FR2796918A1 - AIR CONDITIONING METHOD AND SYSTEM FOR AIRCRAFT CABINS - Google Patents

Abstract

An air conditioning system, in particular for airplanes, is provided in which overpressure air, containing humidity, is treated for the air conditioning of the cabin. The overpressure air is then further compressed in two separate stages, dehydrated in a water separation circuit under high pressure, then expanded in one or two turbine stages (T1, T2). Depending on the design, ice-free air conditioning or a high efficiency of the air conditioning system can be obtained, in particular if two turbine stages are provided and if the energy, recovered respectively in the turbine stages, is used and distributed by regeneration on the compression stages (C1, C2).

Description

I

  Air conditioning system for aircraft cabins.

  The invention relates to an air conditioning system for airliners for the treatment of overpressure air, containing humidity, for the air conditioning of an airliner cabin, comprising: - a compression device for compressing the overpressure air to a higher pressure, - a condenser and a downstream water separator for dehydrating the compressed air, - an expansion device for relaxing the dehydrated air to a lower pressure, the expansion device comprising a first turbine, coupled to the compression device for driving the latter, and - an outlet pipe for transferring the treated air for

  air conditioning of the airliner cabin.

  The invention also relates to a corresponding method for the treatment of overpressure air, containing humidity, for the air conditioning of an airliner cabin, comprising the following phases: compression of the overpressure air at higher pressure,

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  - dehydration of the compressed air, water being condensed and separated from the compressed air, - expansion of the dehydrated air to a lower pressure, process energy, used by regeneration during the compression phase overpressurized air at a higher pressure, then being recovered, and - transfer of the treated air for air conditioning the

airliner cabin.

  The fresh air for the air conditioning of aircraft cabins is treated from the air drawn under high pressure and at high temperature from the powertrain, known as trapped air. Air conditioning systems draw the cooling power required for this purpose from the pressure and temperature potential of the power of the power unit. The trapped air is cooled during the fresh air treatment process, is dehydrated and expanded to the pressure of the bar cabin

  in ground service and / or around 0.8 bar in air service.

  During the treatment of fresh air, a particular value is attributed to the dehydration of the air, to prevent icing of the various components of the air conditioning system and the formation of ice crystals in the fresh air to be treated. Dehydration is actually mainly

  necessary in ground service, because in air service, that is

  say at high altitudes, the ambient air, and therefore the air drawn

  of the powertrain, are in any case extremely dry.

  An air conditioning system, as used in current Airbus and Boeing airliners, for example the A330 / 340 and Boe 757/767, will be described below using

Figure 4.

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  By means of an FCV (Flow Control Valve), the quantity of trapped air ("Bleed"), required to supply fresh air to the cabin, is withdrawn by propulsion group under a pressure of approximately 2 bars and at a temperature of approximately 200 C. In ground service, the trapped air is drawn from an auxiliary propulsion group at approximately 3 bars. The trapped air is first directed through a primary heat exchanger (PHX) and cooled to around 100 C. The trapped air is then compressed to around 4.5 bar in a compressor C (("Compressor"), and again cooled to about 45 C in a main heat exchanger MHX ("Main Heat Exchanger"). The high pressure of 4.5 bar is required to be able to achieve a high degree of dehydration in the downstream water separation circuit. This Air Cycle System is therefore also known as (<

  separation of water under high pressure ".

  The high pressure water separation circuit comprises a condenser CON ("Condensor"), as proposed in the publication EP O 019 492 A3, and a water separator WE ("Water Extractor"), mounted downstream of the CON condenser. The trapped, cooled and compressed air is cooled in the condenser CON by approximately AT = -15 K, the condensed water is then separated in the water separator WE, then the air thus dehydrated is expanded in a turbine T at the cabin pressure of approximately 1 bar, the temperature at the outlet of the turbine being approximately -30 C. Before being mixed in the form of fresh air in a chamber for mixing with the recirculation air of the cabin, the trapped air thus treated is directed in heat exchange through the condenser CON of the water separation circuit under high pressure, to cool the trapped air compressed and cooled to the temperature required for the

