Pneumatic Conveying of Seed Cotton: Minimum Velocity and Pressure Drop

Authors: Hardin Iv, Robert

Submitted to: Proceedings of the American Society of Agricultural and Biological Engineers International (ASABE)
Publication Type: Proceedings
Publication Acceptance Date: 8/4/2014
Publication Date: 8/4/2014
Citation: Hardin IV, R.G. 2014. Pneumatic Conveying of Seed Cotton: Minimum Velocity and Pressure Drop. Proceedings of the American Society of Agricultural and Biological Engineers International (ASABE). Paper No. 131620843.

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. Pneumatic conveying systems are used for most material handling at gins because the layout of these systems is flexible and can easily be changed, installation and maintenance costs are low, and the enclosed conveying system prevents contamination and reduces dust levels inside the gin building. The primary disadvantage of pneumatic conveying systems is their significantly higher power consumption than mechanical conveying systems, such as belt and screw conveyors. When moving air only, the power required by a centrifugal fan varies with the fan speed cubed, while the volumetric flow rate and resulting air velocity varies linearly with the fan speed. Consequently, a small reduction in fan speed and air velocity can greatly reduce energy requirements and could produce significant cost savings at cotton gins. For example, a 20% reduction in air velocity reduces power consumption by nearly 50%. Advances in control technologies, primarily the development of lower cost adjustable-speed drives for AC motors, could make fan speed control feasible in cotton gins. However, the theory and design of pneumatic conveying systems in cotton gins is poorly understood. The goal of this research was to develop an improved understanding of the pneumatic conveying characteristics of seed cotton so that cotton gin conveying systems can be designed that 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, improved gin conveying systems should reduce the likelihood of choking during adverse conditions. Specific objectives of this research were to develop models to predict the minimum velocity needed to convey seed cotton and the additional pressure drop due to conveying solids and to quantify the effects of pipe diameter, cultivar, and moisture content on these models. A conveying system was constructed with a feed control, conveying pipe, separator, and fan. Air velocity was measured at several locations in the system, a differential pressure measurement was taken in the conveying pipe, and temperature and relative humidity were recorded. Two pipe diameters (17.8 and 25.4 cm), two cultivars (DP 161 and ST 4554), two moisture content levels (8.2% and 11.1%), and three seed cotton feed rates (22.7, 45.4 and 68.1 kg min-1) 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 minimum conveying velocity. A segmented linear model was fit to the log transformed data (R2=0.88) to identify the combination of velocity, mass flow rate, diameter, and air density that corresponded to the minimum pressure. This resulted in an equation for the minimum velocity in terms of mass flow rate, air density, and pipe diameter. The pressure drop per length of pipe due to the conveyed seed cotton depended on the mass flow rate and the pipe diameter. This model had an R2 of 0.91 and predicted the pressure drop with 14.2% error. Diameter, cultivar, and moisture content level did not have a practically significant effect on the models developed to predict saltation velocity or the pressure drop due to the conveyed material. These models may be useful in designing control systems for cotton gin conveying systems, resulting in significant electricity and cost savings. 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 dryi

Technical Abstract: Electricity is major cost for cotton gins, representing approximately 20% of the industry’s 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 (8.2% and 11.1%), and three seed cotton feed rates were tested. 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: '=8.8974*10-5*Frmin5.0389, where Frmin is the Fr at the saltation velocity. The solids resistance factor at velocities greater than saltation was found to be 0.1787*Fr-1. This model had an R2 of 0.91 and predicted the pressure drop with 14.2% error. 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. Pneumatic conveying systems may be automated using this model, which could produce significant cost and energy savings at cotton gins.