a and b being approximately 1 25 to 2 0 times the length of the nozzle 20 though this should not be construed as limiting the percentage of filter slot width to orifice slot width is in the range of 83 to 98 for example an orifice slot width range of 0 030 inches to 0 058 inches results in a filter slot range of 0 025 inches to 0 051 inches respectively in fig2 and 3 an end wall 44 of the head 14 is shown and a rod 46 is shown extending through this end wall 44 the rod 46 being formed integral with or connected to the moveable plate 32 this rod 46 can be engaged to any suitable moving means not shown such an hydraulic cylinder motorized screwjack or other suitable device in a manner to cause the moveable plate 32 to move along its elongate axis until a desired filter slot 26 size is achieved stop means 48 such as lock or jamb nuts 48 are used to control the available degree of plate movement by coaction with a bracket 50 defined below it is preferred to set the overlap of slots 26 a and b to a size 0 001 0 005 inch width smaller than the size of the width of the nozzle orifice 18 while filtering is being accomplished with the slots 26 being set to a fully open position fig2 a and 3a for cleaning and flushing or clearing a bracket 50 is proposed for use in limiting the motion of the moveable plate 32 the bracket 50 being engaged to the.