Patent GB-628455-A (GB)
Title: Improvements in or relating to aircraft cabin pressure controllers
IPC: B64D13/02
Filed: 1944-11-09
Abstract: 628,455. Fluid-pressure servo motor-control systems. CLEVELAND, F. J. (Boeing Aircraft Co.). Nov. 9, 1944, No. 22116. [Class 135] Apparatus for controlling the flow of air through an aircraft cabin comprises means to supply to the cabin atmospheric air compressed to or below a predetermined maximum compression ratio and means to control the outflow of air from the cabin, whereby the difference between the cabin pressure and the pressure of the surrounding atmosphere is so regulated that the ratio of the cabin pressure to the pressure of the surrounding atmosphere never exceeds the maximum compression ratio of the air supply means. In this way a desired cabin pressure can be maintained even at the highest altitudes without exceeding the capacity of the blower or other air supply means. The invention may be applied with little modification to the pressure cabin control units described in Specification 621,669 and in the pressure cabin control systems described in Specification 621,670. Normally, as the aircraft ascends, cabin pressure is maintained substantially at atmospheric pressure up to some selected height, say 8,000 feet, i.e. there is no supercharging. Thereafter, supercharging commences and cabin pressure is maintained constant at the atmospheric pressure corresponding to 8,000 feet until the cabin pressure exceeds the atmospheric pressure by a definite maximum amount depending on the design of the cabin. This phase constitutes absolute pressure control. During a further range of altitude, the difference of cabin pressure over atmospheric pressure is maintained constant, the cabin pressure decreasing meanwhile. This phase constitutes differential pressure control. As the atmospheric pressure decreases still further the ratio of cabin pressure to atmospheric pressure increases more and more and eventually a stage would be reached at which this ratio exceeded the maximum compression ratio of the blower. To prevent this happening the difference of cabin pressure over atmospheric pressure is decreased for all higher altitudes so that the pressure ratio equals but does not exceed the maximum compression ratio of the blower. In Fig. 1, the cabin outflow valve 1 is actuated by a diaphragm 4 separating two chambers 41, 42. Cabin pressure is admitted to the latter through a port 40 and to the former, past an adjustable valve 43. Pressure leaks away from the chamber 41 so that the valve 1 is opened more or less by the pressure beneath the diaphragm either through a passage 20 controlled by the absolute pressure control 2 or through an axial passage either in the stem 10 as shown, or in a spindle 12 under the control of the differential pressure control 3. The absolute pressure control comprises a spring-loaded evacuated bellows 21 exposed to cabin pressure through an orifice 26. The base of the bellows is adjustable by a nut 27 to determine the starting point of absolute pressure control and the head of the bellows co-operates with a valve pin 23 having a moving seat 24 operated by the diaphragm 4, as described in Specification 621,669. The differential pressure control 3 comprises a spring-loaded piston 31 which engages a shoulder on the spindle 12. The piston is exposed on its underside to cabin pressure and is connected on its upper side to atmosphere through pipes 35, 36 and the pressure ratio control device 5. The latter has passages 51, 52, 53 of which the two latter are normally connected with one another and the pipes 35, 36 by an adjustable valve 54. At high altitudes the valve 54 is byepassed to modify the action of the differential pressure control by a valve 55a opened by the expansion of a spring-loaded evacuated bellows 6 exposed to atmospheric pressure through the pipe 36. In another form (Fig. 2, not shown), the pressure ratio control is embodied in the differential pressure control 3 and is formed as a spring-loaded evacuated bellows arranged between the floor of the piston 31 and the shoulder on the spindle 12. In the form shown in Fig. 3, the differential pressure control device 3 and the pressure ratio control device 5 are the same as in Fig. 1, but the absolute pressure control device 2 is modified, as described in Specification 622,283 to produce cabin pressures which exceed ambient atmospheric pressure by a fixed proportion of the difference between ambient atmospheric pressure and sea level or some other datum pressure. The absolute pressure control device 2 is divided by a diaphragm 29 connected to the evacuated bellows 21. Cabin pressure acts on the upper side of the diaphragm through a passage 29a and on the lower side thereof through a fixed orifice 26a. The space below the diaphragm may also be connected to a greater or less extent with the atmosphere through an adjustable orifice or valve 28a and a pipe 28c. In Fig. 4, the absolute pressure control is as in Fig. 1 and the differential pressure control is replaced by a pressure ratio control comprising a diaphragm or plate 56a which engages the shoulder on the spindle 12b and surmounts concentric bellows 57a, 58a. Cabin pressure acts on the inner bellows through a passage 59a, atmospheric pressure acts outside the bellows through the pipe 35 and the space between the bellows is evacuated. A valve 28 in a pipe 25 connecting the control valve 23 to atmosphere or other low pressure region such as the venturi 97 is shut down if the absolute pressure control device fails. In another form (Fig. 5, not shown) the ratio control device of Fig. 4 is embodied in a differential pressure control device similar to Figs. 1 and 3 but the piston 31 is replaced by a diaphragm. In another form (Fig. 9, not shown), the byepass valve 55a in the ratio control device 5 of Fig. 1 is operated by a differential bellows arrangement similar to Fig. 4, exposed to cabin pressure and atmospheric pressure. The pressure ratio control device may be associated with the absolute pressure control device. In one such form (Fig. 7, not shown) a pressure ratio control of the differential bellows type is located between the bellows 21 and the valve pin 23 and in the the bellows 21 and the valve pin 23 and in another form, Fig. 8, the moving seat 24 is provided with a seat 24d for a further control valve 23d actuated by a superposed differential bellows 56d exposed to atmospheric pressure through a pipe 50d and to cabin pressure through an aperture 59d. A complete system of cabin pressure control in which in addition to the cabin outflow control, the speed of the air supply blower is controlled according to the air flow is shown diagrammatically (Fig. 6, not shown). Specifications 521,623 and 628,456 also are referred to.