stack temp rule? Jec?

[MapleME]

Is there a rule for stack temps, or is it too "evaporator specific".

Jec, what should I be running my rig at using a probe stack thermometer?

Thanks

[gmcooper]

I have never used a stack temp probe. I fire as steady and hard as I can. I do not have a newer air tight arch and that might make a difference. I know there are some on here that go nuts over keeping the stack temp just where they want it. My guess would be to check with the manufacturer.
Mark

[Jec]

I like to keep mine kind of low. But how I bricked makes it a low temp. I don't have a thermometer. I'm not sure if it matters too much on a non airtight arch.

[northwoods_forestry]

I think what's important with stack temperature is that it a good measure of consistancy. Basically, that you are maintaining a constant temperature and boil rate. Everything I've heard and read says that keeping things consistent helps in avoiding many of the common boiling problems. Every sugarmaker and evaporator will likely have a boiling rate and stack temperature that they find works best for them. For me, I shoot for a stack temp of 900 degrees, though in practice it tends to vary from 800 to 1000.

[RileySugarbush]

I agree it is a good measure of consistency.

There is no correct stack temperature. Higher stack temps can result in higher evaporation rates, but are also an indication of low fuel efficiency....that is waste heat going up the stack. Lost heat is the product of stack flow rate and stack temp.

A real efficient rig would have very low stack temps, but the back of the pan would not be boiling hard and that often bothers the operator so they throw more AUF flow or more fuel on. Late one evening we decided to max our rate and opened up AOF, AUF and fired every 4 minutes or so. I'm pretty confident we were getting 55 to 60GPH out of our 2x6, stack temps (probe) were 1900°F an we were burning wood at an unreasonable rate. Fun for a short while but not sustainable or economical. We don't normally run like that.

Remember when you see temperatures listed here that a magnetic surface thermometer will read much lower than than a probe thermometer that is actually measuring the flue gases.



So it is a balance between production rate and efficiency. You have to find the sweet spot for your rig and your expectations

[nymapleguy607]

My stack temp runs around 1200 degrees. This gives me about 42 GPH and I fire every 6 mins. At the end of the night the stack temp will go up around 1400 because I open up the AUF and AOF to burn up the coals and cool the firebox.

[CharlieVT]

Is there a rule for stack temps, or is it too "evaporator specific".
.... what should I be running my rig at using a probe stack thermometer?....

I heard at a seminar from a Leader guy that a natural draft stack temp should be in the 650-800 degrees F range.
With a forced draft the stack temp should be in the 800-1000 range.

Watch your boil and compare to stack temp. You will probably get a good idea of what stack temp gives a good boil.

I suspect if the stack temp is much in excess of 1000degF then too much heat energy is going up the stack making the evaporator inefficient.

[RileySugarbush]

True. Less efficient, but FAST! It's a trade off.

[Jec]

If I try my hardest I don't think I can get my stack to hit 800F. In raised flue evaporators should the stack temp be lower?

[Vermont Creation Hardwood]

I can't see that a high stack temperature is a good thing. It is a simple waste of heat. It takes a lot of work to get the wood that you burn to evaporate sap. Anything over the boiling temp of sap is waste heat. Of course you need some heat to make a draft in the flue. But 800 degrees is a lot of waste heat. The hard work of felling, cutting to length and splitting is simply going up into the air. At 1200 or 1400 degrees you may be wasting half your wood or more.

[Brent]

LIES MY STACK THERMOMETER TOLD ME
A few months ago I got a Bartlet digital pyrometer. Actually 2. The plan was to put one in the stackand one in the firebox.
These are very high accuracy. I put one in the stack on the opposite side to my trusty $ 19.95 dial type that most of us use with the two stems inside the stack and only a coupl inches appart at the same level.

There was always about 100 degrees difference between them, sometimes at higher temps the dial type would be pointing to 1150 and the digital would say 850.

This is a grand sample of one, so I have no idea if the numbers from the dial types have much variatioin, but it sure makes them suspect.
So if one member says he runs at 1100 and another says he runs at 1300, they could actually be running at the same temp or maybe even a wider spread. When the numbers are unreliable you can learn NOTHING compared to the next guy. You should be able to get a guide as to when your rig boils well and when to fire. Your mileage will almost certainly differ.

Oh yeh - and some guys are measuring stack surface temp, with a hang on sensor, not an internal probe style.

