Post-Mortem on new evaporator's stack temp

[SilverLeaf]

2010 was my first year boiling on a real evap, rather than my prior crude setup, which involved way too many stovepots. It was sure fun to see 'er roar! :D

Wondering though if I can get some feedback on my stack temps. My setup is essentially a 2x6 flat pan (there's no flue pan). I do not have a blower and was just relying on my 10" stack to generate the draft (I kept the firebox door open a crack too to help.) With this setup, I was generally getting 12-14 gallons per hour and as a general rule kept the stack temp around 1200-1400.

What surprised me was that the only way I could get a really good boil all the way to the back of the pan was to get that stack temp up to about 1800. At the 1200-1400 range where I normally kept it, the back two feet of the pan were bubbling a little bit but never giving me that rolling boil that we all lust after.

Everything I've been led to believe is that 1800 degrees in the stack is just plain wasteful because too much heat is being lost. Why, then, would I need to get it that hot to get my entire pan to boil? :confused: Any thoughts?

One other note: The area under the rear 3 feet of my pan is filled with vermiculite to bring the level up closer to the pan. But there's still 4" of clearance between it and that pan. So that leaves 96 square inches of space for the hot gases to make their way to the stack, which is plenty big enough for my 10" stack (which if my math is correct is about 78.5 square inches), so I know that the problem is not that I'm prematurely "pinching off" the hot gases or anything like that.

[red maples]

you would have to talk to the raised flue guys but from what I noticed this year (my first year on a real evavporator too) I am going to cut the space with a peice of metal going up infront of the doors to shorten the space. using wrist and forearm sized wood the space to load is just too large so when the doors open too much cold air draft getting pulled across the bottom of the pans creating mixing between the pans and slowing the boil too much. So at the end of the season I was blocking the space in the door way with larger pieces of wood that would help to keep more draft coming under the fire trought the grates and keepinrg the fire at a more constant temp and didn't mix as bad.

now that I said all that having draft coming in through the doors that you have cracked is possibly the problem if the air is coming in over the fire cold instead of through the grates and warming there before it goes through. I hope that makes sence just my 8 cents a little long winded for 2 cents!!!

and the space under the back of the pan seem to large!!

[3rdgen.maple]

I ran a 2x6 flat pan setup for many many years. It was setup with 4 individual pans and a finishing pan. The back pan never ever boiled and the next pan was lazy. The back pan had cold sap entering it for one and with it being a flat pan setup there just is not enough surface area in the heat to get it to boil. I played every season with making the gap under the pans smaller and larger with ill results. If you are seeing numbers like that you are on target. The best I could ever do was around 15 gph. There is only one way to beat this problem and that is to get a flue pan. And yes there is a point that you are just burning more wood and wasting heat than you are getting in return for doing it. So I think getting the stack temps up to 1800 with not much more boil you are making more work for yourself by cutting and splitting wood. Now with that bit of sad news you can look at other things to make improvements. Preheaters, blower, insulating the arch as best as possible etc. When or if you have done this the next step to get it to boil harder is the flue pan.
Red I found out if you keep that firebox on Patricks rig 3/4 full and when the top layer is burned off and maintain that level your boil will not drop off very much and as soon as you close the doors it is ripping again. So get the wood in your hands, open one door and shove it in as fast as possible. I got carried away this past season and had the stack glowing cherry red and was a little scared.
One last thought might be playing around with where your raw sap enters the pan. I wonder if the sap enters near the front closer to the fire box it would boil better. Then again it all depends on how your rig is setup.

[Jim Schumacher]

Yep, The space under the pan seems too large.