Title: Pneumatic Conveying of Seed Cotton: Minimum Velocity and Pressure Drop Author
Submitted to: Transactions of the ASABE
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: April 10, 2014
Publication Date: N/A
Interpretive Summary: Electricity is a major cost for cotton gins, accounting for approximately 20% of variable costs. Over 50% of the electricity used at gins powers centrifugal fans that are used for pneumatic conveying of seed cotton, lint, seed, and foreign matter, generating air flow that moves material through ductwork. A small reduction in conveying velocity will significantly decrease the energy required to convey material. Advances in control technologies, primarily the development of lower cost adjustable-speed drives for large electric motors, could make fan speed control feasible in cotton gins. However, further research on pneumatic conveying in cotton gins is needed for the development of these control systems. Models were developed that accurately predicted the minimum velocity needed to convey seed cotton and the additional pressure required when conveying seed cotton as a function of seed cotton mass flow rate, pipe diameter, and air density. Cotton cultivar, moisture content (from 8.2-11.1%), and pipe diameter did not have a practical effect on the parameters of these models. For a representative conveying system in a commercial gin, with a pipe diameter of 0.508 m, an air density of 0.95 kg m-3 (air is heated for drying), and a mass flow rate of 3.7 kg s-1 (approximately 21 bales per hour), the minimum conveying velocity predicted by the model was 14.9 m s-1. A typical velocity used in conveying seed cotton in gins is 22.9 m s-1. If the conveying velocity was reduced to 10% above the minimum (to provide a safe operating margin), the air velocity would be reduced 28%. The estimated reduction in the pressure required by the conveying system would be 35%, resulting in a total reduction in energy consumption of 53% for the fan in this conveying system. The models developed in this research will enable the development of fan speed control systems for pneumatic conveying in cotton gins. These systems will use lower air velocities under appropriate conditions, thus reducing energy use and the air volume that must be treated by cyclones to reduce dust emissions. Furthermore, fan speed control systems should reduce the likelihood of choking conveying pipes during adverse conditions, such as seed cotton with high moisture content. The improved understanding of pneumatic conveying theory resulting from this study will lead to more energy-efficient gin designs.
Technical Abstract: 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 theory of pneumatic conveying 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 outlet 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: '=8.90*10-5*Frmin5.04, 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.