Effect of Cloth Underdrainage

Where a cloth is resting on a flat impervious surface, the filtrate and associated particles will have a certain distance to travel before entering a drainage port or channel. 

Thus for a fabric of thickness X cm resting on a slotted surface with drainage ribs SX cm wide, particles may become trapped in the tortuous pores existing between the underside of the cloth and the plain solid support. Generally undrained zones greater in width than four times the cloth thickness should be avoided this principle is inherent in the relatively more ef cient performance of dimpled or pipped drainage plates conqiared with vertical channels in pressure filter plates. 

The presence of a coarse backing cloth, e.g., 14 mesh screen of metal or plastic under a 1/16 in thick cloth wiU improve the escape of fluid and particles fi om the underside of the medimn. 

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