Evaporator Sizing Math

I had a member ask me recently why he shouldn't just buy a big evaporator, even though he was planning to start with only a few trees. He doesn't want to spend his whole life tending a fire, and everyone else is telling him to plan for growth after he's bitten by the maple bug. I thought I'd share my thoughts with all members.

The original question was - why not just get an evaporator that can boil off 30 gals / hr? Then I only need to boil an hour every day or two?
The problem arises when you calculate the minimum amount of sap needed for a boil. At 30 gals / hr, that would be 30 sq ft of surface area, or 7.5 gallons of syrup at 3/8" deep. To make 7.5 gallons of syrup, you need to start with 300 gallons of sap. With 30 trees, that could be 2 full weeks sap, which would have likely spoiled before it was boiled. And you can't start with 150 gals and boil it half way then wait for more. That 50% concentrate is a perfect environment for bacterial growth. So you need a pan small enough that you can boil it to syrup a minimum of once a week.

Below is the logic I use when asked about flat pan sizing. Hope it helps someone.

First some round numbers to work with.

• A typical sugar bush (on buckets) will average about a gallon a tree per running day (maximum of 2 gallons/tree), with 3-5 running days a week. I use 5 gallons a tree a week as a maximum number. Those on tubing and vacuum might have to double those numbers.
• Each tree tapped will produce an average of 10 gallons sap a season (1 qt or litre of syrup)
• A flat pan evaporator boils off ~1 gal / sq/ft / hr with normal firing and no preheat. Add a blower and preheat and maybe 1.5 gal/hr.
• 3/8" deep is as shallow as you can effectively boil.(That's ticking over at the very end - not all the time!!!)
• 1 quart of sap or syrup ~=3/8" on 1sq ft.
• Sap to syrup ratio is 40:1

You can tweak the numbers above if needed, but they give us a baseline to work from.

Everything below is based on a flat pan and batch boiling. I've done similar math for a continuous flow process in the past. Follow the How Much to Sweeten link.

Sap needed for a batch boil - Rule of thumb - pan size in sq feet x 10 gallons.
Sap needed = sap:syrup ratio x pan size x 1/4 gals/ sq ft
For a 2x3 pan = 40 x 2x3 /4 = 60 gallons of sap.
i.e. if you start with 60 gallons of sap, you will have 3/8" left in the pan when it gets to syrup. Any less sap to start and you can't boil it down to syrup without going under 3/8" deep, or pouring it off into a smaller vessel.

Hours per boil - Rule of Thumb - Gallons of sap / sq ft of pan
For a 2x3 pan, and a 60 gallons of sap, Hours = 60 /(2x3) = 10 hours boiling.

Suitable Pan Size - Rule of thumb - 1 sq /ft for every 5 trees.
5 trees x 5 gals/ week = 25 gals/wk.
25 gals = 25 hours boiling @ 1 gal /hr/sq ft. (25 hours ~= 3@8hours)
For 30 trees, a 2x3 pan is perfect.
30 trees x 5 gal/tree / 6sq ft = 25 hours boiling average

Of course, not every week is average, so there will be lighter weeks and heavier weeks. I ran a 2x3 pan with 100 trees for years - but there were many late nights and the occasional time when sap was dumped.

Is it 1 gallon per tree assuming 1 tap per tree, or is it 1 gallon per tap? So, 2 taps on one tree is 1/2 gallon each or 1 gallon each (on average)?

One gallon per tap on gravity per day is the higher end of production except for great runs and all day/night runs.

Quote:

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Originally Posted by Urban Sugarmaker

Is it 1 gallon per tree assuming 1 tap per tree, or is it 1 gallon per tap? So, 2 taps on one tree is 1/2 gallon each or 1 gallon each (on average)?

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I have more trees than I can tap so I only ever put one tap per tree regardless of size. If your trees are large enough to support 2 taps conservatively then count that as 2 trees.

Some trees may give 2 or more gallons on a great day but not all will. An average of 1 gal per tap is a good day. 5 gals per tap over a week is a very good week.

