Balancing AUF/AOF for Maximum Efficiency

[Ghs57]

That is the question: What is the objective when balancing?

Obviously, improved boiling rate/efficient fuel burning is the goal, but what are the incremental steps/benchmarks one takes and aims for in the process?

I relocated my fan (an old bounce house unit found on eBay) outside the shack. Air is now piped in about eight feet through 2" PVC and split off to the AUF (1" black iron pipe in a "U" formation under the grates) and to the AOF manifold (2" round to Rectangular tube feeding air above the fire from behind and on each side through 3/8" nozzles). Both lines have dampers, currently set about even at 3/4 open. I'm burning hardwood (sycamore , popular and some other types at times) and dry white pine. Firing every 6.5 minutes with 4-5 2" sticks. Fan is not shut off during firing. Stack is 8", 16' high, no cap. Stack temp runs about 600-800, Can get above 800 if pushed hard. Fire box is red hot, glowing fire brick after about an hour. 2x6 Evaporator is inside, with a steam hood, although the preheater is bypassed at the moment.

After firing, there is a bit of heavy smoke and during the last nighttime boil I noticed glowing embers leaving the stack and going out within a few feet of the top. I thought I had too much air, so I cut the AOF back a bit. This seemed to reduce the boiling rate. My target is 25gph on this rig, but it seemed to drop below 20 overall for a 6 hour boil.

The fuel this season is substantially the same, split the same, firing consistently. Sometimes the fire can noticeably burn down between firings, but there is no noticeable boiling decrease in the syrup pan. It's very hard to keep the flue section jumping throughout the interval, let alone boiling vigorously front to back. Usually most vigorous at the front and the very rear at the stack base.

So today I'm ready to try to retune this setup. I'd like to get back above 20gph, and 25+ would be better. Thanks.

[madmapler]

It would be hard to help without actually seeing it function. There's so many variables. I know mine likes aof over auf. It sounds like you've pretty much got it nailed down and it's just a matter of fiddling with it. What depth do you run in your pans? Maybe that'll increase your boil rate.

[flyzone13]

What does the heavy black smoke indicate? I have that when I am boiling as well, is that an indication that the wood is not burning properly?

[nymapleguy607]

Your feed pipe from the blower to the inside of the sugarhouse should be larger. A 2" pipe is going to have alot of restriction better bet would be 4". The balancing of the air needs to take place on the fly. On my evaporator I will fire, then open my over fire air through the back and door fully. Watching the stack thermometer the temp will come back up within probably 30 seconds of closing the door and opening the air. I watch my stack temp and gradually open the AUF to hold the stack temp.
Doing this lets me run my stack between 1000-1100 degrees. Under full fire with the steamaway I process sap at 90gph.

[Urban Sugarmaker]

Heavy black smoke is incomplete combustion caused by insufficient air in the fire box right after adding fuel. I have AUF and AOF which helps tremendously with temperature, boil rate, and smoke elimination. But, I still cannot eliminate all embers. Supposedly 2 foot wide shorter rigs don't gasify well but I still think the AOF makes a difference.

For me, embers are largely dependent on the wood species burned. I find that red spruce and red oak are terrible for spitting dangerous embers. When I burn white pine and Norway maple, the embers are few and they only fly a foot or two before going out.

I have more air going to the AOF than from below. I set the AUF blower very low. Seems to work well and I can burn larger pieces of wood without comprising heat.

[flyzone13]

i have AUF but I think my blower may be to small and not moving enough air. I am going to try a larger blower tonight.

[Ghs57]

It would be hard to help without actually seeing it function. There's so many variables. I know mine likes aof over auf. It sounds like you've pretty much got it nailed down and it's just a matter of fiddling with it. What depth do you run in your pans? Maybe that'll increase your boil rate.

Pan depth about 1". This after a few temp spikes in the syrup section. Agreed on the variables, not least of which is my arch design. The firebox is on the smallish side. I will try increasing AOF and see.


Your feed pipe from the blower to the inside of the sugarhouse should be larger. A 2" pipe is going to have alot of restriction better bet would be 4". The balancing of the air needs to take place on the fly. On my evaporator I will fire, then open my over fire air through the back and door fully. Watching the stack thermometer the temp will come back up within probably 30 seconds of closing the door and opening the air. I watch my stack temp and gradually open the AUF to hold the stack temp.
Doing this lets me run my stack between 1000-1100 degrees. Under full fire with the steamaway I process sap at 90gph.

