Improvement of air distribution in a fixed-bed dryer using computational fluid dynamics

Uneven air distribution is a major problem in the performance of batch dryers. Zones receiving a higher airflow rate dry faster, and this heterogeneity reduces efficiency by increasing energy consumption and drying time. A simple box dryer with 36 boxes placed over the plenum chamber was built and tested and computational fluid dynamics was used to simulate its air distribution. Simulations showed that this configuration produced a poor air distribution. Trying to overcome this problem, simulations were conducted with a modified design consisting of a wide inlet into the plenum. Results showed an almost perfectly uniform air distribution. This was considered satisfactory for further study, which consisted of finding a suitable transition between the small cross section of the air ducts and the wide entrance to the plenum. It was seen, from theory and flow simulations, that diffusers need to be prohibitively long to function properly. However, short, wide-angle diffusers can be equipped with air guides and perforated plates to remain effective. Drying trials with woodchips were conducted for the original and modified dryer configurations, during which the drying course and airflow of each box were measured. Results for the original configuration showed, like the simulations, that there was a wide variation in airflow among the boxes, and also the expected wide differences in drying rate. A very significant correlation between these two variables was found. The modified version resulted in much more homogenous air distribution and drying rates and therefore represents a viable approach to improve dryer performance.