Psychrometric Boundary Layer Analysis in Wrapped Paperboard Pallets during Transit
Polyethylene film wraps trap moisture desorbed by paperboard during thermal swings, elevating internal relative humidity and triggering mechanosorptive failure.
Stefan Diffusion
Vapor Migration
Mass transfer equations that describe the diffusion of water vapor through a stagnant gas layer model the drying rate of paper and coatings. Stefan diffusion refers to the process where a vapor moves through a dry gas column from a liquid surface to an open atmosphere, driven by a concentration gradient. This model is used to design paper machine dryer sections, where water is evaporated from the wet paper web and carried away by hot air.
The applicability of this model is restricted to systems where there is no bulk gas flow or convection.
Drying Mechanism
The rate of this vapor migration depends on the temperature, pressure, and the thickness of the stagnant air layer next to the paper web. In Stefan diffusion, the water vapor molecules must push through the air molecules, creating a continuous flux that can be controlled by adjusting the temperature and flow of the surrounding air. This mechanism is essential for ensuring that the paperboard is dried uniformly across its width, preventing moisture streaks that could cause warping or poor printing performance.
Improper drying leads to structural flaws and board curling.
Quality Consequence
Uneven moisture removal during the drying process affects the paperboard’s flat-lying properties and converting performance. By applying diffusion models to the dryer design, engineers can optimize the air hoods to achieve a uniform moisture profile. This optimization ensures that the finished reels meet the strict curl tolerances required by printers.