A NEW MULTISTAGE DRYING SYSTEM
Jukka-Pekka Spets
Helsinki University of Technology,
P.O.Box 4400, FIN-02015 HUT, Finland,
E-mail: jukka-pekka.spets@hut.fi
Keywords: Wood drying, multistage drying system
ABSTRACT
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°C could be replaced with a MSDS with the inlet air
temperature of about 50 to 60°C in each stage.
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