Bidirectional drying of baled hay with air recirculation and cooling

摘要

Twelve batches of 60 small rectangular hay bales were dried artificially in a prototype dryer with air heated at 60°C. Bales were dried according to four treatments: I) unidirectional flow with 50% air recirculation; II) bidirectional flow with 50% air recirculation; III) bidirectional flow with 50% air recirculation plus a 30-min cooling period; IV) bidirectional flow with 90% air recirculation plus a 30-min cooling period. Bidirectional flow was applied by reversing airflow direction once during adrying cycle after 2/3 of the planned propane gas had been combusted. There were two drying cycles, an A-drying period when 100% thermal efficiency was assumed (average time in dryer between 2.5 and 3.5 h) and a B-drying period when the actual efficiency of the previous drying cycle was assumed (average time in dryer between 3.9 and 5.1 h). There was no significant difference between the four airflow treatments in terms of moisture content reduction, bale mass reduction, and thermal efficiency. Overallfor 720 bales, the average moisture content was reduced from 24.0% initially at baling to 20.1 % after A-drying period, 15.8% after B-drying period, and 12.6% after six weeks of storage. On average, 35 L of propane were required to dry a tonne of hay (874 kg dry matter) from the initial moisture content of 24%. The average time in the dryer was 6.8 h. Average bale masses were 31.1 kg at baling, 29.6 kg after A-drying, 28.0 kg after B-drying and 27.0 kg after storage. The thermal efficiency averaged 41.6% during period A and 26.7% during period B. Treatment IV with high air recirculation and cooling tended to have higher thermal efficiencies (45.3 and 30.2% for A and B periods, respectively) than the three other treatments. High density bales dried significantly more slowly (p = 0.028) than low density bales in the range of 161 to 182 kg (dry matter)/m~3. Bales should therefore be grouped as much as possible in a homogeneous batch, both in terms of density and initial moisture. Moisture content distribution was very homogeneous within individual bales after a 10-week storage period. The dryer could produce high quality hay bales for the commercial market. However, further analysis and experimentation are required to determine the optimal conditions for air velocity and temperature, time of drying, and cycles of air inversions.