Patent Publication Number: US-2021188449-A1

Title: Air cycle machines, air cycle machine systems, and methods of controlling air flow in air cycle machines

Description:
BACKGROUND 
     The present disclosure relates generally to air cycle machines, and more particularly to controlling air flow through air cycle machines with flexible turbine arrangements. 
     Air cycle machines, such as air conditioning packs carried by aircraft, are commonly employed to provide conditioned air to environmentally controlled spaces. Such air cycle machines generally employ a centrifugal compressor, two air-to-air heat exchangers and an expansion turbine. Compressed air is communicated to the compressor, further compressed, cooled and thereafter expanded in the expansion turbine. As the air traverses the expansion turbine the air cools to a temperature suitable for mixing with ambient air for introduction into the environmentally controlled spaces. The power for such air cycle machines is generally provided by the differential between pressure of the compressed air provided to the compressor and that output pressure of the air cycle machines. 
     In some air cycle machines performance of the air cycle machine can be influenced by changes in the air cycle machine operating conditions. For example, decrease in pressure of the air input to the compressor can reduce the size of the cooling load that the air cycle machine is capable to carry. And while air cycle machines for a given application are generally selected to operate with acceptable performance at a particular design and/or sizing within a relatively large envelope, performance can drop off as conditions approach the extremes of the operating conditions envelope. 
     Such systems and methods have generally been acceptable for their intended purpose. However, there remains a need in the art for improved air cycle machines, air cycle machine systems, and methods of controlling flow through air cycle machines. 
     BRIEF DESCRIPTION 
     An air cycle machine is provided. The air cycle machine includes a compressor in fluid communication with an output conduit, a first turbine operably connected to the compressor and fluidly coupling the compressor with the output conduit, a second turbine operably connected to the compressor and fluidly coupling the compressor with the output conduit, and a valve. The valve couples the compressor with the output conduit and has an open position and a closed position. In the open position the first turbine and the second turbine are fluidly connected in parallel between the compressor and the output conduit. In the closed position the first turbine is fluidly connected in series with the second turbine between the compressor and the output conduit. 
     In addition to one or more of the features described above, or as an alternative, further embodiments of the air cycle machine may include that the valve is coupled to the output conduit by the first turbine. 
     In addition to one or more of the features described above, or as an alternative, further embodiments of the air cycle machine may include that the compressor is coupled by the second turbine to the output conduit. 
     In addition to one or more of the features described above, or as an alternative, further embodiments of the air cycle machine may include a check valve coupling the first turbine to the second turbine. 
     In addition to one or more of the features described above, or as an alternative, further embodiments of the air cycle machine may include a compressed air source in fluid communication with the first turbine through the compressor. 
     In addition to one or more of the features described above, or as an alternative, further embodiments of the air cycle machine may include a cabin or an overboard duct in fluid communication with the output conduit. 
     In addition to one or more of the features described above, or as an alternative, further embodiments of the air cycle machine may include a first turbine compressor drive shaft operably connecting the first turbine to the compressor and a second turbine compressor drive shaft operably connecting the second turbine to the compressor. 
     In addition to one or more of the features described above, or as an alternative, further embodiments of the air cycle machine may include that the valve is a first valve and that the air cycle machine also includes a second valve, the first valve connecting the first valve to the output conduit and the second valve connecting the compressor to the second turbine. 
     In addition to one or more of the features described above, or as an alternative, further embodiments of the air cycle machine may include a first load cooling heat exchanger fluidly coupling the first turbine to the first valve and a second load cooling heat exchanger fluidly coupling the second turbine to the output conduit. 
     In addition to one or more of the features described above, or as an alternative, further embodiments of the air cycle machine may include a secondary heat exchanger fluidly coupling the compressor to both the second valve and to the first valve through the first turbine. 
     In addition to one or more of the features described above, or as an alternative, further embodiments of the air cycle machine may include a primary heat exchanger in fluidly coupled to the compressor and in fluid communication therethrough with the first turbine and the second valve. 
     In addition to one or more of the features described above, or as an alternative, further embodiments of the air cycle machine may include a bypass conduit coupling the first turbine with the second turbine and a manifold fluidly coupling the compressor with the first turbine and the second turbine, the first valve connecting the manifold to the output conduit and the second valve connecting the compressor to the second turbine. 
     In addition to one or more of the features described above, or as an alternative, further embodiments of the air cycle machine may include a controller operably connected to the valve and having a memory with instructions recorded thereon that cause the controller to receive a bleed air pressure measurement, open the valve when the bleed air pressure measurement is below a predetermined bleed air pressure value, and close the valve when the bleed air pressure measurement is above the predetermined bleed air pressure value. 
