Propane/LNG with drop flue pan

[TheNamelessPoet]

I currently work off a divided flat bottomed 33x17 (roughly) pan on a 3 burner grill (essentially each burner is a turkey fryer).

I am considering upgrading to a 2x4 pan, and will need up upgrade my burners as well. I have ZERO interest in switching to wood; whether it is more efficient or not, for MY use case, it is not something I can really do or even WANT to do.

My question is, if I got a drop flue pan, could I put burners in between the "drops" in the pan in my arch if I got a drop flue pan? Will that have any benefit over having a flat bottom pan with say 6 burners (each 75k BTU's)?

I am looking to make more syrup and do not have the time on such a "small" pan to expand. I get between 2-3 GPH and make about 5-6 gallons a year. My customer base has grown quite a bit and I actually ran out this year already! I won Best in Show / Grand Champion at the Big E this year, and word got out at my previous job and of course my new job and now everyone wants to start getting mine so I am thinking I need to get up to 10 gallons or more per season. Trees and collection is not my bottleneck, I have plenty more I can tap, it is just the processing of it. I use an RO and can just expand my RO or add an additional one to speed that process up, but it all stems down to not having a large enough pan to boil off faster. Even if I can just get up to 8 GPH that should be enough.

This is my current grill (or as close as is currently available)https://www.amazon.com/Coofel-Outdoor-Burners-Pressure-Detachable/dp/B0827R8RZM/ref=asc_df_B0827R8RZM?tag=bingshoppinga-20&linkCode=df0&hvadid=79920843355703&hvnetw=o&hvqmt=e&hvbmt=be&hvdev=c&hvlocint=&hvlocphy=&hvtargid=pla-4583520391341807&psc=1

[DRoseum]

I built a NG arch that has a 20" x 40" drop flue pan set on it. I put multiple pipe burners running across the arch under both the front pan and flue pan (about 1 inch under bottom of flues) for a total of about 200k btuh. I also have a small variable speed blower to help push exhaust gas out/up the stack. I've estimated that i get between 12 - 15 gph on it. I have just over 100 taps on vacuum, 2 ROs, and make about 30 gallons of syrup per year on it at an average ratio of 75 gallons of sap per gallon of syrup (predominantly heavy forested red maples with low sugar %).

https://youtu.be/6FyWa9l-kw0?si=cFrXA1n-Zjjzsz9P
https://youtu.be/4N53Ar0jOdg?si=BlfqNm3j5FP4P4ky

[DanielS.]

Hi DRoseum. I'm not sure if you'll see this, but I'd be curious to know about your NG setup, and I'd love to get your opinion and any critique you might have of mine: I built an NG arch this year to use with a 2X6 pan setup. My reasoning was I just didn't have time to rush home after work, start a fire, let it warm up for ages to start boiling, boil like hell, then do a long shut-down procedure to try and get to bed to be up at 5 the next morning for work. Especially difficult working in the bush with only generator power. I figured NG would be a faster startup/shut-down, more controllable, and it let me boil at home rather than in the shack in the bush.

I'm also probably going back to a flat pan setup (though I built it to handle my 2x6 raised flue/divided syrup pan setup if I ever want to). The reasoning for going to the flat pan is I just don't have time to tap more than 50-60 trees this year - really scaling it back from former years. But that's still debateable - I may tap more and use the flue/divided pan setup from my other evaporator.

I used two 90k H-burners and two 60k H-burners for 300k btu total (though my orifice sizing for my 5" W.C. pressure is a little small - will likely have to open the orifices up a little to get the full BTU potential). Had to re-configure a few of the NG lines at my house to 1-1/4, connected to the 1" flex line and manifold of the evaporator. Built my own venturi air mixers, and still have to do some more tuning to get the flames totally efficient (hopefully I can with just simple venturi mixers. I'm worried that with my burner sizes I may have to use an inspirator to get a fully-efficient flame).

What I'm not sure of is my excess combustion air inlets - currently sixteen 2" holes through the refractory and bottom of the arch centred beneath the burners. Plus holes through the side for the burner inlets. And my stack size: 8" x 5' tall (will likely want it taller, but right now it's short so I can wheel it in my garage fairly easily). I've followed every guideline I can for flue size vs. BTU and free air inlet sizing, but would appreciate your first-hand experience to know if there are other factors I should consider. I've only done a few short test boils thus far - not enough to get a good idea of how efficient it is nor what problems I have.

