Trommel screen is one of the most commonly used sieve equipment in solid waste treatment. Solid waste is classified according to particle size by screen body, and the screen body is installed obliquely when working. The solid waste entering the screen moves helically with the turning action of the sieve body. Under the action of gravity, the solid waste with particle size smaller than the sieve hole passes through the sieve hole and is screened under, while the solid waste larger than the sieve hole is discharged at the bottom of the sieve body.Screening principle of trommel screen
The motion of materials in trommel screen presents three states
1. Sinking state
At this time, the rotating speed of the screen is very low, the particles are picked up due to the circular movement of the screen, and then roll onto the upward movement of the particles, the material mixing is not enough, it is not easy to make the middle of the fine rolling material moved to the edge and touch the screen hole, so the screening efficiency is very low.
2. Dropping state
When speed is high enough, but less than the critical speed, the material particles along the screen wall, to overcome the gravity until it reached its acme screen before, at this time gravity exceeds centrifugal force, particles along a parabolic trajectory fall back into the bottom of the screen, and thus the material particles the most violent rolling degree, accumulation occur rarely, and trommel screen has the highest screening efficiency.
3. Centrifugal state
When the speed of the sieve increases further and reaches a critical speed, the material cannot fall and roll due to the centrifugal effect attached to the cylinder wall, thus resulting in low efficiency.
The trommel screen
for municipal solid waste is to add some sorting or cleaning mechanism on the basis of the ordinary trommel screen
to make it more suitable for the screening of household garbage. There are mainly three kinds of horizontal rotating trommel screen, vertical trommel screen and blade trommel screen. The motion of garbage in the trommel screen can be divided into the motion along the axis of the screen and the plane motion perpendicular to the plane of the axis of the screen. The linear motion along the axis direction of the screen body is caused by the inclined installation of trommel screen, and its speed is the speed of garbage passing through the screen body. The motion of garbage in the plane perpendicular to the axis of the sieve is closely related to the speed of the screen. When the cylinder always rotates at a lower critical speed, the garbage is carried to a certain height and falls in a parabola, which is conducive to the screening. The general rotating speed of trommel screen is 30% to 60% of the critical speed, which is slightly lower than the speed required for garbage material to obtain the maximum drop.
Trommel screen has two important technological indexes. One is the processing capacity, that is, the processing capacity of the screen with a certain aperture in a certain period of time on a certain unit area. he other is the screening efficiency, which indicates the quality index of the screening work. Theoretically, all the fine particles in solid waste with particle size smaller than the size of sieve hole should pass through the sieve hole and become the product under the sieve, while the coarse particles larger than the size of sieve hole should be left on the sieve and discharged into the product on the sieve. However, in fact, due to various factors during the screening process, there will always be some fine particles smaller than the screen hole left on the screen and discharged into the screen. The more fine particles in the screen that do not pass through the screen hole, the worse screening effect trommel screen has.