The Vertical Solar Drier
In 2011, a new dryer model was introduced to the people living in Sahmuratli village and we have used twenty vacuum tubes commercially available for water heating, instead of the solar heating panels. The vacuum tubes are used to heat the air that will circulate inside the drying oven (Fig. 1). In this new design development an electric fan was used to force the internal air circulation. The fan placed on one side of the drying cabinet, near the bottom, is controlled by a set of thermostatic switches to maintain the correct amount of air with correct temperature.
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1. The proposed design improvements for the vertical solar drier. |
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Digital temperature control units to control the air temperatures were placed both inside the oven and at the intake chamber. When the temperature of the incoming air exceeded 40°C (that is at the beginning of the process, in the morning), the fan starts to operate and forces the incoming air to pass through the drying shelves of vegetables or fruit (Fig. 2).
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2. (a) Solar drier cabinet with sleeves on which drying trays are placed.
(b) Digital Temperature Control Unit.
(c) Motor control device. |
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The temperature control device sends the first signal to the motor control device as the temperature, monitored by the first thermocouple, reaches the set temperature of 40°C to initiate the process and with this first signal motor starts turning with its slowest possible rpm. (at the moment it is set to 10% of its maximum speed). But as the inside air temperature reaches to its desired temperature, (30°C for leafy vegetables and 60°C for more massive fruits e.g. apple or pears), this time monitored by the second thermocouple placed inside the oven, a second signal is sent to the motor control unit which causes the motor to speed up. Motor speeds up to a level that is enough to cool the entering air temperature to its desired level. This can be adjusted by the operator.
Metal tubes were used to feed the vacuum glass tubes with fresh air (Fig. 3a). Fresh air, heated while passing through the tubes upwards, sucked by the fan and its heat monitored by the second thermo-couple, enter the oven from the top. Three bimetal thermometers were also used for double-checking the temperature of the entering air at the sides and in the middle of the duct(Fig. 3b). The fan used for circulating the air operates with 220 Volts and 250 Watts, its maximum speed is 3500 rpm. However the fan needs to operate at only 10% of its maximum speed thus consuming about 500 Watts only.
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3. (a) Metal tubes are fitted inside the vacuum glass tubes.
(b) Hot air enter the drying cabinet from the top where three
bimetal thermometers are placed to allow checking of the
temperature. |
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