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  water separation in the WE separator. The cooled and expanded air in the turbine T then heats up accordingly from AT = +15 K to approximately -15 C. The air conditioning is then mixed in a mixing chamber not shown in the cabin recirculation air . The temperature at the outlet of the turbine can be raised by means of a thermal control valve TCV, to obtain an optimal mixing temperature with the recirculation air added from the cabin. Part of the trapped air, precooled in the PHX primary heat exchanger, is diverted for this purpose and returned to the treated air flow downstream of the turbine T. A REH heat exchanger ("Reheater"), mounted upstream of the CON condenser, is provided in the high pressure water separation circuit in addition to the CON condenser. The compressed and cooled trapped air is first directed through the REH heat exchanger, before entering the condenser CON, then the dehydrated air is directed through the REH heat exchanger, before entering the turbine T. The REH heat exchanger then essentially has the function of heating the dehydrated air by about AT = 5 K and evaporating the residual moisture with simultaneous energy recovery, before the air enters the turbine. Residual moisture in the form of fine droplets can indeed destroy the surfaces of the turbine, because the air practically reaches the speed of sound in the turbine T. A second function of the REH heat exchanger is to relieve the condenser CON , the compressed and cooled trapped air being cooled as a result of

  AT = -5 K before entering the CON condenser.

  The fact that the energy recovered in the turbine T is used to drive on the one hand the compressor C and on the other hand a fan F ("Fan") is typical of such an air conditioning system. The three wheels, that is to say turbine / compressor / fan, are arranged on a common shaft and form the Air Cycle Machine ACM, also called a three-wheel machine. The fan F delivers a flow of cooling air, derived from the ambient air, through a cooling duct in which the primary and main heat exchangers PHX, MHX are arranged. In particular in ground service, the fan F must be actively driven by the turbine T. The dynamic air, which can possibly be laminated by a valve at the outlet

  of the cooling jacket, sufficient in air service.

  The entire system is designed for ground service at an ambient temperature of 38 C. To optimize the efficiency of the heat exchange process in the cooling duct, the water recovered in the water separation circuit under high pressure is sent in fine droplets, at a temperature of around T = 20 C and under a pressure of 3.5 bars, at the inlet of the cooling duct to be evaporated there, the efficiency of

  the heat exchanger being thereby improved.

  In the event of total failure of the Air Cycle Machine ACM, since for example the required pressurized air mass flow cannot be reached to fulfill the parameters necessary for the system to operate, a BPV bypass valve ("bypass valve") ") is designed to bypass the turbine T. A CV overload valve (" check valve ") automatically opens in this case, an overpressure, triggering the CV valve, being established upstream of the compressor C fault

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  driven by the turbine T. The compressor C is bypassed and / or <c short-circuited "by the opening of the overload valve CV. In this state, the fresh air is sent directly, through the heat exchangers primary and main PHX and MHX, to the mixing chamber mounted downstream of the air conditioning system for mixing

  with recirculating air from the cabin.

  As mentioned in the introduction, ice formation in the treated fresh air is a problem. An anti-icing valve (AIV), by which a part of the air drawn from the propulsion unit is directly diverted and returned to the treated air flow downstream of the turbine T, is provided to prevent the formation of ice cream. Another possibility to avoid ice is to design the turbine so that no temperature below 0 C occurs at its outlet. However, the latter variant requires much more energy if the same cooling capacity is to be achieved. The arrival of hot air at the

  turbine outlet is therefore preferred.

  An improved variant of this air conditioning system provides for the Air Cycle Machine ACM to be extended by a second turbine. The 3-wheel turbine / compressor / fan machine therefore becomes a 4-wheel turbine / turbine / compressor / fan machine (US 5,086,622). The second turbine is placed with the other wheels on a common shaft, to recycle in the air conditioning system the energy recovered by the turbines, as on the conventional three-wheel system. The second turbine completes the first turbine so that the dehydrated air in the high pressure water separation circuit is expanded in two stages, the condenser of the separation circuit

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  water under high pressure being disposed in heat exchange, with the air line, between the two turbines. This solution is more advantageous from an energy point of view than the conventional structure of the air conditioning system, since the air leaving the first turbine is relatively hot, preferably above 0 C to avoid ice, and this air is heated in the condenser. CON of for example AT = +15 Kelvin at a relatively high energy level, so that the second turbine can use this high level for the

  energy recovery, lost on the conventional system.

