Increasing the drying efficiency of cylinder drying

Abstract

This PhD Thesis concentrates on paper drying: first on the drying rate in a cylinder covered with fabric, enhanced using hot air impingement through the fabric, and its potential effects on paper quality, and second, on the heat transfer rate from hot steam to paper through the cylinder shell. Cylinder drying is a drying method where a heated surface in contact with paper causes water removal from the latter by evaporation. Impingement through the fabric of hot air has been suggested to cause increase in the water removal and drying of paper. Increase in the drying rate by impingement of hot air was confirmed by laboratory tests and by simulations. In laboratory tests the increase in the drying rate was found to be accompanied by improvements in paper quality. The reason for the unexpectedly high increase in the case of combined cylinderimpingement drying of the drying rate was traced down to decrease in the temperature of paper. The thereby increased temperature gradient between paper and cylinder surface caused an increase in the heat flux from the cylinder. A simulation model for the system showed similar increase in the drying rate of paper. Simulations could also be extended to a paper machine scale. Use of the simulation model required further work on the experimental transport of moisture in the thickness direction of paper, with successful results. The potential of impingement drying for improving paper quality was demonstrated, especially for prevention of shrinkage of paper during drying. Shrinkage prevention has a positive influence on the tensile properties of paper. Shrinkage prevention still needs verification in a larger scale, however.