Theoretical Calculation of Heat Transfer Coefficient When Sludge Drying in a Nara-Type Paddle Dryer Using Different Heat Carriers

Heat transfer characteristics of three typical heat carriers, i.e. saturated steam, thermal oil and flue gas, for indirect paddle dryer were studied. The convective heat transfer coefficient (HTC) of the three heat carriers were theoretically calculated. The contact resistance and penetration resistance were also determined by the penetration model. In the examined range, the significance of different heat resistances were investigated. The convective resistance was dominant in the flue gas based drying systems, while the penetration resistance was dominant in the saturated steam based drying system. In the thermal oil based drying system, both the convective resistance and the penetration resistance were significant. The drying rate of sewage sludge in the different heat carrier based drying systems was simulated by the penetration model. The effect of heat transfer resistances on the drying kinetics of sewage sludge was investigated.