Cross flow vs Longitudinal Flow Pan?

[Marvel26]

I'm getting ready to fabricate my very first evaporator and pan set, which is exciting and scary at the same time! In order to get ready for this, I have been readily absorbing ideas from this site....there is a crazy amount of info on here. Madtrapper was nice enough to answer a few dimensional questions for me and gave me a few tips that will be incorporated in my design, thanks for that!

However, I have a sap-syrup flow question for whomever wants to give me their two cents. Is there a difference in GPH or an other aspect of boiling, such as temp control etc, if your channels have cross flow vs longitudinal flow with the direction of the heat? I intend to have a 2 pan setup - a 24X30 flue pan and a 24X24 syrup pan. The flue pan needs to be longitudinal as the flues must run with the direction of the hot air, I understand that. But for my syrup pan or if I can't afford the flues and I decide to make a regular 24X60 flat pan with dividers ....is there a difference?

Also, I have read that you need channels in your flue pan in order to have a gradient and to keep sap from flowing right through to the syrup pan. However, I have seen a few commercial flue pans that don't have dividers such as the smokey lake hybrid pan...I am confused as to why?

https://www.smokylakemaple.com/wp-content/uploads/2014/09/hybrid-2012-10-03.jpg

Thanks in advance for any input and for all the ideas you've given me already!

Rob

[Big_Eddy]

There is not going to be any discernible difference in evaporation rates based on the orientation of the dividers or even the presence or number of dividers. Evaporation rate is primarily affected by firing, surface area and depth. Every divider contributes towards establishing the gradient and reducing the size of each draw. The closer to the the front of the evaporator, the larger the effect. Adding a divider to the flue pan will make a difference, but not as much as the difference between a 2 or a 3 section syrup pan.

Syrup Pan
There are differing opinions on the subject of cross flow versus longitudinal flow in the syrup pan. The typical evaporator is significantly hotter in the middle than along the evaporator edges, and slightly hotter towards the back of the syrup pan than the front. Normally, we would want the highest evaporation rate at the lowest concentration, and vice versa, but if there is too much difference in evaporation, syrup can be made in the wrong compartment.

With cross flow dividers, each section covers both edges and the middle thereby eliminating any effects caused by the hot center and cooler edges. The back (first) section is slightly hotter with the most evaporation but is also the least concentrated, while the front (most concentrated) section is slightly cooler and thereby less "touchy". All consistent with an optimum design.

If the syrup pan dividers are longitudinal and the pan has 3 sections, the first syrup compartment is against the rail and cooler with less evaporation , the middle section is hottest and has the highest evaporation, and the last section is again cooler. There seem to be more issues with making syrup in the middle compartment than there are with 3 section cross flow pans. If the pan only has 2 compartments, then I don't think this is as much of a concern.

The other consideration is whether you want to reverse flow. I designed my evaporator with 3 cross flow sections in the syrup pan. I can reverse flow by draining, lifting and rotating 180 degrees. With 2 longitudinal sections, reversing flow can be as simple as opening and closing some valves and then taking off the other side.

Sap Pan
Sap pan heat distribution is different. In general the sap pan is hottest at the front and cools towards the back. There is still a cooler zone along each rail, but less difference than in the fire box. The sap concentration is never high enough to get "touchy", so orientation is less critical. With flues, the dividers are typically lengthwise to align with the flues. In a flat pan, either is fine, as long as the inlet, outlet and cutouts are designed so that all sap must take the longest path possible.


If I was making a 2'x5' flat pan, I'd probably just make it with 4 cross-dividers 12" apart. Totally reversible that way. A better design would probably be 3@8" syrup sections, and then 2 @18" sap sections at the back. For a flat / flue combination, I would go with a 24" syrup pan with 2 cross flow dividers, and a 36" flue pan with at least 1 lengthwise divider. I have 2 dividers in my 20"x42" flue pan, but I'm not sure one less would make much difference.

[Marvel26]

Thanks Big_Eddie. Great info!

[SeanD]

Syrup Pan
There are differing opinions on the subject of cross flow versus longitudinal flow in the syrup pan. The typical evaporator is significantly hotter in the middle than along the evaporator edges, and slightly hotter towards the back of the syrup pan than the front. Normally, we would want the highest evaporation rate at the lowest concentration, and vice versa, but if there is too much difference in evaporation, syrup can be made in the wrong compartment.

With cross flow dividers, each section covers both edges and the middle thereby eliminating any effects caused by the hot center and cooler edges. The back (first) section is slightly hotter with the most evaporation but is also the least concentrated, while the front (most concentrated) section is slightly cooler and thereby less "touchy". All consistent with an optimum design.

