Underdrain for filter tanks

Claim 1

In a filter system having an underdrain assembly facilitating a backwashing operation, a tank having a cylindrical side wall and a substantially horizontal bottom wall, a bed of filtering material contained in said tank, said assembly being located within said tank near said bottom wall, said assembly comprising a central hub having spaced upper and lower walls defining a chamber therebetween, an outlet sleeve in open communication with said chamber through said lower wall and extending outwardly of said bottom wall, a plurality of radially extending sleeves in open communication with said chamber from between said upper and lower walls, a plurality of elongated and hollow tubular elements connected to said radially extending sleeves and in open communication with said chamber, said elements having capped terminal ends and lying parallel to said bottom wall, said tubular elements being spirally shaped between opposite ends thereof and lying parallel to one another and equally spaced apart between said opposite ends, said elements being of substantially equal length and said terminal ends lying closely adjacent said cylindrical side wall, and said elements having perforations of equal size in lower sides thereof, a first of said perforations of each said elements being spaced first equal distances from inner ends of said elements, the remainder of said perforations being uniformly spaced in said elements from said first perforations thereof so that said perforations in adjacent elements are equally spaced apart from said central hub to the outer periphery of said assembly, whereby backwash water fed through said outlet sleeve and out through said perforations is distributed during the backwashing operation substantially uniformly through said filter bed so that the entirety of the filter bed is completely and uniformly backwashed.