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Rocket Evaporator?

                          


Joined: Nov 20, 2010
Posts: 140
I don't plan on making maple syrup in the near future, but I've been thinking about how one would go about making a rocket evaporator.  Any thoughts?
josh brill


Joined: Sep 06, 2010
Posts: 86
    
    1
Right now we share a sugar shack with two other families but we are planning on building another on our land in the next two years.  I was trying to figure out the most efficient way of building it.  I was thinking of using one of two styles.  Using a biochar producing retort that used the gas to heat the evaporator.  The other is a rocket style heater.  It would be great to hear other peoples take on building one.


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Joined: Nov 20, 2010
Posts: 140
Well, unlike a RMH you will want to harvest all the heat you can off of the heat chamber.  I wonder if you could modify the shape of the top to maximize the area of the evaporator pan?  I also wonder if a small mass area could be made as sort of a pre heater to warm the sap before you put it in the evaporator?  I do have an idea for the pan but it is hard to explain.  Maybe I'll build a model with pop cans and post pics to help explain it.
Joel Hollingsworth
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Joined: Jul 01, 2009
Posts: 2103
Location: Oakland, CA
I wonder if you could get the stove to draw its air from around (or even through) a warm bath of sorghum juice. The rapid flow of air would mean you wouldn't have to heat the maple water to boiling in order to evaporate it, and the pre-heated air might help the fire along a little (although being so moist might not be the best). If you overdid it, though, I could see the sugar spray suddenly lighting on fire...best to have a good metal screen and a good distance between the syrup and the air intake.

Here's a schematic of a low-temperature system: You can see a heat exchanger pipe that draws heat from the chimney, sends hot water, by convection, up to the tub of cane juice, and then flows into a reservoir which doubles as a steam release valve, should things ever start to boil.

A door seals off the air intake to the rocket system after it has really gotten going; a tube in the door draws on the tub of maple water, pulling bubbles up through the snorkel tube. These bubbles are where the evaporation takes place.

This is only a schematic: there have to be a hundred more-elegant ways to arrange it all. It's especially important that the intake air have good contact with the maple water, but I don't think an airstone would be appropriate at all. You'd probably want to minimize the surface area covered in syrup, as well, which would mean a heat exchanger in the walls or floor of the tub, rather than in the sap itself. Finally, the draw of the stove is going to be high-volume, but low pressure, so it might make sense to have the bubbler float on the surface.


[Thumbnail for rocket dryer.PNG]



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Joined: May 26, 2010
Posts: 278
Location: Iowa, border of regions 5 and 6
I don't thing you want a rocket mass heater to do this.  I think you just want a rocket stove to provide the heat through efficient combustion.  If you want to try a proof-of-concept, go back through the posts and find the link to the person who built a rocket stove using concrete blocks.  Cheap, easy, and would be good enough for one season.  His setup got to be over 300C (560F), which would be a tad more than you might need to boil syrup.

So rather than putting the pans right on top of the rocket stove, where you risk not only burning the syrup but burning a hole through the pan, try building two or three of these concrete block stoves.  Then get a hunk of flat scrap steel and place it on the top of these stoves, with spacers (stones) so it's not blocking the chimneys.  Use THAT for your cooking surface, where you put the boil pans.  The steel gets hot but not hot enough to melt, and it transfers the heat to the pans, spreading the heat out so it doesn't burn the pans.
Joel Hollingsworth
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Joined: Jul 01, 2009
Posts: 2103
Location: Oakland, CA
I still think it makes sense to exploit all the air that a rocket stove can move, in addition to the heat it can generate. Plus it's much safer to work with syrup at a relatively low temperature.

But the ugliness of the last design I floated still bothers me, so I came up with this, which still has the same energy flows, but is a lot simpler: a wider, shorter stovepipe is placed around the tall chimney, and a nozzle is plumbed to blow the air heated by this counter-flow setup onto a container of sap. The sap is kept in a box with a gasket on the lid, and the burner draws air from the bottom of this box. The fuel feed also has a lid with a gasket, but I think it makes a lot of sense to separate this from the air feed tube. Some gentle heat will work its way from the fire to this sealed box.

Eliminating water as a heat-exchange medium means that the air from the outer section of tube might heat up to such a high temperature that it ignites the sugar, but I think that risk might not be as bad as I had initially imagined. I think it's definitely worth the simplicity of the new design.


