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

Design and Construction of A Solar Dryer for Mango Slices
EL- Amin Omda Mohamed Akoy a , Mohamed Ayoub Ismail b , El-Fadil Adam Ahmed c and
W. Luecke d .
a Dept. of Agric. Eng., Faculty of Agriculture, University of Zalingei, Zalingei, Sudan.
b Dept. of Agric. Eng., Faculty of Agriculture, University of Khartoum, Khartoum, Sudan.
c Energy Research Institue, Ministry of Science and Technology, Khartoum, Sudan
d Institute of Agric. Eng.,University of Göttingen, Göttingen, Germany
key words: Solar dryer; Design; Construction
Abstract
Based on preliminary investigations under controlled conditions of drying
experiments, a natural convection solar dryer was designed and constructed to dry mango slices.
This paper describes the design considerations followed and presents the results of
calculations of design parameters. A minimum of 16.8m 2 solar collector area is required to dry a
batch of 100kg sliced mango (195.2kg fresh mango at 51.22% pulp) in 20hours (two days drying
period). The initial and final moisture content considered were 81.4% and 10% wet basis,
respectively. The average ambient conditions are 30ºC air temperature and 15% relative humidity
with daily global solar radiation incident on horizontal surface of about 20MJ/m 2 /day. The
weather conditions considered are of Khartoum, Sudan. A prototype of the dryer is so designed
and constructed that has a maximum collector area of 1.03m 2 . This prototype dryer will be used
in experimental drying tests under various loading conditions.
1-Introduction
Sun drying is still the most common method used to preserve agricultural products in
most tropical and subtropical countries. However, being unprotected from rain, wind-borne dirt
and dust, infestation by insects, rodents and other animal, products may be seriously degraded to
the extent that sometimes become inedible and the resulted loss of food quality in the dried
products may have adverse economic effects on domestics and international markets. Some of the
problems associated with open-air sun drying can be solved through the use of a solar dryer
which comprises of collector, a drying chamber and sometimes a chimeny (Madhlopa et al.,
2002). The conditions in tropical countries make the use of solar energy for drying food
practically attractive and enviromentally sound.
Dryers have been developed and used to dry agricultural products in order to improve
shelf life (Esper and Muhlbauer, 1996). Most of these either use an expensive source of energy
such as electricity (El-Shiatry et al., 1991) or a combination of solar energy and some other form
of energy (Sesay and Stenning, 1996). Most projects of these nature have not been adopted by the
small farmers, either because the final design and data collection procedures are fequently
inappropriate or the cost has remained inaccessible and the subsequent transfer of technology
from researcher to the end user has been anything but effective (Berinyuy, 2004).
The total cultivated area and production of mango in Sudan , year 2003, was estimated to
be about 51926 feddan (21,809 hectares) and 442,330 tonnes, respectively(Ministry of
Agriculture, 2004). Mangoes are popular fruits on the world market because of their unique and
attractive flavour, colour and nutritional value. In spite of its excellence, the perishable nature of
this fruit and its short harvest season severely limit utilization, consequently mango has not been
developed as commercial and export crop. Drying may be an interesting method in order to
prevent fresh fruit deterioration. There is spoilage of fruits and other fresh foods that could be
preserved using drying techniques in Sudan and other developing countries. Seasonal fruits like
mangoes are not presently dried for export, or for local consumption during period of scarcity.
Large quantities of the mango fruit spoil under parent tree in remote areas in spite of the
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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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Design Calculations:
To carry out design calculations and size of the dryer , the design conditions applicable to
Khartoum are required. The conditions and assumptions summarized in Table 1 are used for the
design of the mango dryer.
From the conditions, assumptions and relationships, the values of the design parameters
were calculated. The result of calculations are summarized in Table 2.
i- Amount of moisture to be removed from a given quantity of wet mango slices to bring the
moisture content to a safe storage level in a specified time.
