BI-DIRECTIONAL DRYER FOR MID-SIZE RECTANGULAR HAY BALES

Abstract

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