Limit the temperature rate of change at all times to a maximum of 3 degrees per minute F. Faster rates can damage or warp the rolls.
The control system on the TCU should limit the rate of temperature change for each roll water loop to 3 degrees per minute. Faster rates of change do not allow for equalization of mechanical expansion throughout the rolls, bearings, and calender frames. Differential expansion can stress various components and in severe cases actually crack or warp the rolls, or even damage the bearings.
Heating usually is limited by the available heating capacity but the cooling system frequently has the capacity to rapidly cool the rolls and therefore also has the potential to cause damage if not limited to 3 degrees per minute.
As mentioned in a previous post in this series, it is important to keep the calender running during warm-up and during cool down. Stopping the calender and turning off the TCU circulation pumps before the calender reaches ambient is not considered good practice. If the temperature control system has rate of change limits properly set, cool down is simple, since all the operator must do is to lower the temperature set points to ambient and then 60 to 90 minutes later return to turn off the TCU.
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