The amount of amoisture to be removed from the product, mw, in kg was calculated using the
followig equation:
mw = mp(Mi – Mf) / (100- Mf) (1)
Where: mp is the initial mass of product to be dried, kg; Mi is the initial moisature content, % wet
basis and Mf is the final moisture content, % wet basis.
ii-Final or equilibrium relative humidity:
Final relative humidity or equilibrium relative humidity was calculated using sorption isotherms
equation for mango given by Hernandez et al (2000) as follows:
aw = 1- exp[-exp(0.914+0.5639lnM)] (2)
Where:
aw = water activity, decimal
M = moisture content dry basis, kg water/kg dry solids
aw = ERH/100 (3)
iii-Quantity of heat needed to evaporate the H2O:
The quantity of heat required to evaporate the H2O would be:
Q = mw x hfg (4)
Where:
Q = The amount of energy required for the drying process, kJ
mw = mass of water, kg
hfg = latent heat of evaporation, kJ/kg H2O
The amount needed is a function of temperature and moisture content of the crop. The latent heat
of vaporization was calculated using equation given by Youcef-Ali et al. (2001) as follows:
hfg = 4.186*10 3 (597-0.56(Tpr)) (5)
Where: Tpr = product temperature,ºC
Moreover, the total heat energy, E(kJ) required to evaporate water was calculated as follows:
E = m` (hf -hi)td (6)
Where: E = total heat energy, kJ
m` = mass flow rate of air, kg/hr
hf and hi = final and initial enthalpy of drying and ambient air, respectively, kJ/kg dry air.
td = drying time, hrs
The enthalpy (h) of moist air in J/kg dry air at temperature T (ºC) can be approximated as
(Brooker et al., 1992):
h = 1006.9T +w[2512131.0 +1552.4T] (7)
iv- Average drying rate
Average drying rate, mdr, was determined from the mass of moisture to be removed by solar heat
and drying time by the following equation:
mdr = mw/ td (8)
The mass of air needed for drying was calculated using equation given by Sodha et al. (1987) as
follows:
m`= mdr / [wf –wi] (9)
Where: mdr = average drying rate, kg/hr
wf –wi , final and initial humidity ratio, respectively, kg H2O/kg dry air
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From the total useful heat energy required to evaporate moisture and the net radiation received by
the tilted collector, the solar drying system collector area Ac, in m 2 can be calculated from the
following equation:
AcIη = E = m` (hf -hi)td
Therefore, area of the solar collector is:
(10)
Ac = E/Iη (11)
Where E is th total useful energy received by the drying air, kJ; I is the total global radiation on
the horizontal surface during the drying period., kJ/ m 2 and η is the collector efficiency, 30 to
50% (Sodha et al., 1987).
Volumetric airflow rate, Va was obtained by dividing ma by density of air which is 1.2 kg/m 3
v-Air vent dimentions:
The air vent was calculated by dividing the volumetric airflow rate by wind speed:
Av = Va/Vw (12)
Where Av is the area of the air vent, m 2 , Vw wind speed, m/s.The length of air vent , Lv, m, will
be equal to the length of the dryer. The width of the air vent can be given by:
Bv = Av/Lv (13)
Where Bv is the width of air vent, m
vi-Required pressure:
Velocity = Va/A
Va = volumeteric flow rate m 3 /sec.
The pressure difference across the mango slices bed will be solely due to the density difference
between the hot air inside the dryer and the ambient air. Air pressure can be determined by
equation given by Jindal and Gunasekaran (1982):
P = 0.00308 g(Ti- Tam)H (14)
Where: H is the pressure head (height of the hot air column from the base of the dryer to the point
of air discharge from the dryer), m; P is the air pressure, Pa; g is the acceleration due gravity,
9.81m/s 2 ; Tam is the ambient temperature, C.
The prototype solar dryer was sized to have a minimum area of 1m 2 to be used in experimental
drying tests.
Table 1. Design conditions and assumptions
Items Condition or assumption
Location Khartoum (latitude 15º N)
Crop Mango
Variety Kitchener
Drying period April to June
Drying per batch ( 2days / batch), 100kg sliced mango (195.2kg fresh
loading rate (mp)
mango)
Initial moisture content (moisture content
at harvest), Mi
81.4 w.b.