  In specialized circles, this system is known as

name of "condensing-cycle".

  An improvement of the four-wheel machine is described in publication WO 99/24318 and will generally be described as a 2 + 2-wheel system. According to this system, the two turbines are arranged on shafts separated from each other, the first turbine with the compressor and the second turbine with the fan located respectively on a shaft

common.

  The present invention aims to adapt the air conditioning system and / or the method described above, so that they can have a more flexible design and that the overall performance is easier to optimize, in particular so that they are more flexible. by a higher number of freely optional system parameters and that they are therefore better adaptable from an energy standpoint to

respective system requirements.

  The invention is characterized, for the air conditioning system, in that the compression device has a two-stage structure for the subsequent compression of the air in

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  overpressure, and includes first and second compressors

trained separately.

  The process according to the invention is characterized in that the compression phase of the overpressure air at a higher pressure is ensured in a first and in a second

separate compression stage.

  Another objective is to create an air conditioning system and method, which reduce the formation of

ice during air treatment.

  Another goal is to improve overall performance

  compared to known systems and methods.

  Another objective to see elsewhere is to raise the

  overall performance, particularly in air service.

  These objectives are achieved by the air conditioning system and the process with the characteristics indicated in the

  coordinated patent claims and claims

dependent on these.

  On the air conditioning system according to the invention, the other of the two compressors is driven by foreign energy. It is advantageous that the expansion device has a two-stage embodiment and comprises a second turbine, coupled to the other of the two compressors for driving the latter. A water separator is then placed between the two turbines. In a preferred form of construction, a first heat exchanger, through which the compressed air is directed and cooled in heat exchange, is disposed between the two turbines. A second exchanger, through which the compressed air is directed and cooled in heat exchange,

  is then placed downstream of the second turbine.

  It is advantageous that another heat exchanger, through which the dehydrated air is directed and reheated, is arranged

upstream of the first turbine.

  Another judicious measure consists in providing a device

  bypass to bypass the other of the two compressors.

  A bypass device is usefully provided for bypassing

the first turbine.

  It is also useful that a bypass device is provided

  to bypass one of the two compressors.

  In an advantageous form of construction of the air conditioning system, a generator is provided which is coupled to

  one of the two turbines and produces and dissipates energy.

  Finally, it is wise to provide a fan which is coupled to the other of the two turbines and is driven by the latter. According to the process according to the invention, the other of the two compression stages is supplied with energy foreign to the

process.

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  Another solution is to use energy from the regenerated process in the other of the two compression stages. The expansion of the air is then ensured in two separate stages by means of a first and a second turbine, and the energy recovered by the first turbine is used by regeneration, at least in part, in the second stage of compression, and the energy recovered by the second turbine is used by regeneration, at least in part, in the first compression stage, or vice versa. Water is separated from the air after the expansion of the air in the first turbine stage and before the expansion of the air in the second

turbine stage.

  During the condensation phase, the compressed air is advantageously sent, in heat exchange, to the air expanded by the first stage of the turbine, and is cooled. The condensation phase is then carried out on two stages, the compressed air then also being sent, in heat exchange, to the air expanded by the second turbine stage,

and being cooled.

  In a judicious implementation of the process, the air is sent in heat exchange to the compressed air, after the

  dehydration and before relaxing, and is reheated.

  It is useful that at least part of the energy, recovered during expansion in one of the two turbine stages, is

  converted into electrical energy and dissipated.

  The method according to the invention is finally characterized in that at least part of the energy of the process, recovered l 2796918 during the expansion of the dehydrated air, is used for

fan drive.

