I bought a Biodiesel processor over six months ago and now is the time to use it, but I can't figure out the purpose of one of the copper tube with half plastic.

The tube that is on the right side that comes out from top of heater...



Close up... (Also, there is thread by the valve too, as if for the water hose)



Copper goes all the way through on the inside. There is also thread on the plastic side as if a hose is to be screwed on



Rest of the process I understood. Here are more pictures to show you the condition of the system.











Here is where Biodiesel sits, but what is that hose on the inside?