DEVELOPMENT AND EVALUATION OF AN EQUIPMENT FOR THRESHING AND CLEANING LEGUME CROPS

Abstract

The threshing of legume grains is particularly important because the output from the thresher will, to a great extent, affect the subsequent processes and the final quality of the product. A number of equipment developed for cereal threshing which are adapted for legume threshing in African countries are mostly imported. A lot of such machines have not performed well because of poor adaptation. Therefore, an equipment for threshing and cleaning soft-coated legumes (such as cowpea, pigeon pea, and soybean), incorporating a cross flow air stream, was designed and fabricated. It is made up of the conveyor, threshing and cleaning units. Power transmission is achieved with belt-pulley arrangements. The conveyor, threshing unit, transmission system and the centrifugal fan were designed using the conventional design analysis but the aerodynamic theory of separation proposed by Gorial and O'Callaghan (1991 a and b) was modified and utilized for the design of the separation chamber. A theoretical aerodynamic model for predicting particle trajectory in a horizontal airflow was developed during the study using the 'matlab' software. The trajectory model was used for the determination of the dimensions of the cleaning chamber, hence, a design guide. The model incorporates the effect of air velocity, air direction and the terminal velocity of particle on particle trajectory. Preliminary experiments were conducted to determine the physical properties of three varieties of cowpea (IT 812- 994, IT 810- 994 and Ife Bimpe) and one variety of soybean (TGX 448- 2E). The data obtained were reported in the write-up and used for the design of the equipment. From the design analysis, the conveyor unit with a capacity of 350 kg/h could move product through a height of 2.20 m and has a maximum power rating of 650 W. The threshing drum has a diameter of 460 mm and length of 995 mm and requires a maximum power of 1.25 kW to achieve threshing. The centrifugal fan requires a maximum drag force of 72.5 N and power rating of 787 W to clean threshed product discharged at 3.69 kg/s. It also requires fan pressure and airflow rate of 843.30 Nm-2 and 3.25 m/s respectively. The cleaning chamber has a maximum of 1.30 m vertical and 0.99 m horizontal dimensions. Cowpea (Ife bimpe variety) was used for the performance evaluation of the equipment at a conveyor speed of 400 rpm, threshing speed of 600 rpm, fan speeds of 900 and 1,500 rpm, fan angles of inclination of 90, 105 and 120°, concave clearance of 20 mm, pod moisture contents (PMC) of 14.0, 18.0 and 22.0% (wet basis) and corresponding grain moisture content (GMC) of 15.3, 17.2, and 18.8% (wet basis). A wire loop threshing mechanism were also used for the evaluation. A particle dynamics study was conducted using a high speed video camera to verify the feasibility of classifying cowpea grains into size grades. The materials deposited on the trays of the cleaning chamber were collected and the spread patterns of the materials were analysed and evaluated by plotting the percentage frequency distribution curve using the S-Plus and Harvard Professional 3.0 soft wares. The characteristics (axial dimensions and percentage composition) of the materials loaded into the various units of the equipment were determined. The whole seeds had a mean dimension of 7.64, 6.35 and 4.35 mm for big, medium and small size respectively. The pod content of the cowpea loaded into the conveyor and threshing units ranged between 60 and 72% for the three pod moisture contents. The whole seed content ranged between 20 and 22%, while the damaged seeds ranged between 8 and 19% for the three levels of PMC. The damaged done to the seeds before threshing was due to insect infestations, deliberately allowed to ensure a wide range of variety of seed weight and terminal velocity. The conveyor efficiency of the equipment ranged between 56.9% to 74.0%, 53.2% to 73.9% and 60.1% to 79.1% for PMC of 14.0%, 18.0%, and 22.0% respectively with mean values of 64.6% 66.2%, and 70.3%. The threshing efficiency of the equipment also ranged from 67.5% to 97.7%,71.1% to 94.4%, and 83.1% to 97.0% for same PMC. There is significant difference in the conveyor efficiency and threshing efficiency of the equipment at the various PMC for 95% and 99% levels of confidence. This is as a result of the adsorption nature of moisture change from 14.0% to 22.0%. It was inferred from the spread pattern that the cleaning unit is most effective for GMC of J 5.3%, fan speed of 900 rpm and fan angle of inclination of 90° and 105°