2x6 Lightening ... 16gph or can I do better?

[Couchsachraga]

I can't tell if this forum is great, or depressing! Our family has sugared every other year for 35 years or so; two years ago I moved my parents evaporator over to my property (far better sugarbush), and have been working on making the operation more efficient (they tended to make 15-25 gal / year, I made 42 gallons in 2016 with 121 taps (3/16 line for 118 of them, gravity vacuum (I have plenty of elevation change!)).

Anyway, I've been tracking it and it seems I'm boiling about 16gph, which seems quite a bit less than other folks. Wood has seasoned for 2 years, and is a mix of hard and soft wood. I can certainly split it up more and fire more frequently, but I'm guessing there may be more I can do on top of that. From reading here a bit a few things I've noticed and figure I should mention about our operation:
1). I believe this is a raised flue (Small Brother's Lightening from the 80's at least...).
2). Under the rear pan is vermiculite; there are several inches (2?) between the top of the vermiculite and the bottom of the flues / flutes. We basically level off the vermiculite to the top of the arch, and the pan sites on the sides.
3). Plenty of air flow from underneath, but I'm not sure if that is ideal or not. No blower (over a mile from commercial power).
4). We generally run 2" or so in the back pan, and 1.5" or so in the front


I think I've read somewhere that this unit should be good for 24gph, and I see (probably newer) units folks are claiming 30-40 gph.

Once I get it functioning fully I'm thinking of other ways to increase efficiency - steam pan is probably the most likely. RO is intriguing, but given the way our operation functions I'm not sure it makes sense - for now I'd like to get the evaporator happier.

Thank you for any and all thoughts / ideas! I need to be able to keep up better with the flows that I'm getting (150 taps this year, all on 3/16 line... on a good run day it is tough / impossible to keep up with the incoming sap)

[jmayerl]

If it's a raised flue then whatever is under it(vermiculite in this case) should basically be touching the flues after the front 1/4 of the pan and until the back 1/4. You need to force the heat and flames up into a short raised flue. Should get get 30 gph easy.

[Couchsachraga]

Hm... should I be replacing that vermiculite with fire brick? Does anyone still use vermiculite (I'm guessing it was cheap and an insulator, and that is why it was used....)

Thank you for the quick response!

[maple flats]

Vermiculite is used because it is the best insulator for that area. However it should be capped with either fire bricks or just build up a layer of refractory. Using dry refractory cement, start by putting a layer on the surface of the vermiculite, then dampen it with a light mist of water. Much will sift thru, but proceed. Then add another layer, this will work better, and a 3rd or 4th layer may be needed to cap it properly. With each layer fire the arch with a low fire and run it for 45-60 minutes. Then on the final layer, fire it slow for half an hour then a little hotter for the next half hour. Do 4-5 such firings, each a little hotter to set the refractory. When finished the "cap" you made should be 3/4-1" thick and be high enough that there is only 1/4-3/8" under the flues. That forces the heat thru the flues.
Have your wood split "wrist Size" and dry. Add wood by using a timer. Start with every 7 minutes and adjust as needed. When you add wood, be ready ahead, open the door, add the wood and get the door closed quickly. If it has 2 doors, add wood on one side, then next time add it thru the other door.
You should be able to get 25-30 GPH

[Big_Eddy]

.
4). We generally run 2" or so in the back pan, and 1.5" or so in the front




That's super deep in the back pan. Most people just cover the tops of the flies. 1/4" or so. Try reducing to 1".

1" in the front is more typical too but depends on your firing method and attention span.

How often are you firing? Should be less than 10 mins apart. Wood should be small enough a good armful or two are fully consumed in 8-10 minutes

[Couchsachraga]

Firing depends on our energy level - 10-15 minutes usually, sometimes more frequently, sometimes further apart when we're using wood that we should be splitting more (after 10 hours straight I'm inclined to nap in the chair a bit... easy if the back has 2", not so easy at the levels you folks are running!)

I'm hoping for a good run this weekend to try running it as you all recommend - Thank You all!!! I've been running it as we did for 30 years or so from when my parents were shown how to use it when they purchased it used. Before that we used a 55 gallon drum on it's side with home made pans, and before that a wash tub over an open fire (early 80's...).

