Determining Seed Cotton Mass Flow Rate by Pressure Drop Across the Blowbox: Gin Testing

Authors: Hardin Iv, Robert

Submitted to: National Cotton Council Beltwide Cotton Conference
Publication Type: Proceedings
Publication Acceptance Date: 4/1/2016
Publication Date: 5/1/2016
Citation: Hardin IV, R.G. 2016. Determining Seed Cotton Mass Flow Rate by Pressure Drop Across the Blowbox: Gin Testing. Proceeding for Beltwide Cotton Conference, January 5-7, 2016, New Orleans. p.872-880.

Interpretive Summary: An accurate measurement of seed cotton mass flow rate would be useful for gin operators, providing feedback on gin performance. The development of improved process control systems for cotton gins also requires accurate measurement of seed cotton mass flow rate. Previous research developed a mass flow rate measurement system for seed cotton based on the energy required to accelerate the material when introduced into the conveying air stream, but improvements were needed for use in commercial gins. The energy loss of the air stream can be determined by measuring the static pressure difference in the conveying system over the distance the material is accelerated from rest to its maximum velocity. This pressure difference should be proportional to the mass flow rate of material and the air velocity. The feeding system for seed cotton in gins is well-suited to making the necessary measurements. The cause of this increased error in previous testing was identified as a varying volume of air leakage at the blowbox. The volume of air leakage was modeled as a function of the static pressure at the blowbox and the density of the air leaking through the vacuum dropper. This relationship was incorporated into the model for seed cotton mass flow rate. Using the improved system, an experiment was conducted in the commercial size gin at the Cotton Ginning Research Unit to test the effect of cultivar, air velocity, seed cotton mass flow rate, and dryer temperature on accuracy of the mass flow measurement system. Mean absolute error for the improved system in predicting the conveyed seed cotton mass was 5.3% for the first stage conveying system and 2.5% for the second stage system. Dryer temperature had a significant effect on the model parameters, likely due to errors in density from rapid heat exchange between heated air and seed cotton. The system was also tested at a commercial gin in 2014. Although seed cotton was not weighed at this gin, a method for calibrating the system was developed.

Technical Abstract: Accurate measurement of the mass flow rate of seed cotton is needed for control and monitoring purposes in gins. A system was developed that accurately predicted mass flow rate based on the static pressure drop measured across the blowbox and the air velocity and temperature entering the blowbox using a small-scale pneumatic conveying system. An initial experiment was conducted using commercial ginning equipment, but the mean absolute error in predicting the mass of seed cotton conveyed was much higher than in the laboratory. The cause of this increased error was identified as a varying volume of air leakage at the blowbox. The volume of air leakage was modeled as a function of the static pressure at the blowbox and the density of the air leaking through the vacuum dropper. This relationship was incorporated into the model for seed cotton mass flow rate. Using the improved system, an experiment was conducted in the commercial size gin at the Cotton Ginning Research Unit to test the effect of cultivar, air velocity, seed cotton mass flow rate, and dryer temperature on accuracy of the mass flow measurement system. Mean absolute error for the improved system in predicting the conveyed seed cotton mass was 5.3% for the first stage conveying system and 2.5% for the second stage system. Dryer temperature had a significant effect on the model parameters, likely due to errors in density from rapid heat exchange between heated air and seed cotton. The system was also tested at a commercial gin in 2014. Although seed cotton was not weighed at this gin, a method for calibrating the system was developed.