[Thumbnail for rocket dryer.PNG]

                          


Joined: Nov 20, 2010
Posts: 140
Muzhik, That's what I'm proposing, for the most part, a rocket stove.  However, if it had a small mass area, say 4 foot square to utilize heat that would go to waste anyway you could have a pre heater that would warm a pan of sap to 100 degrees or so it would reduce heat loss when adding to the evaporator pan.  I wouldn't worry so much about burning the syrup as you can always add fresh sap if it gets close to burning.  You could also have something to cover the fuel feed tube with if it gets too hot, that would cool it down in a hurry.  I would probably boil off until it was about 80% done and finish up on a more conventional heat source. 

Joel, how do you make and post them little stick drawings?  I could maybe make a pic of what I'm thinking.
Joel Hollingsworth
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Joined: Jul 01, 2009
Posts: 2103
Location: Oakland, CA
I use MS Paint, and save as PNG. If SVG files were acceptable, I might prefer to use Inkscape for some of the drawings.

After you have the file, there's a little triangle under the text box, and a link that reads "Additional Options..." which allows you to upload files as part of your forum post.
Erica Wisner
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Joined: Feb 10, 2009
Posts: 720
Location: Okanogan Highlands, Washington
    
  85
How about some old-school ASCII art?

I haven't done syrup, so please forgive if I don't understand the finer points.

This is a very simple cooker such as I might make for an outdoor kitchen, with bricks and a barrel.  Some insulation would be needed around the bricks.  Exhaust could be contained for a few feet in a rectangular duct that is framed in to support a warming-pot for additional sap.  Depending on the size of the system's interior, and the type of fuel wood, it could be made to just barely simmer the pot on top, or to boil it.

(I'd say 6"x7" could simmer or boil a canning kettle, and 7"x8" or larger could boil a large pot of water or soup, using medium-grade fuels like Doug fir and fruitwoods.  Does syrup takes a lot more heat, and boil a lot hotter, than water? If so, you might need to increase the scale accordingly.)

****************************************
**********| 88 Hot 88 |*******************
**********| 8 Syrup 8 |*******************
**********| 88 goes 8 |*******************
**********| 88 here 8 |*******************
**********\=======/*******************
********|-------------------|*****************
********| ************* |****************
********| ***/]****[\*** |****************
********| ***[]****[]*** |****************
********| ***[]****[]*** |****************
********| ***[]****[]*** |****************
********| ***[]****[]*** |****************
********| ***[]****[]*** |****************
********| ***[]****[]*** |****************
********| ***[]****[]*** |****************
********| ***[]****[]*** |****************
********| ***[]****[]*** |****************
********| ***[]*/\*[]*** |****************
*|Warm|[] ***[]*/\*[][][][][][][][][]****[]***
*|*pot* |[]***[]*/\************* Wood*[]**
*-\===/[]***[]* Fire <<<< ****** fuel*[]**
* exhaust ***[]******************here*[]**
****[][][][][][][][][][][][][][][][][][][][][][]***
***************************************

This is based on a rocket mass heater's J-tube combustion unit, with the kettles replacing the mass.  Burns clean, self-feeds to some degree so you can watch the pot instead of the firewood (or go get more sap, firewood, etc. without stopping the operation).

You could also replace the barrel with a simple piece of sheet metal, as someone suggested, but that leaves the exhaust kinda in your face and not as useful for pre-warming things.

I wouldn't want to count on using the rocket's draft to pull air through anything wet; it could compromise the control you'd have over the temperature of the boil itself. 
A separate chimney or fan could help with creating extra evaporation if needed.  Or just a weirdly-designed pot lid that has a small stack on it but drips outside the pot.  (Kinda like the Tin Man's hat, cocked off to one side on coat-hanger legs?)

Clay brick is about as easy to find as cement block, and lasts much longer as a firebox.

You may also want to modify the firebox to hold bigger batches of wood, like marina has done on her floor-heater:
marina phillips wrote:
...
I'm kinda surprised no one's really asked about the firebox modification.  That's the thing we changed that we weren't planning on.  We were doing vertical feeding at first, but a lot of our wood just isn't perfectly straight, and sometimes the fire would climb up the wood if it weren't touching the bottom of the box, and we weren't able fix it when it happened because we were sitting inside....We needed to do it differently. 

The fire mostly draws from a hole in the front of an elongated fire brick box (around which we will also add more cob for stability), its dimensions are 22"L x 10"W x 9".  You can lay odd shaped pieces of wood in there (this is handy - we have a lot of curvy twisted oak), in a neat but jumbled pile, and the fire burns down.  We place two metal plates on top of the box to limit the draw there.  I still start the fire with vertical kindling up against the burn tunnel entrance, then lay it over when it gets going and add the rest of the wood. 

