DESIGN, DEVELOPMENT AND PERFORMANCE EVALUATION OF A COFFEE SIZE GRADING MACHINE

Abstract

Coffee (Coffee Arabica) is a non-alcoholic stimulating beverage of the Rubiacae family and the genus Coffea. It is well known that Ethiopia is one of the countries that produces, consume, and export Coffee and generates foreign currency. The coffee is processed either by a wet or dry method and this differ according to the operation in the process. In the dry method, coffee cherries are harvested, dried to low moisture content, and dehulled by using a dehulling machine. The existing grading machines were designed and operated based on the clearance between the drums only. In this case, efficiencies of the machines were affected by the cherries size distribution and uniformity, and coffee cherries breakage increases since the drums have fixed clearance between them. To solve this problem, an engine driven coffee cherry size grading machine has been designed, fabricated, and tested to evaluate its performance. . The machine consists of a hopper, grading sieve, frame, outlet, and power transmitting units. The performance of the machine has been evaluated in terms of grading efficiency, grading capacity, sieve clogging, and fuel consumption as dependent parameters. Three levels of feed rate (5 kg/min, 10 kg/min, and 15 kg/min), three level of sieve angle (7o , 9o and 11o ), and three speeds of operation (1.33 Hz, 2.33 Hz, and 3.33 Hz). The experimental design, used for data analysis, was a split-split-plot block. . The results of the performance evaluation revealed that the highest grading efficiency of 88.40% was observed at feeding rate of 15 kg/min, at operating speed of 1.33 Hz/sec, and at sieve inclination angle 7o with a capacity of 137.11 kg/hr. At this highest efficiency, sieve clogging was 4.96%. The analysis of variance (ANOVA) indicated that speed of operation and inclination angle of the sieve had significant effect on the grading efficiency, grading capacity, fuel consumption and sieve clogging at 95% level of confidence. The total estimated cost of the machine is 20,308.81 ETB. Based on the performance evaluation of the machine, it can be concluded that the machine successfully grades the coffee cherries based on their sizes.