[Russell Lampron]

That's super deep in the back pan. Most people just cover the tops of the flies. 1/4" or so. Try reducing to 1".

If his Small Brothers flue pan is like my Algier flue pan he can't run much lower than he does. The bottom of the outlet to the front pan is level with the top of the flues in my pan and if I run it lower than 1 1/2" I risk not having enough sap in the pan to feed the front pan. Algier evaporators were made by Rodney Algier a former partner of Small Brothers and there are a lot of things that are the same.

[Russell Lampron]

From what I'm reading here it looks like you need to split your wood smaller and fire more often. You should lower the depth of your front pan to 1" and lower the depth in your flue pan if you can too.

If I'm reading this correctly your are filling the back of the arch to the rails with vermiculite which is what you want to do. Level it off at the taper in the arch and taper it down at the back far enough forward to allow the smoke to get out. A distance about the size of your base stack collar is all that you need.

A blower would help and what a friend of mine that didn't have electricity did was duct in a back pack leaf blower. He set up the leaf blower outside and used dryer vent pipe to duct it into the draft door of his arch. I think he ran it about 1/4 throttle and it made a huge difference.

[Couchsachraga]

I will be double-checking the outlet from the back pan to front, and see what I can get away with in terms of reduced depth.

Even though the vermiculite is leveled off to the top, the space from there to the bottom of the flues is much more than 1/4 or 3/8", so I'm going to try and reduce that, as well as likely replace the top level so it is fire brick (I have some around; I blacksmith as well and they come in handy).

I'm sure firing more frequently will help, but I don't think alone it will double GPH.

I may try setting up a hand crank blower I have lying around to see if that makes a difference. I want to be sure I'm not just burning more wood though - I've read on here that I should really be adding air to the top of the fire, not the bottom.

Thanks again!

[Russell Lampron]

Ideally adding AUF and AOF is what you want to do but just adding AUF makes a big difference. I've done it without a blower and got 25 to 30 gph. I added AUF and got that up to 35 to 45 gph and then for this year bought an air tight AUF/AOF arch and am getting 65 gph with the same 2x6 Algier pans.

The quality and dryness of the wood and how attentive you are to firing the evaporator play the biggest role in efficiency. Some days the fire just burns hotter than others too. When you add a blower you throw in more wood more often but you empty your feed tank a lot sooner too. When all is said and done you end up making more syrup with less wood.

[mellondome]

If you run your pans shallower .... especially your back pan.. you will be home when its nap time instead of trying to stay awake between firings as your boil rate will increase a lot.

As for a blower, just air into the draft door will be a huge increase... a small squirrel cage blower will run off a small inverter and car battery. Used this 2 years on my 2x6.

[Mikemartin274]

I've got a 2x4 leader pan and it consistently get 23 gph. Need wood about two inches thick and feed it every 7 minutes. If I use bigger wood and fire at 10 minutes I get 15 gph. All about the firing,wood moisture content and depth in pans.

[Couchsachraga]

I've been looking at 12v blowers online, but realized I think I have an old car heater blower sitting in the barn up there, so I may try and wire that up for fun. I have one small solar panel (30w) to keep a battery topped off to run 2 12v LED lights... no reason I can't run the blower off it as well and either charge it up, or get a bigger solar panel for it.

Now to hope for a good run the next 3 days... forecasted high temps have pulled back a bit... we'll see.

[maple flats]

If you split your wood so at least 1 dimension is not over 2" and fire ever 7 or 8 minutes by using a timer, you will likely increase the boil rate enough to boil at almost twice the rate you get now, if you run the back pan shallower. Then when finished, go home and get your nap. Over the years lots of sugarhouses have burnt down because the operator fell asleep.

[Russell Lampron]

Over the years lots of sugarhouses have burnt down because the operator fell asleep.

A lot of pans have been scorched because of that too.

[Couchsachraga]

I tend to stop before I get that tired, thankfully. My parents burned the pans a few times and they had to be repaired. I'm trying to avoid that!

Thank you all again for all the tips - all seemed to help a bit, though what helped the MOST was re-wiring the cap on the stack to open all the way so it drew better. I'll admit that before it might not have made a difference, but with more firings and lower levels (I stuck with around 0.75" in back and 1" in front. I can't go much lower than 1" in front as the thermometer will be out of the syrup then... and it is the type that is threaded in to the pan, so I can't move it much!).