I would only recommend this for outside feed systems.  Especially when things have cooled off for a day or more in between firings, it can be smokey when you're starting a fire or filling it back up with wood - until the metal plates are replaced and the fire shoots sideways again.  It doesn't smoke much at all after the third stoking in a row. 
...


-Erica Wisner


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www.ErnieAndErica.info
                          


Joined: Nov 20, 2010
Posts: 140
Joel Hollingsworth wrote:
I use MS Paint, and save as PNG. If SVG files were acceptable, I might prefer to use Inkscape for some of the drawings.

After you have the file, there's a little triangle under the text box, and a link that reads "Additional Options..." which allows you to upload files as part of your forum post.

Thanks Joel.  This would be a cross section.  A barrel laid crossways above the heat rocket, with a hole the size of the rocket, and welded to it.  A pan welded to the inside.


[Thumbnail for evaporator.png]

                          


Joined: Nov 20, 2010
Posts: 140
Erica, with an evaporator you want to increase the surface area to facilitate evaporation.  This system would give you a much larger surface area.
Erica Wisner
volunteer

Joined: Feb 10, 2009
Posts: 720
Location: Okanogan Highlands, Washington
    
  85
Tinknal wrote:
Erica, with an evaporator you want to increase the surface area to facilitate evaporation.  This system would give you a much larger surface area.


I've rendered down stocks and jams before, but I gather maple syrup is a longer, larger-volume process.  Do you have to boil it, or keep it from boiling?

Open warming pans, troughs, bubblers, big flat saucepans...
I'll let you folks who know what you're looking for design the details.

The one element I'd venture to make a recommendation on:
Lorena, the Good Stove, wood-fired cookstoves, and cooking rocket stoves have in common that they put the pot right over a fast, hot exhaust stream (or in the flame path itself).  Sometimes atop a conductive surface, and sometimes directly.  I think you'll want this kind of heating arrangement to get things up to temperature quickly on a reasonable amount of fuel.
josh brill


Joined: Sep 06, 2010
Posts: 86
    
    1
We haven't decided how large are evaporator will be be but the minimal would be heating 24"x36"x2.  For those folks not similar with what they look like on a larger scale then the old pot on a fire here is a picture.  
The syrup gets heated to almost boiling in a preheating section after it get hot it goes into the syrup pan. The syrup starts at one corner of the syrup pan and flows up and down the tracks till it reaches the other end.  Thats the basic function once you move from the basic single pot technique. 
Joel Hollingsworth
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Joined: Jul 01, 2009
Posts: 2103
Location: Oakland, CA
Erica Wisner wrote:Do you have to boil it, or keep it from boiling?


Those are good questions. I've been working from the assumption that the answer to both, is "no."

Industrial evaporators don't tend to rely on boiling, because it tends to use more energy, and to bring with it some complications. Spray driers are very popular. The ideal case would be to push evaporation from three different directions: gentle warming, reduced pressure, and a flow of gas over the liquid.

In undergrad, I carried out a chemical process where one step called for hours of hot evaporation, to drive off water. I introduced a jet of dry gas to the process, to help carry the water vapor away from the surface, and saved myself a significant amount of time on each batch.
Erica Wisner
volunteer

Joined: Feb 10, 2009
Posts: 720
Location: Okanogan Highlands, Washington
    
  85
Would it be easy to DIY a spray-drier for maple syrup?  I know they can work on very sticky stuff, but I don't know what volume you'd need to make it worth investing.

Thanks for the picture, jbreezy. 
So you are looking at ways to keep a large surface area warm, as well as the pre-heating pot?

Could build a small mass-heater, set the evaporator on semi-permanent supports on the mass itself, and have the pre-heater on the top of the rocket stove barrel.

As for the air movement, it doesn't have to be warm air, it will warm enough from surface contact to carry away moisture.  Using the exhaust itself, or hot chimneys, seems impractical and potentially unhealthy.  If you are building a sugar-shack, then by all means, install a solar-heated clerestory draft tower.  But if this is a back-of-the-truck operation, I'd look at earlier and simpler technologies that are easier to transport and repair.

Low-power technologies include a fan (punkah or pole-lathe style, if you like), or a bellows.  A double-action bellows like big blacksmiths use gives a steady stream of air on simple mechanical motion. 

