dryer is shown in Figure 1. The drying chamber and heat exchanger were connected through a 1hp centrifugal blower to provide necessary air flow to the drying chamber. The preheated air by the solar collector was further heated by the stove-heat exchanger unit to the required temperature prior to send into the drying chamber.
The temperature was controlled with an accuracy of ± 5 C by incorporating a bypass line for supplying ambient air in between the drying chamber and the heat exchanger. Further, a gate valve was fixed at the outlet of heat exchanger to regulate air flow rate through the dryer.
fabricated with L-iron (38.1 mm x 38.1 mm). A 25 mm thick glass wool insulation was provided to all outer areas and to the duct at air outlet. The angle of the solar collector was kept as 10° to the horizontal which is considered to be an optimum angle for Sri Lanka [10]. The collector was oriented to face south to maximize incident solar radiation.
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3 1. Solar air heater 2. Heat exchanger 3. Drying chamber
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Figure 1 - A Picture of Hybrid Dryer A) Solar collector
A corrugated flat plate solar collector with a single glass cover was developed to preheat air as shown in Figure 2. A V-corrugated 1 mm thick sheet coated with dull black paint was used as the absorber plate of the solar collector. The area of the solar collector was 2 m2. The area of the absorber plate was 2.31 m2 since its corrugated shape. The glass cover was made of normal 5 mm thick window glass. Rectangular shape air gap (25 mm) was provided between glass cover and the absorber plate to facilitate air flow. Five V-shaped baffles made out of copper were fixed between glass cover and absorber plate to increase the turbulence of air flow. A 50 mm gap between the absorber and bottom outer plate was filled with sand as heat storage for night and to reduce heat losses. The outer box was made by 1 mm thick iron sheets and the supporting frame and legs were
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1. Glass cover 2. Baffles 3. Air inlet Figure 2 - Solar Air Heater B) Biomass stove-heat exchanger
The stove was consisted with a grate covered by fire bricks. The primary air inlet was located in the door of the stove and secondary air was allowed to flow through the grate. The flame intensity could be controlled by varying the primary air supply. Ash generated during the combustion process was collected underneath of the stove across the grate. This eliminated the accumulation of ash inside the effective burning area in the stove. The stove is shown in Figure 3.
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1. Fire bricks 2. Grate
3. Primary air inlet 4. Door
Figure 3 - Biomass Stove
The heat exchanger was fixed on the top of the stove to facilitate the flue gas flow through the
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1 heat exchanger. Since the air-to-air heat transfer
is very low, the retention time of flue gas in the heat exchanger was increased to gain the maximum heat transfer with minimum loss of heat to the environment. This was achieved by designing the heat exchanger with three sections; center, middle and outer (Figure 4). With this arrangement, the heat exchanger was operated as a three pass shell and tube heat exchanger. Heat was transferred by radiation in the center section while convective heat transfer was dominant in other sections. The outer area of the heat exchanger was insulated by glass wool with a thickness of 25.4 mm.
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meshes and trays could be replaced with desired meshes according to the size of the material to be dried. Each compartment was provided a door with locking arrangement in one side for loading and unloading of materials. A trapezoidal plenum chamber was fixed below the first drying compartment to maintain the proper distribution of inlet air. The outlet of the drying chamber was also fitted with trapezoidal chamber to guide the exhaust air properly into the atmosphere. The outer box of the chamber was made out of 1 mm thick iron sheets and covered with 25 mm glass wool insulation. The supporting frame was made with L-iron (38.1 mm x 38.1 mm). Hollow Aluminium tubes of diameter 10 mm were inserted through the walls of the chamber to measure the temperatures in each compartment using glass thermometers. Even though three compartments were available in the drying chamber only one was used in the present study.
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1. Center section 2. Middle section 2 4
3. Outer section
Figure 4 - Arrangement of Tubes in the Heat Exchanger
Figure 5 - Heat Exchanger C) Drying chamber
A vertical drying chamber (600 x 600 x 1350mm) was used to store copra in three compartments through which the hot air was allowed to pass from the bottom compartment to the top compartment. A picture of the drying chamber is shown in Figure 6. The chamber was designed to store around 420 coconut halves in three compartments. Drying trays were made out of 25.4 mm x 25.4 mm iron
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1. Doors 2. Compartments 3. Trays 4. Measurement points
Figure 6 - Drying Chamber 2.2 Experimental Procedure 2.2.1 No Load Experiments
No-load tests were carried out to examine the performance of the hybrid dryer. Those experiments were designed to find the required biomass feeding rate to the stove in order to keep the outlet temperature of the heat exchanger at 60°C, 70°C and 80°C with and without preheating of air by solar collector. The stove was fired for at least one hour before the measurements were taken in order to allow the shell and tubes of the heat exchanger reach steady state. Since coconut shells could be obtained as by product in the copra drying process, coconut shells were used as the fuel.
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