Most modern calender rolls are the drilled type. This means that water passages are drilled across the roll face under the surface. This effectively forms a radiator-like heat exchanger around the periphery of the roll.
If these passages become plugged, then non-uniform heat transfer will occur and this results in a thermal ‘out-of-round’ condition for the roll or rolls. This has the same effect as if the rolls were machined ‘out-of-round’.
To avoid this condition only treated water should be used and the rolls should be periodically flushed.
How do you check for plugs? Compare the roll run-out at ambient temperature and at operating temperatures and start scouting for anomalies.
What if I have core type rolls, how do I keep a uniformed temperature across the face of the roll?
Whether cored or drilled rolls, temperature across the roll face can be controlled mainly by adjusting the flow through the roll. The larger issue is the change in temperature of the roll surface at the start of a run relative to the end of a run.