Filter unit for use in liquid purification apparatus

Claim 1

A filter unit for filtering a particle-containing liquid, comprising: a casing, arranged with its axis vertical, and defining a first closed chamber and an inlet for said particle-containing liquid; a normally-closed discharge valve located in a lower part of said casing and arranged to discharge liquid from said first chamber; a perforated cylindrical wall located within, and coaxial with, said casing; a non-perforated cylindrical wall having a diameter less than said perforated cylindrical wall coaxially disposed within said perforated cylindrical wall and projecting above said perforated cylindrical wall and said casing; an annular wall closing the upper end of said casing and an annular chamber portion between said cylindrical walls; upper and lower end walls defining with said perforated cylindrical wall, said non-perforated cylindrical wall and said annular wall a second chamber, said non-perforated wall defining an outlet for liquid filtered by said filter unit; a filter element covering said perforated cylindrical wall and arranged to retain particles in said particle-containing liquid fed into the first chamber through the inlet and to allow substantially particle-free liquid to flow therethrough into the second chamber to be discharged through said outlet; a piston slidable axially within said second chamber; a rod connecting said piston to said discharge valve; and a control actuator located above said upper end wall of said second chamber and connected to said piston to drive the latter so as to expel liquid from said second chamber through said filter element, in counter-flow to the normal flow of liquid therethrough, to cleanse the filter element of particles retained thereon, and simultaneously to open said discharge valve to expel particle-containing liquid from said first chamber therethrough; said outlet from said second chamber being connected to a pipe union; a tubular body connected at one end thereof to said pipe union and at the other end thereof to define an opening to the atmosphere; and a non-return valve provided in said tubular body, said non-return valve being openable to allow air therethrough into said pipe union.