Looking at how far back it is boiling in the rear pan I am planning on adding air under fire, and brought an old squirrel cage fan from a old car heater down (in theory I have the speed control, though it is only 2 speed; I need to see if the motor will accept a rheostat of some sort or if I'll burn it up that way). I haven't done a search yet (tonight, I need to wander in to work for a bit; I managed to boil down everything in storage and was home a bit earlier than I thought:) ), but am wondering how folks set up their AUF (I'm assuming metal pipe under the unit, then an elbow straight up, but wonder if it should be split further or not). I can see replacing the grate that is there now (that the wood sits on) with metal pipe with holes drilled in it if I get ambitions this summer.... I"m also interesting in the steam pans and pre-heaters, and will keep my out for used units that might fit mine... seems like a good way to increase productivity for someone off grid as I am (I'd need to run an RO off a generator, and hope the water line line I have from a spring doesn't freeze).

Oh, and one other "safety" I assume most use - I have a separate digital thermometer with a remote probe that I place in the pan, and set the alarm at 219... it tends to bump up against it before it really gets there / to syrup, but it is a good warning and helpful to keep you out of trouble.

Thanks again!

[maple flats]

Just straight in under the grates, no elbow needed.

[Couchsachraga]

Mine is up on cement blocks, as we've always run it... so air straight in will blow all the ash straight out the front most likely.

Seems a lot of AUF posts here; I'm hoping to find some pictures.

Not sure I'll play with the blower tonight... 5 gallons of syrup to bottle tonight (quart and pint jars mostly) from yesterday.

[Couchsachraga]

Running rear pan at 3/4" or less, front around 1", and more frequent firings seems to help. I'm unsure how much yet though (first day it was running in to my storage tank while I was boiling, so tough to tell throughput, next time it was lower in my tank where it is harder to know throughput (upper section is easy - square and 8 gallons / inch).

I've added AUF, and that seems to help quite a bit in my case, and has had the added benefit of much less ash (more complete burn). Only one day so far, but the steam was so thick I couldn't see the other side of the pan sometimes; I'm not sure yet if it was a "good day" or the AUF helped that much. When things finally thaw out we'll see. Right now looks like highs in the 20's and lows in the single digits. Doesn't seem like March.

[steve J]

Small generators are not very costly I would get a blower under that fire. Your boil rate will jump up and you will use less wood too!

[maple flats]

Even a 12V blower run off a deep cycle battery will do well, then take the battery home to recharge, while you recharge.

[Couchsachraga]

Steve J - are you referring to AOF (Air over fire)? As noted I've added a blower under the fire and it has helped, though I've not been able to really quantify by how much yet as it has been too cold (not much sap to boil), but it seemed there was a lot more steam and a lot less wood ash. I'm using a DC fan motor (blower motor from a car) hooked to a deep cycle boat battery; I'll connect to solar panels if I need to.

I'm half keeping an eye out for a steam away too - seems like a good fit for what I have in there and keeping it relatively quiet.

[Sugarmaker]

Steve J - are you referring to AOF (Air over fire)? As noted I've added a blower under the fire and it has helped, though I've not been able to really quantify by how much yet as it has been too cold (not much sap to boil), but it seemed there was a lot more steam and a lot less wood ash. I'm using a DC fan motor (blower motor from a car) hooked to a deep cycle boat battery; I'll connect to solar panels if I need to.

I'm half keeping an eye out for a steam away too - seems like a good fit for what I have in there and keeping it relatively quiet.

With the blower for the steam away, and the noise of the sap bubbling in the steam away, It will add some additional noise to the boiling room.
Regards,
Chris

[maple flats]

For relative quiet with high pressure (HP) AOF or a steam away, the blower must be outside and in an insulated box, only vented away from the sugarhouse. HP blowers are not quiet, but they are very effective in either application.

[mellondome]

Do you have a door on the front of your block arch? How air tight is the firebox area? For a steam away, you will need power @ your boil location for the high pressure blower.

A standard preheater inside a flue pan hood will add a lot to your boil rate without needing any power. As will having an air tight firebox for your auf/aof.

