GB686547A - Improvements in or relating to control systems for compressors more particularly for cabin-pressurising plant - Google Patents

Abstract

686.547. Air-conditioning. BRISTOL AEROPLANE CO.. Ltd. Oct. 5, 1950 [Nov. 1, 1949; Oct. 23, 1950], Nos. 28003/49 and 25808/50. Class 137 [Also in Groups XXVI and XXIX] In an aircraft cabin-pressurizing plant, the output of a centrifugal or axial-flow compressor supplying air to the cabin is controlled through servo-mechanism in accordance with the absolute delivery pressure of the compressor and the differential pressure between the inlet and throat of a Venturi on the delivery side of the compressor in such manner that the ratio of the Venturi differential pressure to the compressor delivery pressure is maintained constant. This, as is shown in the Specification, is equivalent to varying the mass-flow of air passing through the compressor in direct proportion to the absolute delivery pressure of the compressor and in inverse proportion to the square root of the absolute temperature of the delivered air. The apparatus enables the compressor to work at its highest efficiency without surging. The output of the compressor 6 is controlled by a throttle valve 43 in the compressor inlet. If after the valve 43 is fully open, the flow is inadequate for stability, a valve 22 byepassing a refrigerator 17 is opened and if after the valve 22 is fully open, stability is not attained a spill valve 45 is opened. In the plant shown in Fig. 1, the compressor 6 is driven from the aircraft engine 7 through a change-speed gear 8 having actuators 10, 9 for selecting and engaging the different speeds. The actuators 9, 10 are under the control of a plant switch 11. The actuator 10 is additionally controlled by a switch 13 operated by a barometric capsule 14 to select a higher speed at high altitudes. The switch is designed to effect the change at a higher altitude on ascent than on descent. Air passes to the cabin 1 through a Venturi 50, a cooler 15, a heater 16, the refrigerator 17, a valve 25 by-passing a water separator 18, and a non-return valve 2. The valve 25 is controlled by a humidistat 39 and the valves 20, 21, 22 are normally controlled by an actuator 27 under the control of a variable datum thermostat 26 in the cabin. The cabin discharge valve 4 has a pressure control 5. The refrigerator 17 has an additional byepassing valve 40 which is held open when the switch 13 is in its high speed setting. Pressures at the inlet and throat of the Venturi act on a flow control 42 through pipes 49, 55. In Fig. 2, the flow control comprises a closed box 48 containing an evacuated capsule 47, the free end of which operates on a lever 51 having a fixed pivot 52. The opposite end of the lever 51 operates on a further capsule 53 the interior of which is connected through a sliding sleeve 54 and the pipe 55 to the Venturi throat. The end of the capsule 53 remote from the lever 51 is connected through a lever 56 to a piston valve 57 controlling the supply and exhaust of pressure fluid through pipes 60, 61 to a master servomotor 62 (see Group XXIX). The latter controls two further servomotors 36, 67 of the rotary vane type, one 36 operating the refrigerator by-pass valve 22 through a shaft 37 and a lever 38 and the other 67 operating the compressor throttle valve 43 through gear segments 68, 69 and a lever 71. The shaft 37 carries cams 101 which act through a lever 102, gear segments 103, a differential gear 104 and a lever 106 to operate the spill valve 45. The actuator 27 is shown as an electric motor 28 which operates three cams 30, 31, 32. The by-pass valve 22 is operated through a cam follower 33, a shaft 34, a gear segment 35 and a pinion 97 on the control valve of the servomotor 36. The heater and cooler valves 21, 20 are similarly actuated through servomotors controlled by the cams 30, 31. The first Provisional Specification refers to Specification 465,612, [Group XXIV].