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Press filters are used more frequently each time, despite the discontinuous character of their operation and their high investment cost. Their development is explained by the following reasons,
1.-Increasingly mechanization of the appliances up to the point to need a mimimum of manpower or supervision for the tortas discharge operations or for the washing of fabrics. (In situ automatic washing).
2.-Necessity to obtain tortas of great dryness, to allow:
– La autocombustion of sludges in their incineration.
– The reduction if the fuel consumption in the case of thermal dry.
– A more confortable discharge.
– The use of sludges as acarreo material.
– A limitation in the transportation costs of the dehydratated sludges.
The press filter is the only equipment which guarantees very high effective pressures (16 bars and above), with which maximum dryness of torta is achieved. (Generally above 30 %).
Press filters usually used to dry sludge are of the easy mechanization plaques type for the discharge of the tortas.
A filter is usually constituted by a battery of vertical plates strongly supported between them.
Over the two faces of these plaques, filtrating fabrics are applied. Sludges arrive a presión to the cámaras created between two adjacent plaques.
The filtrado recovered by the channels, in the back part of the fabrics, is evacuated by means of conduits arranged through the plaques.
Such plaques also consist of orifices whose alignment constitutes the sludge feeding ducts. These orifices are arranged at the center or at the periphery of the plaques. With the central feeding a better distribution across all the plaque is achieved, of caudal, of pressure and hence, of drainage.
The fabrics generally are tejidos of synthetic fibers. The careful selection of the quality of the fabric has great influence in the yield of filtración a presión installation.
The space that exists between two plaques, in its central hollow part, corresponds to the thickness of the torta. The thickness can oscillate between 30 and 20 mm, depending on the specific resistance, more or less elevated from the sludge and the fixed duration of the cycle.
Filtration cycle:
Filling.- during this phase of relatively short duration (3 to 10 % of total cycle), the filtration cámaras, limited by the outside faces of the fabrics of two adjacent plaques, are filled with sludge to filter. The filling time depends on the caudal of the feeding bomb.
Filtration.- once the cámaras are filled, the continuous introduction of the suspension raises the pressure inside the cámaras due to the closure of the sludge’s pores, which concentrates.
Discharge.- this operation consists in opening the filter. The plaques are separated one from another and the tortas formed between two plaques are successively expelled.

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