Solar Drying: Fundamentals,Applications and Innovations - National ...

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Visavale - Principles, Classification and Selection of Solar Dryers for crop drying are shown in Figure 1.13. Figure 1.14 shows the design by Ortho Grainger & Twidel (1981) for maize drying. The designs generally comprise of an air-heating solar-energy collector, an insulated ducting, a drying chamber and a chimney. Figure 1.13 Typical distributed-type (indirect) natural-circulation solarenergy (Ekechukwu and Norton, 1999) Figure 1.14. A distributed-type naturalcirculation solar maize dryer (Ekechukwu and Norton, 1999) 1.10.3. Direct-Type Passive Solar-Energy Dryers The features of a typical direct-type passive solar dryer are illustrated in Figure 1.15. In these type of solar dryers the direct exposure of the crop to the sunlight enhances the color ripening desired in certain varieties of grapes, dates, coffee and development of full flavour in roasted beans. Two basic types of dryers in this category can be identified as the cabinet and greenhouse dryers. 1.10.3.1. Solar Cabinet Dryers The passive solar cabinet dryers are generally simple and inexpensive units having high applications for domestic purposes. They are suitable for drying of agricultural products, spices and herbs etc., constructed normally with a drying area of 1-2 m 2 and capacities of 10-20 kg. Figure 1.16 shows a typical passive solar cabinet dryer. The heat for drying is transmitted through the glass cover and is absorbed on the blackened interior and crops as well. The required air circulation is maintained by the warm moist air leaving via the upper vent under the action of buoyancy forces and generating suction of fresh air from the base inlet. Pioneering works on solar cabinet dryers were reported by the Brace Research Institute, (1980). A number of other designs of passive solar cabinet dryer in configuration to that developed by Brace Research Institute have been built and tested for a variety of crops and locations. Ezekwe (1981) reported a modification of the typical design shown in Figure 1.17 equipped with a wooden plenum guiding the air inlet and a long plywood chimney to enhance natural circulation, accelerating the drying rate by about 5 times over open sun drying. Figure 1.18 shows the design by Henriksson and Gustafsson (1986) with mesh work floor and a chimney with black PVC foil facing the southwards (sunlight). 22 Drying of Foods, Vegetables and Fruits
Visavale - Principles, Classification and Selection of Solar Dryers The passive solar cabinet dryer have an advantage of being cheap and easy in construction from locally available material, however their major drawback is poor moist removal rates and very high temperature (70-100℃) causing overheating of the product. Figure 1.15. Features of a typical integral-type (direct) natural-circulation solar-energy dryer (Ekechukwu and Norton, 1999) Figure 1.16. A typical natural-circulation solar-energy cabinet dryer (Ekechukwu and Norton, 1999) Figure 1.17. A modified naturalcirculation solar-energy cabinet dryer (Ekechukwu and Norton, 1999) Figure 1.18. A natural-circulation solarenergy cabinet dryer with chimney (Ekechukwu and Norton, 1999) 1.10.3.2. Natural-Circulation Greenhouse Dryers These are also called as tent dryers and are basically modified greenhouses. They are designed with vents of appropriate size and position to have a controlled air flow. They are characterized by extensive glazing by the transparent cover of polyethylene sheet. Drying of Foods, Vegetables and Fruits 23
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