You could even Rube-goldberg it into the process (like, when people bring sap buckets they hang them on one arm of the bellows, and the person tipping them into the pre-heater hangs the empties on the other arm... or make it part of a see-saw for little cousins to use while the big people are handling the heavy work...)
tel jetson
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Joined: May 17, 2007
Posts: 3080
Location: woodland, washington
    
  52
what about something like the rocket bathtub in the RMH book?  that's heated by the stove exhausting along it's bottom side, yeah?  maybe use the top of the barrel for the pre-heater jbreezy mentioned, then use that same serpentine path heated by the stove's exhaust.


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Joined: Nov 20, 2010
Posts: 140
Here is a side view of my previous illustration. 


[Thumbnail for Evaporator side view.jpg]

Erica Wisner
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Joined: Feb 10, 2009
Posts: 720
Location: Okanogan Highlands, Washington
    
  85
tel jetson wrote:
what about something like the rocket bathtub in the RMH book?  that's heated by the stove exhausting along it's bottom side, yeah?  maybe use the top of the barrel for the pre-heater jbreezy mentioned, then use that same serpentine path heated by the stove's exhaust.


yes.  I like it. 
Easier to build the pan atop horizontal exhaust ducting, or with the bottom of the pan exposed to exhaust, and use a barrel to support a simple pot on top of the very hot heat riser.

Putting a wide pan atop a narrow chimney just looks tippy to me, and hard to build around smoke containment and feeding the fire.
                          


Joined: Nov 20, 2010
Posts: 140
Erica, I don't think it would be tippy at all as long as it was supported properly.  I think a mass area at a lower elevation would work for sugaring off.  No hurry and the lower temps would insure that it wouldn't burn.  You could install a gravity drain to a lower pan.
Erica Wisner
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Joined: Feb 10, 2009
Posts: 720
Location: Okanogan Highlands, Washington
    
  85
Tinknal wrote:
Erica, I don't think it would be tippy at all as long as it was supported properly.  I think a mass area at a lower elevation would work for sugaring off.  No hurry and the lower temps would insure that it wouldn't burn.  You could install a gravity drain to a lower pan.


You could build it, sure, and support it.
I'm just elegance-minded, or maybe lazy: why shore up something complicated, if something simpler will work just as well.

I really like putting the wide pan/evaporator tray lower down: easier to handle, easier to keep at the right temperature, easier to watch your syrup flow for QA, and gravity-fed both in and out.

The higher pan would be harder to monitor, and hotter, so it would pay to make it a deeper pot = less likely to burn.  If it does try to burn, you can put a layer of cob (or a copper tray) between barrel and pot, to even out the heat.

A pot up to 24" wide could fit easily onto a standard barrel, saving tons of time in fabricating.
                          


Joined: Nov 20, 2010
Posts: 140
Erica, the highest quality sap gives you about a 40 to 1 return.  That means that you have to evaporate 39 gallons of water to get 1 gallon of syrup.  having a large surface area to evaporate is absolutely essential.  Some saps, in some years can be closer to 80 to 1.
Erica Wisner
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Joined: Feb 10, 2009
Posts: 720
Location: Okanogan Highlands, Washington
    
  85
Tinknal wrote:
Erica, the highest quality sap gives you about a 40 to 1 return.  That means that you have to evaporate 39 gallons of water to get 1 gallon of syrup.  having a large surface area to evaporate is absolutely essential.   Some saps, in some years can be closer to 80 to 1.

No wonder the stuff is spendy.

What about something along the lines of a stainless washtub, flaring wider at the top?  Or a giant wok?  People have been cutting off the top of water tanks to use the wok-shaped bottom as their heating surface for rockets, maybe a larger tank would offer a nice pre-fab surface?

You can make the lower evaporating pans as wide as you want, easily.  And run the sap/syrup along a heated path for 20+ feet, = 80+ feet with baffles.  You could even set two sets of sloped pans to gravity-feed so the syrup comes back toward you at the end, and stay by the fire.

I'm just tossing ideas out in the dark, here, we don't have the trees or the climate for syrup-making in our region.  I've enjoyed seeing the pictures of working equipment that people have posted, tho.

