in a tubular filter bodies with at least one tubular filter structure with a porous wall , through which a liquid passes substantially radially , particles above a given size being held back at the entrance surface , made of porous plastic , ceramic material , metal , graphite , etc , continuous operation at constant pressures is made possible by providing at least one groove which is closed toward the outside and forms a forced flow path leading from an inlet to an outlet in the circumferential entrance surface of the tubular filter body .