Effect of cyclone separator structure and operating conditions on separation efficiency

Thread Content

The efficiency of a cyclone separator is the ratio of the amount of dust removed to the initial amount of dust present, and it is expressed by the following formula: η = (c1 – c2) / c1. In this formula, η represents the dust removal efficiency； c1--The amount of dust per unit volume of gas entering the separator, in kg/h; c2--The amount of dust per unit volume of gas leaving the separator, in kg/h. (1) Influence of the inlet pipe size: The inlet pipes of cyclone dust collectors are generally designed to be rectangular in shape. Some enter in a tangential direction, some enter along an involute curve at 180 degrees (during helical movement), and some enter in a spiral pattern at a certain angle downward (such as 15°). The latter two methods offer better dust removal efficiency as well as lower fluid resistance. According to the centrifugal force formula, F = mu²/r, where F represents the centrifugal force ； m--mass ； u--average gas velocity across the cross-section ； R--Radius of rotation. As the gas velocity in the inlet pipe increases, the rotation speed of the gas in the cyclone separator also increases accordingly. As can be seen from the formula, a higher gas velocity and a smaller diameter of the cylinder result in a greater centrifugal force, which is beneficial for dust removal. Typically, the gas velocity in the inlet pipe ranges from 14 to 25 m/s; when the dust content is high and the dust particles are fine, this range is 18 to 25 m/s. If the gas velocity is too high, the dust removal efficiency does not improve significantly, while the resistance increases greatly, and it may even stir up the dust that has already settled, thereby reducing the dust removal efficiency. When the gas velocity is too low, the centrifugal force acting on the dust is reduced, which lowers the dust removal efficiency; moreover, dust may accumulate in the inlet ducts, posing a risk of blockage. In such cases, if several cyclone separators are operated in parallel, the dusty gas will not be distributed evenly among them, thereby reducing the overall dust removal efficiency of the cyclone separators. The gas flow rate entering the cyclone dust collector is usually known; once the gas velocity is determined, the cross-sectional area of the inlet duct can be calculated. Since the dimensions of the various components of cyclone separators in different forms are in a certain proportional relationship, it is easy to determine the dimensions of each component. (2) Influence of the central exhaust pipe: The size and height position of the central exhaust pipe have a significant impact on the dust removal efficiency. The insertion depth of the exhaust pipe should be small to increase the height between the conical part of the cone and the exhaust pipe. Since the upward airflow at this height is also rotational and serves to re-separate particles, a slightly higher altitude in this region is beneficial for further dust removal. (3) Effect of the cone: The cone is the main component for dust separation. In recent years, long-cone cyclone separators have been developed, which exhibit better separation efficiency. A longer cone facilitates the downward-flowing air currents to separate the dust it contains, and it also helps to re-separate the dust that has been lifted by the upward-flowing air currents. Therefore, increasing the height of the cone improves dust removal efficiency. It is generally advisable to have L2 ≥ 2.5D. (4) Influence of the ash discharge pipe: The ash discharge pipe of the cyclone separator extends into the dust collection hopper, allowing the dust at the bottom of the cyclone dust collector to continuously flow into the dust collection hopper through this pipe. If dust accumulates at the bottom of the device, it will be lifted up by the upward airflow, thereby severely affecting the dust removal efficiency. A negative pressure is created in the central part of the cyclone separator (i.e., the upward swirling air stream area). Therefore, at the connection point between the exhaust pipe and the dust collection hopper, or at the location where the dust collection hopper discharges dust to the outside, it is essential to prevent external air from entering; otherwise, it will blow up the dust inside the hopper, resulting in a sharp decrease in dust removal efficiency. Star valves or augers (screw conveyors) are typically used for continuous ash discharge, which ensures that ambient air does not leak into the collection hopper.