Will try to attach pics of it. It also shows the 24" X 24" cap/trivet I made for the front in case I only want to use the back pan. Any thoughts/advice/criticism is greatly appreciated!

Edit: I forgot - Photo uploading doesn't work on this site. Oh well, I think I described my setup somewhat accurately.

[madmaples]

Hi DanielS, I'd love to know more about your setup! I'm considering building something myself and have been looking around for various kinds of burner tubes and setups.



Edit: I forgot - Photo uploading doesn't work on this site. Oh well, I think I described my setup somewhat accurately.
Perhaps you try imgur.com, it needs no account, just upload and share the link

[DanielS.]

Thanks, we'll see if this works - link to pictures below. I don't have any pictures of the final build. Here it is part-way through the build. It's built to handle my 2x6 flued/syrup pan setup, but in these pictures it's shown with a cheapo Vevor 2X4 flat pan on the back. You can see at the front I have a 2X2 heavy plate cap, with a removable round trivet with a handle and supports beneath it. If I ever use it with a proper 2x6 pan setup, I'll remove this whole plate/trivet assembly and drop the 2x2 syrup pan on there instead.

The burners were stainless fire pit burners. I chose them because they span nearly the full width of the arch to get an even heat from side-to-side, and the 'H' pattern means it spreads the flame more front-to-back than a simple pipe burner would. The problem (which I knew I'd have to deal with when I chose these burners) is that fire pit burners are designed to have a tall, yellow, inefficient flame. Thus, it needs some sort of air mixer to pre-mix the gas before it enters the burners to get nice, blue, efficient combustion. You can theoretically work out the stoichiometric gas/air ratio you need and approximate your air inlet sizes from that, but there are so many other factors at play (shape of the mixing area, draw from the venturi, boundary layer friction around the air inlet holes) it's really more trial and error than anything. All the parts I used for the gas supply, mixer, and venturi are standard gas/plumbing components.

I drew the arch in CAD and had the parts cut on our plasma table at work and formed to suit. Worked out pretty well, with the longitudinal stiffener bars notched to capture and hold the burners (with notched keeper plates bolted over top to keep them in place). The flue size and free-air vents in the bottom were roughly approximated based off whatever resources I could find. Same with the NG orifice diameters (which are just SAE flared caps on bulkhead fittings with holes drilled in them). Getting enough supply was a little tricky for the large number BTU's I could theoretically get out of this thing. Lots of good info on NG pipe sizing charts and line length calculating methods are available to make sure you don't restrict supply nor starve your other appliances (furnace, water heater, etc.) of gas when it's running. If I had to do it again, I'd choose slightly smaller burners or make it for a smaller pan setup. 300k BTU is pretty demanding of gas lines and air requirements, and gets in the realm where you probably shouldn't be doing this kind of thing at-home. Right now, I actually have much smaller orifices than I would if I wanted to get the rated BTU potential out of the burner. The orifices for my 90K burners are only drilled for 75K btu, and my 60K burner orifices are only drilled for 50k BTU. So I'm certainly not using it to its full potential, but it seems to boil just fine. In fact, once it's boiling, I can shut off the back burner and it makes very little difference on GPH. All sized based off my residential pressure, which I've confirmed is between 5" and 6" W.C.

Here's a link to pictures of it part-way through the build. Note the tabs sitting flat in the flue area - they bend up and you use push-on speed nuts to give something to bolt your stack to: https://imgur.com/a/q6AwgLm

Natural Gas Orifice Sizing Chart (you can pretty much assume 1 cubic foot of NG is approx. 1000 BTU/hr.): https://www.gordonpiatt.com/wp-content/uploads/2021/07/Natural-Gas-Orifice-Chart-7.E.80.5-Rev-3-05.01.pdf

One of several pipe sizing charts for NG (note there are other factors you have to consider when checking your line sizing - you can find good resources online about line length calculation for branch appliances): https://fire-boulder.com/wp-content/uploads/2014/08/PipeSizing.pdf

