Natural Gas Stove Top Evaporator

[HowardR]

I tried out my home-made gas-stove-top evaporator today. I pour the sap into a 15 gallon pot with a spout at the bottom controlled by a float in a float pan. The float keeps the sap about 2" inches high in the float pan and in the next three connected pans, each on its own gas stove. I had near-syrup trickling out the end of the last pan into a stainless steel bucket. I can work inside on my computer and just go out and fill up the 15 gallon pot about once every hour or hour and a half.

Here is a picture of the evaporator as a whole:

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Here is the sap entering the evaporator through the float pot which uses a stainless steel float valve that I bought on Ebay:

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Here's the 1/2 inch stainless steel pipe, valve and union as the sap travels between a pair of pans:

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Here is the near-syrup exiting the evaporator:

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My first estimate is that it boils off about 7 gallons of water if the sap entering the pot is already hot.

[maple flats]

Just be careful for foaming. You may not want to let it go unattended that long. As the sap is boiled down the likely hood of it foaming up increases big time. If it starts to foam up, it could quickly ruin a pan and the sap that is left in it. An hour is a long time unless you set up a automatic defoamer, otherwise it's a sure recipe for disaster.

[HowardR]

Dave,

Thanks for the tip. That is certainly something to worry about.

But I may already have that under control. I have a thermometer in the last pan and increase or decrease the trickle rate out, to keep the sap from getting too hot. (I'm not trying to get it all the way to syrup in the evaporator.) Also, I suspect that there is less danger of foaming in a stove-top evaporator, given that the heat under the pans is lower.

[Haynes Forest Products]

Neat set up I take it you have good gas pressure at the house? I didn't see the WiFi camera....................you do have one right.

[maple flats]

Foaming can start well under the 219-220 target temperature.

[HowardR]

Neat set up I take it you have good gas pressure at the house? I didn't see the WiFi camera....................you do have one right.

Yes. The pressure at the gas regulator that feeds my house and garage is set at 7 oz, which is a bit higher than normal. The typical gas stove requires 4 oz. of inlet pressure to operate with full-size flames. My evaporator would work at lower pressures, but it wouldn't boil as much sap away per hour.

I don't have a wifi camera. Never thought of using one. My most important hi-tech app is the countdown timer on my smart phone. My dog starts heading outside just before me as soon as it goes off.

[HowardR]

Foaming can start well under the 219-220 target temperature.

I'm aiming for just 2-4 degrees above boiling in the evaporator. Then after I've collected a day's worth of sap, I finish it off on the kitchen stove. I've got a batch boiling in my kitchen right now.

[HowardR]

Today, I showed my neighbor the sap that was boiling in each of the pans. When I had initially explained my plan to him last summer, he had supposed that since the three pans were connected by open pipes, the sugar density in all three pans would be the same. I explained to him that the sap would always be running in one direction -- from the float pan, to the first pan, to the second, to the third.

Today, I took a clear pitcher and dipped it into the three pans in turn. In the first the sap was perfectly clear. In the second it was golden. In the third it looked just like syrup. Seeing is believing!

[jdircksen]

I'm glad to see it worked for you. I've been thinking of getting a natural gas cooktop and putting a 2x2 pan on it. Maybe the boss will let me put a full size stove out there so I can warm up bottles in the oven.

[HowardR]

I'm glad to see it worked for you. I've been thinking of getting a natural gas cooktop and putting a 2x2 pan on it. Maybe the boss will let me put a full size stove out there so I can warm up bottles in the oven.

If the "boss" lets you, you can get can get a used stove cheap! People discard them because the oven igniters stop working. If you call your local appliance store, you might be able to buy a traded-in stove from them. Look for stoves with some big burners on top. A five burner stove (extra burner in the middle) is perfect.

[HowardR]

I just finished my first season with the stovetop evaporator and it worked great. My only discontent was with the 1/2 inch stainless steel float valve that I bought cheap on Ebay. I may, eventually replace it with a more expensive float valve, perhaps one designed for 3/4 inch pipe, because there was variation of about an inch in the level of the boiling sap in the connected pots. So if you use a cheap float valve, don't try for a boiling level below 2 inches.

I recommend this system for people who like to just check how things are boiling every 90 minutes or so instead of having to be with the evaporator constantly while it is boiling. I'd check the RO and the evaporator at the same time and I'd adjust the concentrate flowing from the RO so that the two would keep pace with each other. I'd also adjust the outflow of the system based upon the temperature of the boiling sap in the last pot and I'd occasionally have to tap the float on the float valve when the level of the sap was getting low.

I'm building a second RO for next year, identical to my first. Each of my RO's will be able to put out 3.5 gallons of concentrate per hour while getting rid of about three times as much water. I'll try feeding the concentrate directly from the ROs to the feeder pot. Even though my evaporator is slow, the combination of my two ROs and my evaporator should be able to process 30 gallons of sap per hour during a big run. And I can continue to process sap day and night when necessary, just checking every 90 minutes or so to make sure everything is proceeding without problem. My other advantages are that I can easily scale everything down for a small batch and also that I don't have to have as much storage capacity because I can process sap as it flows in.

So for about $3,000 of investment ($1,000 each for my evaporator and two ROs), I'll be able to process about 30 gallons of sap per hour!

[jdircksen]

Can you describe your gas supply to the 4 stoves? I am going to try a cooktop connected to a 3/8" hose. I'll have 2 steam table pans and maybe a 5th burner as a pre-heater. I'll make a stand and drop in one like this...

