Bi-Directional Dryer for Mid-Size Rectangular Hay Bales

A pilot scale batch dryer was built with a capacity of six mid-size bales (0.81 x 0.89 x 2.44 m per bale) on one layer or 12 bales on two layers. With a floor area of 2.44 x 4.88 m, the pilot scale dryer included a 102-kW propane burner and a 12-kW blower located at the end of the air duct system, thereby creating a negative air pressure. The side walls were made of plastic film which adhered to the bales because of suction. A re-circulation duct returned a variable fraction of the exhaust air to the input to improve thermal efficiency depending on the level of vapor saturation. Part of the dryer's originality lies in its bi-directionality, i.e. heated air can flow alternately from the top plenum downward or from the bottom plenum upward. Bi-directional airflow was automated by two pairs of gate valves installed in two incoming air ducts and two outgoing air ducts. Results showed that one-layer batches reached an average moisture content of 12% in less than 5, 9, and 14 h with initial moisture contents of 21%, 24%, or 34%, respectively. Two-layer batches reached an average moisture content of 12% in less than 10 and 24 h with initial moisture contents of 21% and 30%, respectively. Total energy efficiency based on combustion heat and electrical energy for water evaporation ranged from 29% to 49% with an average of 38% in the first half of drying time and from 6% to 31% with an average of 17% in the second half of drying time. The difference between moisture content in the upper half and the lower half of bales was reduced with increased airflow inversion cycles. Because of lateral variation in final moisture content due in part to non-uniform initial moisture and bale density, some over-drying would be required to ensure that all bales are dried to a safe storage moisture level.