Patent Publication Number: US-2007113579-A1

Title: Low energy electric air cycle with portal shroud cabin air compressor

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS  
      This application claims the benefit of U.S. Provisional Application No. 60/604,610 filed Aug. 25, 2004. 
    
    
     TECHNICAL FIELD  
      The present invention relates to environmental control systems for various aircrafts. More particularly, the present invention relates to electrically-driven air cycle systems that regulate the temperature of at least the aircraft fuselage.  
     BACKGROUND  
      Passenger aircrafts are typically equipped with an environmental control system, including an air cycle conditioning system for cooling the aircrew cabins, and other aircraft locations and components. One class of air cycle conditioning systems that are widely used in aircraft to provide cooled air takes advantage of a supply of pressurized air that is bled from an aircraft engine, known as bleed air. Other electrically-driven environmental control systems generally operate by receiving fresh ram air from inlets that are located in at least one favorable position near the aircraft&#39;s forward belly fairing leading edge. The fresh ram air is supplied to at least one electric motor-driven air compressor that raises the air pressure to, for example, the desired air pressure for the aircrew cabins. From the at least one air compressor, the air is supplied to an ozone converter. Because air compression creates heat, the air is then supplied to an air conditioning pack in which the air is cooled and then transported to the aircraft fuselage. At least one recirculation system is also provided to recycle air from the fuselage back to the at least one air compressor. The recirculation system may be used at both high and low altitudes, but is particularly useful when the aircraft is flying at high altitudes where the pressure for the ram air is relatively low.  
      The numerous applications and components in a typical environmental control system, including the ram air cycle, the recirculation cycle, heat exchangers, condensers, reheaters, water extractors, and an air cycle machine, can require large amounts of energy to operate. Further, the large number of components in a typical environmental control system tends to be heavy and complex. Hence, there is a continuing need for simplification of environmental control systems for various aircrafts, including a reduction in the number of components, programming, and circuitry. There is also a need for environmental control systems that can be operated with minimized power consumption and weight.  
     BRIEF SUMMARY  
      The present invention provides an environmental control system for an aircraft cabin. The environmental control system comprises a plurality of electrically-driven cabin air compressors, each cabin air compressor compressing ram air received from the aircraft exterior; a heat exchange circuit comprising a primary heat exchanger and a secondary heat exchanger in series, the primary heat exchanger adapted to receive airflow from at least one of the cabin air compressors, and the secondary heat exchanger adapted to supply airflow to the aircraft cabin; and an air cycle machine comprising a compressor, adapted to receive airflow from the primary heat exchanger and supply compressed air to the secondary heat exchanger. At least one of the cabin air compressors comprises a housing having an air inlet, and an air outlet in communication with the primary heat exchanger, the air inlet defining an outer circular wall, an inner circular shroud wall having at least one port extending therethrough, a central channel, and an annular channel disposed concentrically around the central channel and in communication with the central channel by way of the at least one port extending through the inner circular shroud wall. The air cycle machine compressor further comprises a compressor wheel having a plurality of vanes, the wheel being interposed between the inlet and the outlet, and disposed in the central channel.  
      According to one embodiment, the environmental control system further comprises an air recirculation system, having an aft recirculation fan adapted to receive a portion of recirculation air from the aircraft cabin, and a recirculation heat exchanger, disposed in the heat exchange circuit in series with the primary and secondary heat exchangers, and adapted to receive airflow from the aft recirculation fan.  
      Other independent features and advantages of the preferred environmental control system will become apparent from the following detailed description, taken in conjunction with the accompanying drawings which illustrate, by way of example, the principles of the invention.  