Final moisture content (moisture content
for storage) , Mf
10 % w.b.
Ambient air temperature, Tam
30ºC (Average for April)
Ambinet relative humidity, RHam 15% (Average for April)
Maximum allowable temperature, Tmax 70ºC
Drying time(sunshine hours)td
10 hours (Average for April)
Incident solar radiation, I 20MJ/m 2 /day (average for past 30 years)
Collector efficiency, η 30% (Ampratwum, 1998).
Wind speed 2m/s
Thickness of sliced mango 3mm
Vertical distance between two adjacent
trays
15cm
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Table 2. Values of design parameters
parameter value Data or Equation used
Initial humidity ratio, wi 0.0018kgH2O/kg dry air Tam, RHam
Initial enthalpy,hi, 34.5kJ/kg dry air Tam, RHam
Equilibrium relative 51% Mf and isotherms equation
humidity, RHf
(2)
Final enthalpy, hf 65.5kJ/kg dry air wi and Tf
Final humidity ratio, wf 0.014kgH2O/kg dry air RHf and hf
Mass of water to be 79.33kg Equation (1)
evaporated, mw
Average drying rate, mdr 3.967kgH2O/hr Equation (8)
Air flow rate, ma
Volumetric airflow rate,
325.1kg dry air/hr
270.94m
Equation (9)
3 /hr ma, air density (ρ)
Va
Total useful energy, E 201.562MJ Equation (6)
Solar collector area, Ac 16.8 m 2 Equation (11)
Vent area, Av 0.0376m 2 Va, wind speed
Air pressure, P 0.54Pa Equation (14)
Vent length 11.76m
Vent width 0.032m Equation (13)
Construction of Prototype Dryer:
A natural convection solar dryer of a box- type (cabinet) was designed and constructed. The
constructed dryer (cabinet-type) consisted of drying chamber and solar collector combined in
one unit as shown in Fig.1.
50 cm
70 cm
100 cm
5
103cm
76 cm
Fig. 1 Isometric view of the constructed solar dryer.

50 cm
Inlet vent
10 cm
Fig.2 Side view of the constructed dryer.
103 cm
6
vent
Exit
76 cm
10 cm
13 cm
A simple box frame 103cm long, 100cm wide and 76cm high at the back and 50 cm high in
front made of angle irons (2.54cm x 2.54cm) was fabricated. Sheets of mild metal sheet 0.1cm
thick were welded onto three sides and bottom of the fabricated frame.
Glass wool was used as insulator with a thickness of 5 cm and placed above the bottom mild
metal sheet. A 98cm x 99cm corrugated galvanized metal sheet was placed above the glass wool
and fitted to the bottom of the box by nails and rivets in addition to wood. The corrugated metal
sheet then painted in black as absorper plate.
Two tray holders made of angle iron (2.54cmx2.54cm) were welded in such away to hold
tray inside drying chamber. The lower holder was 15cm above the absorber plate and the upper
was 15 cm a part.
Masonite sheets of 0.3cm thick were used as insulators and fitted to the three inner sides of
the frame.
Aluminuim foil sheets were glued to masonite and used as moisture barrier; prevent moisture
from masonite and to reflect incident solar radiations to absorber from sides.
One panel of a 4-mm thick transparent glass (102.5cm x 99 cm) was glued to the top part of
the frame with silicon rubber sealant, which allow the expansion of glass at high temperature.
The glass used was low in iron content (water- white glass) because of its good transmissivity for
solar radiation. The glass was inclined at an angle of 15 due south , which is the angle of the
latitude of the experimental site.
In side the drying chamber there were two movable wire mesh trays that can be placed on
their holders. The frame of each tray was constructed from wood with dimensions of (97cm x
94cm ), while the tray had a surface area of (91cmx87.5cm ) and 7cm deep, with effective area of
91cm x 84cm. Each tray was made of wood and galvanized wire mesh.
