Screens, blinding capacity

Several indicators are utilized to determine the point at which the filter space is filled with solids from the bleached oil when the pressure drop across the leaves reaches a predetermined level, when a predetermined decrease in flow rate occurs, or when a calculated load level is reached. Short cycles or premature filter stoppages are usually the result of (i) inadequate body feed (ii) too high flow rate, which can cause the solids to pack (iii) too low flow rate, which can allow the solids to settle and block the flow rate (iv) blinded screens, which reduce the filter surface area or (v) solid load exceeding filter capacity. The perfect circumstance is when the differential pressure is reached, and the flow rate is severely reduced at the same time that the calculated filter capacity is exhausted. Once the filter cycle is complete, the filter cake must be removed and the process repeated all over again (Butterworth, 1978). 

Rotating microscreen batch, fluid loading 3 to 6 L/s m of submerged area usually 66% area submerged solids loading 0.05 to 0.1 g/s m headless 7 to 14 cm to max. of 45 cm. Clean by backwash at 2 to 5 % volumetric throughput capacity. Screen pack downstream of extruders screen pack area directly proportional to rate of extrusion. Cycle time for the screen to blind > time between shutdowns. For example, screen pack area 0.17 m s/kg and screen pack diameter 20 cm s/kg but must be custom selected for the polymer. 

If damp or sticky, predry or use heater above the screen to reduce moisture to <3%. Avoid resonance frequencies. Usual angle of operation is 12 to 18° for wet, inclined vibrating screen to 7 to 11°. Capacity decreases if the angle of inclination is too high. Blinding is mainly caused by material that is 1 to 1.5 times the hole size. Feed thickness should not exceed 4 X aperture size for 1.6 Mg/m and not exceed 2 to 3 X aperture size for 0.8 Mg/m. ... 

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