suspension to separate the fibrous fraction into an accepts portion and a rejects portion has a housing having an inlet with a heavy and large material trap an accepts outlet a rejects outlet and a dilution liquid inlet a hollow cylindrical screen having an open top and bottom communicates with the fibrous suspension inlet chamber and the rejects outlet chamber accepts fibers in the suspension pass through apertures in the screen to the accepts chamber and are discharged through the accepts outlet rejects discharge through the rejects chamber a rotor having at least four regions a closed top and a bottom mounted drive mechanism is coaxially mounted within the screen hydrodynamic pulses are induced in the suspension in the screening region by a pattern of bumps and or depressions impressed upon the rotor surface the fibrous suspension is maintained at a relatively constant consistency within the screening region by the pumping action of the rotor and by providing dilution liquid to counteract thickening of the suspension a dynamic bar screening device directs stones and other large objects out of the fluid stream and into an annular trap and also deflocculates the suspension application_number US 57085990 A description fig1 and 2 show the housing 60 rotor 30 and screen 10 housing 61 is generally cylindrical with a closed top.