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Aviation History
1948
1948 - 1898.PDF
576 FLIGHT NOVEMBER IITH, 1948 TURBINE COOLER UNIT Airborne Refrigerating Plant for Cabin Atmosphere Conditioning WHAT a good thing it is that, even whilst such animmense event as political revolution is takingplace, the minds of men can be concentrated on smaller but not, in terms of ultimate values, necessarily iess important things. During the turmoil of the French Revolution, Jacques Alexandre Cesare Charles, mathe- matician and physicist, had the qualities of mental im- patience and perception which led him to anticipate Gay- Lussac's definition of the dilatation of gases with heat. Thus, what we now know as Charles' law states that "at constant pressure the volume of a gas increases by equal amounts with equal increments of temperature." It is on this fundamental precept, and its consonant recipro- cals that change of any one of the quantities concerned will bring about change of the other factors, that the Marshall air-cycle turbine cooler unit described in this article oper- ates. By changing the volume of air through compression and expansion, the temperature is caused to change correspondingly. In the early stages of cabin conditioning there was a widespread tendency to regard it as a relatively simple sideline, but with the passing of time it has become in- creasingly apparent that the furnishing of a fool-proof, completely reliable pressurizing and conditioning system in an aircraft is a vastly difficult matter. The complex system of forcing air into the cabin and appropriately cooling it, heating it, drying it or wetting it, is a modern necessity. Certainly, all these conditional processes are not required simultaneously, but the system must embrace them all, so that they may be called upon as and when occasioned by a flight from one climatic region to another. Conditioning Flow Sequence The basic cabin air-conditioning system has been described so frequently in the past, both as an entity and in conjunction with aircraft descriptions, that reiteration would be pointless. Nevertheless, the latest ideas on the RE-ENTRY FROM INTERCOOLER THRUST SCROLL AND SEARING LABYRINTH SEALS TURBINE INFUSEI NOZZLE RING Sectional cut-away of the turbine cooler unit, show- ing details of interior construction. Left : The 13-blade compressor is 6^ in in diameter and employs separate axially-curved inlet blading. Right : The turbine wheel is SJgin in diameter, has SO blades and is fed through a 17-slot infuser nozzle ring. subject have introduced sundry modifications, and, in order to clarify the present state, it is, perhaps, worth while briefly to run through the flow sequence. Air is entrained usually through the wing leading edge and passes through filter units prior to entering the engine- driven blowers. From these, the air successively goes through silencers, spill valves and- non-return valves, to be unified in a common duct in the aircraft fuselage. The accompanying diagrammatic plan layout of a typical air- craft installation defines the general arrangement of con- ditioning units and the arterial system. Adjacent to the lead-in trunk is the mass-flow metering unit, downstream of which the duct is branched, (i) to a choke heater unit and, (ii) to the primary intercooler. The choke heater is a new development and is intended to raise the tempera- ture of the cabin air by choking the blowers and thus making them work harder so that the temperature of the air they deliver is increased. It has been established that the heating obtained in this manner is quite sufficient for all normal pur- poses. When heating is not required, coupled by-pass valves in the duct branches direct the airflow through the primary intercooler and close off the choke unit. For the convenience of installation, the primary and secondary inter- coolers, each of which is a simple air- to-air heat exchanger, are arranged side by side in a ram duct into which the "coolant" air is scooped, passed through the intercoolers, and ex- hausted back to atmosphere. On emergence either from the choke heater or the primary intercooler, the air is delivered to a by-pass valve which, accordingly as heating or cool- ing is required, directs flow, respec- tively to by-pass or to feed the turbine cold-air unit. Downstream of these components, the duct once more branches into a pair of separate ^COMPRESSOR silencers prior to delivery to the cabin. Before going on to describe the cold- air turbine unit, mention should be made of the means of introducing humidification. When flying in con- ditions of low ambient humidity, it B 18
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