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Aviation History
1955
1955 - 0739.PDF
FLIGHT, 27 May 1955 737 JET-AIRLINER SYSTEMS The general principles of this type of system were describedin Flight on October 22nd last year. Cooling air for the two heat-exchangers is taken through twinram-air intakes, located at the base of the fin, and downwards through ducts to the heat-exchangers behind the cabin rear pres-sure bulkhead; it is then released through outlets under the rear fuselage. The air for the brake turbines is drawn from the ductsdownstream of the heat-exchangers and returned there. As can be seen from the diagram, ducts and valves are so arranged thatvarious of the elements in each circuit can be cut out, and a series of temperature-sensers in the ducts supply signals for auto-matic controls and gauges. From the main temperature-adjustment system the air fromeach group of units is led into a common pipe which is then divided again into three branches, two for the passenger cabinand the third for the crew compartment. Where the pipes join, there is a separate intake through which conditioned air can besupplied to the cabin from a trolley while the aircraft is on the ground.The main duty of the system is cooling, because the cabin insulation is good, and the main air supply from the engine com-pressors very hot; and the occupants of the cabin and cockpit themselves provide one of the main sources of heat—S.N.C.A.S.E.have calculated that each passenger will produce 75 calories of heat and each crew member 100 calories. In addition to normal coolingfor passengers, however, the system is designed also to provide cabin temperatures down to plus 5 deg C, this being an acceptablelevel for perishable freight. Incidentally, it is not thought neces- sary to paint the roof of the aircraft white for cooling purposes,since it has been calculated that the average heat-relief on the ground is in the region of only 5 to 9 deg; but the prototypebears S.E. colours which include a white roof. Air leaving the expansion turbines is heavily moisture-laden andthe water separators extract from it about 60 to 70 per cent of the water droplets, thus reducing the likelihood of misting in thecabin, while also acting as conditioning-system silencers. If, however, the charge air passing through the separators is at lessthan 0 deg C there is a likelihood of ice forming there. A sub- sidiary bleed frorn the main charge air supply has therefore beenincorporated which, under thermostatic control, adds fully hot air to that flowing through the separator to keep the charge airalways at about plus 2 deg C. Above 19,200ft, when the water separators are cut out air canbe delivered to the cabin at well below 2 deg C. The water- separator cut-off valves can be switched from both the pilots'and steward's control panels, so that the barometric cut-out can be anticipated should fully cold air be required when not normallyautomatically supplied. Ten to fifteen minutes after a hot- weather take-off the cabin may still berather warm, and it is therefore foreseen that during such periods cold air will berequired. If a ground trolley has been connected forconditioning on the ground, it is possible that the windscreen will mist up during take-off.A separate air intake in the nosewheel bay has therefore been installed in the prototypeto pump air through electric heaters to the windscreen. This was asked for by the testpilot and, in production aircraft, it will be replaced by a secondary bleed from the maincharge-air supply. There are two Teddington Controls on/offcocks by which each system can be completely isolated from its engine bleed; but, in addition,the closing of either throttle actuates the cock for the related engine. In case of an enginefire or a crash landing, therefore, the systems are automatically cut out and fumes or smokefrom the engine are prevented from entering the cabin or cockpit. There is, in the windscreen sandwiches, onefurther small system. Most panels have double layers of Perspex—the inner of which takescabin pressure—but those directly in front of each pilot have the outer panel of glass, toprevent scratching by the windscreen wipers. These glass panels do not take the cabin pres-sure, which is here held by the inner Perspex. This causes the gap between the two, which isat outside pressure, to vary as pressure is released or applied, and, of course, the airspace changes in volume. A breather tube is therefore led to each panel from the nosewheelbay, through a silica-gel capsule which keeps the contained air dry. For conditioning control purposes the cockpit and cabin aretreated as separate units; and the system is intended for automatic operation. There is, however, a full set of warning devices anddirect controls and gauges to enable the crew to deal with unusual situations and possible failures. For both halves of the system there are also cabin and ducttemperature gauges and warning lights indicating excessive tem- perature. Normally the two main bleed-valves are opened tostart the systems and temperature can then be controlled in two ways. First two rheostats can be set at the desired level and thesystem switched to "auto." The rheostat dials are not marked in degrees since a given setting will produce different temperaturesat various altitudes, but it is simple to make adjustments accord- ing to requirements. Secondly, the systems can be switched onand one of two buttons pressed, whereupon the temperature will continue either to rise or fall until the "auto" button is pressed.Temperature is thus easy to regulate, yet there is full indication of the behaviour of the system. In the prototype Caravelle, both control panels are in the cock-pit, but in production aircraft the pilots will control their own (the port) system and the other will be controlled by the stewardsfrom a separate panel in the passenger cabin. Pressurization.—The air-conditioning airflow through the sealedcabin is throttled at the outlet to maintain pressurization, and for this purpose there are three outlet valves; two are automaticallyregulated and the third manually, the automatic ones being in the underside of the tail of the pressure cabin, and the manual oneon the rear top face of the nose-wheel bay. Cabin pressure and the rate of change of pressure can be con-trolled from the cockpit at a panel next to that for the air-condi- tioning system. The indicating instruments are a cabin altimeter,a separate aircraft altimeter and a variometer, these being grouped with warning and control devices for extremes of pressurization.The main control is a dial on which the crew can select cabin altitude and rate of change.The pressurization valves, like the air-conditioning system, are the work of AiResearch. The normal rate of change of cabinaltitude will be set at 2.6 m/sec (500 ft/min), but the available range is from 50 ft/min to 2,000 ft/min. The normal workingcabin differential is to be 8.05 lb/sq in with a regulated maximum of 8.2 lb/sq in. At 8.5 lb/sq in, all three valves will open andhold cabin pressure down to the regulated maximum level, although one of the valves alone has sufficient capacity to meetany over-pressurization case should valves stick shut. They are designed to provide automatic inward pressure-relief as well, andare set to open fully for an excess of pressure of 0.1 lb/sq in. It is intended that, during stacking over an airfield, the cabinatmosphere should be raised to sea-level pressure so that the most rapid possible descent could thereafter be made without causingdiscomfort to passengers. There is an independent pressure- Air bleed from the Caravelle engines is used for air conditioning, pressurization, de-icing and fuel heating as shown in this diagram, which is concerned mainly with air conditioning. (1) ram-air ducts, (2) vent to atmosphere, (3) two-pass heat-exchangers, (4) expansion turbine on the same shaft as (5) brake turbine, (6) automatic (normal) pressurization valves, (7) manual (stand- by) pressurization valve, (8) main on/off cock, (9) engine intake de-icing bleeds, (10) fuel heating air bleeds, (11) system on/off, automatic temperature control and water separator cut-off valves, (12) flow valve, (13) water separators and (14) their de-icing sensers and regulators, (IS) ground conditioning trolley connection, (16) temperature sensers for indication and control, (17) cockpit control panel, (18) steward's control panel, (19) wing and tail de-icing air supply. ARROWS SHOW FULLY COLD FLOW ARROWS SHOWPARTLY COOLED FLOW
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