Trommel screen is one of the most frequently-used sieving equipment in the treatment of solid waste. It adopts cylindrical screen body to classify the solid waste according to different particle sizes. Under the effect of gravity, undersize solid waste (fine material) passes through sieve mesh and drops into the receiving hopper at the bottom, while the oversize waste (coarse material) will go out from the outlet at the tail of trommel screen. 1. The material motion states of common trommel screen can be divided into three types.
1) The state of sink down
In this state, the rotation speed of machine is very low. Due to circular motion, the material tumbles above the upward-moving particles. In this case, the material mixing is inadequacy, which makes it difficult that in-between fine material tumbles and moves towards the screen cloth, leading to extremely low screening efficiency.
2) The state of throw down
When the rotational speed of trommel screen is high enough but lower than critical speed, the material will overcome the gravity and move upwards up to the highest point of revolving drum. In this case, the gravity exceeds the centrifugal force, the material drops along the parabolic trajectory and comes back to the bottom of revolving drum. Since particle tumbling is the most vigorous in this case and material accumulation phenomenon rarely occurs, so the screening efficiency is the best.
3) Centrifugal state
When the rotational speed of trommel screen increases up to critical speed, the material will be attached to the cylinder wall and can't tumble and drop, so the screening efficiency becomes very low. 2. The design of municipal solid waste trommel screen is improved on the basis of common equipment, adding the structure of some sorting or cleaning, which makes the machine more suitable for municipal solid waste screening. The motion of municipal solid waste in the machine is divided into two types.
1) The motion along the axis direction of screen body
The rectilinear movement along axial direction is generated from the slanting installation of screen body, its velocity is the speed of waste passing through the screen body.
2) Planar motion perpendicular to axial plane of screen body
This planar motion is closely related to the rotational speed of screen body. When the screen body rotates at the velocity that lower than critical speed, the waste moves to a certain height and makes parabolic falling, which benefits the screening. The rotational speed is generally 30% ~ 60% of critical speed.
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