Preheater problem

[timbers]

For last season we build a preheater for our 2.5 x 8. It was basically the same design as Brandons and although it preheated sap to about 180 we never could get enough sap moving thru it and end up having to keep our bypass line trickling into the float.
Our feed tank is mounted about 4ft above the preheater, but does have a 15ft run before feeding the it. Any thoughts?
Thanks Ian

[brookledge]

Is it parrallel flow? If so you may want to add another parrallel line.
But the first thing I'd try to determine is where you restiction is. Have you tested the line coming from your feed tank? If you are getting enough there then it is probably your preheater. If not then up size to feed line or raise your head tank to increase pressure. But increasing pressure may cause bleed by in your float when it is not calling for sap.
So take a closer look at everything and that should determine your answer.
Keith

[H. Walker]

If the preheater isn't vented, or if it is fed from the top you maybe fighting a vapour lock. You should have a minimum of a 1" feed line

[Hop Kiln Road]

www.uvm.edu/~uvmaple/sappreheaters.pdf

has a spec charter to size the pipes. Sounds like you're vapor locked. Prepheaters blow off a surprising amount of steam!

[NH Maplemaker]

My preheater is modeled after Brandon's also!(except I used 1"pipe) After the first time we ran it I had to add a vent pipe. I have a temp gage in mine and you would see the temp go way up !If you manually pushed the float down, temp would drop. Vent pipe fixed the problem. Before you start the evaporator, push down on the float and see what you get for flow!! That should tell if it's vapor lock or restriction.
My temp run 200-210 degrees

Jim L.

[3% Solution]

Ok, so let's see if I've got this right.
After looking at the diagram, the vent is at the upper most portion (maniflod) and it is small tube (maybe 1/4") and is allowed to continously run into the flue pan.
This vent keeps the preheater from vapor locking.
Do I have??
Thanks!!

Dave

[Sugarmaker]

Timbers,
I have added three vents at the highest point and these trickle a little hot sap all the time but we get 200 deg. at the incoming sap to the float box. I designed ours after the Leader parallel flow. I did have problems with vapor lock until I got it vented. I think each system is different too. We have a lot of head pressure also and can easily over power our float valve. So I have a valve in the line that actually controls the flow somewhat too. I will go look at your diagram.

Chris

[maple flats]

On my last pre heater (on a 2x6) I had no vent and got 180-200 degree sap but from the high point on the parallel flow double tiered pre heater it was a straight drop into the float box. Doing this I always got good flow. I am building a preheater for my 3x8 and am trying to have the same set up. I have not finalized how to do this yet. Still on the drawing board. My last one used a series of tees to make a manifold, which on 3/4" tube makes a wide set up. Has anyone just drilled into the side of a section on round copper to solder the tubes in directly for a more compact manifold? I am thinking a 1" manifold header with 3/4" tubes, a 1" supply and a 1" outlet dropping directly into the float box. Last year I had a 3/4" feed line from the outside elevated tank and always had good flow. I am also thinking of having a 3/4" bypass from the preheater in feed to the float box for fast flow if needed in an emergency. This would be let run every couple hours to keep fresh sap in the by pass.

[WESTVIRGINIAMAPLER]

I never had any problems with vapor lock even when I only have 20 gallons in the feed tank and less than 18" head pressure. I run 1.5 to 2 inches in the evaporator, so when the level drops say 1/4", the float opens up to a point that any vapor lock or steam has to come out of it. Maybe my float design on my pans is different than most evaporator brands and different than the older Leader pans?? In my last homemade setup a number of years ago, I had a feed tank sitting aprox 18 to 24 inches above the pan and I ran a 1/2" tube down between the back of the pan, thru most of the entire length of the arch and back up the side of the pan and over the top. Needless to say, the sap came roaring out of the preheater tube in bursts, but there was never any problem with vapor lock and the sap was boiling when it came out.

You can use streeted 90 degree elbows on the outside of the preheater and say you a little space on your preheater.

[timbers]

Thanks for all the replys, gotta love this forum. My feed line is 1 inch and prior to adding the preheater has no lack of presure issues. Vapor lock could be it! Where and how do I vent the preheater. Also it is a parallel flow system with 2 x 4 foot sections. thanks Ian

[Sugarmaker]

Maple flats,
Yes I drilled into the manifold and made a more compact unit. I first used a 3/4 copper sleeve and brazed (or some high temp soldering stuff maybe silver solder? does that sound right?) it to the manifold then ran a 3/4 id reammer through that to punch out the hole into the manifold. It was pretty much a royal pain but I am to bull headed to stop. Anyway I ended up with 10, 3/4 tubes across x 4 feet long, so 40 feet of preheater pipe.

Got to go back to work!
Chris