Thermodynamic Relation
Phase transition behavior governs the rate at which liquid water turns into vapor during the industrial drying of paper webs. The clausius-clapeyron relation describes how the vapor pressure of water increases non-linearly with temperature, determining the thermal energy required to drive moisture out of the sheet. This physical principle establishes the limit of evaporation rates at specific cylinder temperatures and hood pressures.
Drying Calculation
Engineers use the differential equation to determine the latent heat of vaporization of moisture bound within the fiber network. Because the water is held by chemical bonds in the cellulose pores, evaporating it requires more energy than evaporating free water. This calculation guides the design of steam-heated drying cylinders, allowing mills to set the correct steam pressure for each drying stage.
As the paper web moves through the dry end, the temperature of the sheet must rise to maintain the high vapor pressure gradient necessary for rapid moisture transport.
Energy Optimization
Papermaking requires substantial thermal energy to remove the final percentages of water from the wet web. By applying the clausius-clapeyron equation to the drying section, mill operators can calculate the optimum balance between cylinder temperature and air velocity in the hood. High hood humidity reduces the vapor pressure gradient, which slows down the drying rate and increases steam consumption.
Adjusting the exhaust fan speed maintains a low relative humidity above the web, maximizing the efficiency of the thermal transfer.
