Evaporation Phase
Time-dependent rates of water and solvent removal from coated paper or wet ink films determine web heating requirements and line speed boundaries. Thermodynamic transport equations govern drying kinetics, controlling the transition from constant-rate surface evaporation to falling-rate internal diffusion during paper coating and printing operations. Initial moisture removal occurs at the wet surface where liquid vaporizes freely under ambient heat supply.
As the saturated surface retreats into the capillary network, internal moisture diffusion limits the evaporation rate.
Mass Transfer
Industrial drying hoods utilize high-velocity hot air impinging on the moving web combined with infrared radiation panels to maximize heat transfer. Precise control over drying kinetics prevents water boiling within paperboard, which forms internal steam pockets that cause blistering or ply delamination. Temperature gradients across the sheet thickness force liquid migration toward the cooler sheet interior before vapor driven by pressure escapes to the surface.
Balancing air temperature and air velocity prevents surface skinning, a defect where a dried outer layer seals moisture inside the coating layer.
Quality Impact
Non-uniform moisture evaporation induces differential shrinkage across web width, leading to sheet curl and lay-flat defects in converting equipment. Controlling drying kinetics maintains uniform binder distribution throughout mineral coatings.
