Demisting theory development and its validation of rotary thread demister
XIANG Xiaodong1,2,,, ZHAN Minjun1, ZHONG Jie1, LIU Tao2 1.School of Resource and Environmental Engineering, Wuhan University of Science and Technology, Wuhan 430081, China 2.State Environmental Protection Key Laboratory of Mineral Metallurgical Resources Utilization and Pollution Control, Wuhan 430081, China
Abstract:Based on the particle capture mechanisms of an isolated cylinder normal to the gas flow and the particle motion equations in vortex flow, the theoretical demisting efficiencies of the interception, the inertial impaction, the centrifugal separation, and the combination demisting efficiency of the independent mechanisms of a rotary thread demister were developed. To test the theoretical demisting efficiencies, a rotary thread demister with diameter of 500 mm was used in this experiment. Two rotary thread layers were arranged in the rotary thread demister. The threads were made from polyester with diameter of 3 mm. The rotary speed of the threads ranges from 250 to 800 r·min?1. The medium diameter of the dorplets is 20.1 μm. The inlet concentration of water sprary is about 4100 mg·m?3. The gas velocity in the rotary thread demister is 2.5 m·s?1. Experimental results indicated that the overall separation efficiency could be greatly enhanced by increasing the thread number. For instance, when the rotary speed was 500 r·min?1, the overall efficiencies of the double rotary thread layer demister with 100 threads per layer and with 200 threads per layer were 95% and 98% respectively. According to the empirical equation of the relation between the overall collection efficiency and the fractional efficiency, the approximate prediction of the fractional efficiency of the rotary thread demister can be found from the experimental value of overall collection efficiency. The comparison results show that the theoretical fractional efficiency of the combination mechanism fitted well with the empirical prediction values. In the range of droplet diameters from zero to 40 μm, the mean deviation was less than 1.4%. From the comparison of the independent mechanisms, it was found that the inertial impaction was predominant, the centrifugal separation was lesser, while the interception was negligible since the interception efficiency was only about 2.5% for 20 μm droplet collection at 200 threads per layer. The centrifugal separation efficiency of the vortex flow created by the fast spinning threads followed the exponential mode which was the function of the square of drop diameter and rotary speed. Thus, the centrifugal separation effect is distinct for larger water droplets. Key words:rotary thread demister/ interception/ inertial impaction/ centrifugal separation/ demisting efficiency.
图1双层旋线除雾器除雾原理示意图 Figure1.Schematic diagram of a rotary thread demister with two layers
图8双层旋线除雾总效率与旋线转速的关系(旋线直径3 mm,长度250 mm) Figure8.Relationship between the demisting efficiency and the rotating speed of rotary thread with two rotary thread layers (diameter: 3 mm, length: 250 mm)
图9双层旋线分级除雾效率理论值与经验值对比(500 r·min?1) Figure9.Comparison of theoretical and empirical fractional demisting efficiency values in a demister with two rotary thread layers (500 r·min?1)
图10双层旋线除雾器拦截、惯性碰撞、离心分离和复合机理除雾效率的比较(500 r·min?1) Figure10.Demisting efficiency comparison of interception, inertial impaction, centrifugal separation, and combination mechanism in a demister with two rotary thread layers (500 r·min?1)
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1.School of Resource and Environmental Engineering, Wuhan University of Science and Technology, Wuhan 430081, China 2.State Environmental Protection Key Laboratory of Mineral Metallurgical Resources Utilization and Pollution Control, Wuhan 430081, China Received Date: 2020-09-29 Accepted Date: 2021-03-09 Available Online: 2021-09-15 Keywords:rotary thread demister/ interception/ inertial impaction/ centrifugal separation/ demisting efficiency Abstract:Based on the particle capture mechanisms of an isolated cylinder normal to the gas flow and the particle motion equations in vortex flow, the theoretical demisting efficiencies of the interception, the inertial impaction, the centrifugal separation, and the combination demisting efficiency of the independent mechanisms of a rotary thread demister were developed. To test the theoretical demisting efficiencies, a rotary thread demister with diameter of 500 mm was used in this experiment. Two rotary thread layers were arranged in the rotary thread demister. The threads were made from polyester with diameter of 3 mm. The rotary speed of the threads ranges from 250 to 800 r·min?1. The medium diameter of the dorplets is 20.1 μm. The inlet concentration of water sprary is about 4100 mg·m?3. The gas velocity in the rotary thread demister is 2.5 m·s?1. Experimental results indicated that the overall separation efficiency could be greatly enhanced by increasing the thread number. For instance, when the rotary speed was 500 r·min?1, the overall efficiencies of the double rotary thread layer demister with 100 threads per layer and with 200 threads per layer were 95% and 98% respectively. According to the empirical equation of the relation between the overall collection efficiency and the fractional efficiency, the approximate prediction of the fractional efficiency of the rotary thread demister can be found from the experimental value of overall collection efficiency. The comparison results show that the theoretical fractional efficiency of the combination mechanism fitted well with the empirical prediction values. In the range of droplet diameters from zero to 40 μm, the mean deviation was less than 1.4%. From the comparison of the independent mechanisms, it was found that the inertial impaction was predominant, the centrifugal separation was lesser, while the interception was negligible since the interception efficiency was only about 2.5% for 20 μm droplet collection at 200 threads per layer. The centrifugal separation efficiency of the vortex flow created by the fast spinning threads followed the exponential mode which was the function of the square of drop diameter and rotary speed. Thus, the centrifugal separation effect is distinct for larger water droplets.