GB811854A - Improvements in or relating to rotary dynamic compressors - Google Patents

Abstract

811,854. Axial flow and centrifugal compressors; gas-turbine plant. POWER JETS (RESEARCH & DEVELOPMENT) Ltd. Dec. 5, 1956 [Dec. 9, 1955], No. 35398/55. Classes 110 (1) and 110 (3). A gas compressor plant comprises a rotary compressor having a pressure tapping at the leading edge of a stationary vane, another pressure tapping at the throat of the working fluid passage between two stationary vanes, and a pressure-responsive element responsive to pressures sensed by the tappings and movable to actuate a control means for varying the mass flow of working fluid through the compressor. The static pressure at the throat of the working fluid passage between adjacent blades in the last row 3 of stator blades of an axial-flow compressor is sensed by a tapping comprising a hole 8 drilled in the inner or outer cylindrical casing wall and communicated through a duct 9 to one side of a piston 7. The dynamic pressure is sensed by a tapping comprising a hole 4 in the leading edge of a blade and communicated through a duct 5 to the other side of the piston 7. The piston 7 takes up a position dependent on the ratio of the dynamic pressure to the static pressure, this ratio being a measure of the non-dimensional mass flow. A servomotor controlled bv the piston 7 actuates a by-pass valve to vary the quantity of gas returned through the by-pass from the outlet to the inlet of the compressor, thereby keeping the non-dimensional mass flow constant. Alternatively, where the compressor forms part of a gas turbine plant, the servomotor may actuate variable turbine nozzle blades. An adjustable stop 12 may be provided to limit the movement of the piston 7 in one direction, thereby preventing excursions into the unstable portion of the compressor characteristic. At pressure ratios or speeds above a predetermined value, the non-dimensional mass flow may be made to depart from a constant value and assume instead values corresponding to peak efficiency. For this purpose a bleed valve 16 may be provided. When the valve 16 is open, gas is bled from the duct 9 through ducts 15 and 17 to atmosphere, so that the static pressure acting on the piston 7 is reduced. The valve 16 may be opened or closed in response to rotational speed. Alternatively, as shown, the valve 16 may be actuated by a spring-biased piston 18 subjected to the static pressure sensed by a tapping 20 in the delivery duct or, as shown, in a throat of the stator blade row 3. The separate tappings 8 and 20 may be replaced by a single tapping. There may be a controlled bleed at the dynamic pressure side of the piston 7 instead of or in addition to that at the static pressure side. More than one bleed valve may be used to permit deviation from a constant operating line at more than one pressure ratio, and one of the valves may be arranged to be gradually closed as the pressure ratio rises. In the case of a centrifugal compressor, the pressure tappings are located at diffuser vanes. According to the Provisional Specification, the pressure tappings may be located in the outlet duct of the compressor. The control may be in accordance with Mach number instead of the ratio of static to dynamic pressure.