that must be some type of constant because in some home HVAC systems or refrigeration systems the air out of the evaporator should be about 10 degrees colder than the air into the evaporator. I am guessing if you had a larger temp. differential that it would require either a larger unit to cool the water if it was reused and if you tried to cool it with a greater temp differential, if the temp flucuated over a larger range it would probably affect the quality of the extruded coating???
The important issue is not the refrigeration equipment at all but rather the thermal control of the plastic (In this case XLPE.) inside the extruder. The whole effort is to reach a thermal steady state condition inside the extruder without burning or scorching the plastic and thereby achieving a controlled and constant melt volume output at a constant temperature for any particular screw RPM. The screw adds shear heat (adiabatic) to the melt and that is the main reason for the screw cooling but at the same time another large concern is that too cool a water/ oil input temperature could result in the plastic freezing on the surface of the screw. That cannot be allowed to happen for reasons to do with the XLPE chemistry.
To clarify what I said previously, here is an example;
The constant water/ oil input temperature could perhaps be set at 75F and the constant output temperature set at 95F. The adaptive controllers then would hold tight proportional bands and thereby provide effective, thermally controlled screw cooling. The whole idea of course is to quickly arrive at a thermal steady state condition inside the extruder.
In other words, the tight thermal control of the plastic completely defines the ancillary chilling and heating equipment and controls.