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The special active agent content makes suitable herbals for healing and health preservation. The key element in herbs and spices processing is drying. The required low moisture content of medicinal plants to storage can be achieved only by applying drying equipment at Hungarian climatic conditions. The extent of the dryer capacity defi nes the processing capacity of the factory. During the daily operation can easily occur bottleneck, when large amount of herbs are transported to the factory, which is above the drying capacity -various plants unexpected simultaneous fl owering, weather emergency, extraordinary collecting, etc. The aim of the manuscript is to determinate operational data at herbs pre-drying with low temperature air by measurements. This work defi nes the drying rate, the volume decrease and the drying air velocity for various medicinal plants at drying with ambient air.

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In literature, there are several Nu=f(Re) equations to specify the heat transfer coefficient between solids and the drying gas, but these equations differ significantly because of the inaccuracy of determining the contact surface between the two phases. A pilot-plant fluidized-bed dryer is developed to study the heat and mass transfer phenomena during the drying process. A volumetric heat transfer coefficient is applied for modeling fluidized-bed dryers. A modified Nusselt number is defined to compare the experimental results and those of the literature. The modified Nu' =f(Re) equation exhibits a proper correlation between the results of the experiments in the literature and those of our experiments.