Design of a convective-radiative dryer and investigating its drying parameters

ABSTRACT Ph.D. Thesis DESIGN OF A CONVECTIVE -RADIATIVE DRYER AND INVESTIGATING ITS DRYING PARAMETERS Duygu EVİN Fırat University Graduate School of Science and Technology Department of Mechanical Engineering 2004, Page: 82 In this study, a novel counter-current type continuous convective-radiative dryer, different from the conventional dryers, is designed and manufactured. Downfall velocity of the particles is controlled by upward air flow through a vertical column composing the main part of the dryer. The drying column, equipped with eight radiative bar-heaters controlled by a PID temperature control system, provides combined convection and radiation heat transfer for drying process. Drying experiments were conducted in open circuit flow and in a closed cycle in which the outlet of the drying column is connected to the inlet. Drying experiments were conducted for sunflower and soybeans in open and closed cycles to determine the drying parameters of the dryer at different drying air temperatures and velocities. 40, 50, 60 and 70°C drying air temperatures and 2.4, 2.8, 3.2 and 3.6 m/s air velocities were used for open and closed cycle sunflower drying experiments; and soybeans were dried at 50 and 70°C drying air temperatures for 2.8, 3.2, 3.6, 4 and 4.4 m/s drying air velocities. As a consequence of high heat and mass transfer rates, high drying rates were achieved owing to the perfect contact between the drying air and the wet solids, similarly in spouted beds. Downfall drying requires less pumping power than spouted beds. As a further advantage, application of combined convective and radiative heat transfer ensured fast removal of surface moisture and provided faster drying rates, especially during the initial stages of drying. Short residence time during the drying periods, provides significant energy savings and permits the usage for heat-sensitive materials necessitating fast drying at high temperatures. Key Words: downfall drying, convective-radiative drying, column dryer. X