Concentrate, flue-to-syrup pan ratio and syrup in the flue pan

[David in MI]

Since I am asking a number of questions, I have italicized them; hopefully that makes it a bit easier to comment.

I would like to glean some experience from those of you who are boiling concentrate as I would like to concentrate my sap a bit more this year but don't want to run into an issue making syrup in the flue pan. For the past couple of seasons, I've concentrated to around 8-10%, although I did run it up to around 12% near the end of the season last year. Here's what I have and the levels I boil at:

Smoky Lake 2x8 pans on a Leader traditional wood fired arch without AUF/AOF and no preheater. My pans are raised flue; the flue pan is 6' and the syrup pan is 2'. I was experimenting a bit last year and would have to check my notes to be sure but I think I wound up running my flue pan around 1.5" and the syrup pan at 2".

I'm looking for input on how high I should run my concentrate with this setup. I heard that Leader has a bit more syrup pan with their 2x8 than I have (3' vs 2') which got me thinking about this.

Also, I never closed the connection between the syrup and flue pans last year and have since read that it's a good idea to do so. What's the harm/negative in leaving it open? How long for the gradient to re-establish itself? I'm planning on closing it this year but am curious about this.

One more thing - I noticed that the boil at the stack-end of the flue pan isn't as strong as it is near the firebox. I have 16' (or a bit more) stack so the length should be ok for a good draw; I leave the draft door wide open so I don't believe it's either of these issues. However, when I pulled the flue pan for cleaning, I noticed that I have a small gap between the top rail and the top of the fire brick in the section of the arch between the ramps. I'm sure the heat chooses the path of least resistance and follows this open space to the stack rather than getting up into the flues. So I'm thinking that this gap needs to be eliminated. Any thoughts on this as well? Gap is probably the width of the steel, so perhaps 1/4". One more note, the wood I was burning may not have been as dry as should be. I'm sure this played into this as well.

Last question - Regarding feeding concentrate to the evaporator: How important is it to consistently feed the same %? For example, is it problematic to use 10% and later go to 14%?

Thanks in advance and I hope everyone has a great season!

[mellondome]

On a raised flue , closing the valve between the pans has no effect. This is only an issue with Drop flue pans as they have no float between pans.
For concentrate, go as high as you like. I have a raised 2x6 with preheater ( used to run aof and auf... but now nat gas) and I run 18%. I also run way shallower in my pans. I just wet the flues in the back, and now run 3/4in in the front. My draws are almost continuous. Running varying % of concentrate will only change how often you draw. But you don't want to go from low to high in the same boil rapidly, it will cause you to maybe make syrup in the back pan. Ie. Feeding with 6% and then suddenly feeding 14%. Gradual increases will work ok, you will just need to adjust your schedule of operations as it will draw more often as the % increases.

You will defiantly need to lower the level in your front pans. The key to not making syrup in the back pan is being able to keep the front pan flowing. The shallower you run it, the faster it boils. Faster boil will give more frequent draws. Faster draw and boil, the faster it will draw from the back pan .

[David in MI]

Thanks, Mellondome. We were definitely doing large draw-offs last year, but not continuous, more like a batch. I'll try running shallower on the front and we'll see how that goes. Any thoughts about the gap between the bottom of the flue pan and the fire brick between the ramps? I'm sure I'm losing heat through there...

I have NG at the house and am seriously considering utilizing it for our arch when we upgrade. I'd appreciate any comments you have on your switch to NG. Did you retrofit your arch or purchase from one of the major manufacturers?

[mellondome]

I have a gap under my flue pan of about 1/4 in. I converted a trafitional wood arch to gas. I bought some burners from another sugarmaker that was switching to pellets. If you swith, you will nred a bigger meter than what you have at the house. I'm running 2 burners in my 2x6 at a rate somewhere close to 700k btu/hr. Will never go back to wood. If you consider switching, make sure the gas company knows how much gas you need. I called leader and asked how many btu my setup required then added any other gas I would need...( ro room heater, hot water heater, finisher,... ) then added 10%.

[ennismaple]

The only problem with varying the % concentrate going to your pans is your drawoff frequencies will change. We'll have high test at the start of the day if we've re-circulated back into the sap tap. Once we fire up we'll match the RO speed to the evaporator to keep the head tank at about the same level. This means the sugar content of the concentrate will slowly drop through the day. If we do a short rinse the 5 minutes we backwash the membranes will result in slightly lower concentrate in the head tank, then it will come back up as we concentrate again. The evaporator will adjust on its own.