Industrial drying is one of the highest energy consuming processes of contemporary engineering. Moreover, drying often finishes an industrial process and controls the product quality. The energy consumed in drying is usually described by some indices, such as the volume evaporation rate, steam consumption, energy efficiency (thermal efficiency), and overall energy consumption. The energy efficiency index is most frequency encountered in engineering literature [1][2][3].New efficiency criteria are needed to work out the optimal performance characteristics of dryers. These criteria will provide recommendations concerning process design, equipping dryers with ancillary facilities, and flow-sheet design with several dryers.
DYNAMIC EFFICIENCY CRITERIAOne commonly thinks in convective drying that the energy supply is consumed in heating of the moist material, water evaporation, and compensation for heat losses. In drying of most materials, however, energy is consumed not only in removing unbound (superficial) moisture but also in removing internal (capillarybound) moisture. Moreover, the energy efficiency, which characterizes the degree of utilization of the energy supply, is defined as the ratio of the heat consumed in moisture evaporation to the overall energy supply. The energy efficiency is a function of the properties of the material (porosity, hygroscopicity, size, shape, and moisture binding type), the embodiment of the process (type and design of the equipment, heat supply, and others), the process parameters (temperature, the flow rate of the drying agent, and others), and the starting and final water contents [1,3].In the engineering literature, the thermal efficiency of drying is usually calculated as an average over time (for batch processes) or an average over the dryer length/height (for continuous processes). Such an approach gives no idea how the degree of energy utilization changes during the drying. Dynamic efficiency criteria make this analysis feasible [4,5]. The energy efficiency, a dynamic criterion, is defined aswhere E ev is the energy consumed in the moisture evaporation in the time interval ∆τ , and E Σ is the overall energy supplied in the time interval ∆τ .Integrating the criterion over the overall drying time τ max or over the dryer length/height ( L max / H max ) gives the overall energy efficiencyThe overall energy efficiency approaches the thermal efficiency in its meaning; however, this is a more exact measure of the efficiency of the energy process. The energy efficiency only reflects the degree of energy utilization. For better understanding, let us introduce another dynamic criterion, namely, the efficiency of drying, to evaluate how easily moisture is removed in the chosen drying process. This criterion is defined asAbstract -A new approach to the evaluation of the efficiency of drying equipment is considered. The approach employs dynamic efficiency criteria that are calculated from time-distributed parameters (for batch drying) or space-distributed parameters (for continuous dryin...
