Design and Construction of A Solar Dryer for Mango Slices

Transcription

1 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 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 1

2 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 kg H 2 O/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). 2

3 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, m w, in kg was calculated using the followig equation: m w = m p (M i M f ) / (100- M f ) (1) Where: m p is the initial mass of product to be dried, kg; Mi is the initial moisature content, % wet basis and M f 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: a w = 1- exp[-exp( lnm)] (2) Where: a w = water activity, decimal M = moisture content dry basis, kg water/kg dry solids a w = ERH/100 (3) iii-quantity of heat needed to evaporate the H 2 O: The quantity of heat required to evaporate the H 2 O would be: Q = m w x h fg (4) Where: Q = The amount of energy required for the drying process, kj m w = mass of water, kg h fg = latent heat of evaporation, kj/kg H 2 O 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: h fg = 4.186*10 3 ( (T pr )) (5) Where: T pr = product temperature,ºc Moreover, the total heat energy, E(kJ) required to evaporate water was calculated as follows: E = m` (h f -h i )t d (6) Where: E = total heat energy, kj m` = mass flow rate of air, kg/hr h f and h i = final and initial enthalpy of drying and ambient air, respectively, kj/kg dry air. t d = 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 = T +w[ T] (7) iv- Average drying rate Average drying rate, m dr, was determined from the mass of moisture to be removed by solar heat and drying time by the following equation: m dr = m w / t d (8) The mass of air needed for drying was calculated using equation given by Sodha et al. (1987) as follows: m`= m dr / [w f w i ] (9) Where: m dr = average drying rate, kg/hr w f w i, final and initial humidity ratio, respectively, kg H 2 O/kg dry air 3

4 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` (h f -h i )t d (10) Therefore, area of the solar collector is: 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, V a was obtained by dividing m a 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 = 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 (m p ) mango) Initial moisture content (moisture content 81.4 w.b. at harvest), M i Final moisture content (moisture content 10 % w.b. for storage), M f Ambient air temperature, T am 30ºC (Average for April) Ambinet relative humidity, RH am 15% (Average for April) Maximum allowable temperature, T max 70ºC Drying time(sunshine hours)t d 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 Vertical distance between two adjacent trays 3mm 15cm 4

5 Table 2. Values of design parameters parameter value Data or Equation used Initial humidity ratio, w i kgH 2 O/kg dry air T am, RH am Initial enthalpy,h i, 34.5kJ/kg dry air T am, RH am Equilibrium relative humidity, RH f 51% M f and isotherms equation (2) Final enthalpy, h f 65.5kJ/kg dry air wi and T f Final humidity ratio, w f 0.014kgH 2 O/kg dry air RH f and h f Mass of water to be 79.33kg Equation (1) evaporated, m w Average drying rate, m dr 3.967kgH 2 O/hr Equation (8) Air flow rate, m a 325.1kg dry air/hr Equation (9) Volumetric airflow rate, m 3 /hr m a, air density (ρ) V a Total useful energy, E MJ Equation (6) Solar collector area, A c 16.8 m 2 Equation (11) Vent area, A v m 2 V a, 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 cm 50 cm 70 cm 103cm 100 cm Fig. 1 Isometric view of the constructed solar dryer. 5

6 103 cm vent Exit 50 cm 76 cm Inlet vent 10 cm 13 cm 10 cm Fig.2 Side view of the constructed dryer. 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. 6

7 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): Basunia, M. A. and Abe.T. (2001). Design and construction of a simple three-shelf solar rough rice dryer. AMA, 32 (3): Berinyuy, J. E. (2004). A solar tunnel dryer for natural convection drying of vegetables and other commodities in Cameroon. AMA, 35(2): 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): Esper, A. and Muhlbauer, W. (1996). Solar tunnel dryer. Plant Res. And development, 44(4): 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): 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: 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):