embodiment show flow away from the axis 12 one skilled in the art can easily convert to flow in the opposite direction the present invention replaces the conventional foils or rotor elements normally employed in such screen rotors with multi element foils mef 14 of the type that will be disclosed in greater detail here in below the function of each the foil 14 is to operate in the conventional manner to facilitate the screening operation as above described one of the principal operations of the foil is to generate a negative pressure pulse at the trailing end of the foil to pull material back through and clean the screen foils also may be shaped to generate a positive pressure adjacent to the leading end of the foil to drive material through the screen the foil 14 in the illustrated embodiments is configured to generate a pressure pulse adjacent to the leading end of the foil 14 the use of the mef 14 of the present invention permits improving the operation of the screen by increasing the magnitude of the pressure pulses particularly the negative pressure pulse generated at the trailing end of the foil 14 the embodiment of the present invention shown in fig2 is a.