Stainless Steel Sap Pan On Natural Gas Burners-Gauge per BTU?

I'm a small producer of 100 to 150 taps. I have a small reverse osmosis system to concentrate my sap before boiling it down. I was using a wood stove and small pans but recently I bought a 3 burner natural gas set up that I want to use to boil my concentrated sap down on. On full open flame each burner produces 100,000 BTU for a total of 300,000 BTU.

I am going to fabricate a 5' x 2' stainless steel pan for it. Not sure yet if I am going to go with a flat pan or a raised flue pan. Either way I go though the worry I have is if I go too thin of stainless steel I think I may melt or warp the pan severely from the high intensity heat from the burners. I'm not sure exactly how many BTU's a regular wood fired evaporator can put out but I have a feeling it isn't near as hot as these gas burners will go. Especially since this pan will be sitting directly on the flame from the burners.

Does anybody know if there is a math formula to figure out what gauge of steel I should use for how hot the flame is. I know the norm is guys are making the pans out of 20 and 22 gauge 304 stainless steel but I am thinking I should go with 16 gauge or so to make sure I don't melt or warp the steel.

Any advice would be appreciated. Thank you

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Your pan will be fine as long as there is sap in it. When I burned wood I was running 1500 degree stack temp so under pans was much higher.

Very true. The gauge dopes not protect the pan from warping, it's the liquid in the pan that protects the pan. If you go dry a 22 ga will warp as will a 18 ga. You need to go quite heavy to appreciably reduce the warp risk and the heavier the pan the poorer the boil. Just keep the boiling sap/syrup at least 1/2" minimum and it will not warp. Most operate at 3/4" up to about 2" deep. If you go with the raised flue design, the tops of the flues need an 1/8"or more over them.
I run my wood fired at 3/4-1" in the syrup pan (flat) and 1/2" over the flues in my raised flue pan. Then just keep track, if anything goes wrong on the fill or draw off, those levels can change real fast. It's best to keep a "O-Sh%#" bucket of sap or even water ready just in case.

When you run that new gas 3 burner it's very unlikely you will be able to burn it at full rate. Those burners will not evenly distribute the heat, you will have hotter and cooler spots. Even a tube burner (which distributes the heat much better) like what is used under a larger finisher with a 2x5 pan on it is not burned that hard.
To compare your BTU's to wood, on my 3x8, with firing every 8 minutes at about 18-20# per fueling (well seasoned wood) I'm adding 18x7,100 BTU or over 950,000 BTU/hr roughly, but it is well distributed under the syrup pan and about the first 2' of the flue pan (it still boils hard all the way to the back) and my pans are 20 ga., I get no warp, I just need to keep the 3/4" in the syrup pan and at least 1/4" on the flues. Some pans are 22 ga. and that works well too.

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Originally Posted by wiam

Your pan will be fine as long as there is sap in it. When I burned wood I was running 1500 degree stack temp so under pans was much higher.

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Thank you Wiam. That makes sense. I'll make sure I never let it get below a certain level of sap in it.

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Originally Posted by maple flats

Very true. The gauge dopes not protect the pan from warping, it's the liquid in the pan that protects the pan. If you go dry a 22 ga will warp as will a 18 ga. You need to go quite heavy to appreciably reduce the warp risk and the heavier the pan the poorer the boil. Just keep the boiling sap/syrup at least 1/2" minimum and it will not warp. Most operate at 3/4" up to about 2" deep. If you go with the raised flue design, the tops of the flues need an 1/8"or more over them.
I run my wood fired at 3/4-1" in the syrup pan (flat) and 1/2" over the flues in my raised flue pan. Then just keep track, if anything goes wrong on the fill or draw off, those levels can change real fast. It's best to keep a "O-Sh%#" bucket of sap or even water ready just in case.

