How to adjust (tune) AOF/AUF

[CharlieVT]

Hi Folks,
Many thanks for all the helpful posts here. This site has been a great help to me. Last year was my third year; I made 138 gallons on 400 taps. You all helped.

My question has to do with how to best adjust an evaporator equipped with Air Under Fire (AUF) and Air Over Fire (AOF) blowers.

For those running adjustable AOF/AUF, what do you look at when making adjustments? Are you looking at stack temperature, appearance of the fire through a sight glass, or what? If there is a message thread on this site that addresses this, please provide a link. I've searched several times and can't find it.

Background:

I have converted a 4x10 oil fired evaporator to an air tight wood burner. Last year I used only AUF and it worked well. This summer I have added AOF for the reported advantages. My guidance on making these modifications was primarily from: Guidelines for the Improvement of Combustion Efficiency for Maple Producers by Harry Atkinson and Lisa Marchetti, Thunderbolt Research Corp. which can be seen here: http://www.uvm.edu/~pmrc/Combustion.pdf

I have a squirrel cage blower for AUF which is basically an old oil burner with the burner components removed and a Grainger # 7C447 blower for the AOF. Both blowers have dampers at the inlet for adjustment.

I have installed a sight glass for the fire box and a draft gauge. I have not yet installed a barometric damper in the stack.

Anyone with a similar setup who has experience runnning it, what are you looking at when adjusting the dampers? Draft? Stack temp? Appearance of the fire through the sight glass?

Thanks in advance for your replies.

[jmayerl]

1st throw out the damper for the stack.
2nd every evaporator is different. Yes watch stack temp, watch and listen to the boil, watch fire through sight window. After mine is up to temperature and I change the blower dampers, the changes can be heard and seen(steam), almost instaniasly.
3rd draft gauge??? Dont know of any evaporator that has one on it.(yeah I know the artical says to have one but pretty sure you won't need it.)

[maple flats]

I use one HP blower to run both. I have no damper on the blower, I go from the blower which is located outside on a shelf under the elevated platform to my head (feed) tank. A 4" PVC pipe goes into the sugarhouse and runs to the arch from the rear, on the floor (actually in a trench in the floor for that purpose). Once under the arch the 4" splits into 2 x 3" at a wye. On each of the 3" pipes I have a 3" ball valve. The under fire air enters thru the same trench in a 4" galvanized smoke type pipe. Once under the grates I have 3 rows of 1/4" holes, and the pipe ends 6" from the front of the arch and is capped there. Then the AOF goes thru another 3" valve and then attaches to the external inlet to the AOF manifold. This manifold has 1/4" black iron pipe nozzles every 6" angled down 12 degrees and it is imbedded into the refractory brick at 6" below the pans. I do not have an airtight front so I can see the flames between the 2 doors. I just started by trying settings until I got a real nice looking and very turbulant fire on top of the fuel wood. I also tried several settings under until I settled on my currant ones. In the beginning I also shut the blower off to fuel but then my grandson forgot to shut it off one time and we discovered that no sparks or smoke etc came out the front. Since that day, we do not shut off the blower to refuel the firebox. My setting for the past 3 or 4 seasons have been 25% open on the under fir and 65% open on the AOF. My stack temps used to run in the 900-1200 range and at night I could see a ball of flame as unburned gasses got fresh air at the top of the stack and some hot sparks. I now get stack temps of 650-800, no ball of flame on top of the stack and no sparks. My boil rate rose about 15-20%. I fueled every 7 minutes before the AOF and since I've tried 7,9,10,12, and 15 minute intervals. We have settled on 9 as best for us. This is not an exact translation, 7 before and 9 after, because we add less wood now at a fueling and we did at 7 minutes before. Basically our wood consumption has fallen about 30% with the AOF. I do not have a barometric nor a sight glass. I have a sight glass to install (from an old commercial boiler) but it runs so well now I just haven't bothered to install it. I am using a HP blower that is likely from the 20's and there is no spec tag but it measures about what the 7C447 blower specs are.

[CharlieVT]

Dave, thanks for the info. I also enjoying looking at the pics of your setup. If I can reduce my wood consumption by 30% I'll be really happy. That's my goal. I work alone, and long boils get tiring between firing and drawing off.
My old oil burner has a squirrel cage blower which I already had set up for AUF. It worked well last year. But everything I read said that it wouldn't be enough blower for AOF.
So I got the Grainger #7C447 which is smaller than the article recommends for a 4' evaporator. But I figured that since I was going to use a separate blower for AUF and the next size up blower was 3 phase which I don't have, I went for the #7C447. I think it is going to be plenty enough.
Your description of how your rig works is helpful. During my short test firing with the AOF going, my stack temps stayed much lower than I used to get with just AUF and the pans seemed to come to a boil really fast. So it looks like I'm on the right track.
So I'll continue to test fire and watch stack temps, look for smoke coming out of the stack and get a feel for how she goes.
Thanks again.

[maple flats]

Just an idea, but if the blower does not seem to give the pressure needed to give the turbulence necessary, you might have too many nozzles on an undersized blower. Try reducing the outlet diameter of the nozzles somehow. It however may not be an issue. Since my blower is reality is an antique and I don't have any specs, I really don't know how it's performance compares with the suggested blower for my 3' wide arch. I just know it works like magic. One thing on mine is under spec for sure. I only had 2" square tube that I made the manifold for the nozzles out of, the plans called for 3". It might work even better with 3" but I am totally impressed with how it works with the 2". The difference in air flow capacity between 2" square and 3" round is very significant.
In the same respect you may be quite alright with 1 size smaller blower. Do you have access to a gauge to read inches of water column? If yes, test the pressure. If that is good as shown in the publication on building this you are good.