Raised flue or drop flue?

[madmapler]

I have a 2 1/2 x 10 with a tin drop flue pan(7 ft.). I am considering changing it to welded stainless and would like to know whats best, Dropped or Raised. What might I get from a good pan set with this size evaporator? Do I have to modify my arch for raised?(I'm rebuilding it anyway) How many taps on vacuum could this potentially handle with a good RO? Last question...what kind of money are we talking for a new pan and from who? Thanks for any guidance.

[delivron]

If you wish more information of my reply please contact me via e-mail.
Most 2.5 x 10 evaporators have a boiling rate of 60 Gallons per hour. Source NA Maple Producers Manual. Your could be above or below that number.
RO to 10 Bricks 1600 gallons of sap it would take you 5 hours to boil on a day where your flow reaches 2 gallons per tap. On a 1 gallon per tap day tap you will boil in 2.5 hours.
I do have a calculator that automatically performs the calculation. The calculator is currently in Beta test.
Some manufactures will add baffles in the long part of the evaporator to direct hot air into the raised flew. Depending the depth of the drop flews you might have to rearrange your bricks under the flew pan.

[lpakiz]

Sean,
While I dont doubt what the cited reference says, that might be a bit low for an estimate.
I have a 2X10 drop flue and we get 90 GPM regularly. Sometimes flirt with 100. No preheater.
It has a 2X2 syrup pan with an 8 foot flue pan behind it. It sits on an Intensofire clone arch. We need to keep stack temps below 800 or sap jumps into the hood drain gutters. This combo is kinda experimental, but I would recommend this flat/flue ratio in a second. The evap rate is probably influenced by the fact that so much of the total length is flue pan, where most of the evaporation really occurs. We are not so sure this would be OK with an RO, as you might be too close to syrup while still in the flue pan.

[maple flats]

My 3x8 raised flue (15 sf raised, 9 sf syrup pan) gets 75-80 gph. I have AUF & AOF on a high pressure blower and 10" tall raised flues, your 2.5 x 10 is 1 SF larger, should be similar or even a little better if you have tall flues and put in AUF & AOF. I hope that helps. Then if you concentrate to 8% you can draw 7.44 GPH, if you go to 12% you can draw 11.16 GPH using the 80 GPH figure. For other rates of boil, just divide the sugar % into 86 for how many gal of sap will make 1 gal of syrup, then divide that answer into the evap rate. I run my % anywhere from 8% up to about 14-15%.
If you are now a drop flue, you will need to raise the area under the flues to force the heat into the flues. Mine is just 1/2" below the flues. To do this, just add vermiculite until almost to the flues, then lay 1 layer of half bricks or a thin layer of dry refractory cement. I laid the refractory about 1/2" thick. Then I lightly misted the surface with water, several times over a 3-4 hr. period. Then I very slowly warmed it to set and cure. I used a weed burner torch, but didn't get very close. I did this each day for about a week, just getting the surface warm to the touch at first, after a few days I got it hotter. After that I turned the torch hotter and applied direct heat. The whole purpose of this is hold the light weight vermiculite in place. To buy the vermiculite go to a commercial greenhouse supply, such as Griffin Greenhouse Supply, the vermiculite will cost a small fraction of what you'd pay buying little bags at Lowes or Home Depot. Calculate the volume needed, I think their bags were 4 or maybe even 6 Cubic Ft. each, but only about $20 each (back in 2005)
I have run both drop and raised flue, I find the raised flue is slightly better efficiency, but you have 2 float boxes to maintain.

[madmapler]

Thanks for taking the time guys. Its a lot of questions for one post. I'm just trying to figure out if I should invest in this set up or hold off for a few years and get a larger one. I have the potential for 4000+ taps easily and I figure it'll take me 3 yrs. to get'em all.

[Thad Blaisdell]

Personally I would buy the H20 version of the max pan. You would get best of both worlds. Call Glenn Goodrich to get a price.

[markcasper]

I had a drop flue for many years and it was fine. As for efficiency, I didn't really see any difference when I went to raised flue this past season. I like that I can boil shallower in the raised flue pan and the float feeding into the front pan worked much better than the ones of years ago. I liked not being worried all the time of banging the raise flue with wood (the main cause of failure on my drop). I liked the bigger taller pans over the old King too. It was always boiling over the sides.

As for converting, just fill your arch with vermiculite and get some cement board from the lumberyard and lay about level with your arch rails. That worked great in my arch. I had two pieces overlapped because they came 3' x 5' i believe. You want two feet open in the front and 1 foot in the back towards chimney.

You asked about price. Mine were Leader pans ordered in May of 2012, picked up in January of this year and was around $9,300 for the set, one 4x4 reverse flow front pan, one 4x8 raised flue pan. This was with the 10% discount, so the list was over 10.