LAUSR

This paper investigates the influence of operating pressure on the drying rate in indirect dryers, focusing on the possibility of using data gathered from atmospheric experiments for the design and operation of a vacuum dryer, thereby simplifying the design process. The penetration model describing indirect drying was used to determine the drying rate for both atmospheric and vacuum conditions. A relation between operating pressure and the overall heat transfer coefficient for indirect dryers was found and described by an equation in the form of a power function. The proposed solution was experimentally verified on an experimental horizontal paddle dryer where a series of experiments were carried out with porous cereal feed pellets with an initial moisture content ranging from 26%db to 35%db for both the atmospheric and vacuum drying. In this way, it is possible to predict the value of the overall heat transfer coefficient, drying rate, and drying curve for vacuum drying according to the results of experiments performed at atmospheric pressure. The number and complexity of experimental verification tests required for the design of a new dryer can thus be significantly reduced. The proposed equation can also be used for optimization of the drying process of an existing dryer, which can reduce drying time and save energy.