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Bala and Hossain - Solar Drying of Fishery Products op forced convection solar driers such as (i) solar tunnel drier (Esper and Mühlbauer, 1993), (ii) indirect forced convection solar drier (Oosthuizen, 1996), (iii) greenhouse type solar drier (Janjai et al., 2008a), (iv) roof integrated solar drier (Janjai et al., 2008b) and (v) solar assisted drier (Smitabhindu et al., 2008). Numerous tests in the different regions of the tropics and subtropics have shown that fruits, vegetables, cereals, grain, legumes, oil seeds, spices, fish and even meat can be dried properly in the solar tunnel drier (Esper and Mühlbauer, 1993; Mühlbauer et al., 1993; El-Shiatry et al., 1991; Schirmer et al., 1996; Esper and Mühlbauer, 1994 & 1996; Bala, 1997b; Bala et al., 1997 & 1999; Bala et al., 2002; Bala et al., 2003; Bala and Mondol, 2001). The purpose of this chapter is to present the developments and potentials of solar drying technologies for drying fishery products 4.2. SOLAR DRYING SYSTEMS The major two categories of solar driers are natural and forced convection solar driers. In the natural convection solar driers the airflow is established by buoyancy induced airflow while in forced convection solar driers the airflow is provided by using fan either operated by electricity/solar module or fossil fuel. Natural convection solar drying has advantages over forced convection solar drying as it requires lower investment though it is difficult to control the drying temperature and drying rate. Due to low cost and simple operation and maintenance, natural convection solar drier appears to be the obvious option and popular choice for drying of agricultural products. Natural convection solar drier can be classified as (i) indirect natural convection solar drier, (ii) direct natural convection solar drier and (iii) mixed mode natural convection solar drier. The limitations of natural convection solar driers prompted researchers to develop forced convection solar driers. Mühlbauer and his associates at the Institute of Agricultural Engineering in Tropics and Subtropics, University of Hohenheim, Germany developed the solar tunnel drier. Solar tunnel drier consists of a flat plate air heating collector and a tunnel drying unit with a small fan to provide the required airflow over the product to be dried. Solar tunnel drier can be operated by a 40 Watt photovoltaic module independent of electrical grid. The collector and drying chamber can be made of plain metal sheets and wooden frames in a number of small sections and joined together in series. These sections can be dismantled easily for transportation from one place to another. Glass wool is used between the two metal sheets at the bottom of the drier as an insulation material to reduce the heat loss from the bottom. Collector size of the drier is 10 m x 2 m. The photovoltaic system has the advantage that temperature of the drying air could be automatically controlled by the solar radiation. Numerous tests in the different regions of the tropics and subtropics have shown that fruits, vegetables, cereals, legumes, oil seeds, spices, fish and even meat can be properly dried in the solar tunnel drier. Bala (2000) has widely demonstrated the potentiality of the solar tunnel drier in Bangladesh for small scale industrial production of high quality dried fish which has a wide acceptance in the supermarkets in Bangladesh. 98 Drying of Foods, Vegetables and Fruits
Bala and Hossain - Solar Drying of Fishery Products Figure 4.2 shows solar tunnel drier in operation for drying of fish at Cox’s Bazar, Bangladesh. Figure 4.2. Drying of fish in solar tunnel drier 4.3. EQUILIBRIUM MOISTURE CONTENT Equilibrium moisture content of a product is defined as the moisture content of the product after it has been exposed to a particular environment for an infinitely long period of time (Bala, 1997a). The equilibrium moisture content is dependent upon the relative humidity and temperature conditions of the environment as well as species, variety and maturity of the product. This can be classified as static equilibrium moisture content and dynamic equilibrium moisture content. Dynamic equilibrium moisture content is obtained by fitting of a thin layer drying equation to the experimental data, whereas the static equilibrium moisture content is obtained after long term exposure of the product to a constant atmosphere. Static equilibrium moisture content and dynamic equilibrium moisture content should be used for storage design and drier design respectively. Figure 4.3 shows the sorption isotherms for shrimp (Tirawanichakul et al., 2008). When equilibrium is approached starting from dry material (MCEMC), water will be evaporated from the material and moisture content will decrease. However, equilibrium moisture content after desorption remains on a higher level than equilibrium moisture content after adsorption. Adsorption isotherms are relevant for the storage process, e.g. to prevent remoistening of dried material by keeping relative humidity in the storage room at a suitable level. For drying, the desorption isotherms are relevant. Sorption isotherms have a lower course at higher temperatures. Consequently, the difference between actual moisture content and equilibrium moisture content as a driving force for drying will be increased at higher temperatures and drying rate will be accelerated. Drying of Foods, Vegetables and Fruits 99
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Solar Drying:Fundamentals,Applicati
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Solar Drying: Fundamentals, Applica
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PREFACE Drying using solar radiatio
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Index Chapter No Title / Authors Pa
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Visavale - Principles, Classificati
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Table 1.4. Quality and other drying
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It was seen that for all fish, hot
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Fadhel - Recent Advancements in Sol
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12. Thangvaravut, H., Chiewchan, N.
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129. Qing-guo, H., Min, Z., Mujumda
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