Heating-Cooling Extruder application

In one of the previous posts, someone talked about extruder control forced air versus water cooled. I have both types of extruders, I am using Micrologix 1500 PID to control the heating and cooling of the barrels. I do not have much trouble with the forced air cooling. I simply wait for the heating output to go all the way to zero, before I start the cooling fan. However, with the water cooling it is much more difficult task. I am applying the same concept, waiting for heating out put to go to zero before water flow to start. The difficulty is because of the latent heat of vaporiztion of water, the barrels cools very quickly. As a result, the heating output comes on. This makes the control very unstable, switches between cooling/heating. I know some of temperature control supplier such as Eurotherm, they do the water cooling very well. Water cooling is considered non-linear and forced air is linear.
Does anyone has done water cooling using PLC PID? Any guidence would be appreciated

Sam

On a pair of Yashino IM machines I used to have we had a hot oil system...the oil flowed thru the heaters all the time, and the heaters would cycle on and off using Fuji Temp controllers...This gave us a very stable,abet slightly wasteful, temp control, and it made for very even heat profiles...you know that shear produces heat in one section, which can be a pain to tune for...the oil would transfer that heat away from there to the areas that needed more...it really worked well. We also ran a oil loop thru the reheat blow mo(u)lding machine to adjust the mo(u)ld temps...that acted as our cooling loop...

Hope this gives you some ideas....

David