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  is a flow chart illustrating the top-level architecture of an environmental control system for an aircraft in which the present invention may be incorporated;  
       FIG. 2  is a flow chart illustrating an air cycle pack for an aircraft according to an embodiment of the present invention;  
       FIG. 3  is a graph illustrating operational relationships between corrected compressor flow and a compressor pressure ratio in prior art air cycle systems and vapor cycle systems for an aircraft; and  
       FIG. 4  is a cross sectional elevation view of a ported shroud air compressor that is incorporated in the air cycle pack of  FIG. 2  according to an embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT  
      The following detailed description of the invention is merely exemplary in nature and is not intended to limit the invention or the application and uses of the invention. Furthermore, there is no intention to be bound by any theory presented in the preceding background of the invention or the following detailed description of the invention.  
      Turning now to the figures,  FIG. 1  is a flow chart illustrating the top-level architecture of an exemplary environmental control system  100  for an aircrew cabin or other area in an aircraft fuselage  30 . The illustrated system  100  includes four electrically-driven cabin air compressors  10   a - 10   d,  each receiving fresh ram air from inlets that are located in at least one favorable position near the aircraft&#39;s forward belly fairing leading edge. Although there are four compressors  10   a - 10   d  in the illustrated embodiment, a system that incorporates fewer compressors may still be used without departing from the scope of the present invention. However, the four-compressor configuration has inherent advantages of maintaining symmetrical loading of the left and right-side electric power buses, and providing redundancy in the case of minor environmental control system failures.  
      The air compressors  10   a - 10   d  raise the ram air pressure to a level that is slightly above the desired aircraft cabin pressure. The compressed air then passes through check valves  12   a - 12   d , and through one of two ozone converters  14   a ,  14   b . If air conditioning is not necessary, bypass valves  15   a ,  15   b  are opened and the compressed air is supplied directly to the aircraft fuselage  30  from the ozone converters  14   a  ,  14   b  . Since some air temperature control is typically required, the air is normally supplied to one of two air conditioning packs  20   a  ,  20   b  , which then transfer the air to an air distribution system  26  that delivers the air about the aircraft fuselage  30 .  
      The air conditioning packs  20   a ,  20   b  may receive about 50% fresh air from the cabin air compressors  10   a - 10   d , and about 50% recirculation air from the aircraft fuselage  30 , although these percentages vary according to a range of factors including the aircraft velocity and altitude. Air that is recirculated to the air conditioning packs  20   a ,  20   b  from the aircraft fuselage  30  passes through valves  22   a ,  22   b  and regulators  24   a ,  24   b . The environmental control system  100  thus uses a relatively cool air supply compared to the conventional system in which a supply of hot pressurized air is bled from an aircraft engine, known as bleed air. In fact, the environmental control system  100  of the present invention entirely eliminates the conventional engine air bleed system.  
      Turning now to  FIG. 2 , details pertaining to an air conditioning pack  20  are illustrated, it being understood that the illustrated air conditioning pack  20  may represent either of the air conditioning packs  20   a ,  20   b  depicted in  FIG. 1 . The air conditioning pack includes the components to the right of the ozone converter  14   a , and to the left of the vertical discontinuous line  58  in  FIG. 2 , the components and flow paths to the right of the discontinuous line  58  being directed into or disposed inside the aircraft fuselage.  
      As previously mentioned, the air conditioning pack  20  receives two air sources, namely, fresh ram air and recirculation air. First, fresh ram air is supplied to the air conditioning pack  20  from the compressors  10   a ,  10   b  powered by motors  11   a ,  11   b . The ozone converter  14   a  removes all or most of the ozone from the compressed ram air, specifically at high altitudes where ozone is included in the air at relatively high concentrations.  
      The compressed ram air passes through a primary heat exchanger  32  that is disposed in a ram air heat exchanger circuit  56 . The ram air heat exchanger circuit  56  has ambient ram air passing therethrough, which cools compressed air in the primary heat exchanger  32 , a secondary heat exchanger  34 , and an air recirculation heat exchanger  36  that are located in the circuit  56 . The ram air heat exchanger circuit  56  receives air drawn through a ram scoop during aircraft flight, and is driven by an electric fan  54  when the aircraft is stationary. In the preferred embodiment illustrated in  FIG. 2 , the electric fan  54  is disposed downstream of the heat exchangers  32 ,  34 ,  36  so the heat from the fan  54  is directed overboard rather than into the heat exchangers  32 ,  34 ,  36 . A portion of the air entering the ram air heat exchanger circuit  56  is diverted upstream of the primary heat exchanger for use as trim air by the cabin temperature control system. Because the ambient ram air in the circuit  56  is cooler than the air passing through the heat exchangers  32 ,  34 ,  36 , the ambient ram air serves as a heat sink before the air is expelled using the electric fan  54 .  
