Newby is back with boiling questions.

[wmick]

Hi Folks
I haven't been hanging out here much, for a while... Good to be back. .....an of course, I'm back with some questions for the seasoned maple junkies...
2 years ago I built an evaporator and got a very last minute run with it, (outside in the pouring rain.).. So I really wasn't in the mood to do any fine tuning...
Last year, I had spine surgery, and wasn't allowed to work in the bush, so I just boiled a few buckets worth (that my wife and kids tapped for me) on a propane stove....

I got back at it this year. Turned a used 10x16 garden-shed into a shack.. and finally got to do a half decent boil on the new (home-made evaporator) ....
24 x 72, with a 2x4 drop-flue and a 2x2 syrup pan. (both with dividers for continuous flow..).
Boiled down about 265 gallons of sap this weekend, in about 24 hours... and had no luck in getting a consistent flow or gradient...
It seemed to me, like every time I opened up the door to add wood, I would lose some boil, temperature in the syrup pan would drop, levels would change and syrup would migrate...
I did not actually get syrup from the outlet until about 20 hours in... 3 gallons of syrup... so I think my pans are too sweet at this point, but not sure??...

1. I feel like maybe I am feeding the fire too infrequently, and creating my own fluctuations.. Maybe every 15 minutes or so... (certainly not on a timer) How often do you folks feed the fire? Is this likely to be my problem?
2. Based on my loose description of my setup.... How long after starting a fresh boil, would you expect to be drawing syrup?
3. Can anyone direct me to some information on the relationship between liquid depth and boiling rate?? I know shallower is better.... but cant seem to find any scientific explanation why?

Just a FYI, my sketch shows the syrup pan lower than the flue pan... this is not actually the case.... I raised the syrup to the same level, with no float between the pans.

Thanks - to anyone that takes the time to read or respond to my long-windedness...

evap.jpgshack.jpgArch.jpg

[DRoseum]

Quick answer to you question about liquid depth and evaporation/boiling rate:

"It takes approximately 8.3 BTU to heat 1 gallon of water 1°F. The more sap you start with in your pan, the more BTUs will be needed to bring it to a boil and sustain it. However, the surface area and volumetric evaporation rate remain constant for a given surface area. Therefore the most fuel efficient approach will limit the number of BTUs required, hence boiling thiner layers of sap"

https://www.sugartree.run/p/sugaring-diy.html

[wmick]

Quick answer to you question about liquid depth and evaporation/boiling rate:

"It takes approximately 8.3 BTU to heat 1 gallon of water 1°F. The more sap you start with in your pan, the more BTUs will be needed to bring it to a boil and sustain it. However, the surface area and volumetric evaporation rate remain constant for a given surface area. Therefore the most fuel efficient approach will limit the number of BTUs required, hence boiling thiner layers of sap"

https://www.sugartree.run/p/sugaring-diy.html

Thanks DRoseum... I am a bit confused, yet... I do understand "8.3 BTU to heat 1 gallon of water 1°F". It's the "and sustain it" part that I'm not clear on. My question is specifically about throughput, after the system is boiling.
EG... Lets say pans are completely up to temperature and things are flowing.... All other factors being equal, would there be any difference in rate of evaporation (gph) between 1" of liquid in the pan vs 2" ?
....and if so, why?

[DRoseum]

Yes. At steady state, it's all about balance of heat in vs heat lost. The larger volume of sap (deeper in your pan) will have a greater heat loss from the sides than a thinner layer. This is because you have a far greater surface area around the outside of the pan exposed to ambient air temps. In your example of 1" vs. 2" you have double the surface area of heat loss thru the pan walls without gaining any evaporation surface area.

So you will need to be putting more heat in (e.g., BTUs) to sustain that boil. Therefore a less efficient evaporation rate per BTU.

Does that help?

[wmick]

Yep - That makes sense. Thanks.... and it did occur to me, but I just assumed that the extra inch of heat loss from the pan sides would be too insignificant to create a noticeable difference in throughput..

[BCPP]

1" around the perimeter of a 2x6 pan is about 1.6 sq ft so about a 10% increase on the 12 sq feet of pan area!

[Russell Lampron]

You are firing too infrequently. You should fire every 5 to 8 minutes depending the type of wood you are burning. With a natural draft you will probably get the best results with only 2 or 3 layers of wood in the firebox. I did with mine before I went with forced draft. Your wood should be split to about wrist size, no bigger. An evaporator that size should be boiling off about 25 to 30 gallons per hour and your only getting about 11 the way that you're doing it now.

[DRoseum]

Yeah and remember, in a continuous flow pan, you have the same amount of new sap coming in to offset what you are drawing off plus evaporating out and you have to heat that up too.

[bj's sugaring]

Fire quick and often, and make sure that the fire doesnt clog itself with those frequent firings. Keep your levels the same for now, until you get used to reacting to the faster evap rate.

[Trapper2]

Quick answer to you question about liquid depth and evaporation/boiling rate:

"It takes approximately 8.3 BTU to heat 1 gallon of water 1°F. The more sap you start with in your pan, the more BTUs will be needed to bring it to a boil and sustain it. However, the surface area and volumetric evaporation rate remain constant for a given surface area. Therefore the most fuel efficient approach will limit the number of BTUs required, hence boiling thiner layers of sap"

https://www.sugartree.run/p/sugaring-diy.html

I just read your complete article and found out I've been boiling incorrectly for 40 years. I usually start boiling at 4:00 pm on Fridays and continue straight thru until 10:00 am or so on Sundays. I don't do a draw off until then. I just keep on trickling sap in until that time. I'm using a 36X42 flat pan. I cant imagine syrup much lighter in color than this.
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