Method of cleaning a filter surface in situ in a pressure filtering apparatus for liquids and pressure filtering apparatus for carrying out the method

In a pressure filtering apparatus for liquids the combination of

• (a) a filter housing,
• (b) a filter element having a fine porous filter surface, said filter element being mounted in said filter housing in a stationary position in a manner to have its filter surface substantially vertically oriented and to divide the interior of said filter housing into a suspension chamber and a filtrate chamber, the filter surface of said filter element facing the suspension chamber,
• (c) each of said suspension chamber and filtrate chamber having individual outlet openings with at least the outlet opening of the suspension chamber being disposed at a level below the filter surface of said filter element,
• (d) means for supplying under pressure a liquid suspension to said suspension chamber,
• (e) an tangential inlet opening to said suspension chamber disposed in the upper part thereof,
• (f) means for alternately supplying to said inlet opening a heated gaseous drying medium under a relatively lower pressure and a gaseous cleansing medium under a relatively higher pressure, and
• (g) means for causing both of said media to flow through said suspension chamber along a path extending substantially parallel and close to the filter surface of said filter element so as to come into superficial contact with any coating formed on said surface.

A method of cleaning a filter surface in situ in a pressure filtering apparatus for liquids of the kind in which the filter surface is at least generally vertically oriented and stationarily mounted in a filter housing for dividing the interior thereof into a suspension chamber and a filtrate chamber, between which chambers a substantial pressure difference is maintained during each filtering procedure in order to separate a filtrate from a liquid suspension supplied to the suspension chamber while leaving on the filter surface a moist coating at least mainly consisting of finely divided solid substances, the cleaning operation comprising the steps of

• (A) first draining off both the liquid suspension from the suspension chamber and the filtrate from the filtrate chamber,
• (B) subsequently passing through the suspension chamber a gaseous drying medium which has been heated to a temperature ensuring rapid vaporization of the liquid content of the coating retained on the filter surface while causing said heated drying medium to flow at a velocity of at most 6 meters per second along a path extending substantially parallel to the filter surface in a manner to let said drying medium come into superficial contact with the coating thereon in order to thereby effect successive layerwise drying of the coating,
• (C) promoting detachment of dried coating portions from any moist rest of coating still remaining on the filter surface as well as from the filter surface itself by temporarily causing a gaseous cleansing medium to flow through the suspension chamber along substantially the same path as said drying medium but at a velocity of at least 30 meters per second,
• (D) removing at least a major portion of detached dried coating from the suspension chamber before again admitting liquid suspension thereto for resumed filtering, and
• (E) said filter surface at least generally taking the form of a cylindrical surface of revolution having a substantially vertical axis, wherein at least said heated drying medium when being introduced into the suspension chamber is imparted a whirling motion in order to flow over the filter surface along a substantially helical path.