Abstract: An air moving device in the form of an axial flow fan is provided that is adapted for use in substantially enclosed high-density electrical systems, e.g. computers and the like. The fan includes an annular wall extending about the blades thereof and in which pressure equalizing openings are formed for increasing the maximum pressure in the preferred operating area of the fan and just before the stall effect occurs due to increasing system pressure resistance. A method is also provided for optimizing the number and size of the slots to obtain a desired maximum pressure increase. The slots are optimized so that there is no appreciable airflow loss in early stages of the preferred operating area while providing the desired increase in maximum pressure. The slots preferably have an elongated shape, e.g. rectangular or oval, with a major dimension extending generally axially and parallel to the axis of rotation of the blades, and are spaced from each other circumferentially about the annular wall.