Evaluation of a multi-stage airflow control-algorithm for near-ambient drying of stored wheat in Manitoba
Epp, David Arlen
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Near-ambient drying of grain is an effective method of reducing the moisture content of freshly harvested grain to a safe storage level, but use of continuous fixed airflows can result in high airflow rates in some years, causing over-drying of the grain as well as consuming excessive energy. This contributes to high costs of drying in some years. One method of optimizing the cost of drying is to effectively utilize the fluctuations in ambient weather conditions on the Canadian Prairies to control the drying process. An airflow controller which optimizes cost of drying by varying the airflow through the grain based on diurnal fluctuations in ambient air and predicted progress of drying and spoilage was modified to use airflow rates typical of those recommended for near-ambient drying in Manitoba, and to facilitate drying over a range of harvest dates and initial moisture contents. The resulting multistage controller (MC) simulated drying with four possible airflow rates (0, 15, 25, and 40 L.s-1.t1) produced by two drying fans operating singly or in parallel. the performance of the MC was simulated using 30 yr of hourly Winnipeg weather, 1961 - 1990. The multistage controller with two optimized control parameters (MC2P) was successful in simulating drying of wheat for seven out of nine combinations of harvest dates ranging from Aug. 15 to Sept. 15, and initial moisture contents ranging from 16.0% to 19.0% (wet mass basis)... Practical considerations may prevent the MC2P from being developed into a marketable controller for near-ambient drying. When the additional capital cost needed for the MC2P system were included with the cost of drying, the MC2P had a cost of drying of 12.78$/t, compared with the continuous-airflow cost of 12.11 $/t (based on drying 115 t of wheat in an 8.2 m diameter bin). Also, the economy of the MC2P is dependent on grain prices, which can fluctuate widely from year to year. Finally, although the MC2P shows great success in reducing the operating costs of near-ambient drying compared with continuous-airflow, it has not yet been validated experimentally.