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ARS Home » Southeast Area » Stoneville, Mississippi » Cotton Ginning Research » Research » Publications at this Location » Publication #310757

Title: Determining seed cotton mass flow rate by pressure drop across a blowbox

Author
item Hardin Iv, Robert

Submitted to: Applied Engineering in Agriculture
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/16/2015
Publication Date: 3/1/2015
Citation: Hardin IV, R.G. 2015. Determining seed cotton mass flow rate by pressure drop across a blowbox. Applied Engineering in Agriculture. Vol. 31(4): 581-587.

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. 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. In previous testing, this prototype was only evaluated using small-scale equipment and the conveying air was not heated for drying purposes. An experiment was conducted with the mass flow rate measurement system installed in the first and second stage drying system of commercial ginning equipment. Drying temperature, seed cotton mass flow rate, air velocity, and cotton cultivar were varied to determine their effect on the accuracy of the mass flow measurement system. Mean absolute error in predicting seed cotton mass was 11.3% for the first stage system and 10.4% for the second stage system, worse than the 7.4% error obtained during previous testing with the small-scale conveying system. Cultivar had no effect on the model regression coefficients. Significant differences existed between the regression coefficients of different air velocities and feed rates for both first and second stage systems and dryer temperatures for the second stage system. The commercial ginning system utilized a combination negative and positive pressure conveying system, while the small-scale system used negative pressure conveying. Additionally, air velocity was varied in the commercial ginning system by adjusting slide valves. These factors likely affected the actual air velocity through the blowbox and the system accuracy. Modifications to the mass flow rate measurement system have been made and further testing will be conducted.

Technical Abstract: A seed cotton mass flow rate sensor would offer useful feedback for gin managers and provide a critical input for advanced process control systems. Several designs of seed cotton mass flow rate sensors have been evaluated in the laboratory, but none have found acceptance in commercial gins. The objectives of this research were to develop a system for predicting seed cotton mass flow rate based on the pressure drop measured across a blowbox; investigate the effect of duct diameter, cultivar, moisture content, feed rate, and fan speed on this relationship; and provide recommendations for developing a prototype system for testing in commercial gins. A negative pressure pneumatic conveying system was constructed, with a variable-speed feed control and fan. The inlet air velocity, blowbox pressure drop, temperature, and relative humidity were recorded during testing. A model was developed to predict the seed cotton mass flow rate based on the blowbox pressure drop, air velocity, air density, and duct cross-sectional area. The model was calibrated by conveying a known mass of seed cotton through the system and integrating the model over this time. Mean absolute error in predicting seed cotton mass was 7.35%. Cultivar and moisture content had no effect on the model regression coefficients. Significant differences existed between the regression coefficients of different feed rates; however, these only occurred at mass flow ratios much larger than achieved in commercial gins. The effect of fan speed was small and likely not practically significant. Measuring the pressure drop across a blowbox is a suitable basis for further development of a seed cotton mass flow rate sensor for commercial gins.