  It is essential for the invention that the compression of the trapped air be ensured in two stages. One of the two compression stages conventionally receives the energy required for compression by regeneration of the energy recovered during the expansion of the dehydrated air. One of the two compressor wheels is then for example arranged with a turbine wheel on a common shaft, so that the compressor wheel and the turbine wheel, possibly in addition with a fan wheel, form a (first) Air Cycle Machine with two or three wheels. The compressor wheel of the first compression stage is preferably arranged with the turbine wheel on a common shaft, but it can also be the compressor wheel of the second compression stage. The other compressor wheel can for example be driven by energy foreign to the system. It is therefore possible to design the (first) Air Cycle Machine so that the compressors and the turbine, arranged on a common shaft, have a relatively high efficiency. First of all, the

  compression power of the Air Cycle Machine is

  below the power that would be required to bring the air from the power unit to be treated to the pressure required for air dehydration. The compression power still missing is therefore provided by the additional compression stage. The air conditioning system can therefore have a flexible design and overall performance is easy

to optimize.

  Due to the second compression stage, it is in particular possible to produce air conditioning free of

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  ice cream. The invention then takes advantage of the fact that the quantity of water, condensed from the air, rises to a predefined temperature with increasing pressure. Since the temperature, for systemic reasons, can only be influenced within certain limits, in particular because the air in the compressed powertrain, cooled in the main heat exchanger, cannot be cooled in the cooling duct below the ambient temperature (calculation for an ambient temperature of 38 ° C.), a relatively high compression pressure 2 4.6 bars can be produced by the additional compression stage, to obtain the desired high degree of condensation, in the high pressure water separation circuit. The absence of ice is for example obtained at -10 C and under 1 bar with

  less than 1.8 g of water per kilogram of dry air.

  Instead of a source of energy external to the system for the additional compressor, the latter can also be exploited by regeneration, not only the compression of the trapped air, but also the expansion of the dehydrated air, being ensured on two stages, for example in two separate turbines, and the energy supplied by the turbines being used on the one hand for the first compression stage, and on the other hand for the second compression stage. The air conditioning system then comprises two machines, which respectively have at least one compressor wheel and one turbine wheel on a common shaft. In addition, the fan can be arranged on a shaft, and a motor on the other shaft, the motor can also be designed as

generator.

  The arrangement of the compressor and turbine wheels on two separate shafts and / or in two separate machines allows

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  Essentially more flexible design of the entire system compared to conventional air conditioning systems. Optimal design, especially of

  compressor and turbine, is obtained.

  The absence of ice can then be obtained in particular without problem if not only the compression of the trapped air, but also the condensation of the humidity contained in the air, are ensured on two stages in the water separation circuit. under high pressure. A first condenser of the high pressure water separation circuit for heat exchange with the dehydrated air is then placed before entering the turbine, before entering the second turbine for two-stage expansion, and a second condenser of the high pressure water separation circuit for the heat exchange with the dehydrated and expanded air is disposed after the outlet of the turbine, the compressed air then being directed in heat exchange through these condensers to condense the then separate it. The efficiency of dehydration is essentially high by two-stage condensation. This applies in particular if relaxation is also ensured on

  two stages in two turbine stages.

  If small amounts of ice in the cooling air are not a relatively large problem, but if a high efficiency of the whole system is in the foreground, it is advantageous to combine two-stage compression with a two-stage expansion , the compressed air being directed in heat exchange, for the separation of moisture, through a condenser arranged between a first and a second turbine. The efficiency can be further improved if the dehydrated air, before admission to the first stage of

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  turbine, is sent in heat exchange in a "Reheater" on the compressed air, not yet dehydrated. The condenser is relieved by this fact, on the one hand, the compressed air being precooled before admission to the condenser. On the other hand, any residual moisture contained in the dehydrated air is evaporated before admission to the first turbine, so that the surfaces of the latter are protected from destruction by drops of water. This variant should be classified as the most advantageous of the

performance point of view.

  The invention offers the other advantage that a disconnection of the additional compression stage, and possibly of the turbine stage driving this stage, is permitted by suitable bypass arrangements. This possibility is particularly useful in air service, where the humidity of the air and the absence of ice in the cooling air play no role, high compression for the cooling water circuit under high pressure does not is therefore not necessary. In air service, one of the two machines can then be completely disconnected by opening and / or closing valves, unnecessary losses being avoided thereby and the performance in air service can be reduced.

increased consequence.