Last year the fan was under the arch, two feet away. I can see that a 2" feed pipe at this distance might be constricting. There are also two 45 degree sweeps in the line. I have larger pipe here.


Heavy black smoke is incomplete combustion caused by insufficient air in the fire box right after adding fuel. I have AUF and AOF which helps tremendously with temperature, boil rate, and smoke elimination. But, I still cannot eliminate all embers. Supposedly 2 foot wide shorter rigs don't gasify well but I still think the AOF makes a difference.

For me, embers are largely dependent on the wood species burned. I find that red spruce and red oak are terrible for spitting dangerous embers. When I burn white pine and Norway maple, the embers are few and they only fly a foot or two before going out.

I have more air going to the AOF than from below. I set the AUF blower very low. Seems to work well and I can burn larger pieces of wood without comprising heat.

Last year I was burning more ash and other woods and the wood was several years dry. No glowing embers as I recall. The current wood was split last summer from mostly standing dead trees. It's not that it necessarily burned better last year with a higher GPH, but that was my first season with this pan. Coming from a 2x4 flat pan, this was a huge leap ahead.

This is all good info, and has me wanting to try making some changes.

[CharlieVT]

I found this helpful when I converted my 4x10 oil fired to AUF/AOF:
http://www.uvm.edu/~pmrc/Combustion.pdf

Size of the firebox will limit benefits of AOF as others have noted.
Not sure if your blower and pipe size are optimal. It is best to be able to get more AUF/AOF air flow into the box and then throttle it back as needed for tuning.
I found a firebox sight glass and draft gauge helpful. I refer to the draft gauge often, saves me walking outside to look at the stack. Assuming you don't have these, stack temp, stack smoke, and appearance of the boil are your indicators. The appearance of the fire without a sight glass isn't worth a thing, as soon as you open the door everything changes. :)

Based on a few years experience with my rig, here's what I would do to tune your rig:

Make all adjustments a few minutes after firing. You want the new wood in the firebox to come up to combustion temperatures before making changes. Wait for the black smoke belching out of the stack to die down after firing. (If you later want to get into adjusting air at firing and then turn it back again you'll want easily accessed controls at the front of the arch. I don't bother changing AUF/AOF settings for firing.)

Start with just AUF and no AOF. With a forced draft, a stack temp in the range of 800-1000 degF is good. Way more than that you are probably wasting heat by forcing up the stack. With a good fire going, adjust AUF for stack temp in the 800-1000F range. (You can increase your evaporation rate with higher stack temps, but your fuel efficiency will suffer; evaporation rate vs fuel efficiency is a trade off and you have to find your comfort zone.)

Then look at the stack smoke. You'll probably see some black smoke, (but not as bad as you'd see just after firing). Add a little AOF and check the stack smoke again. Your goal to to eliminate all smoke coming out the stack; you want to see just the heat waves above the stack. (If you firebox is small and your AOF nozzles and AOF pressures are insufficient to provide for good mixing of gasses above the fire, you won't achieve full gassification and you'll still see some smoke.)

As you add AOF you may find your stack temp going up with the additional air being forced into the firebox. If the stack temp goes way up above 1000degF, you are probably pushing too much heat up the stack (wasteful) and should adjust the AUF back a little to compensate for the increased AOF. Up to a point, too much air will increase stack temp but decrease boil rate, especially in the syrup pan.

When you add wood, you'll see black smoke; never mind. Just wait a few minutes and then check stack smoke, looking for no (or minimal) smoke. If you are blowing embers and smoke up the stack several minutes after firing, then you probably have too much air being forced into the firebox. Back off the AUF a little.

Once you have found a AUF/AOF setting where you have a good stack temp and minimal stack smoke a few minutes after firing, you have established a baseline setting. You can then look at the boiling rate in the pans.

Too much AOF into the firebox will cool the syrup pan and decrease the boil there.
Too much total air can increase your stack temp but not increase your boil rate; again, you don't want the highest possible stack temp you can achieve.

Some folks will have a setting for just after firing, and a setting for when the new wood is up to combustion temperature. That will decrease the amount of smoke at firing time and increase efficiency, but I would worry about doing that until you get your steady state settings. As I said, I don't bother with doing this, I just wait a few minutes after firing then check the stack smoke before making adjustments.