For loading and unloading of material to be dried ,a hinged door was made for this purpose.
The hinged door was constructed from galvanized metal sheet (0.1cm thick), 3cm thick cork was
used as insulator. The door was sealed to prevent air leakage between the surroundings and the
drying chamber.

Two air vents for ventilation were provided. Inlet air hole (front air vent) located above the
base of absorber plate; 70cm length and 4cm width, provided with adjustable cover that has tow
level of opening; full and half opening for dryer temperature control. The outlet vent (rear air
vent); 100cm x 10cm was located 10cm below the back top edge and provided with adjustable
cover for dryer temperature control, it has two levels of opening; full and half opening.
The dryer was set on four casters to make it mobile.
Conclusion:
A solar dryer was designed and constructed based on preliminary investigations of mango
slices drying under controlled conditions(laboratory dryer).
The constructed dryer to be used to dry mango slices under controlled and protected conditions.
The designed dryer with a collector area of 16.8m 2 is expected to dry 195.2kg fresh mango
(100kg of sliced mango) from 81.4% to 10% wet basis in two days under ambient conditions
during harvsting period from April to June. A prototype of the dryer with 1.03m 2 solar collector
area was constructed to be used in experimental drying tests.
References:
Ampratwum,D.B. (1998). Design of solar dryer for dates. AMA, 29(3): 59-62
Basunia, M. A. and Abe .T. (2001). Design and construction of a simple three-shelf solar rough
rice dryer. AMA, 32 (3): 54-59
Berinyuy, J. E. (2004). A solar tunnel dryer for natural convection drying of vegetables and other
commodities in Cameroon. AMA, 35(2): 31-35.
Brett, A; Cox, D.R. ; Simmons, R. and Anstee,G. (1996). Producing solar dried fruit and
vegetables for micro and small-scale rural enterprise development. Hand book3:
Practical aspect of processing. Natural Resources Institute, Chatham, UK.
Brooker, D. B.; Bakker-Arkema, F. W. and Hall, C. W. (1992). Drying and storage of grains and
oilseeds. Avi, Van Nostrand Reinhold. USA.
El-Shiatry, M.A.; Muller, J. and Muhlbauer, W. (1991). Drying fruits and vegetables with solar
energy in Egypt. AMA, 22(4): 61-64.
Esper, A. and Muhlbauer, W. (1996). Solar tunnel dryer. Plant Res. And development, 44(4):16-
64.
Hernandez, J. A.; Pavon, G. and Garcia, M.A. (2000). Analytical solution of mass transfer
equation considering shrinkage for modeling food-drying kinetics. Journal of food
engineering, 45(1): 1-10.
Jindal, V. K. and Gunasekaran, S. (1982). Estimating air flow and drying rate due to natural
convection in solar rice dryers. Renewable energy review, 4(2): 1-9.
Lambert, J.M.; Angus, D.E. and Reid, P.J. (1980). Solar energy applications in agriculture. The
dried vine industry. University of Melbourne, Australia.
Madhlopa, A.; Jones, S. A. and Kalenga Saka, J. D. (2002). A solar air heater with compositeabsorber
systems for food dehydration. Renewable energy, 27:27-37.
Ministry of Agriculture and Forestry (2004). Department of Horticulture. Khartoum, Sudan.
Sesay, K. and Stenning, B. C. (1997). A free-convective fruit and vegetable hybrid tray dryer for
developing countries. Cited by Berinyuy, J. E. (2004).
Sodha, M. S.; Bansal, N. K.; Kumar, A.; Bansal, P. K and Malik, M. A. (1987). Solar crop
drying. Vol. I and II. CPR press, Boca Raton, Florida, USA.
Youcef-Ali, S.; Messaoudi, H.; desmons, J. Y.; Abene, A. and Le Ray, M. (2001). Determination
of the average coefficient of internal moisture transfer during the drying of a thin bed
of potato slices. Journal of food engineering, 48(2): 95-101.
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