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Aviation History
1956
1956 - 1692.PDF
854 FLIGHT, 30 November 1956 CONTROLLED COMFORT A Survey of the Problems of Air Conditioning ALTHOUGH announced as a section lecture, the paper pre-sented to the Royal Aeronautical Society at HamiltonPlace on November 13 was not, in the event, a lecture atall. Dr. E. W. Still, Ph.D., B.Sc, A.C.G.I., A.M.I.Mech.E., F.R.Ae.S., took the unusual step of devoting the complete sessionto a discussion of his paper Air Conditioning in Aircraft—pre- viously circulated and containing much useful data, tables andappendices. Extracts from it form the subject of this article. An aircraft air-conditioning system is required to ensure thatall equipment, and the compartments that require heating, cool- ing, etc., are maintained in the correct condition to ensure thatthey function efficiently under all aspects of operational use. By proper centralization of the various aspects of aircraft conditioningin the initial stages it is possible to achieve a system that is a com- plete entity which, as well as ensuring greater utility and sim-plicity, results in a very considerable saving in weight. This is the first time that such a proposal has been expounded in thiscountry. . Safety Considerations. In these days of high-altitude flying,the interpretation of the "fail safe" requirement is a major issue. How many safeguards should be built into a pressure cabinsystem? The Comet has two sets of series discharge valves, and these are paralleled by a safety valve as a final safeguard. Sucha system is naturally heavier and more complicated than would at first seem necessary, but this has been dictated by the needro ensure maximum safety. The Americans demand that emer- gency oxygen is laid on to every passenger seat, with a supplysufficient to cover flight from the point of no return. Differential Pressure. Another matter that requires clarifica-tion is the need for aircraft interiors to be maintained at sea-level pressure in order to allow military aircraft to climb or descendat very high rates without violent pressure changes occurring inside the cabin. If the cabin is controlled at varying pressure valuesup to an altitude equivalent of 25,000ft, any rapid descent must subject the crew to extremely high rates of pressure change.Alternatively, if the cabin pressure is not increased at a fast rate, the cabin is at a high negative differential pressure on landing—afactor involving a drastic revision of the stressing case. Air Supply. A final item to include in any list of requirementstoday is passenger reaction. In order to attract business, airlines are now forced to study passenger comfort.In the field of aircraft conditioning the passenger-comfort aspect arises in a number of ways, of which two are worthy ofmention. Is cabin-air recirculation desirable? It may cause the air to become stuffy and to require filtering. On the otherhand, recirculation ensures that the minimum of fresh air is taken from the engine, and hence the minimum of power is absorbed.Should a supply of fresh air be used, plus the same amount obtained by recirculation? Or should the entire supply be obtainedfrom the main engines at the cost of extra horse-power absorption? Secondly, what about humidity? The problem is twofold inthat while it is necessary to reduce humidity when on the ground, the opposite is the case at high altitude where the air is verydry, and it is therefore necessary to increase it to approximately 30 per cent R.H. The vapour-cycle system is installed to reduce the humiditywithin the aircraft while standing and taxying and, where neces- sary, during flight as well.Cooling Systems. The two main types of cooling systems in use are the air-cycle system and the vapour-cycle system.The first can be in one of three forms, each based on a cold- air unit. In one form the unit is used on the bootstrap principle,in another it is used as a turbine fan, and in the third case the cold air unit can be applied on the regenerative cycle principle. The bootstrap system (Fig. 1) was the first to be used, and itoriginated from the need to pressurize the cabins of piston-engined fig. 1. The "bootstrap" system cold air unit. aircraft. The cabin is pressurized from engine-driven blowers ofeither the centrifugal or Roots types. The pressure rise across the engine-driven blowers in this case is relatively low, with theresult that insufficient cooling would be caused by passing the air through a turbine. By making the turbine drive a compressorand by using the compressor to compress further the air from the engine-driven blower and then passing this air through the cool-ing turbine, a sufficiently high pressure-ratio across the cooling turbineis achieved, thus giving a much greater temperature-drop.It should be stressed that the cooling effected by the turbine, i.e., the refrigeration, is brought about not merely by the expansion ofthe high-pressure air in the turbine; it is fundamentally the work done by the turbine in driving its compressor that is responsiblefor the very considerable temperature-drop across the turbine— hence the bootstrap principle. With progress from piston to turbine engines there is nowusually sufficient air available from the engine at a high enough pressure not only to pressurize the cabin but to provide theenergy for cooling. The turbine fan system (Fig. 2) comprises a heat exchangerwhich provides the initial cooling of the air from the engine com- pressor. The air passes from the heat exchanger to the air turbine,and the refrigeration is provided by making the turbine drive a fan or ejector-pump compressor, either of which can be made toboost the flow of coolant ram air through the heat exchanger. Owing to the fear of air contamination by oil, etc., our Ameri-can friends are using a cabin compressor which takes the air from the wing leading edge, and passes it through a centrifugalcompressor and into the cabin. This system has not been found FROM ENGINE PRIMARY HEAT EXCHANGER COMPRESSOR INTERCOOIER TURBINE HEAT EXCHANGER AND TURBINE JET PUMP Fig. 2. Cold air unit operating on the jet-pump system. necessary where British engines are employed, owing to the veryefficient turbine compressor bearing seals. The engine-compressor bleed method of cabin pressurization and the turbine fan coldair unit are the lightest and simplest ways of cabin pressurizing and cooling, and it is with some surprise that one still finds newdesigns of turbine or turbo-propeller engines which do not make this provision and so necessitate going back to the old and heavyengine-driven compressors. The bootstrap and jet-pump cold air unit described above aresuitable for aircraft operating up to Mach 1, but with the speeds which are being considered today it is obvious that heat exchangerswill cause considerable drag if they are to be ram-cooled. At attitudes above 45,000ft there is not much air available for cool-ing in any case. These factors have led to the development of the regenerative systems which can employ either fan or bootstraptypes of unit. In the regenerative system (Fig. 3), cabin air is bled from theengine compressor, passes through an air-to-air heat exchanger, and then goes through the turbine of the particular type of coolair unit chosen before finally entering the aircraft cabin. The discharge air from the cabin is passed through the heatexchanger and is finally expelled through a nozzle, to obtain the maximum use of the energy left. The air from the cabin to theheat exchanger can be given impetus by the fan or compressor coupled to, and driven by, the cooling turbine. All the availableenergy has been extracted from the air before it is finally thrown overboard, and the ram-air heat exchanger losses have beeneliminated. This system is particularly applicable to small military aircraft.Vapour-cycle Cooling. Instead of using air-cycle cooling it is possible to make use of the refrigeration obtained by the evapora-tion of liquids: either by an open-circuit method whereby the refrigerant is lost or by a simple air-cycle system modified byplacing a water boiler between the ram-air heat exchanger and the turbine. But for the long-range Mach 2 or 2.5 aircraft, andalso to a certain extent for shorter-range aircraft and missiles, the closed-circuit vapour-cycle refrigeration system is the most •®
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