Maybe I'll stop speculating, and look forward to seeing what the do-ers come up with.
tel jetson
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Joined: May 17, 2007
Posts: 3080
Location: woodland, washington
    
  52
Erica Wisner wrote:
I'm just tossing ideas out in the dark, here, we don't have the trees or the climate for syrup-making in our region.


don't be so sure.  there have been some folks on Vancouver Island using big leaf maples for syrup production with good results.  I believe the sap flows when evenings are below freezing and days are above.  that's a substantial portion of our winters most years.
Erica Wisner
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Joined: Feb 10, 2009
Posts: 720
Location: Okanogan Highlands, Washington
    
  85
tel jetson wrote:
don't be so sure.  there have been some folks on Vancouver Island using big leaf maples for syrup production with good results.  I believe the sap flows when evenings are below freezing and days are above.  that's a substantial portion of our winters most years.


we had a friend in the Coquille area who was watching the temperatures for syrup-making, there were about 2 days that winter that would have been good sap flows, and he missed one of them.  Much milder in the rainforest.
Might be possible elsewhere in the region, but I don't think it will ever challenge the Canadian and New England traditions.
tel jetson
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Joined: May 17, 2007
Posts: 3080
Location: woodland, washington
    
  52
Erica Wisner wrote:
Might be possible elsewhere in the region, but I don't think it will ever challenge the Canadian and New England traditions.


I expect you're right.  but with weird winters in syrup country becoming more common, it wouldn't surprise me if a market for big leaf syrup developed.
                          


Joined: Nov 20, 2010
Posts: 140
The last time I made syrup I had 80 taps that gave me 50  to 200 gallons a day.  A 24 inch pot just wouldn't have the needed surface area.  I used a water heater tank, cut lengthwise on an iron stand surrounded with concrete blocks on 3 sides.  It worked fairly well but used a ton of fuel (which was ok because I had a ton of scrap lumber to burn) and needed constant tending.
Erica Wisner
volunteer

Joined: Feb 10, 2009
Posts: 720
Location: Okanogan Highlands, Washington
    
  85
Tinknal wrote:
The last time I made syrup I had 80 taps that gave me 50  to 200 gallons a day.  A 24 inch pot just wouldn't have the needed surface area.  I used a water heater tank, cut lengthwise on an iron stand surrounded with concrete blocks on 3 sides.  It worked fairly well but used a ton of fuel (which was ok because I had a ton of scrap lumber to burn) and needed constant tending.

Got it.  So maybe we are looking for ways to get the same amount of heat into your big trough, out of less wood, for future years?
                      


Joined: Nov 30, 2010
Posts: 53
here is my noob idea can be modified as in the "preheater mass" can continuse downcould hold the finishing pan up with a brick wall with many air gaps .. ie 10 1 - 2 inch per side to let the drum cool and cause suction . i am working on the assumption forgive me if i forgot but the 'closer to finished product' has to be hotter as the boiling point rises during concentration. i used the the exess flue heat to warm a "mass" of sap from inside storage putting ice cold sap would probally cause a back rush nd might want to vent the preheater incase there is any water vapor


[Thumbnail for finishing pan.jpg]

Micky Ewing


Joined: Dec 12, 2012
Posts: 55
Location: Merrickville, Ontario
    
    7
I wonder how people have been doing with this concept. It's been a few years since this conversation started and stalled: lots of time for folks to experiment and refine their ideas. Does anybody care to make a progress report?

I would like to try making some maple syrup next spring. I've never done it before, nor have I had any opportunity to experiment with rocket stoves or RMHs yet, but syrup making seems like the perfect application for rocket tech because:
  • It's very energy intensive, so high efficiency is a major benefit.
  • It's not about heating a structure, so code compliance isn't an issue (I think).
  • It's out in the woods, so wood is the most readily available fuel.

  • If I understand correctly, the two magic features of RMHs that make them so efficient are the heat riser, which yields near complete combustion, and the mass, which captures heat that would otherwise be lost through the exhaust gases. I think a rocket evaporator could be similarly efficient, with some kind of sap pre-warming stage replacing the mass for exhaust heat recovery. I'm only now getting out the sketch pad, so I don't have anything to share yet, but I hope some of you do! I'll try to put my own "artist's conception" drawing up within a couple of days.


    Mike
    http://tenderfootfarmer.ca
    Matthew Groves


    Joined: Apr 23, 2013
    Posts: 4
    We've been making syrup here in residential missouri for 5 years.

    I'm looking forward to digesting all of RMH principles to apply it to our sugaring, but what hasn't come up in this thread yet is this very necessary concept in making syrup:

    Evaporation rate is tied predominantly to surface area, not to heat.

    You wanna sit all day to make a pint of syrup? Turkey pot is your container.

    If you need to boil off 50gallons of water to make a gallon and a half of syrup? You need as large of a surface area as you can. I'll post pictures later.

    Gotta run.
     
     
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