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-conte...ev-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/...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/literatu...g_Handbook.pdf
Propane/LNG with drop flue pan
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Originally Posted by DanielS.
