All establishments where food is prepared, served or kept shall have wire screens at all outside openings for the purpose of excluding flies and other insects. All screen doors must be self-closing and outside doors shall open outwards only. All screening used in such places shall be not less than 14 meshes to the inch. Fly fans may be substituted for screen doors and when used shall be installed inside the building over the door opening so that the airflow is directed downward and outward. Fly fans shall produce an airflow with a minimum velocity of 750 feet per minute over the entire door opening, from the top thereof to a point three feet above the floor. (Ord. 855 § 2; prior code § 15.48(F)).