Hood Question

[Farmboy]

Im resetting my head tank this year. I want to plan for the future. How much higher will the sap inlet on my evaporator be if I add a hood and preheater. There will probably be a hood in the future and I want to set my head tank once. My head tank is outside and I poured a concrete pad for it. Im going to build metal legs for it and I only want to do it once. Thanks

[adk1]

How big is this tank! must be bigger than my 200 gallon truck tank!

[wiam]

I have not measured and pans are not on my rig. I would guess on mine I raised inlet about 12".

[Maplebrook]

Hello Farmboy,
The preheater I built sits on a frame on the top rail of my flue pan. The inlet is about 5" above the top of the pan. The outlet of your tank should be at least 18" higher than the preheater inlet. Keep in mind that the sap enters the lowest part of the preheater and travels upward to the exit.
Darren

[Tweegs]

Apologies in advance Farmboy, I really don’t intend to highjack the thread, but one of the replies has me a bit confused.

First though, I’ll offer my thoughts on the tank height. My shack (if the blasted thing ever gets built) is planned to have 10 foot walls, therefore I plan to put my tank up at least 6 feet, maybe go as high as 8, and here’s why:

Last year I had my tank set about 2 feet above the inlet to the float box. When I got below 1/3 of my tank capacity, I noticed I no longer had enough pressure left to operate the inlet against the float. This did cause a scorched pan at the end of the season (well, along with a heaping helping of stupidity on my part). The thought is that the higher the tank, the more gravity is going to work for you and against that float.

I have a plan for a float system to keep an eye on the feeder tank level. It’s a take-off on an ancient design so the theory is sound, it should be dirt cheap to construct (under $20), and if it works well in practice, I promise to post the instructions on youtube.

Now for the confusion clarification part:



Keep in mind that the sap enters the lowest part of the preheater and travels upward to the exit.
Darren

This runs counter to what I would have expected.
I’m not saying it’s wrong, I’m just confused as to why it should run uphill to the exit.

[waysidemaple]

The reason sap enters the low spot of the per heater is so the colder sap stays low in the perheater. Heat rises so the hotest sap is at the top of the perheater if the sap entered the top of the perheater the cold sap would just flow to the bottom and trap the warmer sap in the top allowing cold sap to enter the float. This is my understanding of it anyway

[Ausable]

Yep - I agree - Being that heat rises - the fluid to be heated - enters at the bottom and exits at the top. The same holds true - even in an enclosed system - like a boiler. -----Mike-----

[Tweegs]

That makes sense based on one particular design, but it means I need to do some more reading.
I’ll ask any further questions in a different thread.

Thanks

[RUSTYBUCKET]

As I'm just about finished with setting up my DG 2x6 dropflue, this post caught my attention:

"Last year I had my tank set about 2 feet above the inlet to the float box. When I got below 1/3 of my tank capacity, I noticed I no longer had enough pressure left to operate the inlet against the float."

Isn't float box operation related to sap level with a simple gravity feed to the box.


- Russ

[Ausable]

I agree Russ - Water seeks it's own level - Unless it meets resistance of some kind - Then Standing Head Pressure would come into play. Something sticking at the float valve or something else going on? ---Mike--

[bees1st]

My 500 gal.feed tank is 10 ft. away from my evaporator,and is 6inches higher than the inlet to the preheater on the steam hood .never a problem right down to the last drop in the tank.

[Tweegs]

On my rig, there is a vertical stem on the float with an adjustable collar. A forked horizontal arm rests on the collar (adjusts sap level in the pans) on one end, the other end of the arm has a stopper which restricts the sap inlet to the float box.

The end of the horizontal arm that has the stopper is heavier than the forked end that connects to the float. Since the mid point of the arm is used to secure it to the float box rather than the balance point, the stopper, being heavier on that end, will restrict the inflow of sap if there isn’t enough head pressure from the feeder tank to work against it. And that, friends, is what got me in trouble.

I’ll confess:
I was watching how fast the sap was entering the float box and mistook the slow down for needing more fire. I didn’t have enough sap coming into the pans to replenish what I was boiling off as it was, and here I chuck in another armload of wood.

On the plus side, I learned just how quickly things can go from smooth sailing to catastrophically wrong, especially at the end of a long day.

[maple flats]

quote : Tweegs:
Last year I had my tank set about 2 feet above the inlet to the float box. When I got below 1/3 of my tank capacity, I noticed I no longer had enough pressure left to operate the inlet against the float. This did cause a scorched pan at the end of the season (well, along with a heaping helping of stupidity on my part). The thought is that the higher the tank, the more gravity is going to work for you and against that float.


Tweegs, you might have a faulty float design. My tank only sets 12" above the float inlet, and my preheater rises to within 4" of my tank bottom, but I never had any problem getting sap to my pans. Mine runs well to the last drop.

[Maplebrook]

Clarification:Keep in mind that the sap enters the lowest part of the preheater and travels upward to the exit.
If the sap entered the preheater from the top, it would dribble down to the bottom , trapping some air along the way and there is a good chance of a vapor lock. Since a parallel flow preheater is a series of tubes all connected to the same inlet and outlet manifold, sap flowing downward would travel the path of least resistance and possible not fill all the tubes, thereby not gaining as much heat and not working to full efficiency.
Sap travelling upward will allow trapped air in the line to rise to the highest point in the preheater where - important part here - there is a vent to let it escape. Once the sap passes the vent, the plumbing allows it to drop back down to continue on to the float box.
The vent - my vent is a ball valve tee'd into the highest point on the preheater. Connected to that is a length of sap tubing. Some are vented back to the head tank, mine is run to the float box so when I open the valve to let air out (every 30 minutes or so) I can see what is there for air. (also a great source of hot sap for making coffee!)
On tank level and float operation - I have noticed that when the tank is getting low, the flow slows down a bit. It will still drain completely. I have a diverter valve right behind the float box (where sap enters/leaves the preheater) to bypass the preheater when draining the last of the tank.
Good discussion guys!
Darren

[Tweegs]

Ok, you guys just averted another problem I was about to face.
I’ll tinker with that float some, maybe add a little weight on the float side of that arm. It stands to reason that the inlet should be wide open if there is no sap in the pan, instead of closed as it is currently set up. It’s a new evaporator and my first year with it. I took it for granted that was they way it was supposed to be.

Avoiding vapor lock seems to be the key reason for the pre-heater design you folks are describing. I was thinking along the lines of several back and forth passes of copper, two, maybe three tiers…first sap in would be the first sap out…but there is no way to be sure you wouldn’t get vapor lock in that type of system. What I had in mind for a pre-heater lead to the confusion about an uphill climb. It’s clear now.

[RUSTYBUCKET]

Tweegs -

I remember seeing that float box design at Leader's Open House this year. My feed tank outlet sits about an 1" above the float box inlet and never had a problem. I'll score it as difference in float box design.

Thanks

- Russ

[Farmboy]

How big is this tank! must be bigger than my 200 gallon truck tank!\

The tank is a 200 gallon stainless steel bulk tank. Last year I had it up on 8' construction staging. It sits outside my sugar shack on a concrete pad because it is a little big to be in the sugar shack. Thanks for all the Info. I will probably be putting it on 6' legs which will be more than enough.