Filter

A backwashable filter comprising:

• a vessel including a bulkhead forming an upper container and a lower container, a generally upright flow tube depending from the bulkhead into the lower container and terminating in a lower end spaced from the bulkhead, a generally upright outlet pipe depending from the vessel into the upper container and having a free end located above the bulkhead, the lower container including an inlet so that liquid can flow from the inlet through the first container, the tube and the second container into the outlet, thereby forming liquid levels coincident with lower end of the flow tube and the free end of the outlet pipe and entrapping gas in a first section in the lower container above the liquid level therein and in a second section in the upper container above the liquid level therein;
• screen means attached to the flow tube for filtering liquid flowing from the lower container to the upper container, whereby a filter cake of contaminants builds up on an upstream side of the screen means;
• a vent valve fluidly communicating with the first section via a port opening into the first section and located above the liquid level in the lower container;
• and
• means for feeding a pressurized gas into the first section so as to maintain the first section substantially filled with compressed gas irrespective of the pressure of the liquid in the vessel and the liquid level substantially commensurate with the lower end of the flow tube;
• the flow tube, the upper container and the outlet pipe being arranged and dimensioned so that upon the opening of the vent valve and the exhaust of the compressed gas from the first section the liquid level in the first section remains at all times substantially below the port;
• whereby the filter can be backwashed by opening the vent valve and permitting the compressed air in the second section to rapidly force liquid in the flow tube and in the upper container through the screen means into the lower container, thereby dislodging the filter cake from the screen means and suspending it in the surrounding liquid while substantially no liquid is discharged through the port and the vent valve.

An automatic strainer for filtering a contaminated liquid comprising:

• an inlet container defining a first compartment for holding a first volume of compressed air and having an outlet through which contaminated liquid can be flowed into the container;
• an outlet container defining a second compartment for holding a second volume of compressed air and having an outlet;
• means for fluidly connecting said inlet container and said outlet container;
• screen means for filtering contaminated liquid flowing from the inlet to the outlet container;
• an outlet valve fluidly connected to said outlet;
• an exhaust valve fluidly connected to said first compartment via an exhaust port in the first compartment for venting the first volume of air to the exterior, whereby the second volume of compressed air energetically forces a column of liquid between the compartments through said screen means when the outlet valve is closed to thereby dislodge the contaminants deposited on said screen means;
• means limiting the volume of liquid which is available to enter the first compartment when the exhaust valve is opened so that substantially all of the liquid in the containers remains therein when the exhaust valve is open;
• and
• means for removing liquid within said inlet container after the exhaust valve has been opened.

A method for operating and backwashing a filter having an inlet fluidly connected to an inlet chamber holding contaminated liquid and an outlet fluidly connected to an outlet chamber holding filtered liquid, the inlet and outlet chambers including respective first and second compartments for holding volumes of a compressed gas during normal filtering, portions of the first and second compartments being defined by surfaces of the liquid in the inlet and outlet chambers, respectively, during normal filtering, and liquid filtering screen means disposed between the chambers, the compartments being arranged so that a liquid column is disposed between them which must be moved in an upstream direction before gas can flow from the outlet chamber to the inlet chamber, the method comprising the steps of:

• flowing contaminated liquid from the inlet to the outlet to thereby filter the liquid with the screen means by depositing the liquid contaminants as a filter cake on the screen means;
• periodically backwashing the screen means to remove the filter cake therefrom by substantially instantaneously discharging compressed gas in the first compartment from a port therein which is located above the contaminated liquid surface, whereby the compressed gas in the second compartment rapidly forces the liquid column in an upstream direction through the screen means, thereby dislodging the filter cake from the screen means and suspending it in the contaminated liquid disposed in the inlet chamber;
• maintaining the compartments substantially completely filled with compressed gas irrespective of pressure of the liquid in the chambers;
• and
• sizing the volume of the liquid column so that it is less than the volume of liquid necessary to raise the contaminated liquid surface to reach the port, whereby the escape of substantially any liquid from the port is prevented.