A NEW MULTISTAGE DRYING SYSTEM

Most of the energy consumption in industry takes place in different drying operations. This article describes a new multistage drying system (MSDS), in which the drying air and wet matter to be dried is led through a series of direct-contact drying stages. The multistage drying system enables remarkable savings in the energy costs of drying. It also offers the possibility to use secondary energy from (i. e.) a plant facility as the drying energy. The MSDS is suitable for drying materials, which can not stand high drying temperatures. The absorption capacity of the drying air is maintained by heating up the moist drying air in between each stage. The multistage structure makes it possible to use lower inlet temperatures or lower mass flows of drying air than in a single dryer system. The specific energy consumption of the multistage system is also smaller compared to conventional single dryer systems. In this paper calculations are applied to wood drying. In the specified case of a 4-stage drying system, the MSDS specific primary energy consumption (kJ/kgH2O) is over 10 % lower compared to a single stage drying system with the same inlet temperature of drying air. At the same time, the MSDS decreases the mass flow of fresh air into drying to 72%. Considering the previous, a single drying stage with an inlet air temperature of 106 o C could be replaced with a MSDS with the inlet air temperature of about 50 to 60 o C in each stage.