Patent GB-985197-A (GB)
Title: Improvements in or relating to multi-stage axial flow compressors
Inventor: CREUZBURG FRIEDRICH
IPC: F04D27/02
Filed: 1961-05-11
Abstract: 985,197. Axial-flow compressors. GASTURBINERBAU UND ENERGIEMASCHINENENTWICKLUNG PIRNA VEB. May 11, 1961, No; 17135/61. Heading F1C. [Also in Division G3] In a multi-stage axial-flow compressor forming part of a gas turbine engine, air is blown off between two consecutive stages through a ring of openings in the periphery of the compressor casing under the control of a radially movable band actuated by fluid-operated pistons, the band being guided by pins attached thereto and sliding in cam slots disposed on both sides of the ring at different points on its periphery whereby the radial height of the blow-off cross-section is uniform around the entire circumference of the band in any position of the band. The band may be divided into several parts. The band 20, Fig. 3, is movable between the fully closed position shown in full lines and the fully open position shown in dotted lines, the two ends of the band being attached to pins 78<SP>1</SP> ,  88<SP>1</SP> slidable within cam slots 79<SP>1</SP> ,  89<SP>1</SP>. Rollers or sliding blocks may be mounted on the pins. Further pins and cam slots are provided around the circumference of the band 20 as shown, the cam slots 79 diametrically opposite the ends of the band being disposed radially while the other cam slots are inclined at successively greater angles to the radial direction, whereby the band remains concentric to the compressor casing in all positions. The band 20 is actuated through linkages 74, 84, Figs. 2 and 3, by means of pistons 72, 82 in accordance with engine speed (with a correction for temperature), whereby air is blown off at low speeds to prevent stalling of the compressor. For this purpose, a pressure proportional to the square of the engine speed and developed by an engine-driven pump is applied through pipe 17 and duct 42b to a piston valve 41 whereby the latter takes up a corresponding position against the action of a spring 45 having an adjustable abutment 46. Oscillations are damped by a dashpot plunger 43b. Fluid at a constant pressure upstream of a throttle 19 is supplied through a pipe 18 to act on the face 42c of an annular servo-piston 42 and passes via an annular chamber 41a and a duct 42d to a return pipe 30<SP>1</SP>. Displacement of valve 41 alters the degree of throttling exterted by the control edges of valve 41 and piston 42, therby altering the pressure downstream of throttle 19, whereby piston 42 moves to take up a new equilibrium position corresponding to the new position of valve 41. Piston 42 acts through a double-arm lever 22 to actuate sleeve valves 52, 62 each provided with ports 52a, 52b co-operating with lands 51a, 516 on follow-up piston valves 51, 61, whereby pressure fluid supplied through throttle 25 and pipes 26, 27 is directed through pipes 28, 28<SP>1</SP> or 29, 29<SP>1</SP> to act on one side or the other of the actuator pistons 72, 82, whereby the band 20 is operated. Follow-up motion is transmitted from the linkages 74, 84, to the valves 51, 61 through rods 57, 67. To prevent the blow-off band 20 from remaining in the open position when the engine is stopped, a valve 91 is connected as shown to the shaft 81 of the manual control lever of the engine so as to be opened when the lever is moved to the shut-off position. When open, the valve 91 relieves the fluid pressure downstream of the throttle 19 whereby the band 20 is moved to the closed position. It is stated to be known to provide ramps instead of cam slots for guiding the blow-off band. The use, instead of a flexible band, of a rigid ring movable in a radial plane is also known. Reference has been directed by the Comptroller to Specification 869,576.