into it, the stack temperature goes up and the boil in the front pan goes down.

I installed a draft gauge when I did my conversion. I don't think you need a draft gauge, stack temperature, boil rate, and the amount of smoke coming out of the stack tell you a lot. However, I have learned from watching my draft gauge; when too much air is forced in, the draft in the fire box decreases, the boil rate falls off dramatically, the stack temperature goes up, and black smoke comes out the stack. With more experience, I learned to just watch the stack temp. After I fire I might step outside and check the smoke coming out of the stack, I might adjust the AOF a little until the newly added wood is burning well.

I'd suggest you go for the AOF. It is kind of fun to learn to "tune" it, and you will boil faster and use less wood.

Quote jmayerl: "Just because someone doesn't manufacture it doesn't mean it is a great idea." I think that statement is spot on.

I suspect that a manufacturer would consider the cost of the additional construction requirements and the fact that there is a smaller percentage increase in efficency when adding AOF to a small arch compared to the gains associated with a larger arch. Building a small arch with AOF might increase the price point of a small arch to the point that is wasn't competitive. I got lots of ideas during a Leader open house when I walked around to the fabrication area and chatted with a welder who was fabricating an AOF evaporator. The guys in the Leader shop were very helpful and friendly during that open house, but I wasn't talking to a sales guy about building my own evaporator. ;)

[nymapleguy607]

Asking a salesman about an AOF/AUF arch wont get you real far. I built my 2x6 arch as an Intensofire clone as well, it might look a little funny with the pans off considering the firebox is 4ft long but laughs quickly subside once they see it boil. I first boiled on standard 7.5" raised flue pans my average was 48GPH figuring in time to start and time to shutdown. I timed the flow of sap once in the middle of a burn and sap was feeding in at 60GPH. Last year I put a Steamaway on it and now average 75gph. I think the biggest thing to consider when building one is the size of the high pressure blower, and the design of the dampers. I am currently changing the dampers over to blast gates so air can be fully shut off in the different areas.

[madmapler]

If you're building your own, you can do some sort of AOF preheater. The UVM site has some suggestions. I made one for my 4x12 over the summer. I posted some pics on a thread I started. Search under "AOF preheater. Be sure to use the ". It should come up.

[maple flats]

I added AUF/AOF on mine several years ago. My boil went up, my stack temp fell and my wood use dropped. All of this on a 3x8. Just size it right and then have valves to regulate the air flow. Once I found the sweet spot on my valves they have not been changed since the early part of my second season with it. Try it, you will be impressed.

[Robert K]

The laughter comes from you when others look at all the sap you are boiling with that little arch. Smokey lake and Patrick phaneuf are building small AOF arches and they are getting great boil rate and efficiency. Properly controlled it will work well. I am hoping to install blast gates on mine before the season starts. Preheating has been documented by a few and seems to help.
Robert

[DocsMapleSyrup]

Thanks to all who have offered their experience and ideas. I will be adding AOF/AUF to my arch as you have changed my mind. I'm trying to get a materials list together and figure out how to match the AOF blower to match the high pressure blower. Unless I hear otherwise, I will probably follow the guidelines commissioned by UVM from Thunderbolt Research Corp with the exception of using a 1hp blower. Why are most of you using blast gates or diverters for you high pressure rather than an electric rheostat to control the blower speed?

[maple flats]

Cheaper and easier on the motor.
When you say you want to match the AOF blower to match the high pressure blower, please explain. The AOF blower must be a high pressure blower, the AUF blower can either be the same HP blower if big enough but must be valved to regulate air flow, or it can be a much cheaper squirrel cage blower. What is it you are matching?

[nymapleguy607]

If your building a 2x8 arch a 1/2hp high pressure blower should give you plenty of air for both the AOF and AUF. Before building my arch I tried using separate blowers, it worked okay but the problem I saw with that system was that if the under fire air blower was turned down low the blower wanted to stall out. If you use a single blower for both systems I would use the blast gates to control the air because they can fully shut off the flow of air.

