MODELING OF WEB DRYING

摘要

A model for the entire paper web contact drying machine has, of course, to be based on simplifying assumptions and approximations. On the solid surface there exist equal rates of transport of dry air both toward, and from, the surface. Heat and mass flows between a wet surface and the gaseous phase in contact with it are conveyed by these air movements. For the sensible heat transfer (convection) a heat transfer coefficient, α′, is defined. It obviously is a function of the drying intensity. With no mass transfer, it is equal to the "dry heat transfer coefficient", α, and with evaporation taking place at boiling temperature egual to the evaporation intensity (and thus being dependent of the intensity of the heat flux from the outside heat source). Recently C.-G. Berg has made preliminary measurements on this dependence. The study was carried out with an apparatus in which the test section was composed of a 42 mm permeable tube of alumina. The range of mass transfer rates was 2,5...30 kg/m_2h. At the highest intensities the obtained values of α′dropped nearly 50 % relative to the dry heat transfer coefficient, α. Making use of this transfer coefficient α′, thermodynamic properties of moist air, simple energy balance considerations, and of the generally accepted fact that the air state change direction on the psychrometric chart is always towards the state point of saturated air in equilibrium with the evaporating surface, the law of evaporation will be as given in Eq.