Fundamentals and Practical Experiments of Heat Transfer in Calenders, 2003 Advanced Coating Fundamentals Symposium Proceedings

During supercalendering or soft calendering, an extreme pressure is applied on the paper under a certain temperature gradient. Heat applied to the paper softens the fibers, enhancing the mechanical effect. To find the best balance between thermal and mechanical energy, it is worth understanding how heat is transferred from the rolls to the paper web. This requires fundamental work on heat transfer and practical trials to validate theory. The purpose of this work is to furnish certain results validating theory and to observe the effects of certain calender parameters (temperature of the heated rolls, dwell time, pressure and number of nips) and of certain paper properties (coated, uncoated and moisture) on heat.

Two means were used to evaluate heat transfer between the rolls and the paper:

• The first involved the fundamentals of transient heat transfer,
• The second involved energy balance considerations.

The validity of the assumptions used in these theories was tested and certain criteria were established to determine whether paper can be regarded as a semi-infinite slab or not, and to determine whether convection and radiation must be taken into account in heat exchanges.

The validity of the models was tested using three pilot-calendering trials:

• Firstly, a single nip, supercalender trial was performed with 4 papers: one 60 g/m², wood-free substrate at two levels of moisture, and one 80 g/m² paper coated on both sides at two levels of moisture, with the same fiber used as the uncoated paper,
• Secondly, a single nip, soft calender trial performed with an 80 g/m², wood-free, pigmented paper.
• Thirdly, an iterative 2-nip, soft calender trial performed with an 80 g/m², wood-free pigmented paper, coated on one side with 12 and with 24 g/m².

The main conclusions of this work are that it is quite possible to model heat transfer in calenders accurately.