Design and Construction of A Solar Dryer for Mango ... - Tropentag

enormous potential for the utilization of solar energy for drying and other applications. It is ,
therefore , envisaged that the design of a simple solar dryer could contribute greatly in solving
this problem.
Solar dryers are usually classified according to the mode of air flow into natural
convection and forced convection dryers. Natural convection dryers do not require a fan to pump
the air through the dryer. Therefore research efforts will be focused on designing and
constructing a simple natural convection dryer. Since the rural or remote areas of Sudan are not
connected to the national electric grid and remote areas of Sudan facing energy crisis, especially
West Darfur state. The use of solar technology has often been suggested for the dried fruit
industry both to reduce energy costs and economically speed up drying which would be
beneficial to final quality (Lambert et al., 1980). El- Shiatry et al. (1991) dried grapes, okra,
tomato and onion using solar energy. They concluded that drying time reduced significantly
resulting in a higher product quality in terms of colour and reconstitution properties. They also
believe that as compared to oil or gas heated dryers, solar drying facilities are economical for
small holders, especially under favourable meteorological conditions.
The specific objectives of this study were:
1- To design a natural convection solar dryer for mango slices
2- To construct a prototype of the dryer for drying mango slices
Design Features of the Dryer:
The solar dryer has the shape of a home cabinet with tilted transparent top. The angle of
the slope of the dryer cover is 15º for the latitude of location (Sodha et al., 1987). The dryer is set
on casters to make it mobile. It is provided with air inlet and outlet holes at the front and back,
respectively. The outlet vent is at higher level. The vents have sliding covers which control air
inflow and outflow.
The movement of air through the vents, when the the dryer is placed in the path of
airflow, brings about a thermosiphon effect which creates an updraft of solar heated air laden
with moisture out of the drying chamber. The source of air is natural flow.
Solar Dryer Design Considerations:
A solar dryer was design based on the produceure described by Ampratwum (1998) for
drying dates (a cabinet type) and procedure described by Basunia and Abe (2001) for drying
rough rice (natural convection a mixed-mode type). The size of the dryer was determined based
on preliminary investigation which was found to be 2.6kg per m 2 (tray loading). The sample
thickness is 3mm as recommended by Bret et al.(1996) for solar drying of mango slices.
The following points were considered in the design of the natural convection solar dryer system:
a- The amount of moisture to be removed from a given quantity of wet mango.
b- Harvesting period during which the drying is needed.
c- The daily sunshine hours for the selection of the total drying time.
d- The quantity of air needed for drying.
e- Daily solar radiation to determine energy received by the dryer per day.
f- Wind speed for the calculation of air vent dimensions.
Design procedure:
The size of the dryer was determined as a function of the drying area needed per kilogram of
pulp of fruit. The drying temperature was established as a function of the maximum limit of
temperature the fruit might support.
From the climatic data (meteorological station, Shambat, Khartoum North) the mean average day
temperature in April is 30ºC and RH is 15 %. From the psychrometeric chart the humidity ratio is
0.0018kg H2O/kg dry air . From the result of preliminary experiments on the crop, the optimal
drying temperature was 70ºC and final moisture content of mango for storage is 10% w.b. the
corresponding relative humidity is 51%(sorption isotherms equation).
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