The five basic discharge types are:

• SCRAPER DISCHARGE
• BELT DISCHARGE
• STRING DISCHARGE
• ROLLER DISCHARGE

• PRE COAT DISCHARGE
  

SCRAPER DISCHARGE

The LOVELY PROJECT AND ENGINEERING. Scraper discharge RVDF medium is usually caulked into grooves in the drum grid, with cake removal facilitated by a scraper blade just prior to the re-submergence of the drum.

The scraper serves mainly as a deflector to direct the cake, dislodged by an air blowback, into the discharge chute, since actual contact with the medium would cause rapid wear. In some cases the filter medium is held by circumferentially wound wires spaced 50 mm (2 in) apart, and a flexible scraper blade may rest lightly against the wire winding.

Excellent for heavy solids and typical slurries are easy to filter.

BELT DISCHARGE

The LOVELY PROJECT AND ENGINEERING. Belt discharge RVDF carrying a fabric that is removed, passed over rollers, washed, and returned to the drum. A special aligning device keeps the medium wrinkle-free and in proper line during its travel. Thin cakes of difficult solids which may be slightly soluble are good applications. When acceptable, a sluice discharge makes cakes as thin as 1.5 to 2 mm (about gin) feasible.

STRING DISCHARGE

The LOVELY PROJECT AND ENGINEERING. String discharge RVDF system of endless strings or wires spaced about 13 mm (0.5 in) apart pass around the filter drum but are separated tangentially from the drum at the point of cake discharge, lifting the cake off as they leave contact with the drum.

The strings return to the drum surface guided by two rollers, the cake separating from the strings as they pass over the rollers. If it has the required body, a thinner cake (5 mm) than can be handled by drum filters is feasible, allowing more difficult materials to be filtered. This is done at the expense of greater dead area on the drum. Success depends on the ability of the cake to be removed with the strings and must be determined experimentally.

Applications are mainly in the starch and pharmaceutical industries, with some in the metallurgical field.

ROLLER DISCHARGE

A roll in close proximity to the drum at the point of cake discharge rotates in the opposite direction at a peripheral speed equal to or slightly faster than that of the drum.

If the cake on the drum is adequately tacky and cohesive for this discharge technique, it adheres to cake on the smaller roll and separates from the drum. A blade or taut wire removes the material from the discharge roll.

This design is especially good for thin, sticky cakes. If necessary, a slight air blow may be provided to help release the cake from the drum. Typical cake thickness is 1 to 10 mm (0.04 to 0.4 in).

PRE COAT DISCHARGE

The LOVELY PROJECT AND ENGINEERING Pre-coat drum filters are coated with a bed of diatomaceous earth or similar material. During cycling a clear filtrate is obtained. When vacuum is applied, the liquid is drawn through the pre-coat material, and solids deposited on the pre-coat surface are removed along with a thin portion of the pre-coat by an advancing scraper blade, as the drum revolves.

The blade operates independently of the drive to provide a “precision lathe: movement. Depending on the movement of the blade the drum can operate up to a week without retracting the blade to apply a new pre-coat bed.

Pre-coat filters are recommended when small amounts of solids are to be removed from large volumes of liquids, for clarification or polishing of effluent, or for the filtration of material that produces sticky solids.

They provide a high clarity of filtrate when the filtrate is the valuable product and the cake is to be discarded.