Refrigeration – Processes – Reducing pressure on compressed gas
Reexamination Certificate
1999-07-30
2001-03-13
McDermott, Corrine (Department: 3744)
Refrigeration
Processes
Reducing pressure on compressed gas
Reexamination Certificate
active
06199387
ABSTRACT:
This invention relates to an air-conditioning system for conditioning moisture-containing, pressurized air for air-conditioning a room, in particular for air-conditioning airplane cabins, and to a corresponding method.
Fresh air for air-conditioning airplane cabins is conditioned from the air (known as bleed) bled off the engine at high pressure and high temperature. Air-conditioning systems draw the necessary cooling power out of the pressure and temperature potential of the engine air. In the course of the fresh-air conditioning process the bleed is cooled, dehumidified and expanded to the cabin pressure of 1 bar in ground operation or about 0.8 bars in flight operation. Special value is attached in fresh-air conditioning to dehumidification in order to prevent icing of individual components of the air-conditioning system and ice crystallization in the fresh air to be conditioned. The necessity of dehumidification exists mainly in ground operation. however, because in flight operation. i.e. at high altitudes, ambient air and thus the bled-off engine air is already extremely dry.
With reference to
FIG. 4
an air-conditioning system will be described in the following as is used in present-day Airbus and Boeing passenger airplanes, for example the A330/340 and Boe 757/767.
Via flow control valve FCV the amount of bleed required for supplying fresh air to the cabin is bled off an engine at about 2 bars and 200° C. In ground operation bleed is withdrawn from an auxiliary engine at about 3 bars. The bleed is first passed through primary heat exchanger PHX and cooled to about 100° C. Then the bleed is compressed further in compressor C to about 4.5 bars and 160° C. and cooled again to about 45° C. in main heat exchanger MHX. The high pressure of 4.5 bars is necessary to be able to realize a high degree of dehumidification in the following water extraction cycle. This air cycle system is therefore also known as a “high-pressure water extraction cycle”.
The high-pressure water extraction cycle comprises condenser CON, as proposed in EP 0 019 493 A3, and water extractor WE following condenser CON. Compressed, cooled bleed is cooled in condenser CON by about &Dgr;Y=−15K, condensed water is then extracted in water extractor WE, and the thus dehumidified air is subsequently expanded in turbine T to the cabin pressure of about 1 bar, the temperature at the turbine outlet being about −30° C. Thus conditioned bleed, before being mixed as fresh air with recirculated cabin air in a mixing chamber, is passed through condenser CON of the high-pressure water extraction cycle in heat-exchanging fashion in order to cool the compressed, cooled bleed to the temperature necessary for water extraction in water extractor WE. Air expanded in turbine T and cooled is thereby accordingly heated again by &Dgr;T=+15K to about −15° C.
The conditioned air is then mixed with recirculated cabin air in a mixing chamber (not shown). Temperature control valve TCV can be used to increase the temperature at the turbine outlet to obtain an optimum mixing temperature with the admixed, recirculated cabin air. For this purpose part of the bleed precooled in primary heat exchanger PHX is branched off and resupplied to the conditioned air stream after turbine T.
The high-pressure water extraction cycle has. in addition to condenser CON, heat exchanger REH (reheater) preceding condenser CON. Compressed cooled bleed is first passed through heat exchanger REH before entering condenser CON, and subsequently the dehumidified air is passed through heat exchanger REH before entering turbine T. Heat exchanger REH has substantially the function of heating the dehumidified air by about &Dgr;T=5K and vaporizing residual moisture while simultaneously recovering energy before air enters the turbine. Residual moisture in the form of fine droplets can destroy the turbine surfaces since air almost reaches the speed of sound in turbine T. A second function of heat exchanger REH is to relieve condenser CON by cooling compressed, cooled bleed before it enters condenser CON by &Dgr;T=−5K.
It is typical of such an air-conditioning system that the energy gained in turbine T is used to drive compressor C, on the one hand, and fan F, on the other. All three wheels, that is turbine/compressor/fan, are disposed on a common shaft and form air cycle machine ACM, also known as a three-wheel machine Fan F conveys a cooling air stream branched off from ambient air through a cooling shaft in which primary and main heat exchangers PHX, MHX are disposed. Fan F must be driven actively by turbine T in particular in ground operation. In flight operation ram air suffices, it being optionally throttled by a valve at the cooling shaft inlet.
The overall system is designed for ground operation at an ambient temperature of 38° C. In order to optimize the effectiveness of the heat-exchange process in the cooling shaft, water gained in the high-pressure water extraction cycle is supplied at a temperature of about T=20° C. and a pressure of 3.5 bars in the cooling shaft inlet in fine droplets to be vaporized therein, thereby improving the effectiveness of the heat exchangers. In case air cycle machine ACM fails completely, for example because the necessary mass flow rate of compressed air is not attainable for fulfilling the parameters necessary for the system to work, bypass valve BPV is provided for bypassing turbine T. In this case check valve CV opens automatically since an overpressure triggering check valve CV builds up before compressor C as turbine T is not driven. The opening of check valve CV causes compressor C to be bypassed or “short-circuited”. In this state, fresh air is supplied directly through primary and main heat exchangers PHX, MHX to the mixing chamber following the air-conditioning system to be mixed with recirculated cabin air.
As mentioned at the outset, icing in the conditioned fresh air is a problem. In order to avoid icing, anti-icing valve AIV is provided for directly branching off part of the air bled off the engine and resupplying it to the conditioned air stream after turbine T. A further way of avoiding ice is to design the turbine such that no temperatures below 0° C. occur at the turbine outlet. However, this latter variant requires much more energy if the same cooling power is to be reached. Therefore, it is preferable to supply hot air at the turbine outlet.
An improved variant of this air-conditioning system provides that air cycle machine ACM is extended by a second turbine. This makes the three-wheel machine, turbine/compressor/fan, into a four-wheel machine, turbine/turbine/compressor/fan (U.S. Pat. No. 5,086,622). The second turbine is disposed on a common shaft with the other wheels in order to recycle the energy gained by the turbines into the air-conditioning system, as in the conventional three-wheel system. The second turbine supplements the first turbine such that air dehumidified in the high-pressure water extraction cycle is expanded in two stages, the condenser of the high-pressure water extraction cycle being disposed with the air pipe between the two turbines in heat-exchanging fashion. This is more favorable energetically than the conventional structure of the air-conditioning system because air exiting the first turbine is comparatively warm, preferably above 0° C. to avoid ice, and this air is heated in condenser CON by for example &Dgr;T=+15 Kelvin to a comparatively high energy level, so that the second turbine can utilize this high energy level to gain energy which gets lost in the conventional system. This system is known in expert circles as a “condensing cycle”.
The problem of the present invention is to adapt the above-described air-conditioning system or method so that it can be designed more flexibly and the overall efficiency optimized more easily, in particular to make it adaptable to the particular system requirements more flexibly and therefore better energetically through a greater number of freely selectable system param
Christie Parker & Hale LLP
Drake Malik N.
Liebherr-Aerospace Lindenberg GmbH
McDermott Corrine
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