AOF Design

[treehugger]

Im going to make an AOF system. I already have a high pressure AUF system in place. I would appreciate some thoughts on my proposed design. It's pretty standard from the UVM website's instructions with one exception. I have made the entire manifold a little longer. It extends 16" under the flue pan. I did this partially due to ease of retro fitting it to my arch but also due to that sunrise evap video with the air injection system directly under the flue pan. That boil is insane when he turns the blower on. Thanks for any input. 14249

[treehugger]

I plan on 2" square tube. 6" from top of arch. Will put a valve to control AOF flow rate.

[nymapleguy607]

Be sure to insulate the steel tubing very well ot it will warp shortly after the first boil. I cant remember the nozzle size recomended, I think it was 1/2". I would consider using 1/2" material for the nozzle tube but then reducing it to 3/8" at the end. I have 1/2" on my evaporator and I plan on changing to 3/8", this will give you higher air velocity from the nozzle and wont have as much volume. The hardest part with AUF/AOF on smaller evaporators is balancing the air for maximum evaporation without forcing all the heat out the stack.

[treehugger]

So cone down the injectors? Weld a 3/8" to a 1/2"? Glad you said something about insulation. I was going to leave some open to the heat to allow for the air to heat up.

[Ghs57]

I added AUF/AOF this year following the UVM recommendations. This included the specified 3/8" nozzles, and powered by a high pressure fan. It all worked as expected. Stack temp was 400-500 with no smoke or sparks.

My manifold is behind arch board, which helps insulate it, along with the cooler air moving through the system, but still the temps in the fire box are extreme. The nozzles glow red hot after running for a short time. I suppose it could warp, but I don't see any obvious signs. I had no problem adjusting the airflow between the AUF/AOF, but this is based on the boil rate and stack temp. Mostly trial and error there.

Since we are talking unique setups specifically designed for different arches, our fine tuning and performance is probably going to vary. I think your design is fine. It's fundamentally the same as many I reviewed here, and the UVM recommendations. Good luck.

[nymapleguy607]

My nozzles were cut 3" long with the 15 degree down angle. I then used 3" of ceramic fiber insulation. What I plan on doing to reduce the nozzle opening is weld washers over the end to reduce to 3/8". You could start with 3/8" and not have to do anything to the nozzle then too. My thought when building my arch was to oversize any ductwork I could to improve airflow into the evaporator. I would say you want a minimum 2" of insulation over your 2" tubing.

[RileySugarbush]

One thing to keep in mind is to not throttle the AOF flow anywhere between the blower and the nozzle. If you need less flow, change the nozzle size. That preserves the velocity of the air jet velocity like Jeff mentioned. Jet velocity is proportional to the pressure drop through the nozzle. If you put in a blast gate or throttle valve the flow will be reduced but the mixing of the AOF air will be compromised too.

[lpakiz]

I am not sure I would reduce the nozzle with a washer welded over the opening. You need a "lazer jet" of air at high velocity to penetrate into the gasses. I'm thinking of an acetylene cutting torch, which must have a "smooth bore"" to get the air into a concentrated jet, rather than a puff which disperses too soon. Another analogy might be a rife verses a shotgun.
If you have a machinist friend, he could make 3 inch long "bushings" which you could insert into the bigger nozzle. Could be welded in or even glued in with furnace cement.

[Ghs57]

The UVM publication has the dampers located either between the fan and manifold, or on the supply side to the fan. As an alternative, or in addition to the the damper, they also recommend a reostat on the fan to control the velocity. In the description of the operation of the forced air, it states that the air flow should be adjusted over the course of the burn. Any of these methods is going to change the force of the air at the nozzle.

My dampers are between the fan and the manifold. Finding the optimum volume of air is part of the fine tuning the system. There are probably other methods to control velocity which work just fine, but I deferred to the publication, and am satisfied that the UVM method works well on my arch.

[RileySugarbush]

The UVM publication has the dampers located either between the fan and manifold, or on the supply side to the fan. As an alternative, or in addition to the the damper, they also recommend a reostat on the fan to control the velocity. In the description of the operation of the forced air, it states that the air flow should be adjusted over the course of the burn. Any of these methods is going to change the force of the air at the nozzle.

My dampers are between the fan and the manifold. Finding the optimum volume of air is part of the fine tuning the system. There are probably other methods to control velocity which work just fine, but I deferred to the publication, and am satisfied that the UVM method works well on my arch.

I know that is what they say. I just don't completely agree with them. The advantage of adding a damper or throttle is you can adjust on the fly. The disadvantage is you lose velocity and mixing. Both ways work.