HTH

[Ghs57]

Changes and Results from Last Boil

I adjusted the air, increasing the AOF, leaving the AUF alone. Well, that resulted in a very hot fire. Lowered the pan level to slightly under 1 inch, and tried to leave it there even after a moment of shear panic due to a runaway syrup pan boil. Did about 150 gals straight sap in 6 hours. That's an improvement. With the door open (no sight glass), I could see the effect of the AOF on the fire. There was a noticeable hole blown in the flames of the fire by each nozzle which was surrounded by a blueish aura. I remember seeing this last year. I also tried some other hardwood, like oak,ash and hickory, and loaded more wood each time. That seemed to make a difference too. Stack temp was still low at 600-800.

It also seems like the float box level setting is tricky as the evap rate goes up. There is only one float on this pan. Too low, and there is not enough sap in the syrup pan, too high, and there goes the boil rate. I'm also a one man show, and with firing, stocking wood, splitting wood, transferring sap, watching the manual draw-off, and monitoring all the other stuff, it doesn't leave much time for making adjustments on the fly. Air dampers are under the arch, and can be difficult to reset without crawling under.

Thanks CharlieVT, I'm still reviewing your comments. I'm familiar with the UVM paper, but will reread it also. It may be some days before the sap runs and I can try additional adjustments.

[maple flats]

When I installed my high pressure AOF/AUF I used just one blower. It is likely over 100 years old and is belt driven, thus I have no idea what volume or pressure I'm running but the price was right, free.
I run the air into a 4" PVC pipe with 3 long sweep elbows, then it splits into 2 lines in 3" PVC. The AUF goes straight thru and the AOF splits off at about a 30 degree angle then another 30 to then go parallel to the AUF line. Each has a 3" brass ball valve that I initially used to regulate the balance of flow between the 2 lines. My first season I tried many different settings but finally settled on the AOF having the valve at about 3/4 open and the AUF valve at about 1/3 open.
Once my fire is going well after the initial lighting, I see no smoke out the stack and my fire is super hot. I can see thru a gap between the 2 fueling doors (just under 1/4") and there is an extreme turbulence in the firebox. My stack temps run between 600-700 generally.
When first installed I was shutting off the blower to add wood, but then one time my grandson fueled and forgot to shut it off. There was no smoke nor sparks coming out the front so since then the blower has been left on from right after lighting until after the last wood is added and the coals have been consumed.
My AUF goes into a single 4" heavy galv. pipe as it comes out of the 3" ball valve. My AOF is not ideal but it works great. My air after the ball valve on the AOF goes into a 2" Square tube, up into the slope on the arch and enters a 2" sq. tube which then splits and goes both directions, first from center along the top of the rear wall to the firebox , then turns and goes along each side and at the front it turns towards the doors for 1 last nozzle on each side of the door. According to the http://www.uvm.edu/~pmrc/Combustion.pdf my air duct should be larger. It may well not be optimal but it made a huge improvement when installed and all I had to pay for it was to buy the black pipe the nozzles were made of.

[Ghs57]

Thanks Dave.

I need to check the outlet on my fan. I know I used a rubber Fernco reducer to connect the fan to the PVC, and that the fan outlet was larger than the 2" PVC. So when I have that size, I will trying switching to a larger inlet pipe and a couple of long sweeps. I will then have to step it down to 1" for the AUF and 2" for the AOF. After yesterday's run, I am encouraged that I can still do better with this rig. But it has its design limitations, and after the season I'm pretty sure I'm going to enlarge the fire box. It's 24 inches deep on the outside, but much less on the inside after allowing for the firebrick and insulation. I also noticed that the bottom of the ramp is sagging, so the gap between the flues and the ramp is now larger than 1". The bottom is split fire brick on cement board. I will try to push the bottom back up and support it so it stays put until I can fix it Don't want it to fall out while running.

Another thing I noticed was that my grates are warping under the heat. These were made from an old road sing post. What does everyone do for grates? I was thinking cast iron, but the UVM papers says to cast one out of fire clay.

[maple flats]

My grates are still the original cast ones from when the arch was new in 2001. If I ever need new, I'll be casting the floor like UVM suggests. Even if you use steel angle, if you can position it so the V points down and then you have the V filled with ash it should work fine. If you point the V up it will fail because there will be almost no ash to protect the steel.

[Ghs57]

My grates are still the original cast ones from when the arch was new in 2001. If I ever need new, I'll be casting the floor like UVM suggests. Even if you use steel angle, if you can position it so the V points down and then you have the V filled with ash it should work fine. If you point the V up it will fail because there will be almost no ash to protect the steel.

The sign post is more like -u- (flat, channel, flat). Must be too much metal showing, not enough channel to hold ash. They are bending. I had some old heavy duty shelving brackets before, and they held lots of ash, but they rusted away in the off season.