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Aviation History
1956
1956 - 0057.PDF
FLIGHT, 13 January 1956 Fig. 5a. Type FOW temperature-control valve; Fig. 5b, the FOW in longitudinal cross-section; Fig. 5c, a development of the original FOW. HOT AND COLD ... valve. This exceptionally neat, light and simple device (firstrevealed at last year's S.B.A.C. show) embodies a temperature- sensitive element A comprising an Invar rod inside a brass tube.Upon a reduction in throughput temperature this element con- tracts, opening the bleed valve B. The change in pressure dif-ferential across the face of the main valve C then suffices to open the latter against the internal pressure of the aneroid stack. Theinput then escapes through the annular gap of the main valve, while the increase in area of the needle bleed D raises the internalpressure behind the valve C, the opening movement of which is then arrested. It will be observed that, in operation, such a valve is not merelyan on /off control. The design is such that there is a closed-loop servo system at work, and feed-back gives a modulating charac-teristic to the valve, in which the position of the main valve is normally a function of the sensing-element temperature. The valve shown in Fig. 5c is a simplified development inwhich the sensing head can be remotely mounted, the appropriate signal being transmitted through the socket A. At B is a sealingring in the actuation gallery, provided to permit the valve to be dismantled. This particular unit was developed to control fuelde-icing, the sensing head being mounted in the fuel filter and the main flow of hot air being passed to a heat exchanger towhich the unit is directly attached. From these relatively simple valves can be evolved a varietyof wholly pneumatic control systems with a high degree of inbuilt intelligence, capable of being tailored to govern temperatures,pressures, pressure ratio or mass flow in a variety of installations. Pneumatic control has not been widely adopted in the past, partlyowing to the difficulty of eliminating severe hunting. To take the most basic form of pneumatic temperature controlit is possible to actuate the valve controlling a flow of hot air by attaching it directly to an aneroid. The latter can then be actedupon by air pressure from a tube to which constant-pressure air is applied through a bleed orifice, the flow escaping via a valvewhose orifice area varies according to the temperature of a bi-metallic sensing unit. Clearly, however, any system in whichthe_ valve is directly moved by an aneroid has very limited poten- tialities and it is accordingly advantageous to make the aneroidmove a small selector valve governing the supply of air pressure to a double-acting piston, which can then in turn be attached tothe main hot-air butterfly. A schematic representation of a pneumatic cabin-temperaturecontrol—designated FOY—is Fig. 6. This system requires no external power source and can be engineered into a very compactsteel assembly capable of operation at temperatures as high as any which are at present foreseen in supersonic aircraft. No lubricat-ing oil, or other fluid, is employed anywhere in the system. The sensing element comprises a steel tube and an Invar rod,the differential expansion between the two materials being em- ployed to adjust the bleed orifice at the end of a pipe to which issupplied constant-pressure air, via a restriction. The variation with temperature of the flow through this orifice in relation tothat through the restriction causes a changed back-pressure which is sensed in the controller aneroid and applied to a selector valvewhich, in turn, admits full-pressure air to one end or the other of a double-acting piston. By suitable mechanical linkage the pistoncan then be employed to actuate the main butterfly valve (denoted by the letter A). The valve A could be situated in a by-pass linearound an air cooler and a cold-air unit, the back pressure through these units being sufficient to cause nearly all of the flow to passthrough A when the latter is open. The only work done by the controller is the actuation of the selector valve, which requires aninsignificant effort. The actuator itself can be made of any required size, and thus the effort required to move the main butterfly valve can be exactly matched by the output of the ram.Going into more detail, the diagram shows that a tapping from the engine compressor, upstream of any cold-air or refrigerationunits, is applied to the pressure-reducing valve, and is then bled through a small restriction. Thus, the pressure applied to one ofthe aneroids in the controller is equal to 5 lb/sq in above that in the cabin, less a slight pressure due to the leak through the tempera-ture-sensing element. The other aneroid is subject to full cabin- pressure and the two aneroids are linked by a steel beam, thuscancelling out the cabin pressure on either side and leaving only the resultant, which is a function of the sensing-element leak andthus of temperature. Full bleed-pressure is applied to the selector valve and thence to the large piston which is sealed by carbonrings. As the piston moves, cam grooves along its length alter the setting of the main valve A. At the same time, the movement ofthe piston alters the fulcrum of the linkage between the controller and the selector valve to cancel the original call. The control is, therefore, of a proportional nature, analogousto a follow-up resister in an electronic system and, being a band control, is susceptible to off-set. The circuit could be developedto act as a point control, by deriving the offset term and feeding a signal back to eliminate offset within the band. An ancillary unit is the manual close solenoid—a glass-insulatedunit capable of operation at 300 deg C—which shuts the valve in emergencies. In operation, the selector valve is never fully closedexcept when forced shut by the manual solenoid following engine failure. If either, or both, of a pair of such controllers were tofail, the appropriate circuit could immediately be isolated. It can readily be seen that the system, although arranged tocontrol temperature, is really a control of pressure; and similar circuits can be devised for a variety of purposes. As a temperaturecontrol the circuit shown should be capable of controlling cabin temperature within plus or minus 5 deg C, although sensitivityand operating range can readily be adjusted to alter either the band width or the desired temperature level. Units of this nature arenow being developed for some of our fastest projected aircraft, in which insensitivity to environment will be all-important. RESTRICTION CABIN LEAK VARIED BY TEMPERATURE CHANGES \ EXHAUST TO fj ATMOSPHERE ,™ THERMAL SENSING ELEMENT AND SELECTOR Fig. 6. The Type FOY all-pneumatic control system,shown here arranged to govern temperature.
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