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STUDY ON THE REVERSAL TIMING FOR THE SRA REVERSIBLE DRYER #
Phan Hieu Hien, Nguyen Hung Tam, Nguyen Van Xuan # #
# Paper for presentation at the Seminar on “Agricultural Engineering and Agro-products Processing towards Mechanization and Modernization in Rural Areas“ at Nong-Lam University, HoChiMinh City, 11-12 December 2003.
# # Lecturers, Faculty of Agricultural Engineering and Technology, Nong-Lam University HoChiMinh City, Vietnam. E-mail phhien@hcm.vnn.vn
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INTRODUCTION
In Vietnam, over the past 20 years, the simple flat-bed dryer has been accepted and developed into the foremost dryer for reducing post-harvest paddy losses due to adverse rainy weather, with about 3000 units of capacities from 4 to 8 tons per batch. Good features of these dryers are: fairly high capacity, good grain quality, low drying cost, easy to install and operate. Nevertheless, as simplest dryers with a major drawback of non- uniformity in final grain moisture content, they have to be changed to meet the needs for more-mechanized dryers at rice milling centers.
The series of SRA dryers (RA is abbreviation for Reversible-Air, S is drying in Vietnamese) have been designed to serve the above change. The research began in 1999 at Nong-Lam University (NLU, formerly University of Agriculture and Forestry) with a laboratory model for basic information about drying characteristics of various crops with reversible airflow. Next, a pilot 1.5-ton/batch SRA dryer was designed and tested with paddy and coffee. Finally, the dryer was scaled up to different models, of 2; 4; 6; 8; 10; and 12 tons per batch. Twenty five SRA units have been applied successfully in various Provinces of Vietnam; of which 8 units have each dried 500- 1500 tons in the past 3 years. Features drawn from testing and using these dryers are: a) Saving of land space; b) More mechanized, meaning less use of manual labor; c) Multi-crop use, including high-moisture products such as coffee, sliced cassava, longan… , and d) The investment and drying cost are not higher compared to a flat-bed dryer of similar capacity. Successful applications of these SRA have been reported elsewhere (Nguyen Hung Tam et.al., 2002; Phan Hieu Hien, 2003).
The unique feature of these dryers, is that the drying is reversed only once, which makes the process different from other dryers reported in the literature with periodic air reversal, say, every 2 hours. Less air reversal means less intervention by manual labor, which in turn contributes to lower drying cost. But this once-only air reversal should ensure moisture uniformity for users’ acceptance.