Fabrication and Detailed Instrumentation of the Solar Dryer
4.1 Working Principle of the Solar Dryer
4.2.3 Fabrication of the shell and tube energy storage
The energy storage module is basically a shell and tube heat exchanger. It consists of 9 stainless steel tubes of 25.4 mm in diameter and 2 m in length, and the tubes are enclosed by a shell of diameter 0.18 m. The arrangement of the tubes is shown in Fig. 4.8 and the CAD view is depicted in Fig.4.7. One tube is placed along the axis of the shell, and the remaining eight tubes are arranged in a pitch circle of diameter 0.11 m at an equal distance. The shell was made of mild steel sheet of the thickness of 2.5 mm in a rolling machine. Two flanges or headers of the
equal size with 9 holes of 26 mm in diameter were welded to the both ends of the shell, and the tubes were then inserted through the holes and welded to the flanges. Another two short shells of 0.3 m in length and 0.18 m in diameter were welded to the both ends of the main shell to distribute the air. The shell side is insulated with polyurethane foam of thickness 25 mm to reduce wall heat losses.
Fig. 4.8 Front view, left view, and sectional view of the energy storage.
Fig. 4.9 Positions of the thermocouples in the energy storage (Not in scale).
Six thermocouples (TC) are provided in the storage to measure the temperature of storage material at different locations. The thermocouples TC - 1, TC - 2, and TC - 3 are located near the inlet of the storage unit, and the thermocouples TC - 4, TC - 5 and TC - 6 are located near the outlet of the storage as illustrated in Fig. 4.9.
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A blower of the capacity of 0.417 kW (0.56 HP) was placed between the thermal energy storage and the drying chamber to circulate the air in the drying system.
The detailed of the materials used in the fabrication and the specification of the main components of the dryer are given in the table below.
Table 4.1
Materials and specification of the different components.
Main
components
Components Materials Specification
Solar air heater
Absorber Galvanised (GI) sheet
Size (1 m × 2 m) , thickness ( 0.85 mm) Glaze Plastic sheet Size (1.04 m × 2.04 m), thickness (4
mm)
Base plate Aluminium Size (1 m × 2 m), thickness (0.2 mm) Insulation Polyurethane foam Thickness (25 mm )
Box Wood and plywood Thickness (20 mm) and (6 mm) Drying
chamber
Wall Mild steel sheet thickness (1.2 mm) Tray Wood and GI mesh Size (0.8 m × 0.6 m) Insulation Polyurethane foam Thickness (25 mm ) Shell and
tube energy storage unit
Shell Mild steel Thickness (2.5 mm), diameter (0.18 m), length (2 m)
Pipe Stainless steel Diameter (25.4 mm), length (2 m) Insulation Polyurethane foam Thickness (25 mm)
Blower 0.56 HP (0.417 kW)
4. 3 Measuring Equipment
The dryer was installed on a roof top of the institute’s building. The instruments necessary for the measurement of the operating parameters were connected to the drying system. The following equipment were used in the drying system for the measurement of the solar radiation, temperature, humidity, mass flow rate, mass loss of the product, and the velocity.
(i) Pyranometer: One of the important measuring equipment is the pyranometer. It is used for the measurement of the solar radiation intensity incident on the surface of the air heater panel.
The pyranometer was mounted on the solar air heater. A pyranometer of the Make: Apogee;
Model: SP - 101; Calibration factor: 5 W/m2 per mV; Non - linearity: 1 % (up to 1750 W/m2) was used to measure the solar radiation intensity (Fig. 4.10).
Fig. 4.10 Pyranometer. Fig. 4.11 T - type thermocouple.
(ii) Thermocouples: T - type thermocouples (Fig. 4.11) with accuracy ± 0.2 ºC were used to measure the temperature at different locations of the drying system. The thermocouple wires were calibrated in constant temperature bath before installing in the drying system.
Fig. 4.12 Hot wire anemometer. Fig. 4.13 Electronic weighing balance.
(iii) Hot wire anemometer: The velocity of air inside the drying chamber was measured with a hot wire anemometer (Make: TESTO; Reading accuracy ± 0.01 m/s; Measuring range 0 - 25 m/s) (Fig. 4.12).
(iv) Electronic weighing balance: The loss of mass of the agricultural product was measured with a digital weighing balance (Make: K. Roy: Model: DJ 602A, Reading accuracy: 0.01 g, Measuring range: 0‒600 g) (Fig. 4.13).
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(v) Power meter: The power consumed by the blower was measured with a power meter (Make: MECO meters Pvt. Ltd; Model: PG09/PG09H, accuracy ± 1 W) (Fig. 4.14).
Fig. 4.14 Power meter. Fig. 4.15 U - tube differential manometer.
(vi) Orifice meter: An orifice meter was designed and fabricated for the measurement of the flow rate of air. It was placed at the inlet of the first air heater. The pressure drop across the orifice plate of the meter was measured with a U - tube differential manometer (Make: Bombay Instrument Ltd. Measuring range 150‒0‒150) (Fig.4.15). The specification of the orifice meter is given in Table 4.2 and the schematic of orifice meter is shown in Fig. 4.16.
D
D D/2
d
Upstream tap Downstream tap
Orifice plate
Fig. 4.16 Schematic of the orifice meter.
Table 4.2
Specifications of the orifice meter.
Diameter of pipe (D) 101 mm
Diameter of the orifice(d) 60.6 mm d β
D 0.6
Position of the upstream tapping D Position of the downstream tapping D/2 Coefficient of discharge 0.63 4.4 Summary
The various components of the developed dryer were fabricated from the locally available materials such as wood, mild steel, polyurethane foam, plastic, and plywood. Two double pass solar air heaters of the gross size of 2.04 m × 1.04 m, a shell and tube heat exchanger of the shell diameter of 0.18 m, and 2.6 m in length with 9 tubes of diameter 25.4 mm, and a rectangular drying chamber of the size of 2 m × 0.85 m × 0.4 m were assembled together to develop the dryer.
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