In filtering, many arrangements have been proposed employing hydrostatic pressure and gravity to force a liquid through a porous septum or medium. The prior art in this particular field is probably best represented by U.S. Pat. No. 3,523,077; British Pat. Nos. 1,298,581 and 1,270,842; and an article in Filtration and Separation, Nov-Dec 1971, page 782. This last-cited reference broadly teaches the use of hydrostatic pressure and gravity for filtering wherein two vertically disposed filter cloths are joined at their edges to form a funnel-like arrangement (Manor-Guva tower press).
In this last-cited reference, the seal for the edges of the filter cloths is accomplished by pressure clamping. The entire filter includes top, center and bottom sections. The cloths are sandwiched with belts or clamped together. The center cloth section is supported from the back but filled with fluid (feed) in the center. This Guva tower traveling vertical double belt filter requires a balance of height (hydrostatic pressure) and belt speed to match a given feed so that a filter cake will be produced at the bottom section. The cake in the discharge gap of the bottom section must be stiff enough to resist the hydrostatic pressure and not be extruded or blown out the bottom. Variable gap pressure rolls at the bottom section of the Guva tower press can compress a cake of variable thickness depending upon the plastic flow characteristics of the produced cake. Broadly, the lower portion of the Guva tower press employs a parastolic action with the varying compression of the rolls. It appears that the Guva tower press has no problem discharging the cake, but it must insure that there is no liquid core present in the bottom section. If there is, liquid will be discharged with the cake. Therefore, Guva teaches the compaction due to hydrostatic pressure and gravity upstream of the bottom section utilizing the pressure rolls which squeeze the porous cake. The steadily increasing pressure of Guva which forms a compacted ribbon of cake is limited by the height of the chamber above the pressure rolls and the ability of the edge seals to hold. The British patents broadly refer to inherent hydrostatic pressure in forcing the liquid through the filter cloth. Further, the essence of the British patents is directed to a press zone that tapers.
The reason that the British references and the Guva tower press require this tapering and/or squeezing action is that the liquid core in the center of the column of feed to be filtered cannot be allowed at the discharge end. That is, the cakes formed within the filter must be pushed together to form a stiff cake that does not flow axially. This is done to prevent blow-out at the bottom. However, it also prevents any transmission of hydrostatic pressure from the point where the cake becomes stiff and stops flowing. No further pressure build-up can occur if the cross-sectional area of the filter remains the same. With these prior art devices, with a constant cross-section there is no further filtering or pressing of the cake. Therefore, the solution of the prior art is a decreasing taper or pressure rolls so that additional pressure can be applied. The tapering of the filter and/or the use of external pressure rolls add to the cost and difficulty of filtering with such apparatus.