Using the stack to preheat - ideas

[Clan Delaney]

Been thinking a lot about how to reclaim more of the heat going up the stack. I've seen/heard of guys who've had success by wrapping copper tubing around their stack. What I haven't heard of is anything that involved pipe/tubing/coil inside the stack.

I know what you're probably thinking, so hear me out. I've already ruled out the idea of heating the sap directly. The heat in the stack is just too, well, hot. The chances of the sap boiling or burning is too great.

What about indirect heating, though? Take a look at the (crudely drawn) pic below. This setup would heat the sap in the holding tank. The system would be entirely self contained - sets of coils in both the stack and the holding tank, connected with pipes, likely copper. Water in the system would be heated and rise in the stack coil, and cool and fall in the tank coil as it released it's heat to the cold sap. Theoretically, this would create circulation without a pump.

So. Trouble shooting. Good idea or bad idea? Do you think it would even work? Would having a coil in the stack negatively affect your draft? Are there serious downsides to heating the sap in the holding tank? Anything else I missed?

[sapsick]

the holding tank would need to be really close so that your cu pipe doesnt cool down while its in free space before it enters the tank. however that said what if you were to use solid cu like a building ground wire coiling it inside the stack as well as in the tank. the solid conducter should hold the hat longer.

[RileySugarbush]

Assuming you are going to used coiled copper with water as the heat transfer medium, this is just like my hydronic heater at home. A boiler heat the water that circulates through a coil in a big tank which is circulated to four zones in my house and is used for hot water too.

Things missing from your sketch are a safety relief valve to prevent steam explosions, an expansion tank and a circulation pump. Relying on convection circulation may not be sufficient , and slow flow in the stack coil would flash that to steam.

The old tried and true hood and preheater sure works good......

[Dennis H.]

Yeah I think you would need something for expansion. unless it will be an open(unpresserized) system, but then you would have to have a way to keep adding water as some will always be evaporating out.

With the temps that can be in the stack, man I can see the water going to steam real quick, bam!

I agree also that it would work even better with a circulator pump. It would allow a little more freedom in the location of your tank.

I remember someone on the trader here has made a system just as you are talking about, he aslo uses a pump. I just can't remember who it was was.

[Haynes Forest Products]

That might be me. Last couple of years I used a pool heater for the heat exchanger and then a 10 plater exchanger and now this year I upgraded to a 50 plate 5"x12" its a monster and I started over with the total design. CLAN D. what you drew is a crude boiler but as was stated you need a pressure relief and expansion tank. It was mentioned that you could use an open tank method and that works but evaporation will catch up with you .....but you dont need the expansion tank or relief valve. Now you need a circulator pump or you will make steam in about 2 min. Most hydronic heat systems have alot more water circulating in them so they dont over heat. A small closed system like I have creates problems and the bigest is when I run out of sap. The water that is going threw the circulator pump and flue stack coils super heats and without the cool sap in the plate heat exchanger it starts to pop the relief valve and then things get scary so what I have done is with the auto fill always suppling water 14 PSI I open a drain valve and let cool water in to the system till the flue cools down about 5 min and all is well. I KNOW...... WHY SUCH A COMPLICATED SYSTEM? Because its got motors valves and its shiney and I could. Its long winded but boilers are either right or its a trip to the burn ward. I will post picks if i can figure out how.

[WF MASON]

As long as there is volume of sap moving in the stack like 3/8"tube would steam versis 1"+ wouldn't and the length of the run in the stack it would work. I think CDL makes the water jacket basestack preheater that is simple and works well.I had one guy tell me he ran stainless pipe from inside the arch down the length and plumbed it to the float box , he kept lengthing it finally crossing behind the dropflues and out to get his 200 degrees at the float. Like many growing up we had a waterjacketed kitchen stove that was plumbed to the copper hot water tank and it would just heat and circulate to and from the tank on its own.

[Grade "A"]

The only thing about put warm/hot sap back into your holding tank is that it will grow bacteria faster. I have a friend that put 4 passes of 1" copper pipe through his stack and lets it trickle into his pan, it gets very hot. From seeing what my friends preheater will do I think you don't need a big coil to have it work. His stack is 8" with 4 passes across it so thats 32" of 1" copper and works great.

[RileySugarbush]

playin' with fire.....

[WESTVIRGINIAMAPLER]

The last homemade setup I had, I had a 3x7 flat pan on a firebrick arch and I had a piece of 1/2" copper flexible pipe that entered the arch in the firebox area and ran back to the back wall of the firebox, up and into the pan. I controlled the incoming flow manually and the steam and hot sap jumped out of the pipe into the pan boiling hot. At the end of the night, I would pour aprox 1 quart of water which was several times the amount of liquid the pipe would hold at any given time and flush it out. I just unscrewed the fitting by hand from where it came out of the feed tank aprox 3 feet higher than any other area on the pipe and poured the water thru it. The water that I poured thru it, of course most of it ended up in the pan, but the remainder steamed out after I was done boiling until the pipe was dry. I never worried about scrorching anything inside the pipe because water or raw sap can't get any hotter than the boiling point of water and the pipe had a downward turn where it came out of the sap tank and an upward turn where it went into the pan which forced the pipe to remain full of liquid at all times which is the key in my opinion.

[Beweller]

You can reduce the heat going up the stack with any method of heat transfer. All you need (well, most of what you need) is surface area. The true question becomes cost of the surface. One way of getting more surface is using a larger evaporator--but firing it at the rate costumary for a smaller evaporator. Is that surface cheaper or more expensive that a preheater working on the stack?

A preheater based on condensing evaporated water is simpler that one in the stack. You can't do both, as the cold sap has a limited capacity for absorbing heat and there is more heat available than can be absorbed.

The larger evaporator can deliver more heat to the syrup because the boiling syrup continues to absorb heat even after reaching the boiling point. The larger evaporator increases efficiency by reducing stack temperature and still allows preheating the sap either in the stack or by condensing vapor.

[RileySugarbush]

Well stated, Beweller!