Guidelines of flue sizing for NG appliances (see pages 8-9). Note that for excess combustion air inlet, I made the rough approximation that my excess air holes in the arch should sum to about the same opening area as the flue: https://www.selkirkcorp.com/literature/Chimney_Venting_Sizing_Handbook.pdf

[madmaples]

Thanks, we'll see if this works - link to pictures below. I don't have any pictures of the final build. Here it is part-way through the build. It's built to handle my 2x6 flued/syrup pan setup, but in these pictures it's shown with a cheapo Vevor 2X4 flat pan on the back. You can see at the front I have a 2X2 heavy plate cap, with a removable round trivet with a handle and supports beneath it. If I ever use it with a proper 2x6 pan setup, I'll remove this whole plate/trivet assembly and drop the 2x2 syrup pan on there instead.

The burners were stainless fire pit burners. I chose them because they span nearly the full width of the arch to get an even heat from side-to-side, and the 'H' pattern means it spreads the flame more front-to-back than a simple pipe burner would. The problem (which I knew I'd have to deal with when I chose these burners) is that fire pit burners are designed to have a tall, yellow, inefficient flame. Thus, it needs some sort of air mixer to pre-mix the gas before it enters the burners to get nice, blue, efficient combustion. You can theoretically work out the stoichiometric gas/air ratio you need and approximate your air inlet sizes from that, but there are so many other factors at play (shape of the mixing area, draw from the venturi, boundary layer friction around the air inlet holes) it's really more trial and error than anything. All the parts I used for the gas supply, mixer, and venturi are standard gas/plumbing components.

I drew the arch in CAD and had the parts cut on our plasma table at work and formed to suit. Worked out pretty well, with the longitudinal stiffener bars notched to capture and hold the burners (with notched keeper plates bolted over top to keep them in place). The flue size and free-air vents in the bottom were roughly approximated based off whatever resources I could find. Same with the NG orifice diameters (which are just SAE flared caps on bulkhead fittings with holes drilled in them). Getting enough supply was a little tricky for the large number BTU's I could theoretically get out of this thing. Lots of good info on NG pipe sizing charts and line length calculating methods are available to make sure you don't restrict supply nor starve your other appliances (furnace, water heater, etc.) of gas when it's running. If I had to do it again, I'd choose slightly smaller burners or make it for a smaller pan setup. 300k BTU is pretty demanding of gas lines and air requirements, and gets in the realm where you probably shouldn't be doing this kind of thing at-home. Right now, I actually have much smaller orifices than I would if I wanted to get the rated BTU potential out of the burner. The orifices for my 90K burners are only drilled for 75K btu, and my 60K burner orifices are only drilled for 50k BTU. So I'm certainly not using it to its full potential, but it seems to boil just fine. In fact, once it's boiling, I can shut off the back burner and it makes very little difference on GPH. All sized based off my residential pressure, which I've confirmed is between 5" and 6" W.C.

Here's a link to pictures of it part-way through the build. Note the tabs sitting flat in the flue area - they bend up and you use push-on speed nuts to give something to bolt your stack to: https://imgur.com/a/q6AwgLm

Natural Gas Orifice Sizing Chart (you can pretty much assume 1 cubic foot of NG is approx. 1000 BTU/hr.): https://www.gordonpiatt.com/wp-content/uploads/2021/07/Natural-Gas-Orifice-Chart-7.E.80.5-Rev-3-05.01.pdf

One of several pipe sizing charts for NG (note there are other factors you have to consider when checking your line sizing - you can find good resources online about line length calculation for branch appliances): https://fire-boulder.com/wp-content/uploads/2014/08/PipeSizing.pdf

Guidelines of flue sizing for NG appliances (see pages 8-9). Note that for excess combustion air inlet, I made the rough approximation that my excess air holes in the arch should sum to about the same opening area as the flue: https://www.selkirkcorp.com/literature/Chimney_Venting_Sizing_Handbook.pdf

Woah, thank you for sharing, this is gold! I've been digging through some of the same literature trying to pull something together.
I'm currently boiling on a 16x30" vevor pan over a pair of "Martin" 65K burners, which fits perfectly, but I'm loosing a ton of heat out the sides and any breeze really dampens the flames. I was thinking about a similar setup to what you have, but with round burners underneath.