[TimJ]

I can't see that a high stack temperature is a good thing. It is a simple waste of heat. It takes a lot of work to get the wood that you burn to evaporate sap. Anything over the boiling temp of sap is waste heat. Of course you need some heat to make a draft in the flue. But 800 degrees is a lot of waste heat. The hard work of felling, cutting to length and splitting is simply going up into the air. At 1200 or 1400 degrees you may be wasting half your wood or more.

Wait a minute - so you are saying that anything over 219 degrees F is a waste? Please tell me you are kidding.

Perhaps you have months and months to simmer away the extra water, but many of us don't have the luxury of all that time, so we boil hard and fast.

Perhaps inefficient with the wood, but my time is valuable, and I don't mind "wasting" some wood that is easy to find and cut so that I can go to bed by midnight instead of 5 am...

[RileySugarbush]

The difference between a surface thermometer and a probe can be hundreds of degrees! Plus both are not the most accurate types of thermometers. All these comparisons should be tempered by that knowledge, as Brent points out.

Vermont creation hardwood.....Can you share with us what your set up is and how productive it is? If I ran 400°F stack temperatures I would need to be boiling (simmering) night and day. Certainly high temperature flue gas up the stack is waste heat, but heat transfer to the sap is directly proportional to the difference in temperature between the sap and the flue gas. At 400°F, that is pretty low and I can't imagine much gets accomplished. Running too low as tack temp can also be inefficient, unless the reason the stack temp is low is because you have like a 20 foot long arch, sucking all the heat out of the combustion products. Most of us don't have that.

[Brent]

We are all just beginning to get into stack temp vs heat transfer vs heat waste vs air over fire vs air under fire.

We've got a lot to learn. That's why I got 2 pyrometers. One to put at the front edge of the flue pan and one for the stack. Our rig was a Leader Inferno that got modified to reduce the air under so it now blows some air through a channel that wraps around the door. I had intended to put in my stainless tubes to give me more air over the fire but the season started much too soon. Next year ( sounds like a promise from the head office of the Toronto Maple Leafs ) next year I'll get those tubes and the second pyrometer in and get some data about air flow, temps and boiling rates.

I can say that on our previous 2x6 rig, the more air we put over the fire, instead of under the grates, the better the boil was and we got a HUGE reduction in the wood consumed, with little or no change in the stack temps. Something to look forward to next year.

[Vermont Creation Hardwood]

The point is to get the heat into the sap. Heat going up the stack is not getting into the sap. One price of too much air movement in the arch is that much heat is carried away from the pans and straight up the arch. Anything that increases transfer of heat into the pans increases the efficiency of the boil. Increasing the rate of the fire does not in itself increase the rate of heat transfer to the sap efficiently. If it's simply heading out the stack, and a stack temp of 1200 to 1400 is an indication of that, then the heat is wasted.

What I've been commenting on is the focus on increasing the roar of the fire rather than a focus on increasing heat transfer to the pans. Yes, roaring the fire will mean more heat transfer usually, but at what cost? If 50% of the increase of roar is simply heading up the stack, that's quite a cost for a modest increase in the transfer of heat.

The time making the wood you use to feed the fire is also part of the equation of the time it takes to evaporate sap. Waste a lot of wood and you waste a lot of time.

[twofer]

We have a modified Inferno with AOF and the highest the stack temp gets is ~600 degrees.

[MapleME]

This is a very interesting discussion- I hope others will chime in. Lots to learn about this...probably lots we dont really know. Too many variables in this business.

[tuckermtn]

last night was the first night I have run my new rig (intensofire copy) with one of the leader probe dial stack thermometer. we would refire at 900 and the dial stayed at 1000 or over (not sure if it stops reading at 1000 or that is just as hot as we could get it) We made 15 gals. in 2.5 hrs boiling 6% sap. The cool thing was watching how quickly after firing and adjusting the air dampers open again that the temp would jump from 875 right back up to 1000. Would take maybe 15-20 seconds to go right back up. Makes me appreciate the materials and insulation my arch is made of so as to take those sudden temperature spikes. Lots to learn- but love the arch and love the stack thermometer.

One thing I noticed was judging by how the back pan was boiling and matching that with the stack temp I am guessing the hotest we were running our old arch last year was around 500-600 deg.