  The realization of the air conditioning system with two machines separated from each other, respectively comprising a compressor and a turbine wheel on a common shaft, one of which can be disconnected in air service, offers other advantages which result that a higher pressure ratio than in air service is available in ground service due to the system. It is therefore advantageous from an energy point of view to provide a ground nozzle for

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  relatively small turbine (cross section of the directional grid). This small nozzle is produced by the series mounting of the two turbine stages, a "total nozzle", smaller than each individual nozzle, resulting therefrom. But in air service, despite a lower pressure ratio available, approximately the same volume flow is required for the air conditioning of the aircraft cabin, so that a large nozzle would be necessary for about the same air flow in air service. A machine, and thus a turbine stage, being disconnected for the entire system in air service, this results in a large nozzle for the entire system due to the only turbine remaining in the second stage, and therefore the efficiency in air service can be high. efficiency gain is preferably used to configure the primary heat exchanger and the main heat exchanger with the lowest possible construction dimension and weight, subject to the limits es that the required volume flow has just been filled. In fact, the measure consisting in providing two machines instead of a single machine can therefore make it possible to obtain a smaller construction dimension, and thus a total weight.

  weaker air conditioning system.

  Another advantage, obtained if the air conditioning system has two separate machines with a compressor and a turbine respectively coupled, is that at least the other machine can still operate in the event of a machine failure, and the air conditioning can have unlimited later operation. With the redundancy required for aircraft, this means that one less air conditioning unit ("Pack") is required per aircraft, for example two packs only instead of three. The consequence is that the

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  weight is reduced due to the number lowered accordingly

  of components, the reliability of the installation increases, and maintenance and repair costs are reduced.

  Finally, it should be noted that, both in ground service with two machines and in air service with a disconnected machine, the energy acquired during expansion in the turbine (s) is respectively largely recovered via the two stages of compression (ground service) or through the single compression stage

remaining (air service).

  The invention will be described by way of example using FIGS. 1 to 3. The drawings correspond to: FIG. 1: a diagram of an air conditioning system according to the invention, FIG. 2: a diagram of a improved construction form of the system of Figure 1; in particular for the production of air conditioning free of ice, Figure 3: a diagram of an improved form of construction of the system of Figure 1 with improved efficiency, and Figure 4: an air conditioning system according to the state of l 'art. FIG. 1 represents an air conditioning system, which differs essentially from the air conditioning system described in FIG. 4 with reference to the state of the art in that two compressors C1 and C2 are provided to supply the trapped, cooled air. in the PHX primary heat exchanger, at the

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  pressure required for separation of water under high pressure. Compressors C1 and C2 must be designed according to the fact that the absence of ice or that the efficiency

  high air conditioning system lie in the foreground.

  In FIG. 1, the compressor C1 of the first compression stage forms with the turbine T and the fan F a three-wheel machine ACM. That is to say that the compressor C1 and the fan F are driven by regeneration by the energy recovered in the turbine T. The compressor C2 of the second compression stage is driven by a separate motor M, that is to say by foreign energy. The overload valve CV2 opens automatically in the event of a blockage of the compressor C2 or if the motor M of the compressor C2 is not disconnected, for example in air service. The overload valve CV1 opens automatically in the event of a blockage of the Air Cycle Machine ACM or if the bypass valve BPV2 is

actively open.

  The air conditioning system shown diagrammatically in FIG. 1 corresponds, moreover, in principle, in its structure and its function, to the system of FIG. 4, having to then take into account the fact that the <Reheater "is not absolutely essential, but is d '' a big advantage in particular

  for the case where the absolute absence of ice must be obtained.

  Another configuration of the invention is shown in Figure 2. In the air conditioning system shown schematically in this figure, the dehydrated air is expanded over two stages by means of turbines T1 and T2. The energy recovered during expansion in the turbine T1 is used by regeneration for driving the compressor C2, while the energy supplied by the turbine T2, as before, is used by regeneration by the compressor C1. In

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  additional condenser CON1 in the high pressure water separation circuit, through which the compressed trapped air is sent in heat exchange to the dehydrated air, expanded in the turbine Tl, a second condenser CON2 is provided, through from which the pre-cooled air in the condenser CON1 is sent in heat exchange to the air expanded by the turbine T2. CONI and CON2 condensers are particularly advantageous if the absence of ice in the air conditioning is required. It is otherwise possible to give up the condenser CON2, solution chosen in particular if a

  high efficiency of the whole system must be obtained.