[asknupp]

Yep, what nymapleguy said. I use one high pressure blower for both auf/aof with blast gates. It took a couple boils to figure out sweet spot. I usually open gates 75% until fire is raging(90 seconds at most) and then leave aof blast gate at 50% open and shut auf gate down considerably low. If stack is turning purple there is too much heat being lost.

[lpakiz]

One drawback of a single HP blower is that when you adjust the grate air, you influence the AOF also. My grate air is adjusted several times each loading, to keep stack temps consistant.

[maple flats]

I use one HP for both, but once I found the "sweet spot" I have never adjusted the 2 valves again. That has been 5-6 or maybe even 7 seasons now. I run my 4" HP blower outside, under my head tank. the HP air is ducted thru a 4" PVC pipe with long sweep ells. Once it is under the arch, the 1- 4" PVC splits to 2- 3" and each has a 3" ball valve (they were free to me, from an old factory that was throwing them away, as was my 4" HP blower). I have the AUF open about 30% and the AOF open about 75% and they stay there. I do not even need to shut the blower off to add wood, I just pull one door open using my poker, then I fill that side to about 2/3 full and close the door. At my next fueling I do the same with the opposite door. Over the years since I put in the AOF I have tried everything from 7 minutes cycles up to 20 minute, I have settled on 9 minute if I have help and 10 minute if I'm working alone, I use a timer. Before I AOF I often had stack temps up to as much as 1500 degrees, now my usual temp is between 600 to maybe 700, and I boil faster than before. My original fueling cycle was 7 minutes.

[DocsMapleSyrup]

[QUOTE]I'm trying to get a materials list together and figure out how to match the AOF blower to match the high pressure blower./QUOTE]

Maple Flats:
My fault here. I meant to say that I wanted to figure out how to match the AOF blower to the high pressure coming out of the nozzles. Is there some measurement or math formula for this?

My question came up when I read the post from you that said, [QUOTE]Just size it right and then have valves to regulate the air flow./QUOTE] What does it mean to "size it right"?

If I can get by with one high pressure blower, I will, but don't want to be undersized.

nymapleguy607:
Thanks for the 1/2 HP high pressure motor being enough to handle both AOF/AUF. I often overbuild and will listen to experience.

I like the idea of dialing in the valve adjustments and rarely having to readjust them.

Now the big question: If you were to build an arch with a 2x8 pan set, how long would you make the combustion chamber knowing I will be using AOF/AUF?

[nymapleguy607]

[QUOTE]I'm trying to get a materials list together and figure out how to match the AOF blower to match the high pressure blower./QUOTE]

Maple Flats:
My fault here. I meant to say that I wanted to figure out how to match the AOF blower to the high pressure coming out of the nozzles. Is there some measurement or math formula for this?

My question came up when I read the post from you that said, [QUOTE]Just size it right and then have valves to regulate the air flow./QUOTE] What does it mean to "size it right"?

If I can get by with one high pressure blower, I will, but don't want to be undersized.

nymapleguy607:
Thanks for the 1/2 HP high pressure motor being enough to handle both AOF/AUF. I often overbuild and will listen to experience.

I like the idea of dialing in the valve adjustments and rarely having to readjust them.

Now the big question: If you were to build an arch with a 2x8 pan set, how long would you make the combustion chamber knowing I will be using AOF/AUF?


Rule of thumb for a gasification arch is 2/3rds the length of the arch putting you at 6ft. I would say 5ft would give you a little more area under the flues and possibly help keep the stack temp lower.

[DocsMapleSyrup]

Thanks for all the replies. I am going to take your advise and lengthen the firebox. I've been looking over the photos of the builds for those that have replied to my posts and what you have said makes sense. I'm planning to get the steal ordered this coming week and once the cuts and welds start happening, I'll try to post some build photos.

My current plan includes AOF/AUF and a fire box of 54" slanting back slightly to 60" under the flues, then flat. 1/2 HP HP dayton blower for AOF. Air tight door. 10" stack. Pans have been ordered from Smoky Lake. 2x2 divided same side draw off and 2x6 drop flue. 4" of total insulation in the firebox. 2" of blanket and 2" firebrick. No natural draft. Thanks again everyone. You've been very helpful!