Experimental Study On Gas-solid Separation Characteristics Of Rotary Air Classifiers

Rotary air classifiers are a kind of separators which separate particles withsuitable particle size away from powder material using centrifugal force andcollision effect. Until now, rotary air classifiers have been widely used in coal,cement, food and pharmaceutical industries. Rotary pulverized coal classifiersare important auxiliary equipments of coal pulverizing systems in power plants,the classification performance of rotary air classifiers directly affects theoutput and economic operation of the coal pulverizing system. Therefore, thestudy on rotary air classifier classification performance is necessary.This thesis designed two rotary air classifiers with different structure (RigsI and II), and built a test system to study the respective separationperformances of these two rotary classifiers. Experiments found the influencesof the structure and operating parameters on the separation performance.Based on the experimental measurements, the mechanism of the influence ofon the classification performance was discussed.The velocity field between two adjacent blades in rotary classifiers wasmeasured using the particle image velocimetry (PIV) technique equipped witha self-developed synchronizer. It was found that the vortex swirling flowexisted between the two rotary blades or in the near annular region of Rig Iclassifier and in the two adjacent stationary blades of Rig II classifier. Thevortex center and swirling intensity depended on the impeller rotational speed, the air flowing rate and the location of measurement plane. The vortex centerin the two rotor blades of Rig I classifier would move regularly with thechange of the impeller rotation speed, and the air flow through rotary bladeschannel first decreased and then increased when the impeller rotation speedincreased. There was no vortex in the rotary blades channel of Rig II classifier,while the change of S influenced the movement direction of airflow in thechannel. When the impeller rotation speed increased, the air flowing throughrotary blades channel decreased.The classification performance was characterized by the escape efficiencyη and the cut size d50of the classifiers. The results showed that in Rig Iclassifiers, the overall escape efficiency ηefirst decreased with the increasing Sand then increased with the further increase of S. The minimum was35.5%.Similar non-monotonic variation trend was found for d50of the escapedparticles. The minimum of d50was30.6μm. It was found that when S increased,the variation of ηeand d50of the escaped particles for Rig I classifiers agreedwith the change of air flowing through the blade passage. This was also true inRig II classifiers. Both of its ηeand d50of the escaped particles monotonicallydecreased with S. The minimum of its ηeand d50was43.8%and53.7μmrespectively.Both ηeand d50of the escaped particles decreased significantly with theincreasing inlet solid concentration Cswhen Cswas smaller than a criticalvalue and then barely changed or even slightly increased when Cskeptincreasing. This conclusion was suitable for both Rig I and Rig II classifiers.The grade escaped efficiency ηe,gof Rigs I and II classifiersmonotonically decreased with the increasing Cswhile variednon-monotonically with S. The result of the grade escaped efficiencydemonstrated that in different Cs, there would be an optimal S for airclassification. The relationship between Csand S should be obtained before the air classifier is put in industrial application for the optimal performance.The classification performance could be well explained by the velocityfield changes near the entrance of the impeller and particles aggregation, underspecific operational conditions in the classifier. The experiment resultsprovided references for further optimizing the rotary air classifier structure. Atthe same time, the results of the classification performance experiments couldbe used to guide the actual operation in certain degree.