Abstract:
An air conditioning pack for an aircraft provides primary and secondary heat exchangers arranged in a ram air duct. A third heat exchanger is arranged in the ram air duct and is fluidily connected to the primary and secondary heat exchangers. The overall or combined size of the primary, secondary, and third heat exchangers is less than that of the typical primary and secondary heat exchangers of the prior art. A combination of heat exchangers is used to provide cooling during worst case scenarios enabling efficient use of the overall heat exchangers configuration to provide the desired cooling capacity.

Description:
This application claims priority to U.S. Provisional Application No. 60/504,671, which was filed on Sep. 22, 2003. 

   BACKGROUND OF THE INVENTION 
   This invention relates to a heat exchanger configuration for an air conditioning pack of an aircraft air conditioning system. 
   Aircraft air conditioning system packs have ram air cooled heat exchangers that provide the heat sinks for the air flow which is being cooled prior to being supplied to the aircraft cabin. A typical pack has a primary heat exchanger that rejects heat generated by compressing the ambient air to the required pack inlet pressure. The pack also includes a secondary heat exchanger that rejects heat generated by the air cycle machine (ACM) compressor. 
   Each of the heat exchangers are sized for worst case conditions, which do not apply throughout most of the operation of the air conditioning pack. For example, the primary heat exchanger is sized for a high altitude/cruise condition, and the secondary heat exchanger is sized for a hot day/ground operation. Sizing the heat exchanger for these worst case conditions results in much larger heat exchangers than is necessary for most pack operating conditions. As a result the overall heat exchanger configuration is inefficient in that it takes up more space and provides more weight than is necessary. 
   Therefore, what is needed is an improved heat exchanger configuration for an aircraft air conditioning system pack. 
   SUMMARY OF THE INVENTION 
   The present invention provides an air conditioning pack for an aircraft comprising primary and secondary heat exchangers arranged in a ram air duct. A third heat exchanger is arranged in the ram air duct and is fluidly connected to the primary and secondary heat exchangers. A valve system selectively fluidly connects the third heat exchanger with at least one of the primary and secondary heat exchangers in response to a command from a controller. The controller determines when the primary and/or secondary heat exchangers requires supplemental cooling from the third heat exchanger. 
   The valve system fluidly connects the third and primary heat exchangers to provide a first pack cooling capacity, for example, sufficient for high altitude/cruise conditions. The valve system also fluidly connects the third and secondary heat exchangers to provide a second pack cooling capacity that is different than the first pack cooling capacity, for example, sufficient for hot day/ground conditions. 
   The overall or combined size of the primary, secondary, and third heat exchangers is less than that of the typical primary and secondary heat exchangers of the prior art. A combination of heat exchangers is used to provide cooling during worst case scenarios enabling efficient use of the overall heat exchanger configuration to provide the desired cooling capacity. 
   These and other features of the present invention can be best understood from the following specification and drawings, the following of which is a brief description. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a schematic view of the inventive air conditioning pack. 
       FIG. 2  is a schematic view of the inventive air conditioning pack during a hot day/ground condition. 
       FIG. 3  is a schematic view of the inventive air conditioning pack during a high altitude/cruise condition. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
   An example of the inventive air conditioning pack  10  is shown schematically in FIG.  1 . The pack  10  produces conditioned air that is provided to a distribution system  12  that delivers the air to the aircraft cabin. A supercharger  14  provides compressed air having a pressure desired by the pack inlet. The supercharger  14  is used as an alternative to bleed air from the aircraft&#39;s primary engines to provide more efficient operation of the aircraft. 
   The pack  10  includes a ram air duct  15  providing air flow through the heat exchangers  16 , which provide the heat sink for the air as it is being conditioned within the pack  10 . A fan  17  moves air through duct  15 . An ACM  18  compresses and expands the air flowing through the fluid lines to produce conditioned air. The ACM  18  is a three wheel machine including a compressor  44  and first  46  and second  48  turbines. The ACM is of a conventional type, and is well known in the art. A valve system  20  is connected to a controller  23  and selectively opens and closes valves to manage the air flow as desired through the different flow paths. A humidity control system  22  removes moisture from the air at desired portions of the pack  10 . 
