Copper tubing Pre- Heater Question

[JDP]

I have fabricated a pre-heater using some soft copper tubing I had in the barn. We have it hooked up to a head tank with approx. 6ft of head from the tank to the top of the pan. Sediment faucet at the tank outlet and a ball valve nearest the evaporator. Ball valve adjusted for flow at about half way to reflect our boil rate (in lieu of a float box). Copper is coiled on both sides of oil tank flue surfaces to absorb heat. Our test run increased sap from 33 degrees to 120 degrees. (Will get it higher if all copper is pressed in contact with steel but we are still a work in progress.)

PROBLEM: We seem to keep getting an airlock or something and flow will trickle to almost a stop. If we pump sap through the line rather than gravity, we have no problem. If we hook it back up it runs full for another half hour or so. But we get spurts of air when we purge with the pump.

Is water boiling inside tubing and push sap back to the head tank thereby leaving some air in tubing?

Will increasing the flow eliminate this?

Is there a max temp we should adjust ball valve to deliver to eliminate air gaps??

Any advise is appreciated!

[smokeyamber]

From everything I have read the preheater needs a vent stack that goes up above the head tank. Putting it close to the outlet would be my option. I have a flat preheater that I am building that I am not sure needs one, but may get one anyway. I believe the only time you need the vent is if you get the sap hot enough to boil in the pre-heater ( though it doesn't sound like that is happening in your case).

Hope this helps and have a great season !

[Maplebrook]

Reverse your flow through the preheater so the sap travels uphill. With 6' of head this shouldn't be a problem. Weight of the sap in the tank will keep the air out of the system. Have a bypass geared up so when the tank level gets low and won't push sap anymore, you can open the bypass and flow directly into your pan.
I used the copper tubing wrapped around the stack preheater on my home made evaporator for a few years. Another thing is mutiple runs of tubing if your flow won't keep up with boil. I had 50',but it was cut it into 2 - 25' lengths and wrapped around the pipe.

Have fun!,

Darren

[MT Pockets Producer]

I agree with Maplebrook. We built our own preheater and all of the set ups we saw fed the sap in at the lowest point of the preheater and pushed the sap up so the air can seek its way to the top. I also agree that venting is necessary at some point in the system and the vent outlet does need to be higher than the head tank to prevent your sap from being forced out of the vent. Our preheater is in a hood and built with four pipes and a manifold on each end with the vent coming off the uphill manifold outlet that goes to our float box. Based on this I would think you could do the same thing by venting your coil at its highest point before it begins it's bend down to your pan. The vent does not have to be much, just big a big enough tube to let the air escape. We initially had no vent in our system and experienced the same issues with loosing flow. We have never had an issue since adding the vent. I believe that some of the issue is caused as much by steam or vapor buildup as air due to the sap warming. That's why it will run for a period of time before locking up.

[JDP]

thanks guys. i was a plumber before getting on the fire dept. shouldve thought about a vent. i will modify the coil to vent at the highest point. that should cure the problem.

thx again