Today, at the local maple tour, I saw an interesting propane fired evaporator that has a single forced air blower/burner which throws a flame toward the front, the flame then turns 180 degrees and loops back under the pans. Unfortunately I didn't get the make/model off of it, here's a low-effort sketch: https://imgur.com/a/ptRuCVd

I thought this was an interesting idea as well, but I'd imagine figuring out the right parameters of that would be quite difficult.

[DRoseum]

Looks like am awesome build! You did your homework and it will probably work nicely for you. Definitely do some tests and be ready to make minor adjustments.

The main thing I learned is that with multiple burners all in a row, the exhaust gas from the front ones can seem to effect the ones towards the back. I have a nice long insulated exhaust stack (more than 2x length of arch) and even with that drawing a nice draft it still seemed to occur.

My back burners are lower than my front ones (due to me running a drop flue pan) , so I did 2 things that completely addressed the issue: (1) added the variable low speed blower to just slightly help move that exhaust gas out and (2) i actually segmented my front burners from my back burners and each set has its own for air intake and the front ones have an insulated "shelf" that runs under them and then 90° up to 1.5 inches below the pan. This forces the hot exhaust gas to go out right up against the top part of the flues and then doesn't effect the lower burners under the drop flues.

I also ran 1.25" gasline of my 2 inch main out to the back of the house (and out to my pool heater). I tapped off that with a 1 inch csst flex line to the final manifold that's all 1 inch. I also have considered changing the venturies or going to a inspirated design similar to many of the forge burner designs out there if i ever extended the arch to have a longer front pan. So far this has worked great though amd the flames are all bright blue. I get minimal soot build up on the bottom of the pans, so it's not a perfectly clean burn, but its pretty darn close.

Please keep us posted as to how yours works out and what you learn. Best of luck on your season and great job on your build!

[DRoseum]

Today, at the local maple tour, I saw an interesting propane fired evaporator that has a single forced air blower/burner which throws a flame toward the front, the flame then turns 180 degrees and loops back under the pans. Unfortunately I didn't get the make/model off of it, here's a low-effort sketch: https://imgur.com/a/ptRuCVd

I thought this was an interesting idea as well, but I'd imagine figuring out the right parameters of that would be quite difficult.

This is a reverse fired arch design. I believe the benefit of these over one where the burner is mounted on the front is that this will ensure all the heat will be under the entire bottom of the front pan (the very front and spread out to the sides) and you won't have a giant burner sticking out off the front of your arch.

[maple flats]

You can either make your own pipe burners or order them ready made. Google " custom made pipe burners"

[DanielS.]

Looks like am awesome build! You did your homework and it will probably work nicely for you. Definitely do some tests and be ready to make minor adjustments.

The main thing I learned is that with multiple burners all in a row, the exhaust gas from the front ones can seem to effect the ones towards the back. I have a nice long insulated exhaust stack (more than 2x length of arch) and even with that drawing a nice draft it still seemed to occur.

My back burners are lower than my front ones (due to me running a drop flue pan) , so I did 2 things that completely addressed the issue: (1) added the variable low speed blower to just slightly help move that exhaust gas out and (2) i actually segmented my front burners from my back burners and each set has its own for air intake and the front ones have an insulated "shelf" that runs under them and then 90° up to 1.5 inches below the pan. This forces the hot exhaust gas to go out right up against the top part of the flues and then doesn't effect the lower burners under the drop flues.

I also ran 1.25" gasline of my 2 inch main out to the back of the house (and out to my pool heater). I tapped off that with a 1 inch csst flex line to the final manifold that's all 1 inch. I also have considered changing the venturies or going to a inspirated design similar to many of the forge burner designs out there if i ever extended the arch to have a longer front pan. So far this has worked great though amd the flames are all bright blue. I get minimal soot build up on the bottom of the pans, so it's not a perfectly clean burn, but its pretty darn close.

Please keep us posted as to how yours works out and what you learn. Best of luck on your season and great job on your build!

Thanks kindly DRoseum - that's all great info to note. I confess I found your website with your pictures/videos while I was building mine, and spent quite a lot of time studying your setup with your burner and air intake configuration. Your comment about the gases from the front burners affecting the back burners was actually what I'm most worried about - especially with my burners spanning almost the full width of the arch. I wouldn't be surprised if I have to add a blower like you did.