Preheater.

[vach50]

I have a 2x6 homemade evaporator with a rectangular chimney that I would like to solder 3/4" copper pipe with elbows to wrap the pipe around the chimney for a preheater. Anyone else try this? Any idea how much pipe will be needed to preheat cold sap to 170-200 degrees F? For my setup this is a better option than a hood and preheater. Thanks.

[Haynes Forest Products]

This is what I think I know. Don't try and be so concerned about contouring the preheater to the stack. Elbows take time and money and cause restrictions. Direct contact of the copper to the flue stack can cause scorching and over heating. You don't want it to be so efficient that you boil the sap in the stack. Cleaning after cooking is also important so soldered fitting will trap bacteria so keep it one piece soft copper with the ability to flush it as the last of the sap passes thru it. Make it easy to drain and maintain. The larger the tube with more length will allow the sap to flow freely and heat more sap. Have a bypass valve so you can dump water into the tube as the last sap enters so you don't taint your syrup with burned sap. Wrap the stack with the pipe and insulate it with blanket or a outer skin of tin to keep the heat in. There are to many variables to accurately calculate what you ask.

[maple flats]

I sort of agree with Haynes but I suggest you shape a surpentine zig zag to huge just one flat side (away from pans will make the shield easier, try to space it about 1/2" from touching the stack and likely use 1/2" ID or larger, no elbows. For the rest, do as Haynes said. For length, try 30' and test it one year then decide if you need more. Part of that will be based on how many gal of sap you run/hr in your rig.

[RileySugarbush]

Another downside of soldering is that if you do go dry in the preheater, you have a chance of melting your solder joints. If you have an external shield and no liquid in the tube, you may exceed the solder melt point, which can be less that 500°F for some lead free solders.

[vach50]

Another downside of soldering is that if you do go dry in the preheater, you have a chance of melting your solder joints. If you have an external shield and no liquid in the tube, you may exceed the solder melt point, which can be less that 500°F for some lead free solders.

Never thought of melting the solder. When finishing a batch or final cook at the end of the season there would be times the pipe would have to be empty. Might have to rethink this.

[Tweegs]

I get why folks see a stack type preheater as attractive. I mean, the stack gets really hot and why waste that heat, right?

Couple of problems though.
I know my stack gets really hot long before I get to a boil in the pans. If my pans aren’t at a boil, I’m not bringing sap into them.
If sap is stagnant in a stack type preheater it may come to a boil, depending on the size of the copper.
If the copper is too large you don’t get enough heat transfer. Too small and you boil/vapor lock/scorch/melt solder.

Even if you got the size of the copper just right, every point of contact between your preheater and the stack is going to corrode.
All you need are two dissimilar metals in contact with each other combined with a little moisture and you’ve got galvanic corrosion.
At the very least, you’ll shorten the life of the stack.

Steam is pretty hot, too. We get an abundance of that, amiright?
Long ago folks figured out how to make a parallel flow preheater that gets sandwiched between your flue pan and a hood, right in the steam path.
A damper in the steam stack helps regulates the temp of your incoming sap. Lots of write-ups in here on how to construct one.

Steam won’t melt solder (not in this application anyway), no corrosion, less fuss, less math (always a good thing), hood over the flue pan keeps the creepy crawlies out, steam gets vented outside…what’s not to love?

The preheater for my 2x6 uses five ¾” parallel runs and 1” manifolds.
The runs are the length of the flue pan, or about 60” down and back.
I set the damper so that the sap hitting the float box is about 160*.
No vapor lock that way.

[Sinzibuckwud]

You can run it on 3 sides and put in a compression fitting for adding or removing the unit. Mines right after the feed valve.

[RileySugarbush]

I agree with Tweegs. Recovering the heat of vaporization is better in almost every way. I love the hot distilled water we collect.

But if you don't have a steam hood, it's tough to make one.

I've said it here before, so pardon for repeating myself, but preheaters are attractive but not the best investment in increasing production. In very specific conditions where you have a great preheater and near freezing sap it might give you 15% increase, but probably more like 5%. If you don't have forced draft, it is simpler and many times more effective to add an under fire blower. Or even add some height to your stack or a slightly longer pan or some drop tubes.

If you have a hood, a pre heater makes lots of sense. I have one and I'm glad I do. But I would never try one on the stack. I have enough trouble with things I have to keep track of and no't need something else that can scorch or spray me with hot sap.

[Johnny Yooper]

I have a 2x6 homemade evaporator with a rectangular chimney that I would like to solder 3/4" copper pipe with elbows to wrap the pipe around the chimney for a preheater. Anyone else try this? Any idea how much pipe will be needed to preheat cold sap to 170-200 degrees F? For my setup this is a better option than a hood and preheater. Thanks.

vach50,

I have a 2x5.333 homemade evaporator and been using a copper tubing preheater around the chimney pipe for several seasons; brings 40F sap to 160F; I could increase the output temperature but wanted to see results of the initial build before I started tweaking. Haven't had any vapor lock or scorching problems so far. I have a length of tubing coming off horizontally that swivels so I can swing that arm to divert hot sap into another container when we remove the pan. You and I are in the same town so if you want to stop out sometime let me know.

[vach50]

I added 20' of 1/2" pipe not wrapped completely around the stack. I will be able to lift the preheater and remove it anytime I want. Can't wait to try it out. Thanks for all the advice, even those who said don't do it. This is what will works best for me and my setup. Am working at 12 volt fan for AUF. Still looking for a good fan. image.jpgimage.jpg