   A relatively small primary heat exchanger  24  is arranged in the ram air duct. The primary heat exchanger  24  is sized to provide the necessary heat rejection at the ground or climb out condition to reach a nominal compressor outlet temperature. In current systems, the primary heat exchanger is sized at the high altitude cruise condition to be much larger than what is the required size for the low altitude condition thereby resulting in operating inefficiencies. The inventive secondary heat exchanger  26  is also arranged in the ram air duct  15 . The secondary heat exchanger is sized to provide the necessary heat rejection at the cruise condition where its hot side flow is relatively low since most of the pack supply air is bypassed to the pack outlet. Typically, the secondary heat exchanger is sized at the hot day ground auxiliary power unit condition and is much larger than required at the cruise condition. 
   Since the primary  24  and secondary  26  heat exchangers of this invention are downsized as compared to prior art systems, supplemental cooling is needed to address the worst case cooling conditions. To this end, the invention utilizes a third heat exchanger  28  that may be selectively fluidly coupled to the primary  24  and/or secondary heat exchangers  26  to provide the desired cooling during the worst case conditions. Various combinations or configurations of the primary  24 , second  26 , and third  28  heat exchangers may be used to provide the desired cooling capacity for the cabin. 
   The valve system  20  includes a first valve  30  selectively permitting fluid flow between the primary heat exchanger  24  and/or the third heat exchanger  28  and humidity control system  22 . A second valve  32  selectively permits fluid flow between the primary heat exchanger  24  and pack outlet. The second valve  32  is commonly referred to as the ACM bypass valve. A third valve  34  selectively permits fluid flow between the secondary heat exchanger  26  and the humidity control system  22 . A fourth valve  36  selectively permits fluid flow between the second heat exchanger  26  and third heat exchanger  28 . A fifth valve  38  selectively permits fluid flow between the third heat exchanger  28  and the pack outlet. A sixth valve  40  selectively permits fluid flow between the third heat exchanger  28  and the humidity control system  22 . A seventh valve  42  selectively permits fluid flow between components within the humidity control system  22  such as a water collector  52  and a condenser  50 . 
   The distribution system  12  receives conditioned air from the pack outlet and distributes it throughout the aircraft as desired. The distribution system  12  includes a mixer  54  that receives recirculation air from the cabin and mixes it with the conditioned air from the pack  10 . The air from the mixer  54  and pack  10  travels through acoustic treatment devices  58  and  60 , as well known in the art, to reduce the noise generated by the conditioned air flowing through the system. 
     FIG. 2  schematically depicts the pack  10  during a hot day/ground condition. The connections between the controller  23  and valves is omitted for clarity. Compressed air from the supercharger enters the primary heat exchanger  24 . The second valve  32  may be opened or closed as necessary to provide the desired temperature air at the pack outlet. The first valve  30  is closed so that the air from the primary heat exchanger  24  flows into the compressor  44 . The compressed air from compressor  44  enters the secondary heat exchanger. The third valve  34  is closed and the fourth valve  36  is opened so that the air from the secondary heat exchanger is directed to the third heat exchanger  28 . The fifth valve  38  is closed and the sixth valve  40  is opened such that air from the third heat exchanger  28  flows into the condenser  50 . From the condenser  50 , the air flows into the water collector  52 . The dehumidified air from the water collector is modulated through the seventh valve  42  so that the dehumidified air flows through the turbine  46  and/or back to the condenser  50 . From the condenser  50  the air flows to the secondary turbine  48  where it exits through the pack outlet. 
   A high altitude/cruise condition is schematically depicted in FIG.  3 . Compressed air from the superchargers enters the primary heat exchanger  24 . A desired quantity of air from the primary heat exchanger  24  flows through an opened or at least partially opened second valve  32  to the pack outlet. Air from the primary heat exchanger  24  flows through the opened first valve  30  into the third heat exchanger  28 . The fifth valve  38  regulates the amount of fluid flow from the third heat exchanger  28  to the pack outlet. 
   A portion of the compressed air from the primary heat exchanger  24  also enters the compressor  44 . Air from the compressor  44  flows into the secondary heat exchanger  26 . The fourth valve  36  is closed and the third valve  34  is opened permitting fluid to flow from the secondary heat exchanger  26  into the humidity control system  22  where it flows into the first turbine  46  and then the second turbine  48 . From the second turbine  48  the conditioned air exits the pack outlet. 
   Although a preferred embodiment of this invention has been disclosed, a worker of ordinary skill in this art would recognize that certain modifications would come within the scope of this invention. For that reason, the following claims should be studied to determine the true scope and content of this invention.