[n8hutch]

I thought only enhanced steamaways had blowers is that right? Either way they are pricey.

If your pan is boiling really hard at the back near the stack then you may not have it restricted enough so your blowing heat right past the pans. What kind of tap between the flue pan and the bottom of arch, what kind of gap at the flue exit do youhave.

[Couchsachraga]

Let's see:

-Arch is not air tight. I'm added a bit of gasket material to make the cracks smaller at least, mostly to cut down on sparks and ash escaping. I'd prefer to keep my sugarhouse standing!
-There is 8" or so between where the fire brick under the flue pan ends and the flue exit. I do not have a really hard boil going on back there (sometimes no boil or only a slight one)
-I've used a infrared thermometer on the stove pipe, but do need to track down one to put in the pipe to have a much better idea what the temperature is. I gather I'm aiming for 700-800 degrees F?
-As recommended here I've reduced the gap between the flues and the arch to 1/4" to 3/8"

I do have a small generator, but am trying to keep things simple (the is 1.25 miles from grid power). The AUF running off a deep cycle battery is working well so far; I'm glad you folks recommended it and I added it. The fan has 2 speeds, I'm still experimenting to see which is best. So far high definitely boils a bit better, but I do want to be sure I'm not just blowing a lot of hot air up the stack!

It sounds like a sap pre-heater may be my first step. I'm not sure that will get put in this year. If a used steam away or sap raider appears I may look to add it.

We make enough for ourselves and gifts, and sugar every other year; if I can make 30 or 40 gallons I'm happy (and aiming for 50 this year, though so far it's been a weird season... looks like it may finally start running again at the end of this coming week. Low of 4F tonight (-9F with windchill). doesn't seem like mid march!

[mellondome]

Reduce the rear flue exit to 4" of exposed flue area for the gases to drop out of the pan. This will help to hold the heat in the pan longer and slow the rate at which the heat passes throught the flues. Also, dont be afraid of stack temps over 1000. It is "wasteful" but you will get higher boil rates.

[Russell Lampron]

If you're not getting a full boil at the back of the flue pan with the blower on speed one put it on speed two. You're definitely not blowing too much heat up the stack.

Of course when you have your flue pan off again narrow up that exit at the back of the flue pan to 4" like mellondome said. The area back there only needs to be as wide as the area of the opening for your base stack.

[Flat Lander Sugaring]

2x6 king leader, firing every three minutes i used to be 25/30 gph. Couple times 35 to 44. with AUF.
Now I have AUF/AOF 2 to 3 inches of 2600 fire blanket 4 post 4" RO. last week we were doing 1.2/1.3 GPM of 12% concentrate and we were keeping up. That's like 72 GPH. Some people will say BS but until you see it run it is unbelievable. I have made 110 gal syrup on 3 cords of wood. I have had sugar makers come by that have been sugaring for 50+ years and they say they have never seen a 2x6 boil like this.
That was an outstanding night, on a more typical night we do around 50GPH. I am not sure why of the huge difference but maybe barometric pressure and a little different wood mixture.

[Russell Lampron]

2x6 king leader, firing every three minutes i used to be 25/30 gph. Couple times 35 to 44. with AUF.
Now I have AUF/AOF 2 to 3 inches of 2600 fire blanket 4 post 4" RO. last week we were doing 1.2/1.3 GPM of 12% concentrate and we were keeping up. That's like 72 GPH. Some people will say BS but until you see it run it is unbelievable. I have made 110 gal syrup on 3 cords of wood. I have had sugar makers come by that have been sugaring for 50+ years and they say they have never seen a 2x6 boil like this.
That was an outstanding night, on a more typical night we do around 50GPH. I am not sure why of the huge difference but maybe barometric pressure and a little different wood mixture.

I'm surprised that you are using that much wood. I've got an airtight WF Mason arch with AUF and AOF with my Algier 2x6 pans hood and pre heater. I boil 14% and am making 100 gallons of syrup per cord of wood. There has been a lot of waste this season too. It is the first year with my new arch so I had to do a test boil before the season and at the start of the season and had to dial in the blower and learn how to fire the new set up, both of which cut into my gallons per cord. I'm sure that I can improve that number now that maple Weekend is over. I've gotten as much as 65 gph out of my evaporator but normally get about 50 gph. I draw off 8 gallons of syrup an hour which is fun. I've made 140 gallons of syrup so far including maple weekend where I burned a 1/2 cord to make 30 gallons and I've only used 1 1/2 cords of wood so far this season.