I agree with lpakiz and suggest a sleeve instead of a washer. I use stainless steel hydraulic tubing for my nozzles.

[lpakiz]

John,
You mentioned that you utilized steel hydraulic tubing for your nozzles. Do you know the exact ID of that tubing? Or OD and wall thickness?
I used 3/8 schedule 40 pipe for nozzles. I get LOTS of sparks out the stack and I would like to reduce them. They are dangerous in dry conditions.
Do you suppose the nozzles are too big and I'm getting too much air volume and not enough ""speed" out the nozzles? Should I reduce the amount of AOF volume and see if sparks diminish?
It takes about a full cord of good hardwood sawmill waste (heavy edgings, not slabs) to make 20 gallons of syrup, no RO.

[RileySugarbush]

I'll have to check dimensions. I know my first setup had 16 nozzles made of half inch hyd tubing. As I recall the ID was about 0.40". I just had a bunch of scraps left from work. When I upgraded to a bounce house blower with more pressure I got way too much volume. I sleeved them with 3/8 tubing and that helped. I still get too much air volume and will need to reduce a bit more. I use too much wood and get sparks when I add in shop scraps of Baltic birch.

[nymapleguy607]

I am not sure I would reduce the nozzle with a washer welded over the opening. You need a "lazer jet" of air at high velocity to penetrate into the gasses. I'm thinking of an acetylene cutting torch, which must have a "smooth bore"" to get the air into a concentrated jet, rather than a puff which disperses too soon. Another analogy might be a rife verses a shotgun.
If you have a machinist friend, he could make 3 inch long "bushings" which you could insert into the bigger nozzle. Could be welded in or even glued in with furnace cement.

Interesting analogy, I never thought about it that way

[nymapleguy607]

On my arch I find that the amount of sparks are dependent on how hard I am running the under fire air. Also burning soft wood gives of alot more sparks as well, what I have done is added an 1/8" screen to the stack collar on my evaporator. It helps keep most of the larger sparks from floating out. The next step would be to make a step down similar to what the vortex evaporator has to trap the sparks. As far as dampers go, I switched over to blast gates on all the ducts. The rear air and door air are usually ran at full force when running and the grate air is adjusted depending on stack temp. The nicest part with the blast gates is I can fully shut down the air when I fire without switching the blower on and off.

[RileySugarbush]

Jeff,

I agree that blast gates are perfect for the under fire air, where pressure and velocity are not important or desirable. My old blower setup was a low power squirrel cage blower for under and a separate radial vane high pressure for over. I adjusted or shut off each with inlet dampers. Last year I changed to a bigger high pressure bounce house blower, and pull off some of the air for under and regulate that with a blast gate. New blower has twice the pressure, (8" of water). So I had to reduce nozzle size. Over air goes straight to the nozzles. For firing, I use a quick flapper that shuts off the inlet air and therefore completely stops both flows while the door is open.

[RileySugarbush]

Larry,

I checked my pile of scraps and measured: hydraulic tubing comes in a lot of sizes with different pressure ratings. What I used is stainless sized for 3000 psi systems:

1/2" tubing is .500 OD and .400 ID
3/8" tubing is .375 OD and .270 ID.

If your know what pressure you have in the manifold just upstream of your nozzles (it's easy to check) you can calculate jet velocities and flow rates with the following equations:

P is pressure in inches of water
D is nozzle diameter in inches
V is velocity in MPH
Q is flow rate per nozzle in CFM

Note that a good design has a big enough blower that the flow through the nozzles does not drop the pressure very much. So no restrictions between the blower and the nozzles. That is why I don't like throttling to control AOF. If reduces CFM by dropping pressure before the nozzles, but that also reduces velocity and affects mixing.

Q = 13.75 x D^2 x sqrt(P)
V = 28.7 x sqrt(P)

For my current system with 16 nozzles and 8" pressure:

Jet velocity = 28.7 x sqrt(8) = 28.7 x 2.83 = 81 mph I think this is good velocity for mixing in the air.
Nozzle flow = 13.75 x .27^2 x sqrt (8) = 13.75 x .073 x 2.83 = 2.84 CFM
Total AOF flow = 16 x 2.84 = 45 CFM

I think that is too much flow and I run very high stack temps, 1800 °F with a probe stack thermometer. I will reduce the nozzle size some more for next season in the hope to increase wood efficiency without sacrificing evaporation rate.

In 2015 I had larger nozzles, .400 diameter. Same pressure, so the jet velocity was the same, but the total AOF flow was 55 CFM
In 2014 and earlier I had a lower pressure blower, 4.0 inches of H20 with the .400 nozzles. Velocity was 57 MPH and total flow was 70CFM