There are two coal crushing systems, which are open-circuit crushing system and closed-circuit crushing system. The first system is often used for those that have not serious requirement of end product' s granularity or those that will not produce the particle too large than imagined. While the latter system is also called recycling-circuit crushing system, which is used for crushing the material with end product smaller than a given size under the situation the crusher itself cannot achieve this requirement. In the closed-circuit system, the end product should be sieved through the screen, those coal which cannot through the screen should be transported into crusher cavity for secondary crushing until through the screen. The coal selecting plant often use open-circuit crushing system.