what causes an air lock in preheater?????slows the feed into flue pan

[grimmreaper]

always patroling the sap house ,,,checking floats,levels in pans ,and sap flow in from head tank.my sap has been slowing down to a trikle and didnt know why,so i payed close attention to it and notice a air bubble float to the top of my head tank sight gage(there is a air releif pipe above the feed tank. i do have a temp gage on the sap coming out of the preheater (did notice it hit 170-175 degrees)normally runs 140-160

[nymapleguy607]

If the preheater airlocks it is usually caused by the sap geeting hot enough in the preheater pipes to make steam. You have a vent on the upper manifold of pipes to prevent it from vapor locking.

[Dennis H.]

It doesn't even need to be hot enough for sap to trun to steam.
Ever watch water in a glass bowl when hot? There are tiny bubbles that form on the sides of the bowl.
I put one of those saddle valves that you use for ice makers in your refrigerator. I then run a 1/4" tubing from that down to the float box. I adjust it so it drips all the time.
No problems.
My preheater is a parallel flow style with 4 pipes coming out the back of the hood. I feed sap in the one side lower pipe, the upper above this one is where I put the saddle valve. The hot sap comes out the other side upper side.

How many feet of pipe is in your preheater? It sounds kind of low temp wise. My sap is flowing into the float box around 190. Maybe this all depends on how fast the feed rate is. I have about 60 feet of 1/2" copper in the hood.

[Super Sapper]

A liquids capacity to dissolve gasses is higher at lower temperatures. If a liquid is saturated with gasses (air) at a low temperature it will release the gas as the temp. increases. A lot of agitation when filling the head tank could help saturate the sap with air at the lower temperature and cause it to release it in the preheater.

[grimmreaper]

mine is also a parallel system,made of 1/2'' stainless (around 60').so if i understand you right 1 maple...you take raw sap with a 1/4'' line and tap into the top of preheater??? i also have a vent tube from the top tube that goes 6 ' above evaporator and feed tank

[bowhunter]

Wood sapper is right. It's most likely dissolved air being released as the sap is heated and the solubility drops.

[Dennis H.]

The sap flows into the pre heater from the head tank by a 3/4" line. There is also a 3/4" line that feeds the flue pan float box.
All the 1/4" tubing is for is to keep air from forming at the high point of the pre heater, vapor lock.
I have the saddle valve adjusted so as it always drips, if it is dripping there will be no air at high point of pre heater.

[grimmreaper]

thanks for all you help guys and happy boiling.....