  Before admitting the expanded air into the condenser CON1 into the first turbine stage, a water separator WE2 is advantageously provided in addition to the water separator WE1 provided in the high pressure water separation circuit. The separated water is sent to the dynamic air heat exchangers MHX / PHX to be evaporated there. The WE2 water separator is advantageous in particular in the event of blockage of the Air Cycle Machine ACM, because the efficiency of the first separator

  of water WE1 is then strongly limited.

  The high pressure water separation circuit can also be disconnected by opening the ECV "economy valve", which is particularly useful in the event of failure of the Air Cycle Machine ACM and insufficient pressure for energy efficient use of the high pressure water separation circuit. Water separation is then ensured at low pressure by the WE2 water separator. The condensers CON1 and CON2 are in this case out of function.

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  As on the air conditioning system previously described, the machine comprising the turbine T1 and the compressor C2 can be disconnected, in particular for air service, the bypass valve BPV1 being open. The Air Cycle Machine ACM can also be bypassed, in particular in the event of

  failure, by opening the BPV2 bypass valve.

  An optional motor M, by which the efficiency of the system can be optimized, is shown in dotted lines in FIG. 2, on the shaft which mutually assembles the turbine T1 and the compressor C2. Either additional energy is supplied to the system. Either, and in particular, the motor can be used as a generator, to supply the network of

edge in excess energy.

  While the air conditioning system shown in Figure 2 is particularly adapted to provide air conditioning without ice, an air conditioning system shown schematically in Figure 3 has a particularly favorable performance. As described with reference to FIG. 4 (state of the art), a REH "reheater" upstream of the turbine T1 and a condenser CON downstream of the turbine T1 are arranged in heat exchange for the passage of compressed air and the condensation of the moisture contained therein. The REH "Reheater" can in principle be omitted, but it is advantageous for the reasons already mentioned. Contrary to the state of the art described in FIG. 4, the condensation in the condenser CON of the humidity contained in the compressed air is ensured at a relatively high energy level, this energy can be used in the turbine. T2, like

  basically proposed in the publication US 5,086,622.

  In the publication US 5,086,622, however, the turbines T1 and T2 are arranged jointly with the compressor C1 and

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  fan F on a common shaft in a single ACM Air Cycle Machine. According to the invention, the compression being distributed in two stages, and the turbine T1 with the compressor C2, as well as the turbine T2 with the compressor C1, forming respectively separate machines, the efficiency can be still high, because the design of the

  air conditioning system is on the whole more variable.

  As described in FIG. 2, the (economys valves "ECV1 and ECV2 are used for the optional bypass of the high pressure water separation circuit. By opening the bypass valve BPV1, the machine comprising the turbine T1 and the compressor C2 is bypassed in air service. The BPV2 bypass valve is therefore used to bypass the Air Cycle Machine ACM in the event of its failure. The two bypass valves are also

  optionally usable as thermal control valves.

  The TCV2 thermal control valve is also optional, while the TCV4 thermal control valve should preferably be included in the air conditioning system. As previously mentioned, it is also possible to opt out of the REH "Reheater". But it is advantageous for the reasons

cited in the introduction.

  Depending on the system requirement and / or to simplify the system, individual valves, as already mentioned, can be omitted or be partially grouped. The ECVl, BPV1 and ECV2 taps, for example, can in particular be grouped together in a pipe with a single tap,

  so that in total this results in a less complex system.

  The installation is then optimized for air service

  with a disconnected machine (turbine T1 / compressor C2).

