- Bed Depth of material in a screen for efficient screening:
The bed depth of the material on the screen affects the efficiency and the performance of a screen. The feed end of the screen surface is overloaded while the rest of the screen surface is thinly spread with the material. The depth of the bed, the rate of feed and the inclination of the screen are of major importance to the screen operation.
- Too thick a bed will tend to delay stratification.
- Too thin a bed reduces the efficiency as it allows unconstrained movement of particles on the screen.
Thumb Rules for Bed depth:
Relation of bed depth to Bulk density of material w.r.t. screen Aperture:
- For Bulk density of material above 1.6 t/m3 , the bed depth is less 3 times the aperture size. Example: If we are screening Lime stone with BD of 1.6t/m3 in a screen of size 16x16mm then the bed depth should be less then 16 x 3 = 48 mm.
- For Bulk density of material between 0.8 to 1.6 t/m3, the bed depth is less 2 times the aperture size. Example: If we are screening fine powder with BD of 1.0 t/m3 in a screen of size 5 x 5 mm then the bed depth should be less than 5 x 2 = 10 mm.
Relation of bed depth to slope of the screen:
- If the screen is inclined at an angle below 15 degree to horizontal and if the screen length is less than 4 meters, then bed depth should be 2 times the aperture size, if the same screen the length is increased to 6 meters then bed depth can be increased to 3 times the aperture.
- If the screen is inclined at an angle above 15 degree to horizontal and if the screen length is less than 4 meters ,then bed depth should be 1.5 times the aperture size, if the same screen the length is increased to 6 meters then bed depth can be increased to 2.5 times the aperture.(Example: In a screen the length is 5.5 meters and inclined to an angle of 17 degree to horizontal having a mesh of 10 x10mm .The bed depth should be 10 x 2.5 = 25mm approximately for efficient screening.)
Relation of bed depth with particle MPS (Mean Particle Size)
- For screens of short length below 4meters the bed depth is 1.5 times the MPS and for screens above 2.5 times the MPS.
