Maximumminimum Pressure Rise And Motor Speed

The pressure rise limits of the pump are defined by the pump specification and are determined by airframe and engine system needs.

The motor speed (that is, the number of poles) is selected by the pump designer to be compatible with the overall requirements of the pump, in particular the altitude requirements. Because of the need to minimize size and weight, the maximum possible speed is employed. This has, in recent years, resulted in the almost exclusive use of axial inducers in airframe boost pumps. They are used either alone, as purely axial pumping elements, or in conjunction with mixed-flow sections depending upon the flow and pressure requirements. Axial inducers are a pumping element that have proven to provide the best suction performance because of their low blade loading and gradual pressure rise characteristic. The inducers on airframe boost pumps incorporate features such as blade sweep back, blended suction surfaces, and thin blade inlet edges. The inlet blade angle at the tip usually ranges from eight to ten degrees measured from a plane perpendicular to the axis of rotation. These pump elements have demonstrated the capability of operating at suction-specific speeds in excess of 40,000 measured in units of gpm, feet of fluid, and rpm (that is, the universal suction specific speed Q,ss exceeds 14.6).

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