[Flat Lander Sugaring]

I'm surprised that you are using that much wood. I've got an airtight WF Mason arch with AUF and AOF with my Algier 2x6 pans hood and pre heater. I boil 14% and am making 100 gallons of syrup per cord of wood. There has been a lot of waste this season too. It is the first year with my new arch so I had to do a test boil before the season and at the start of the season and had to dial in the blower and learn how to fire the new set up, both of which cut into my gallons per cord. I'm sure that I can improve that number now that maple Weekend is over. I've gotten as much as 65 gph out of my evaporator but normally get about 50 gph. I draw off 8 gallons of syrup an hour which is fun. I've made 140 gallons of syrup so far including maple weekend where I burned a 1/2 cord to make 30 gallons and I've only used 1 1/2 cords of wood so far this season.
my AOF/AUF is not correct. Im pushing to much heat out my stack. I have 2 blowers and I need to get off my procrastinating *** and just use my HP blower for both. I have been using pine exclusively but last few nights I have been mixing a lot of hard wood in and my fire times have jumped from 3 minutes up to 8 to 10 with the heat at rear flu being around 570C.

Maybe you would not mind if I stop some time see how you have you system set up, I am always ready to learn and be more efficient.

[Couchsachraga]

So my Lightning is driving me crazy. So far from comments I have:
-Changed my floats around so I am running 1" (or just a hair over) in the front pan, and 1/4"-1/2" in back (when it does start rolling good at the front 1/3 of the rear pan occasionally bare metal)
-Added fire brick in back with 1/4 clearance to the flues (Note: Left 6-8" in back for the smoke to get back down and out - someone replied recently and said that should be 4"... with any luck that will happen tomorrow morning, as well as cleaning the flues while I have it tilted up)
-Added AUF (more steam, seems to go from 16 GPH to 20GPH
-Started firing more frequently - last night I used my watch and put in wood every 5 minutes (3 pieces or so, less than I was). Still seemed to boil about the same mount
-Added a flue thermometer (probe type); temp seems to hold around 700F

At the previous sugar house I remember a good roiling boil all he way to the back - I"m not getting that, just the first 1/3rd usually. I'm hoping tweaking the "outlet" helps. I'm almost out of my "good" wood (2 year seasoned ash and poplar), and have a bit of hardwood left (same drying time), and 1 year somewhat dry other hardwood. I'm making more than planned (a good thing!) but if my sugarbush keeps producing more than I planned I clearly need to add another wood shed (or reverse osmosis or a steam away;))

I'd really like to see 25-35 GPH some time this year. Firing every 5 minutes and not seeing much improvement was pretty depressing.

[mellondome]

What is the cfm of your air under?
Do you clean the flues before each boil?

[Russell Lampron]

my AOF/AUF is not correct. Im pushing to much heat out my stack. I have 2 blowers and I need to get off my procrastinating *** and just use my HP blower for both. I have been using pine exclusively but last few nights I have been mixing a lot of hard wood in and my fire times have jumped from 3 minutes up to 8 to 10 with the heat at rear flu being around 570C.

Maybe you would not mind if I stop some time see how you have you system set up, I am always ready to learn and be more efficient.

The way that my arch is set up is that I have about 25% of the air blowing up under the fire and about 75% blowing back into the fire from the arch front. Not a true AUF/AOF with air coming in to the sides of the fire too but it works. It only uses one squirrel cage blower for both.

If you're in the area stop in. I like showing off my stuff so others can learn.
18386 18387 18388

[Couchsachraga]

Not sure of the CFM of my blower -it is out of my old Land Rover ('68 :) ). Two speed, and I run it off a battery... so I've tried no blower, low speed, high speed, and low and high speed with a generator charging the battery (high definitely kicks up more steam... that is when I think I'm hitting 20).

Flues haven't been cleaned all season... my parents cleaned them once a year (end of season), so. that was my plan. As you can tell from my last post I"m wondering if that is part of the problem!