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Thank you Maple Flats. Do you think it is more risky for me to go with a raised flu pan? What I mean is, does the same size pan need more sap in it with a raised flu pan vs a flat pan because of the increased surface area of the flues and the faster evap rate? Like is it harder to stay on top of the sap level in a raised flu vs flat pan? I'm asking cause I'm not gonna have anything automated on this years tapping season. Just a pre warmer pan above the sap pan and a manual draw off tap on the sap pan. I will do my finishing afterwards on a single burner and smaller finishing pan.

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Originally Posted by maple flats

When you run that new gas 3 burner it's very unlikely you will be able to burn it at full rate. Those burners will not evenly distribute the heat, you will have hotter and cooler spots. Even a tube burner (which distributes the heat much better) like what is used under a larger finisher with a 2x5 pan on it is not burned that hard.
To compare your BTU's to wood, on my 3x8, with firing every 8 minutes at about 18-20# per fueling (well seasoned wood) I'm adding 18x7,100 BTU or over 950,000 BTU/hr roughly, but it is well distributed under the syrup pan and about the first 2' of the flue pan (it still boils hard all the way to the back) and my pans are 20 ga., I get no warp, I just need to keep the 3/4" in the syrup pan and at least 1/4" on the flues. Some pans are 22 ga. and that works well too.

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Thank you Maple Flats. Last season I did do a batch of syrup on the three burner natural gas using a smaller pan on two of the burners. I was surprised how well the burners covered the whole bottom of the pan. The flames reach each other cause the hit the metal and spread a bit. I will make sure I don't make the new pan two wide or too long so that the flames cover almost all the metal. I'll try to get some pictures of the burners running full open with a pan over it. The sap I had in the pan last season was deep cause I just did it as a one batch test run with the new burners and it had a crazy strong boil going on with 7 inches of sap in the pan. The pan I had was 10" high and the 7" of sap was boiling so hard it was rolling right up to the 10" top of the pan. Saying that though it wasn't a large pan in width and length so things will change somewhat when the new larger pan is on them.
I had the gas company come and swap out our home meter for a higher flow one and put a separate regulator on the line that is specifically set up for the three burner. So I can crank those burners full open and get the full BTU's out of what they are capable of.

Before fabbing the new pan I am planning on putting a full sheet of steel on top of the burners and cranking them open on full for a minute and make note of how wide and long the flames fan out to on the metal. I'll mark the steel and make the pan that size so that I don't waste any space where the flames don't reach and will make sure I don't have any flame going up the side of the pan at all so I don't risk sap burn on the sides or warping on the sides.

Really appreciate all the advice. Thank you. I'll take some pics as the progress goes on.

You shouldn't have a problem with a raised flu pan but I would run it a little deeper if you do not have a float setup. I would also have more than 3 inches of freeboard above the sap. My sides are on my drop flu are 14 inches above the flues and I will get an occasional splash out the door of my hood when I am looking in.

Mine also, 24" sides with 10" raised flues (so I have 14" above flues) and a little still lands in the gutters around the hood and goes to drain

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Originally Posted by Super Sapper

You shouldn't have a problem with a raised flu pan but I would run it a little deeper if you do not have a float setup. I would also have more than 3 inches of freeboard above the sap. My sides are on my drop flu are 14 inches above the flues and I will get an occasional splash out the door of my hood when I am looking in.

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Thank you Super Sapper. After a lot of thought about what would be involved with trying to fabricate my own raised flu pan I have decided to go a different route and I have ordered a new custom hybrid raised flu pan with a float box, warming pan, draw off spout, temp gauge etc..Decided to bite the bullet and just get everything I need all at once. Using natural gas I have to try to get my pan as efficient in evaporating as possible and I think this is the way to go. Hoping I can get a decent 20 gal/hr or so out of it which will really help with paying for the gas. The RO system helps with that a lot too.

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Originally Posted by maple flats

Mine also, 24" sides with 10" raised flues (so I have 14" above flues) and a little still lands in the gutters around the hood and goes to drain

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Thank you Maple Flats, yup the new raised flu hybrid pan I ordered will have high sides to make it safe from splash up above the flu's. Can't wait to make syrup from a nice efficient pan this year. I may have to put a lot more taps in this season than last :)

good idea! You will like it.