HEAT AND MASS-TRANSFER IN IMPINGEMENT DRYING

In industrial drying applications, efficient transfer of heat and mass between a drying medium and the material being dried is very critical for the overall economics of the operation. Impinging jets are theref ore widely used for their enhanced transport characteristics, especial ly for drying of continuous sheets of materials such as paper and text iles. In such applications, a thin sheet of material, as wide as 6m in cross machine direction, speeds at velocities as high as 90 km/hr und er high velocity jets emerging from a confining surface parallel to th e material surface. Many variables and effects need to be considered f or proper design of such impinging jet systems: nozzle geometry and si ze, nozzle configuration, location of exhaust ports, nozzle-to-surface separation, jet-to-jet separation, cross flow, jet exit velocity and surface motion. For permeable materials, additional enhancement of hea t and mass transfer that occur when some of the impinging gas is remov ed through the material makes this option an attractive one. Here, we review the above effects and offer predictive correlations from litera ture which may be used in the design of high velocity impinging jet sy stems.