PDA

View Full Version : low stack temp?????



JandBsugarbush
02-04-2015, 12:16 AM
just purchased a new set of drop flue pans for our 2.5 x 8' arch. auf/aof but can only reach 600 stack temp??? need more wood?

maple flats
02-04-2015, 05:54 AM
How hard is the boil when you are at 600?
Before I put AUF/AOF on my rig, stack temps generally ran 850-1100 and at times went even higher. Then after the changes my stack temps are from 600-700 most often, in the midst of a real hard boil. That just means you are getting the heat to the pans rather than up the stack. As long as the boil is good the aof/auf are working good.

jimbison
02-04-2015, 08:11 AM
I am running a 4x12 raised flue burning 30 inch wood with AUF with a full boil front to back on both pans and only getting 550-600 degrees on the stack via infrared measure can get higher by increasing the blower speed but will I gain any gallons per hour or just more heat up the stack ? is stack temp that so important or is a full boil all anyone can get regardless of stack temp could someone explain the emphasis on stack temp to me thank you

sugarsand
02-04-2015, 08:38 AM
Jimbison, that is about the same temps at the stack that our 4x12 raised flue gets, with on occasion to 700 or 750. Hardly any smoke or sparks other than when we fire up. I'm thinking of adding AOF for next year to hopefully cut down on wood and lenghen firing time.

sugarsand

red maples
02-04-2015, 09:04 AM
Stack temp means different things depending on your set up.

Straight natural air flow (no AOF or AUF) will usually give you higher stack temps because your air intake is 100% reliant on heat and draw from the the hot stack. more air=hotter fire=hotter stack temps.

AUF will do similiar but you are now heating directly under the fire instread of through it the heat of the stack is not as important but you will usually get a better boil with the similar stack temps. but go through slightly more wood but your boiling rate will increase as well

Now AOF combined with AUF: you are creating a hotter fire by the AUF but you are also creating turbulance in the fire box by the AOF keeping the heat under the pans instead of sending them out of the stack. This helps to burn the gases given off by the wood not just the wood itself so this will increase your boil cut down the amount of wood you are using and get a better boil with lower stack temps. You should see a big decrease in smoke and sparks out of the chimney with AOF bacause the gases are burned up in the firebox making the arch more efficient as well as better for the envirenment.

OK that being said now what is the gap between the bottom of the flue pan and the arch. I have a 2x6 drop flue originally natural air flow and traditionally the arches have a pretty big space between the back of the firebox to the back of the flue pan. which doesn;t make any sense to me because the the fire is so far away from the pans. I added AUF and I built up the arch so that it is only an 1 to 1.5 inches from the pans so the fire must go through the flues I get a MUCH better boil from front to back and it increased my average boiling rate as well.

Play around with the size of the wood, how often you fire and how much wood you put in at each firing, and monitor the air flow balance between the AOF and AUF to see what ratio works best. Monitor GPH boiling rate and adjust so that you can be as efficient as possible. Its pretty easy to build up the arch as well. bricks vermiculite and ceramic blanket.

Its all trial and error...Good luck

maple flats
02-04-2015, 11:07 AM
On my raised flues I only have about 1/4-1/2" clearance under the flues, that forces the heat into the flues and boils even faster. You should also have a similar clearance under drop flues. Make the heat go where it does the most good.
To answer the question about stack temp. importance, the lower the stack temp while you have a real hard boil indicates that you are getting better efficiency. High stack temps just demonstrate wasted heat.

jimbison
02-04-2015, 02:41 PM
thanks for the info as I have virtually no smoke looks like I am burning propane most of the time, at night sparks are visable am firing at 12 minute timer, wood split no larger than 4 inches average 150 gallons per hour intake from match light to shut down am I about right for a 4x12

JandBsugarbush
02-04-2015, 04:22 PM
I have 1inch of clearance under the flues but I'm not getting a hard boil from front to back on my flue pan. We only get a hard boil in the the front 18" of the pan but the syrup pan is going nuts.

mellondome
02-04-2015, 05:18 PM
I have 1inch of clearance under the flues but I'm not getting a hard boil from front to back on my flue pan. We only get a hard boil in the the front 18" of the pan but the syrup pan is going nuts.

The heat is not staying up in the flues. I try for 1/4in clearance under my flues. You will get more boil with heat in between the flues then on just the bottoms. 1" is lot of space for the heat to just cruse on by. How are you firing? How often? Wood type and size? try adjusting your air flows. You might be blowing in too much cold air.

JandBsugarbush
02-04-2015, 05:52 PM
we fire with a mixture of hard and soft wood mosty slab. We try to fire at 10 min in small amounts. We also have played with the air,when we add more air the we get a little more boil but not a full boil from frount to back seems like we can't get the heat too the back of the arch

Sugarmaker
02-04-2015, 09:03 PM
Sounds just about right to me. 600 F is about what we get with AOF and AUF, on the old 3 x 10. That rear pan will probably not boil real hard in the back, but the front of the rear pan should be rolling about 10 inches deep. And if the front pan is a hard boil in all compartments then your doing real good!
Regards,
Chris

JandBsugarbush
02-04-2015, 10:06 PM
well we'll have to do a little more experimenting with it, thanx for all the advise
thanks Barry

mellondome
02-04-2015, 10:56 PM
How deep do you run your pans?

nymapleguy607
02-05-2015, 06:33 AM
I have 1inch of clearance under the flues but I'm not getting a hard boil from front to back on my flue pan. We only get a hard boil in the the front 18" of the pan but the syrup pan is going nuts.

What are you running for stack temps, I have some trouble with my evaporator not boiling in the back of the flue pan very well but my stack temp is fairly high. So my problem is pushing the heat out to quickly. Low stack temp might be an indicator you don't have enough stack draw to pull the flames and heat back.

mellondome
02-05-2015, 07:57 AM
With forced air, stack height is not an issue.

JandBsugarbush
02-05-2015, 12:38 PM
We run are pans around an inch and a half of sap over the flues. Are stack temps are high 5's to low 6 hundreds. I think im going to drill my holes out to a larger size for the air under and see if that helps any

mellondome
02-05-2015, 02:02 PM
Try reducing your depth.. that is lot of water to heat. The shallower the sap, The better the boil rate.

If your flue pan is 30 x 48, then reducing from 1.5 in to 3/4 in will reduce the amount of water to heat by almost 3.5 gal. This is just the back pan. If your front is reduced the same amount, you will be reducing water needed to heat and keep boiling by almost 7 gal.. that is a huge difference in how much heat you need to boil .

Other things to consider..
any doors or windows that will allow a draft to hit the back pan?
Preheater?
Where does fresh sap enter your flues?
How high does it boil? Jump out of the pan with violent geisers? or just right for making mac n cheese?
How air tight is your arch?
How are your rails/pans gasketed?
How high do you keep the firebox loaded?
do you let foam build up before defoaming?

Sugarmaker
02-05-2015, 03:35 PM
just purchased a new set of drop flue pans for our 2.5 x 8' arch. auf/aof but can only reach 600 stack temp??? need more wood?

What was the stack temp before the new pans? Did you have AOF and AUF prior?
Regards,
Chris

JandBsugarbush
02-05-2015, 03:49 PM
We had flat pans before...went to drop flues this year. Running a pre heater and auf/aof just as before. Did a test burn with new set of pans last weekend and the front pan goes nuts but in the flue pan only the first 18 inches boils violently the rest just slow boils. We add sap via float box now and I didn't have a stack prob last year so not sure of temps then. The arch should be air tight and gaskets all in good shape..