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Novel dryers and recent developments
Fluid Bed Drying
This type of dryer in which individual grains are suspended and sometimes transported by air moving at high velocity, 2-3 m/s, can produce very evenly dried grain. Recent research in the Philippines (Sutherland & Ghaly 1990; Tumambing & Driscoll 1991) has indicated that the fluid bed dryer has promising potential for the rapid first-stage drying of paddy to 18% moisture in two-stage drying (see above). Paddy at a bed depth of 100 mm can be dried from 24% to 18% moisture in 15 minutes with air at 100°C and a velocity of 2 m/s, with no adverse effects on quality. However, due to the high air velocities required to fluidise the paddy, power requirements for the fan are high and the thermal efficiency is low compared to conventional (fixed bed) drying. Re-cycling of the exhaust air was identified as a potential means to improve the thermal efficiency.
Conduction Drying
Work at IRRI (Stickney et al. 1983) has investigated the use of a heated floor dryer. This consisted of a metal floor heated to 50-90°C by circulation of water heated by a furnace burning agricultural wastes. Paddy at depths of up to 60 mm could be dried from 22-26% moisture to 18% moisture in 1-2 hours depending on the floor temperature. Frequent raking was necessary but no parboiling effects were recorded and grain breakage was generally lower than that of sun dried paddy. This method is also considered as an option for the first (rapid-drying) stage of two-stage drying.
The Warehouse Dryer
This dryer has been developed (Jeon et al. 1984) for use with a wide range of crops including maize and paddy. Its particular feature is the use of a wind-powered vortex flow inducer as an alternative to conventionally powered fans for generating increased airflow over and around the drying grain. The flow inducer is mounted centrally on the roof of the dryer building and draws air, heated by a furnace and heat exchanger, through the drying bins or trays positioned in the middle of the dryer. The performance is governed by the velocity of the prevailing wind.
Rotary Drying
This method of drying has been researched at the IRRI in Philippines and also at the AIT in Thailand. Small dryers for farm use were developed at the IRRI as reported by Espanto et al. (1985). A directly-heated version consists of a perforated iron drum (0.6 m in diameter and 0.9 m long) mounted over a portable stove. The interior of the drum is fitted with flights to facilitate mixing and uniform heat transfer. The drum is rotated manually.
An indirect-heated dryer is constructed from a 2001 oil drum also mounted over a stove. Air is passed through the drum by a fan.
The performance of both dryers was very similar. Batches of 25 kg of paddy at 28% moisture can be dried to 18% moisture in 50-60 minutes and batches of maize from 33% moisture to 18% moisture in 80 minutes. Larger versions of the indirectly- heated dryer have been built (Jeon et al. 1990). With paddy, milling quality was improved relative to sun drying but the viability of the grain was greatly reduced due to the high temperatures attained at the drum surface. Similar results were obtained with the AIT dryer as reported by Jindal & Obaldo (1986).
Microwave and Infrared Dryers
When grain is irradiated by electromagnetic energy high temperature potentials are generated between the interior and surface of individual grains. Moisture therefore migrates to the surface where it evaporates to the surrounding air. The rate of airflow necessary is that required to absorb the moisture and not as the provider of latent heat. This reduction in airflow would minimize the dust and other pollutants discharged to atmosphere. More uniform drying is possible compared with conventional heated-air drying. However the capital cost and energy consumption of the microwave equipment necessary is considerable. Radajewski et al. (1988), in Australia investigated, using simulation techniques, the use of microwave heating as a means of pre-heating wheat before drying and concluded that the reduction in drying time could not offset the power consumption required for microwave heating. Infrared heating systems are similarly expensive, and since infrared radiation only penetrates superficially it is necessary to agitate the grain thereby exposing all the surface area to the radiation, thereby incurring additional cost.