We're in the midst of bottling the 13 gallons we made this week:)

[mellondome]

Cleaning the flues between every boil will make a difference. Get a flue brush made for your pan and a length of chimney brush handle.

[Couchsachraga]

I hope so - I'm running out of things to try!

The frustrating bit is recalling it boiling better than it does now, and not having figured out why yet.

Thank you all for your thoughts and help!

[mainebackswoodssyrup]

I was boiling with some friends who run a 2x6 lighting last weeeknd. They have a hood and preheated setup in the hood. No aof or auf and they were getting 40 gph so I would say you can do much better. They were at 28 gph or so before hood and preheater.

[Flat Lander Sugaring]

So my Lightning is driving me crazy. So far from comments I have:
-Changed my floats around so I am running 1" (or just a hair over) in the front pan, and 1/4"-1/2" in back (when it does start rolling good at the front 1/3 of the rear pan occasionally bare metal)
-Added fire brick in back with 1/4 clearance to the flues (Note: Left 6-8" in back for the smoke to get back down and out - someone replied recently and said that should be 4"... with any luck that will happen tomorrow morning, as well as cleaning the flues while I have it tilted up)
-Added AUF (more steam, seems to go from 16 GPH to 20GPH
-Started firing more frequently - last night I used my watch and put in wood every 5 minutes (3 pieces or so, less than I was). Still seemed to boil about the same mount
-Added a flue thermometer (probe type); temp seems to hold around 700F

At the previous sugar house I remember a good roiling boil all he way to the back - I"m not getting that, just the first 1/3rd usually. I'm hoping tweaking the "outlet" helps. I'm almost out of my "good" wood (2 year seasoned ash and poplar), and have a bit of hardwood left (same drying time), and 1 year somewhat dry other hardwood. I'm making more than planned (a good thing!) but if my sugarbush keeps producing more than I planned I clearly need to add another wood shed (or reverse osmosis or a steam away;))

I'd really like to see 25-35 GPH some time this year. Firing every 5 minutes and not seeing much improvement was pretty depressing.
is your arch airtight?
preheater?
hood?
how often do you use defoamer.

go find an old oil gun from a furnace, strip it out pipe that into to bottom of fire box.

[maple flats]

I'll bet cleaning the under side of the flues will make a big difference, but also cleaning the sap side helps too. For the under flue brush handle I far prefer a pipe for a handle, that way you can get up into the flues better, the chimney brush handles are flexible. I use a 3/4" pipe and then use a coupler to get down to the brush fitting size. Black iron pipe is cheapest. I think I may have had some 3/4" is why I have 3/4" but 1/2" would work just as well.

[Couchsachraga]

Wood fired, old Small Brothers Lightining (not air tight... from the 80's...)
No preheater (yet!)
No hood (yet!)
Defoamer in front as needed (usually once or twice before a draw off. We use margarine.

In theory this evaporator is supposed to be good for 24-30 GPH with no AUF.

I'm hoping decreasing the smoke outlet at the back of the arch will help, as well as cleaning the underside of the rear pan (I've cleaned the inside twice this year simply as I drained it to work on the fire brick under it...).

I'm confused on what I'm supposed to do with the old oil gun - air? Right now I have a 4" duct with an elbow pointing up from where the ash pan would be (never had one, and it is up on cinder blocks (a throwback from increasing the draft by allowing more air in...).

[Russell Lampron]

You should be defoaming the back pan too. The foam holds the steam in which also reduces the evaporation rate.

[Flat Lander Sugaring]

i think the old oil gun give you mot=re air than what you have.
put 3 drops of defoamer in float box and 3 drops where sap goes from rear pan to front every time you fire. This is especially true if you wouls have a rear steam hood.
keep sap to lowest level you feel comfortable at. When all I had was steam hood, pre heater and oil gun I used to run 5/8" maybe 3/4.
ok time to go gather sap start RO, GOOD LUCK

[maple flats]