Thanks. I thought I read some time ago that each tap produces up to a gallon per run on average. I've also read that 10 gallons per tap per season is a good rule of thumb. I have some good trees with large crowns that I'm hoping will produce well if the weather cooperates this spring. I'll be keeping track, so I look forward to continuing this discussion.

In the other thread, I wanted food for thought as I think about what we can do easily in our first year wearing training wheels. This thread that Eddy started is some real meaty info. Although it goes hand in hand with what I was looking for, it was great to put into it's own thread. I'm anxious to see how this progresses and I hope it gets a lot of input from members. Thanks Eddy
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Big Eddy and everyone else,

I'm about 5 years late to the party, but thanks for all this amazing information. I have been poring over all of your posts and just soaking it all in. Can I ask you some more about pan size and GPH rates?

I tapped for the first time last year, 5 trees in my yard, 4 of which poured out something like 110 gallons of sap and produced just shy of 4 gallons of syrup. It was pretty magical. I cooked on a cinder block arch with three steamer pans, and by my 4th and final week boiling, had gotten close to 6 GPH. The most sap I collected and boiled in any given week/batch was 50 gallons.

This year, I am planning on buying a flat pan with handles and a draw-off valve, mostly because I never want to watch anyone lift a boiling pan of "oh my God it's almost syrup" off of the fire with oven mitts again. Especially not with all of our young children watching us. *shudder* Since 3 steamer pans is roughly 2'x3' when you put them all together, I figured that a 2x3 pan was the way to go. I only have my 5 trees right now, and I know that last year may have been an exceptionally good year as far as sap volume. However, after the excitement and success of last year, I have several friends and neighbors who have expressed an interested in tapping their trees and boiling off at my place, so I would imagine that there could be some weekends where we have far more than 50 gallons to get through.

Here is my worry. According to your calculations, having less than 60 gallons of sap makes the 2x3 pan sound too big. On the other hand, 6 GPH sounds slow if I end up needing to boil off more than 50 or 60 gallons. (If I were still living the single life, I'd have no problem sittin' by the fire for 10-12 hours...but now that I've got little mouths to feed and bedtimes to worry about...well, different story.
So, in your seasoned opinion, what's a gal to do?!?

Thanks, everyone! Hope there's someone around still reading this thread! :)

Amy

Amy.
If you have neighbors and friends with an interest, get them involved in the operation, or you will be spending lots of hours using a 2x3 pan to boil down with. I started with a 2x3 flat pan and 30 taps. Then bought a barrel evaporator and divided pan 2x3 and went to 50 taps. One year we even had 72 taps, that was to much. I also had three older boys who didn't mind staying up all night tending the evaporator. Make sure you have lots of wood and keep it mahageable for you to handle. Grow as you feel comfortable. By the way, once the maple bug bites, your infected.

Amy,
As busy as you are with kids, this boiling thing may get old in a hurry. especially if you get more sap from neighbors. So if you have made the dissision to go with the new flat 2 x 3 and draw off port do it! Nice upgrade from your old set up and safer too!
How many taps do you have on the 5 trees? With 110 gallons of sap and 4 gallons of syrup it says the sugar content is around 4%. Which is great! normally sap on good trees is around 2% average. And one tap should produce an quart of syrup on average on gravity system, if you stay with it all season.
Welcome to the trader!
Regards,
Chris

A 2x3 is not going to do much better than 6 gph. You can add a blower and push it up a bit but you won't be able to double it. Ever.

With <10 taps you are fine. But if the whole neighbourhood starts bringing sap you will be swamped. If that is the case though, set the rules up front. You bring sap, you better also bring wood, and be prepared to stay a while.

If you have a draw off valve on the pan then the minimum volumes are less important. You can always boil it 90% down then draw off and either finish on propane or freeze / refrigerate and then combine with the next batch to finish.

I tend to lean towards smaller pans and frequent / longer boils, but if you have cold storage available for sap weekend only boiling on a bigger pan will work too.

(In our sugar house 8-10 hours would be a short day)


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I boil on a D&G 18x48 which is a 2x3 and I get about 8-10gph with preheat and a blower. I should probably do less then the 60 taps I do but I cant. I'd do more if time premitted. And ya I have some long boils and get swamped pretty quick. Lucky to have retirement friends boil while I'm at work.