1.  Field of the Invention
This application relates broadly to improvements in automatic backwash filtering systems.  More particularly, it concerns improved filter cell sheet installations in such systems. 
2.  Description of the Prior Art
Automatic backwash filter systems are known in the art as exemplified in U.S. Pat.  Nos.  3,239,061; 4,133,766; 4,617,131 and 4,764,288 currently assigned to Infilco Degremont Inc., the assignee of the invention disclosed herein.  Such systems are further exemplified in Infilco Degremont's brochure DB-370, dated May, 1989, entitled ABW.RTM.  Automatic Backwash Filter.  The disclosures of these patents and the brochure are incorporated herein by reference. 
In the backwash filter systems, cell sheets that divide the total filter bed into compartmented filter cells have a tendency to deflect (bow) when the cells are being backwashed.  The resulting movement can cause seals associated with porous rigid plates, that are fixed at a level between the upper and lower ends of the cell sheets and transverse thereto, to fail allowing a media leak.  In other cases, the cell sheets may bow towards each other which causes high backwash flow rates that, in turn, promotes media loss through the washwater systems.  When the cell sheets bow away from each other, the backwash flow rate is reduced causing insufficient media expansion, hence allowing the media to become dirty.  Of course, poor installation caused by difficulties in handling of parts forming the systems can also result in cell sheet bowage. 
In an effort to mitigate the bowage problems with cell sheets in the subject filter systems, filter manufactures and fabricators, have resorted to use of nut and bolt tie rod assemblies.  Each such assembly requires two holes in the cell sheet, a rod, four washers and four nuts.  Each assembly must be put together one piece at a time, a precise measurement must be made and all nuts must be locked.  Any such assembly which is not completely tight can be loosened and negate its purpose.  Also if strict attention is not paid to measurements, compressive forces can allow sheets to bow. 
The present invention addresses the problem of cell sheet bowage in automatic backwash filter systems and provides improved installations for the cell sheets that greatly simplify cell sheet installation, reduce the number of required parts, mitigate measurement requirements and reduce assembly costs.