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Claims (22)

Claims.   1. Method for the treatment of overpressure air, containing humidity, for the air conditioning of an airliner cabin, comprising the following phases: - compression of the overpressure air to a higher pressure , - dehydration of the compressed air, water being condensed and separated from the compressed air, - expansion of the dehydrated air to a lower pressure, of the energy of the process, used by regeneration during the phase of compression of the overpressure air to a higher pressure, then being recovered, and - transfer of the treated air for the air conditioning of the airliner cabin, characterized in that the compression phase of the overpressure air at higher pressure is ensured in a   first and in a second separate compression stage.   2. Method according to claim 1, energy foreign to the process being sent into the other of the two compression stages. 3. Method according to claim 1, the energy of the regenerated process being used in the other of the two compression stages.   4. The method of claim 3, the expansion of the air being provided in two separate stages by means of a first and a second turbine, and the energy recovered by the first turbine being used by regeneration, at least in part, in the second stage of compression, and the energy recovered by the second turbine being used 22 2796918   by regeneration, at least in part, in the first stage compression, or vice versa.   5. Method according to claim 4, water being separated from the air after the expansion of the air in the first stage of the turbine and before the expansion of the air in the second stage turbine.   6. Method according to one of claims 4 and 5, air   compressed being sent in heat exchange, during the condensation phase, to the air expanded by the first   turbine stage, and being cooled.   7. Method according to claim 6, the condensation phase being carried out on two stages, the compressed air also being sent in heat exchange to the air.   expanded by the second turbine stage, and being cooled.   8. Method according to one of claims 1 to 7, air   being sent in heat exchange, after dehydration and before expansion, on the compressed air, and being heated.   9. Method according to one of claims 4 to 8, at least   part of the energy recovered during expansion in one of the two turbine stages being converted into energy electric and dissipated.   10. Method according to one of claims 1 to 9,   characterized in that at least part of the process energy, recovered during the expansion of the dehydrated air, is used for the drive of a fan (F). 23 - 2796918   11. Air conditioning system for airliners for the treatment of overpressure air, containing humidity, for the air conditioning of an airliner cabin, comprising: - a compression device (Cl, C2) to compress the pressurized air to a higher pressure, a condenser (CON; CON1, CON2) and a downstream water separator (WE; WE1, WE2) to dehydrate the compressed air, - an expansion device ( T; T1, T2) for expanding the dehydrated air to a lower pressure, the expansion device comprising a first turbine (T; T1, T2), coupled to the compression device (Cl) for driving the latter, and - an outlet pipe for transferring the treated air for the air conditioning of the airliner cabin, characterized in that the compression device (C1, C2) has a two-stage structure for the subsequent compression of the air overpressure and includes first and second   compressor (C1 and C2) driven separately.   12. Air conditioning system according to claim 11, the other of the two compressors (C2) being driven by foreign energy (M).   13. Air conditioning system according to claim 11, the expansion device having a two-stage embodiment and comprising a second turbine (T1), coupled to the other of the   two compressors (C2) for driving the latter.   14. Air conditioning system according to claim 13, characterized in that a water separator (WE2) is arranged between the two turbines (T1, T2). 24 2796918   15. Air conditioning system according to one of claims   13 and 14, a first heat exchanger (CON; CON1), through which the compressed air is directed and cooled in heat exchange, being arranged between the two turbines (T1, T2).   16. Air conditioning system according to claim 15, a second heat exchanger (CON2), through which the compressed air is directed and cooled in heat exchange, being   disposed downstream of the second turbine (T2).   17. Air conditioning system according to one of claims   11 to 16, another heat exchanger (REH), through which the dehydrated air is directed and heated, being   arranged upstream of the first turbine.   18. Air conditioning system according to one of claims   11 to 17, a bypass device (CV2) being provided for   bypass the other of the two compressors (C2).   19. Air conditioning system according to one of claims   13 to 18, a bypass device (BPV1) being provided for   bypass the first turbine (T1).   20. Air conditioning system according to one of claims   11 to 19, a bypass device (CV1; BPV2) being provided   to bypass one of the two compressors (C1).   21. Air conditioning system according to one of claims   11 to 20, a generator (M), coupled to one of the two turbines and which produces and dissipates energy, is provided. 2796918   22. Air conditioning system according to one of claims   11 to 21, a fan (F), coupled to the other of the two   turbines (T2) and driven by the latter, being provided.