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The effect of drying air temperature and relative humidity on the rate of drying and damage to navy beans was investigated. A thin-layer apparatus was developed to dry beans from an initial moisture content of around 25% to 14% wet basis (w.b.) using temperatures between 328 and 62° C and relative humidities between 7.5% and 45%.

A method of drying grain with alternate periods of blowing hot air through the grain bed (bin) followed by rest-periods is considered. An attempt is made to give the basic relationship between the “blowing ratio”, that is, the ratio of the cycle time to the blowing time, and the efficiency of drying for various temperatures and air velocities through the grain. The effect of this drying method on the final quality of the grain is also discussed. The practical application of the above drying system was considered in a multi-bin drier where the number of bins was equal to the value of the blowing ratio. It was established that this system, operating at its optimum blowing ratio, was more economical than the conventional system with continuous blowing.

Field losses of wheat grain and sprouting can be reduced by harvesting at higher moisture content than is currently practised. When drying from high moisture content the throughput of continuous flow driers can be increased by preheating the grain at the inlet into the drier, using a microwave heating system. A simulation model was developed to establish the performance and the cost of drying for such a system. The effect of the specific costs of the microwave source, electricity and fuel on the overall cost of drying were investigated. Sensitivity analyses were carried out for drying air temperatures in the range 50 to 90°C and for initial grain moisture contents of 16% to 24% w.b. (wet basis). The most critical factors affecting the cost of drying were the specific costs of the microwave source, electricity and fossil fuel. With the present costs of fuel and electricity of around 0·05 Australian dollars per kWh, the maximum economically viable cost of the microwave component of the drying system is 480 $/kW. The highest saving on the cost of drying was achieved when a microwave-supplemented drier operated at an air temperature of about 75°C and dired 150 t/d, reducing grain moisture from 18% w.b. to 12% w.b.

The optimum method of using a solar air collector combined with a grain drying system was investigated. Two types of solar collectors, integrated into a roof are considered; one uncovered (flow under absorber) and one covered (flow over absorber). A computer model was developed to minimize the cost of drying and in doing so optimize the geometry of the collector and the specific rate of air flow through the collector. The ratio of the cost of drying in a solar-supplemented system to the cost of drying in a conventional system was minimized. This ratio represents the maximum possible savings for the given drying conditions. Sensitivity analyses were carried out for drying air temperatures in the range 40 to 90°C and for grain initial moisture contents of 16 to 35% d.b.(dry basis). The effect of daily drying times, the length of the drying season and the specific cost of collector and fuel was also investigated. Two types of farm management systems were considered for various drying conditions: use of the dryer to operate for a fixed number of hours per day and, alternatively, drying a given quantity of grain per day. The results show that there is an optimum number of hours of operation per day, optimum drying temperature and optimum initial moisture content for any combination of the other variables. The most critical factors affecting annual savings are the length of the drying season and the specific cost of the collector. The optimum geometry of the collector and the specific flow rate are also functions of these factors.