INTERNATIONAL JOURNAL OF AGRICULTURE & BIOLOGY ISSN Print: 1560–8530; ISSN Online: 1814–9596 07-284/AWB/2008/10–1–61–65 http://www.fspublishers.org Full Length Article To cite this paper: Yadollahinia, A.R., M. Omid and S. Rafiee, 2008. Design and fabrication of experimental dryer for studying agricultural products. Int. J. Design and Fabrication of Experimental Dryer for Studying Agricultural Products A.R. YADOLLAHINIA, M. OMID 1 AND S. RAFIEE Department of Agricultural Machinery, Faculty of Bio-Systems Engineering, University of Tehran, Karaj, Iran 1 Corresponding author's e-mail: omid@ut.ac.ir ABSTRACT Because of the importance of rice cracking in milling process, precision control of drying conditions is important. For this purpose, it is necessary to determine drying kinetics and obtain the moisture change during the drying process. In this paper, design, fabrication and testing of an automated thin-layer dryer is presented. Experiments on the drying kinetics of rice paddy (Fajr cv.) were conducted at five drying air temperatures, ranging from 30 to 70 , in four air velocities, ranging from 0.25 to 1.0 ms C o -1 and three replicas (60 runs altogether) with initial moisture content of 25% (d.b.) at the start of all runs. During drying, the mass loss was measured continuously. Experimental curves of the drying rate versus time grouped by air temperature showed the strong dependence of drying rate with temperature revealed that: (a) drying rate increased when air temperature increased and (b) moisture transfer occurred during the falling rate period of drying. However, when the curves of moisture ratio verses time were grouped by air velocity, very weak dependence of moisture ratio with velocity was observed. Increase in air velocity from 0.25 to 1 ms -1 had little effect on the drying period paddy. Drying curves obtained from the experimental data, fitted to eight thin layer models and compared with three statistical parameters, showed that two terms model can predict moisture change with grater accuracy than other models. Key Words: Thin layer; Drying; Paddy; Regression; Mass loss; Temperature; Moisture content INTRODUCTION Rice is one of the most important food crops, as 80% people around the world consume rice. World rice production in 2004 was just less than 610 million tons and Asian farmers produced about 90% of this total. In the same year, Iran rice production was about 3,400 tons and rice yield was calculated as 5.96 tons ha -1 . Currently, Iran ranks 23 rd in terms of rice production and 26 th in terms of the areas under the crop's cultivation, but is among the top 10 states in terms of average rice productivity (faostat.fao.org). Thin-layer drying is the process of removal of water from a porous media by evaporation, in which excess drying air is passed through a thin layer of the material until the equilibrium moisture content (EMC) is reached. Moisture removal from an agricultural product depends on their drying temperature, velocity and relative air humidity, variety and maturity. Hence, various isolated and combined parameters are involved in moisture removal from a grain (Couto, 2002). The simplest situation is when the drying resistance (isothermal process) lies on the grain surface, for which the decay of moisture with time follows approximately an exponential law. Lewis was first to recognize the moisture transfer from the solid materials is analogous to that of the flow of heat from a body immersed in cold fluid. Comparing the drying phenomenon with Newton’s law of cooling in heat transfer, the drying rate will be approximately proportional to the difference in moisture content between the material being dried and EMC at the drying air state. Theories have been proposed to describe mechanisms of moisture movement in solids during drying in the falling rate period. Drying of agricultural materials such as grains is a non-linear process with long time delay and considerable complexity. Therefore, it is very difficult to establish a precise mathematical model for grain drying control (Cao, 2002). Although some mathematical models of drying process itself have been established, their structures are often too complex to be used for control model and hence effective control is very difficult to be realized (Marchant, 1985; Courtois et al., 1995). Akpinar (2002) developed a cyclone type thin-layer dryer for drying agricultural materials. This system was introduced in the literature for drying study of some vegetables and fruits including potato, red pepper, apple, strawberry, pumpkin and eggplants slices (Akpinar et al., 2003, Akpinar, 2005). Byler et al. (1989) constructed a microprocessor-based system to control experimental conditions in the study of the moisture content of agricultural products. The authors described the hardware and software, which were developed for used in the study of Agri. Biol., 10: 61–65