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

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Visavale - Principles, Classification and Selection of Solar Dryers Figure 1.36. Pictorial view of Solar Cabinet Dryer at ICT-Mumbai (Visavale, 2009) The solar cabinet dryer consisted of an indirect forced convection solar dryer with a solar air collector, a blower for air circulation and a drying cabinet The solar air collector had dimensions of 4 × 5 m. A plain copper sheet painted black was used as an absorber plate for incident solar radiation. It was oriented southward under the collector angle of 31°. The collector and blower system was well insulated to prevent heat losses. The drying cabinet dimensions were length 1.20 m, breadth 0.76 m and height 0.40 m constructed with insulated walls and had 16 shelves. A centrifugal blower (capacity 1000 m 3 /h) connected to the drying cabinet provides a maximum air velocity of 1.0 m/s. The circulation fan that supplies fresh air has a power of 1.5 kW. During experiments the air velocity, temperature and relative humidity in the cabinet were in the range 0.9-1.0 m/s, 40-60°C and 50-65%, respectively. The sample data of solar intensity and respective inlet air temperature in the cabinet were recorded during one of the drying experiments and shown graphically in Figure 1.37. Geographical location of Mumbai is at 18.96º N and 72.82º E on the sea coast at the average height between 10 to 15 m over the sea level. Summer is hot and humid, while winter is cool and dry. The tray loading density was 4 kg/m 2 . To collect the drying data, tray at the centre was selected and weight loss was recorded at predetermined time intervals. The product was uniformly spread on 16 mesh trays which were placed in the drying cabinet. In solar drying processes, the drying air temperature can vary based on the magnitude of the solar radiation. However, the recycle damper was used for controlling the drying air temperature to some extent. The amount of solar radiation was measured with a pyranometer. Temperature and relative humidity were measured by thermocouple and RH-meter, respectively (Jadhav et al., 2010a; Mathew et al., 2010). Table 1.3. Solar cabinet dryer configuration Drying Cabinet Gross dimensions 1.12 × 0.76 × 0.40 m Spacing between two trays 0.025 m Tray material Al/ SS-316 Tray Dimensions 0.56 x 0.76 x 0.025 Space between two trays 0.025 m Tray type mesh tray Number of trays 16 Tray thickness 0.002 m Outlet pipe diameter 0.20 m 32 Drying of Foods, Vegetables and Fruits
Visavale - Principles, Classification and Selection of Solar Dryers Mode of air flow Forced convection Air flow rate 1000 m 3 /h Air velocity in cabinet ~1m/s Geographical Location Mumbai, India. (18° 53”N & 72°50"E) Collector Area of absorbing surface 20 m 2 Number of absorbing surface 10 Absorbing material Black painted copper sheet Insulation thickness 0.05 m Insulation material Plexifoam Transfer fluid Air Collector tilt angle 25°, Facing south Cover plate material Glass Dimensions of each panel 1 m x 2 m Absorber plate thickness 20 G Air passage Rectangular aperture of 0.125 m2 Mode of air flow Forced convection Insolation, (W m -2 ) 800 700 600 500 400 300 200 100 Insolation Temperature 70 60 50 40 30 20 10 Temperature at the dryer inlet,°C 0 8 9 10 11 12 13 14 15 16 17 18 0 Time of the day, (h) Figure 1.37. Variation of solar radiation throughout the day during day of drying experiment (March 23, 2008) (Visavale, 2009) The plot for drying rate versus average moisture content for six types of fish using different dryers are shown in Figure 1.38 - 1.43. The specific energy consumption was estimated for the three types of dryers viz., SCD, FD and HAD (Visavale et al., 2011, Visavale, 2009), considering the total energy supplied to dry the fish from initial moisture content of about 74-87% to a moisture content of 5-6 % (approx.). Open sun drying was considered as a non-energy intensive process as only solar radiation was used to dry the samples. The specific energy consumption, H, is expressed as follows (Sharma and Prasad, 2004). Drying of Foods, Vegetables and Fruits 33
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