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[HowardR]

All of my gas pipes are iron screw in pipes. My gas supply gets reduced to a 3/4 inch pipe when it enters the garage. (I'm planning to change that some year and keep it up at 1" in the garage.) I tap into the 3/4 inch pipes separately for each stove, usually reducing in order to connect a flexible metal gas stove feeder line (the ones that you use to connect a gas stove in the kitchen). None of my pipes is as narrow as a 3/8" hose.

[HowardR]

This year I replaced the cheap Ebay float valve that I used last year with a Blichman AutoSparge (https://www.blichmannengineering.com/autosparge.html) purchased from a beer supply store. The Blichman float valve is the same 1/2" pipe size and it is definitely a better valve, but it didn't solve the problem. I still have to tap the float sometimes to get the sap to flow faster through the valve.

My problems don't occur when I am running the output of my ROs right into the feeder pot. Whenever I am doing that, the level in the feeder pot doesn't change much and the float valve works perfectly. My problems occur when I am filling the stove top evaporator without the ROs. At such times, I have to fill the feeder pot every 80 minutes or so to keep it from running out of sap. As a result the feeder pot goes from nearly empty to nearly full and gradually back to nearly empty. As a result, I often have to tap the float to get the feeder valve flowing faster right after I fill the feeder pot.

All of this means is that the level of the sap in the stove-top evaporator can vary by as much as 3/4". So when you plan your system, don't aim for a 1" height in your pans. That could be cutting it too close. Aim for a 2" height.

[HowardR]

Maple Flats wrote last year:


Just be careful for foaming. You may not want to let it go unattended that long. As the sap is boiled down the likely hood of it foaming up increases big time. If it starts to foam up, it could quickly ruin a pan and the sap that is left in it. An hour is a long time unless you set up a automatic defoamer, otherwise it's a sure recipe for disaster.

Now that I have has almost 2 seasons experience with the Stove Top Evaporator, I can definitely say that there is no danger of foaming below 218 degrees. I even let the temperature get up to 219 in the last pot one time by mistake, and yet there was no foaming.

Foaming is caused by a low sap-volume-to-heat-source ratio. You can prove this by finishing up a batch of syrup on your kitchen stove. When it starts to foam at about 217 or 218 degrees, turn down the flame and it will stop foaming while you gradually heat it up to 219. My Stove Top Evaporator has a much higher sap-volume-to-heat-source-ratio than a standard evaporator.

[DRoseum]

I built a natural gas evaporator and RO this year and it worked great. Details are here:

https://www.sugartree.run/2019/03/custom-hobby-maple-syrup-evaporator.html

[jdircksen]

Thank you for sharing that. I’m hoping to build something similar this summer.

[DRoseum]

No problem. Let me know if you have any questions about items I used. The steam table pans work great, and I use them to draw off into, to preheat sap above main pan, and to finish boil over one of the U-burners. Had 35 taps this year and made 6.25 gallons so far.

[Cedar Eater]

This really makes me wish that I had natural gas to my house. Propane destroys the economics of doing this. Good job!

[HowardR]

D. Roseum,

Your evaporator looks beautiful and professional! How much did it cost you in parts to build it?

[jdircksen]

DRoseum - I'm looking at your burners trying to see if I can apply that to my setup.
Was it difficult to set up those Dante NG air/gas mixers? I found documentation that their mixer is rated for an input of 25,000 Btu. is that all you're getting out of your 3 burners? I was hoping for double that per burner.
Do you think I could make my own burner using pipe, a T, 2 elbows, and 2 caps? Not sure how I'd know how many and what diameter holes to drill in the pipe.

[DRoseum]

Setting those up was easy. NPT fittings and adjusting the air shutter is very simple. As for building your own, you definitely could. I was very close to doing exactly that, but for cost of materials, time drilling out all the holes, etc, I decided to go with the the stainless U-burners from Hearth Product Controls. They were about $37/burner for the 18" ones. Spotix online distributor sells them in a good variety of sizes and you can get an LP orifice if you prefer that. Hard to beat that price for stainless.

As for BTUs... the burners are rated for 90k BTU and the gas line I ran and manifold I built supplies enough for the full 270k (all 3 burners), but i am not sure how much that is getting throttled by the orifice in the air mixer. I saw the same documentation you did but went with them anyhow. A venturi mixer runs very expensive (hundreds of $) compared to these.

What I will say is that you can get a VERY clean burning blue flame with them, and nothing close to it without them. Blue flame is going to be significantly hotter than the yellow or orange, and your fuel efficiency will be far better. Also, no soot being deposited on the bottom of the pan. I also wanted as many BTU as I could possibly get, and while these might slightly limit it (doubt it's down to 25k BTU based on how quick it comes to a boil), the tradeoffs were worth it.

You could always try to ream out a very small amount of the orifice inside the mixer to increase flow rate but you have to be careful because these work on pressure differential. Probably would want to run some calcs on that first.

[DRoseum]

Thank you. It was a very fun project to design and build it (yeah I am a bit of an engineering nerd). But it was even more fun using it and enjoying the syrup it helped produce!

I will total up list of materials and costs at the end of the season and post it. Been meaning to do that anyways.

[Daveg]

"Sap foaming" and "syrup foaming nearing the finishing point" seem to be two different things. At times, I have to put defoamer in the flue pan (212°F), but rarely in the finishing pan (217°F, 218°F, 219°F). You mention turning the heat down when it foams at 217°F. What happens if you don't turn the heat down? Does it foam even more? If you put less sap in The Stove Top Evaporator you can have a lower sap volume to heat source ratio. What guage metal are your pans? The thicker the metal, the poorer the heat transfer. 20 and 22 guage are the most common in the industry.