Pre heat??

[325abn]

If one could get the sap flowing into the pan up from 65 to 180 or so what would be the resulting percentage of evaporation/hr increase.

I know there are several varible factors but there must be a general rule of thumb.

Any thoughts?

[RileySugarbush]

The heat required to warm water (or sap) from 65°F to 212°F is 82 calories/ gram.

Heat required to warm water from 180°F to 212°F is 18 calories per gram.

The heat required to vaporize water at 212°F ( like sap in the flue pan) is 539 calories per gram.

Don't worry about the units, the efficiency increase with your preheater is the ratio of the two cases:

Without preheater 82+539=621 calories per gram
With preheater 18+539= 557 calories per gram.

557/621=.90 1/.90=1.11
So your preheater would let you use about 11% less fuel in this case.


Maximum improvement with a preheater is if your sap was near freezing and your preheater gave you sap at near boiling. Raising water from 32F to 212F is 100 calories per gram.

In that case, the savings would be 539/639=.84

1/.84 =1.19

So the maximum improvement with a simple preheater is a bit less than 20%.

[sweetwoodmaple]

Good job on the calcs.

The harder question is the heat transfer between the steam and preheater.

Getting the sap to come out near boiling is difficult as the difference in temperature is real close between steam and say 200 degrees. You also have to manage increased pressures. There have been posts about that recently.

The best source of heat for the preheater is the outgoing exhaust from the stack. It's also the most difficult to manage.

[WESTVIRGINIAMAPLER]

Some say you can get 15 to 20 percent increase with a preheater, but I think more along the lines of 10 to maybe 12 percent, with the ultimate max of 15% and I don't think that is likely. Either way, it is all free recoup of heat and every bit helps, but I can't see anyway possible that a preheater is going to give anyone 20%.

[RileySugarbush]

I agree, Brandon. Theoretical max is about 20%, practical max is much less, probably around 10%. But that is nothing to sneeze a!

There may be other advantages that are hard to calculate, like not introducing cold sap and killing the boil near your inlet. And the added advantage of the nice hot distilled water for clean up and preheating bottles (or hot chocolate for my son!)

Pre heating with the stack can't do any better unless the sap is boiling in the preheater, and then it's not a preheater, its another pan!

[tom jr.]

It all depends on how large your preheater is. My neighbor custom built his and was getting 140 gph on a 3x10 grimm oil fired no joke!! Previously around 80 gph with out. you have to remember steam in a reasonably tight hood will boil water,also depends on how hard you fire evaporator.

[tessiersfarm]

Just throwing this out there. I have a flat pan set-up to pre heat my sap I wrapped 6 coils of 1/2" soft copper tubing around the smoke stack. This was supplied by my gravity tank and a ball valve. I adjusted the flow to the boiling rate and the sap was coming out at 120 to 180 degrees depending when in the heat cycle I tested. I thought that was a pretty good temp rise. After the sap was gone I ran clean water tru it and heated it to wash with. A little crude but it worked for my set-up.

[Harken]

To Tessiersfarm : What size is your pan (evap rate)?? Also, what dia. is your stack ? I would like to build a similar setup for my 2x4 flat pan, but not sure what length of pipe would be required...too short, not hot enough.....too long,boiling in preheater!

[325abn]

I have 10Ft of 1/2 copper tubing wrapped around my smoke stack as my feed pipe. With the flow set at what my pan can keep a steady level I manage to get the sap @ around 65 - 68 based on the temp taken at the end of the feed pipe.

[sweetwoodmaple]

Actually, if you run the coils in the flame, you have a steam boiler. Good point, I shouldn't have used "preheater".

My point was...wood @ 20 million BTU/cord is alot of stored energy. With a normal wood fired evaporator, I doubt you are even using 50% of that between the pan and sap, let alone what is wasted by going up in steam.

Us sugarmakers need to find a way to tap into that amount of energy, no pun intended.

As "The Governor" pointed out, we need a system that is more universal to different evaporators, similar to an RO in that regard. Perhaps with most of the components coming from a steam boiler system.

A project for me next year.

[Sugar Daddy]

I agree - there's a lot of heat going up the stack and any that can be captured is a bonus.

Last week I put a piece of rectangular heating duct in-line in my stack - with elbows on both sides, so that it sits horizontal (supported by a section of adjustable metal shelving). On that flat surface I put a steam pan as a pre-heater. Filled it with about 3-4 gallons of sap and dumped that into the main pan about every 20 minutes. Sap went into the pre-heater at the freezing point (ice floating in it), and warmed to between 60 and 90 degrees depending upon the volume in the pan, how long it was in there and how hot the fire was.

Overall I was pleased with the outcome of the inaugaral run of this batch system pre-heater. Especially when I had some concerns about how much heat transfer would occur from a relatively small surface area and also whether putting a couple of 90s and a horizontal section of ductwork in the stack would affect the draft. The draft didn't seem to be affected. And there seemed to be a good amount of heat transfer to the steam pan as the duct was noticeably cooler on the outlet side than the inlet side. It still needs some tweaking - primarily insulating.

We heat our cabin with an air-to-air heat exchanger (Magic Heat) in the chimney, which uses tubes that run perpendicularly through the stack. Wonder if anyone has devised a similar pre-heater for sap with tubes that run through (not around) the stack?

[Brent]

Preheating off the stack makes me think of the old expression
"pigs get fat, hogs get slaughtered"

If you take out too much stack heat you will lose draw, which could be overcome with a blower

and you will get more creosote condensing, especially on the coldest surfaces.

[tessiersfarm]

I have a 27" x 42" Flat Pan, with an 8" smoke stack. I don't know how many feet of tubing I have but there are 6 turns of tubing around the chimney. My square inches is about the same as 2x4. I was getting temps as high as 200 deg when the flow was cut back and as low as 70 when the flow was higher. Most of the time was in the 120 to 180 range. I use a valve on the gravity pipe to control the flow to match the evaporation rate. I started with three wraps but it wasn't enough.