      After the compressed ram air passes through the primary heat exchanger  32 , the air is supplied to a bootstrap air cycle machine, referring specifically to a compressor  40  and turbine  42  that either share the same rotating axis or are otherwise powered and rotated together. The compressor  40  further pressurizes and heats the ram air. The compressed air is then supplied to the secondary heat exchanger  34 , causing the compressed air to cool. During normal operation, an altitude valve  60  is closed, causing the air to pass through a re-heater  44  and a condenser  46 , and then through a water extractor  48 , which substantially dries the air. From the water extractor  48 , the air is again heated in the re-heater  44 , and then the hot and dry air is supplied to the turbine  42 . The turbine  42  forwards the air to the condenser  46 , which cools the air further and supplies the air to the aircrew cabins in the aircraft fuselage  30 . At high altitudes, the altitude valve  60 , and also a compressor bypass check valve  62 , is opened, causing air from the secondary and primary heat exchangers  34 ,  32 , respectively, to bypass the bootstrap air cycle machine and revert to the ram air heat exchanger circuit  56  for cooling. This bypass mode of operation minimizes the supply pressure to the air conditioning pack  20  and reduces the required input power to the cabin air compressors  10   a - 10   d.  At low elevations, a recirculation heat exchanger bypass valve  64  is opened, allowing the recirculation air from an aft recirculation fan  52  to bypass the recirculation heat exchanger  36 .  
      A forward recirculation fan  50  mixes some recirculation air from the aircraft fuselage  30  into the fresh air supplied from the air conditioning pack  20  before the fresh air reaches the aircrew cabins. However, a majority of the recirculation air is transferred back to the air conditioning pack  20  using the aft recirculation fan  52 , which supplies the recirculation air to the recirculation heat exchanger  36  for cooling. The cooled recirculation air leaves the recirculation heat exchanger  36  and is then mixed with the fresh air being supplied to the aircraft fuselage  30 . Thus, the air conditioning pack  20  delivers a dry, subfreezing supply of air to the air distribution system  26  with a significant portion of the ventilation air entering the aircrew cabins being recirculation air.  
      In the embodiment illustrated in  FIG. 2 , the recirculation heat exchanger  36  is located with the primary and secondary heat exchangers  32 ,  34  in the ram air heat exchanger circuit  56 . Additional heat exchangers may also be located in a series arrangement with the primary, secondary, and recirculation heat exchangers  32 ,  34 ,  36 . For example, motor cooling heat exchangers for the cabin air compressor motors may be located in the ram air heat exchanger circuit  56 . Also, a power electronics chiller, which is a liquid-to-air heat exchanger, can be located in the ram air heat exchanger. In an alternate embodiment, the motor cooling heat exchangers and the power electronics chiller are disposed in series in a separate, parallel ram circuit that is powered by a separate electric fan.  
      In a preferred environmental control system  100 , the compressors  10   a - 10   d  are ported shroud compressors. One example of a suitable ported shroud compressor is illustrated in  FIG. 4 , although various other designs may also be incorporated. The compressor  10  includes a housing  162  with an outer wall  164  defining an inlet  166 . The inlet  166  includes an outer portion  167  and an inner portion  168 . The compressor housing  162  also defines an outlet  186 . Within the outer wall  164  is a shroud  170  that is defined by an inner compressor wall  172  having an inner surface  174  and an outer surface  176 . In one embodiment, the outer wall  164  defined by the housing  162  is circular, and the shroud is defined by the circular inner compressor wall  172  concentric to the outer wall  164 .  
