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Two precoat filter techniques are currently dominating the global scene. Both originating in the USA, for decades they have been – right or wrong – the Bible of plant design. The first technique, which we shall refer to as A, is almost the only type used in the ENEL (Italian Electricity Board) plants. This filter has an extremely compact design concept in which the fluid to be treated enters the filter at the bottom and the treated fluid also exits from the bottom. In this type of filter, the filter elements, which represent the heart of the filtering process, are screwed or clumped to the bottom plate. Coating is normally performed useing a single tank in which the precoat suspension is prepared prior to being pumped out onto the filter elements. The latest backwashing systems for this type of filter operates on the principle of impulses which require an external compressed air storage and dosage line which forces water through the filter elements.

The second method, which we shall refer to as B, consists of a filter whose elements are anchored to the top plate in different ways. The fluid to be treated enters the filter at the bottom while the treated fluid exits from the top. Backwashing of the filter elements is considerably more straightforward than in the previous method, it doesn't require external equipment other than the filter itself, in which water and compressed air are stored in the upper head.

precoat-filter-system precoat-filter-system

There are also considerable differences in the coating process of the two different types of filter. Coating of type B filters is normally performed useing two auxiliary tanks: the first providing circulation of the suspension transport fluid by means of a high-service circulation pump and the second in which the suspension is prepared before being dosed at a constant rate through the suction intake of the circulation pump. This dosage system ensures exceptionally accurate coating. The constant-concentration dosage of the suspension containing powdered resins and/or inert products ensures perfect distribution of the coating on the filter elements and thus a regular, homogeneous thickness of the filter elements coat with several benefits: optimum filter element performance, longer life of filter elements, excellent chemical-physical results from the precoat layer.

The results obtained through B-system technology, especially in nuclear power plants, are exceptional both in terms of capture of suspended solids and dissolved radionuclides, which often is a crucial factor. This is unquestionably due to the different flow distribution in system B, ensuring a more homogeneous, immediate involvement of the entire filter elements bundle.