Until you figure out the issues with your set up you now have, a steam away or piggy back should not be in your plans. Get this running right before going that route.
I never ran a block arch, but I wonder if that is part of the problem, too many uncontrolable air leaks maybe. Before next season either look for an arch to set the pans on or build one. There are lots of threads on this forum of home built arches. Air is key in achieving a hot fire. You mention reducing the opening for the stack, likely not a good plan unless it is wide open. If you look at pictures of any 2x6 you will see that it has a large base stack which then tapers down to the final smoke stack size, which on older arches was often 7-8", newer ones more often have a 10" stack on a 2x6. Then do not ever put a damper in the stack on a wood burning evaporator. Typical boil rates with no improvements for a 2x6 drop flue are about 25 gph and for raised flue more like 30 gph. Most of that is just from splitting the wood right, fueling at the right interval, and most importantly having the arch set up correctly, especially level and with gasket between arch and the edge of the pan and having the right space under the flues, which should never be more than 1/2" and 1/4" is better. The insulation under the flues should also be protected, you can use half brick, or even a layer of refractory.
My raised flue has vermiculite under the flues and that then has a layer of half brick (1/2 thick fire brick) and there are voids in the brick, (spaces), those voids were filled with dry refractory cement, then I lightly sprayed a mist on them, 4-5 times over 3 days, then I put water in the pans and started a slow fire for a few hours, the next day I fired it a little hotter and the 3rd day I fired it hot enough to boil the water in the pans. That was sufficient to cure the refractory.
If you don't cover the vermiculite it will not stay in place, your flue brush will mess it up and some will actually go up the stack.
Back before I had power at my sugarhouse my original set of pans on my 3x8 evaporator got 55-60 GPH, with no blower at all. It had a 17' high stack, 12" diameter after the tapering base stack and I had the draft door (14" x 8") open at about 45-50 degrees. My arch was not and still is not air tight. I used a mix of about 70% hard wood/30% soft, all split wrist size and fueled it every 7 minutes by a timer. I did however have a homemade aluminum hood. I haven't run an evaporator without a hood only 1 year, and that was the first year I had on a 2x6 drop flue.

[NhShaun]

I'd hope to see you getting more than 16gph. I'm getting 20gph on my 2x6 flat pans with dividers. Using super dry pine slabs split to 2inch wide pieces, maybe that helps. Hoping to upgrade my pans next season, 20gph is just not fast enough for my schedule.

[Couchsachraga]

I think I hit over 30 this weekend, but with sap coming in it is hard to tell.

I changed a few things, and noticed a big difference in the boil:
1). Cleaned the bottom of the pan (cleaned out the flues) from the under side. Not sure how much it helped, but certainly didn't hurt
2). The big one - though it was recommended I make the gap smaller at the back, I increased it, cleaning out the vermiculite that was back there, and ramping the brick over it back down to allow for more flow. This summer I'll likely add the refractory cement to smoothen the flow over the whole thing. It clearly draws MUCH better now, and I had a good boil all the way to the back, such that defoamer was necessary in the flue pan (I haven't been added it as it hasn't been foaming up!). Before the best boil was at the front of the flue pan, now the best is over the middle (spitting sap out a times it is boiling so hard (after defoamer), running 1/4-1/2" or so in back (+ /-)), but a good boil front to back.

I'd like to seal my front door up a bit - still won't be air tight, but keeping the ash from blowing all over the place would allow me to run the blower higher at times. Stack temp typically was around 700-750. My wood pile is getting down there, so mostly 2 years seasoned ash, maple and hop hornbeam left. I need to put up more poplar next time....

I split what I had wrist sizes or smaller in BOTH dimensions, and kept up firing every 7 minutes or so. My rough calculation was 24-28 GPH at one point (over a 60 min measurement) PLUS whatever was running in at that time (which was over 5 GPH, but I'm not clear how much). Better wood would have kept it up a bit higher.

So, for this summer I'm planning on:
-Cleaning out the back of the arch even more thoroughly, and adding refractory cement over item fire brick to fill in the cracks(right now 1/4" from the bottom of the flues)
-Try and seal up the front doors better - perhaps weld on studs, and use metal over ceramic blanket? I seem to have read on here folks have done that....

After running my small generator out there to charge the fan battery up I'm also wondering about a small RO... reading about the Leader MicRO 2 right now which doesn't look unreasonable cost wise at least (I don't sell what we make, just give use it and give it as gifts (we seem to make 40-50 gallons each year, and sugar every other year), so this more to save time / sanity / marriage than anything else;) ).

[maple flats]

It sounds like you are now on the right track. 25-30 gph is respectable without extra enhancements.