Pneumatic Conveying of Seed Cotton: Minimum Velocity and Pressure Drop
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Electricity is a major cost for cotton gins, representing approximately 20% of variable costs. Fans used for pneumatic conveying consume the majority of electricity at cotton gins. Development of control systems to reduce the air velocity used for conveying seed cotton could significantly decrease electricity use and cost. A greater understanding of the pneumatic conveying characteristics of seed cotton is necessary for development of these systems. A negative pressure conveying system was constructed with a feed control, conveying pipe, separator, and fan. Air velocity was measured at the system inlet and in the conveying section when testing with air only. A differential pressure measurement was taken in the conveying pipe, and temperature and relative humidity were recorded. Two pipe diameters, two cultivars, two moisture content levels, and three seed cotton feed rates were included in the experimental design. Seed cotton was fed into the conveying system, and the fan speed was decreased until choking occurred. The minimum differential pressure measurement indicated the saltation velocity. A segmented linear model was fit to the log transformed data to identify the mass flow ratio and Froude number (Fr) corresponding to the minimum pressure. This model accurately fit the data (R2 = 0.88) and resulted in the following equation for finding the saltation velocity: φ = 8.90 × 10-5 × Fr min5.04, where Frmin is the Fr at the saltation velocity. The solids resistance factor at velocities greater than saltation was found to be 0.179 × Fr-1. This model had an R2 of 0.91 and predicted the pressure drop with 14.2% error. Pipe diameter, cultivar, and moisture content level did not have a practically significant effect on the models developed to predict saltation velocity or the solids resistance factor. These models may be useful in designing control systems for cotton gin conveying systems, resulting in significant electricity and cost savings. © 2014 American Society of Agricultural and Biological Engineers.
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