      A compressor wheel  180  is rotatably mounted within the shroud  170 . In one embodiment, the compressor wheel  180  includes a plurality of vanes or blades  182 . The compressor wheel  180  is located so the shroud inner surface  174  is adjacent to the compressor wheel blades  182 . The wheel  180  is coupled to a shaft  184 . As the compressor wheel turns, air is drawn into the compressor  10  through the inlet  166 , through the blades or vanes  182  of the compressor wheel  180 , and then forced out through the outlet  186 .  
      The shroud inner wall  172  defines a central channel  188 . An annular channel  190  is defined between the outer surface  176  of the shroud inner wall  172  and an inner surface of the housing wall  164 . The central channel  188  and the annular channel  190  form the inlet inner portion  168 . At least one port  192  extends through the shroud inner wall  172 , allowing communication between the annular channel  190  and the compressor wheel blades or vanes  182 . In one embodiment, the at least one port  192  comprises a series of apertures through the shroud inner wall  172 . However, slots or other methods of allowing flow through the shroud inner wall  172  may also be incorporated.  
      Ram air enters the ported shroud compressor  10  through the inlet outer portion  167 . The air then passes through the central channel  188 , into the compressor wheel  180 , and is forced to the outlet  186 . A surge condition may exist at low altitudes, in which the volume of air entering the compressor exceeds the compressor requirements. In order to avoid a surge condition, air also bleeds from the compressor wheel  180  through the at least one port  192  and flows through the annular channel  190  back to the inlet outer portion  167  where the air re-enters the central channel  188 . This bypass action allows the compressor to reach an equilibrium state.  
      A choke condition may exist at high elevations, in which the compressor&#39;s requirements exceed the volume of air entering the compressor. In order to avoid a choke condition, air enters the compressor  10  through the inlet outer portion  167 , where a portion passes through the central channel  188  and into the compressor wheel  180 , and another portion bleeds through the annular channel  190  and directly into the compressor wheel blades or vanes  182  through the at least one port  192 , with both portions then forced to the outlet  186 . This inward flow bypass action allows greater airflow into the compressor wheel  180 .  
      Referring to the graph of  FIG. 3 , the data represented by line  80  denote an operation range in which conditions are optimal, meaning that the compressor  10  is neither at risk of a choke condition or a surge condition. The data represent a relationship between a compressor pressure ratio, with values for such on the Y-axis, and a corrected flow per compressor, with values for such on the X-axis. If the compressor pressure ratio for a given flow rate is to the left of line  80 , there is a risk of a surge condition since the air exceeds the requirements of the compressor  10 . As seen by the data set for a conventional air conditioning pack that includes electric compressors, the data set represented by line  90 , at a high altitude of 43 kft the conventional air conditioning pack operates well within optimal range. However, for low aircraft velocities at which the airflow per compressor approaches 1 lb/s, there is a risk of a surge condition.  
      Some conventional ways to correct this condition include turning off one or more of the compressors  10   a - 10   d  at low velocity or when the aircraft is not moving, thereby increasing the airflow per compressor and shifting the line  80  to the right. However, automating a power shut-off requires additional programming and circuitry, and can consequently be inefficient in terms of cost and complexity. Other conventional ways to correct this condition include installing a more complex variable diffuser compressor as part of the bootstrap air cycle machine. However, a variable diffuser compressor is costly as it requires its own actuation mechanism, and includes a large number of moving components that introduce the possibility for compressor leakage and increased maintenance.  
      Utilizing the ported shroud air compressor  10  to receive the ram air for the environmental control system  100  overcomes the problems of operating with a risk of a surge or choke condition by enabling operation within at least a 10% to 15% margin between line  80  and line  90  in  FIG. 3  by effectively shifting the line  80  to the left in the low pressure, low flow region.  
      Thus, the previously-described environmental control system  100  provides a low energy consumption cycle that minimizes the expenditure of power and reduces the weight of the overall system. While the invention has been described with reference to a preferred embodiment, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt to a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this invention, but that the invention will include all embodiments falling within the scope of the appended claims.