If the syrup pan dividers are longitudinal and the pan has 3 sections, the first syrup compartment is against the rail and cooler with less evaporation , the middle section is hottest and has the highest evaporation, and the last section is again cooler. There seem to be more issues with making syrup in the middle compartment than there are with 3 section cross flow pans. If the pan only has 2 compartments, then I don't think this is as much of a concern.

The other consideration is whether you want to reverse flow. I designed my evaporator with 3 cross flow sections in the syrup pan. I can reverse flow by draining, lifting and rotating 180 degrees. With 2 longitudinal sections, reversing flow can be as simple as opening and closing some valves and then taking off the other side.


I'm mulling over the same question. I currently have a front pan that is set up for reverse flow (longitudinal dividers) but I'm exploring a cross flow set up. The only thing I don't like is the turning of the pan. I know for most it's no big deal, but I always boil alone and I'm always squeezed for time. Could I get the best of both worlds by having a float on either side in back and a draw from either side up front? Then I could reverse if I want, but still have the benefits of a cross flow setup. Is that right?

The other thing I wondered about is the problem with making syrup in the middle channels. I currently run into it from time to time and I've learned to avoid it with more consistent draws and firings. I can avoid it more when I fire off to the side where I'm drawing from. If I fire it up in the middle, then the liquid is drawn to the hotter part of the pan where the evaporation is highest. It seems like this would happen with a cross flow too. Doesn't the hotter part of the pan near the middle and back pull liquid from the cooler part near the door?

I welcome input.

Thanks,
Sean

[Schiefe4]

Smoky Lake Maple has a new type of same side reverse draw off pan that is cross flow (transverse) Jim is currently making a 2'x4' front pan for me. I can send photos when I receive the pan. 9980

[Marvel26]

Ahhhh TRANSVERSE! That was the word I was looking for....

The good thing about a 24X24 pan is that it will fit in the hole anyway you turn it.

Pictures would be great!

[SeanD]

Okay. If I reverse a typical cross flow, the syrup stays in the same channel and just goes 2' to the other side. There's probably no benefit to switching sides, then. It would need to be the transverse model you have to take advantage of reversing across the whole length of all the partitions.

[Schiefe4]

Honestly when I had 200 taps on buckets I never needed to reverse flow. One season I drained the pan the day after a boil a few times, cleaned the pan and then put the concentrate back in.

[maple flats]

Another idea is how mine is made. While I didn't make it, the idea is the same. My flue pan has just 1 center divider, the incoming sap (concentrate from RO) enters at the back corner, runs inside a SS tube to the center of the section, then turns and goes to the front of the pan, then the sap hits the pan. It travels to the rear of that section, crosses at the back and moves forward to the exit at the front side of the 2nd section. From there I have a manifold to feed either of 2 float boxes, controlled by 2 valves. My front pan has 4 sections cross flow, if I open one valve with the other closed, the flow enters the rear float box, then enters the syrup pan and flows til it reaches the front box which is now my draw off box. If I close the first valve and open the second valve the flow is reversed. I switch by closing both valves, move the float valve arm and float the the other box (takes about 15-20 seconds) and then open the second valve. I really like that design. M manifold is designed so from the flue pan connection (raised flue), it attaches with a clamp, goes out maybe 4" to a T, the flow can either turn to the rear float box or proceed to an elbow the takes the pipe about 1" outside the outer face of the 2 float boxes. The second valve is at about midpoint along that pipe, which proceeds forward to the front side of the front float box, to an elbow, then another elbow and into the front float box. When I reverse the flow I close both valves, then push down a few seconds to drain the active float valve, lift on the float control arm to remove it, reverse how it faces and position it on the opposite float valve. Then I lift out the float, and while holding up on the control arm, I tilt the float about 45 degrees to sneak it under the control arm, then set it in the float box, hook the arm to the float and then open the proper valve. As I said, the process takes about 15-20 seconds at a comfortable pace. Each float box also has a draw off valve. When I reverse flow I usually have the new flow gradient establishes in about 10 to a max. of 15 minutes and at that point my first draw is faster , as the temp gets right, I slow the draw (I do continuous draw as much as possible) to a steady slow flow. On my 3x8 evaporator I draw between 5-7 gal/hr, running about 8-14% concentrate.

[Marvel26]

The holidays were good to me....I found a welder to fabricate my pan! it's going to be a 2'X5' setup. 24" syrup pan with 3 transverse dividers and a 36" flue pan. I have one last question for the group....which is better for the flue pan design: 2 12" wide flue sections or 3 8" wide sections?

All input is appreciated!

[roads-end]

First day sap ounpl

[Marvel26]

What's an ounpl?