     An air cycle machine system is also described. The air cycle machine system includes an air cycle machine as described above, the valve being a first valve and the air cycle machine system including a second valve connecting the compressor to the second, a compressed air source in fluid communication with the first turbine through the compressor, and a cabin or overboard duct in fluid communication with the output conduit. 
     In addition to one or more of the features described above, or as an alternative, further embodiments of the air cycle machine system may include that the output conduit is coupled to the first turbine by the first valve, wherein the compressor is coupled to the output conduit by the second turbine, and further comprising a check valve coupling the first turbine to the second turbine. 
     In addition to one or more of the features described above, or as an alternative, further embodiments of the air cycle machine system may include a controller operably connected to the first valve and the second valve, the controller including a memory having instructions recorded thereon that cause the controller to receive a bleed air pressure measurement, open the first valve when the bleed air pressure measurement is below a predetermined bleed air pressure value by moving the first valve to a first valve open position, open the second valve when the bleed air pressure measurement is below the predetermined bleed air pressure value by moving the second valve to a second valve open position, close the first valve when the bleed air pressure measurement is above the predetermined bleed air pressure value by moving the first valve to a first valve closed position, and close the second valve when the bleed air pressure measurement is above the predetermined bleed air pressure value by moving the second valve to a second valve closed position. 
     In addition to one or more of the features described above, or as an alternative, further embodiments of the air cycle machine system may include that the instructions cause the controller to move the first valve to the first valve closed position and the second valve to the second valve closed position coincidently with one another. 
     In addition to one or more of the features described above, or as an alternative, further embodiments of the air cycle machine system may include that the instructions cause the controller to move the first valve to the first valve open position and the second valve to the second valve open position coincidently with one another. 
     A method of controlling flow through an air cycle machine is additionally described. The method includes, at an air cycle machine as described above, fluidly coupling the first turbine and the second turbine in parallel with one another between the compressor and the output conduit by moving the valve to the open position, and fluidly coupling the first turbine to the second turbine between the compressor and the output conduit by moving the valve to the closed position. 
     In addition to one or more of the features described above, or as an alternative, further embodiments of the method may include receiving a bleed air pressure measurement, comparing the bleed air pressure measurement to a predetermined bleed pressure measurement, opening the valve when the bleed air pressure measurement is below a predetermined bleed air pressure value, and closing the valve when the bleed air pressure measurement is above the predetermined bleed air pressure value. 
     Technical effects of the present disclosure include the capability to change the flow path of air within an air cycle machine. In certain examples the present disclosure provides the capability to connect turbines of air cycle machines in series with one another when pressure of air provided to the air cycle machine is relatively high, and to further connect turbines of the air cycle machine in parallel with one another when pressure of air provided to the air cycle machine is relatively low. In accordance with certain examples air cycle machines are provided with relatively low hydraulic resistance over a relatively wide range of input pressures by changing connectivity of the air cycle machine turbines between serial and parallel arrangements according to pressure of air input to the air cycle machine. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The following descriptions should not be considered limiting in any way. With reference to the accompanying drawings, like elements are numbered alike: 
         FIG. 1  is a schematic view of an air cycle machine constructed in accordance with the present disclosure, showing an air cycle machine having a low input pressure mode of operation and a high input pressure mode of operation; 
         FIG. 2  is a schematic view of the air cycle machine of  FIG. 1 , showing a compressor in fluid communication with an output conduit through a first turbine and a second turbine, the compressor connected to the output conduit through a first valve and a second valve; 
         FIG. 3  is a schematic view of the air cycle machine of  FIG. 1 , showing the air cycle machine operating in the low input pressure mode of operation, the first turbine and the second turbine connected in parallel with one another between the compressor and the output conduit; 
         FIG. 4  is a schematic view of the air cycle machine of  FIG. 1 , showing the air cycle machine operating in the high input pressure mode of operation, the first turbine and the second turbine connected in series with one another between the compressor and the output conduit; and 
         FIG. 5  is a block diagram of a method of controlling air flow through an air cycle machine in accordance with an illustrative and non-limiting example of the method. 
     
    
    
     DETAILED DESCRIPTION 
     Reference will now be made to the drawings wherein like reference numerals identify similar structural features or aspects of the subject disclosure. For purposes of explanation and illustration, and not limitation, a partial view of an example of an air cycle machine constructed in accordance with the disclosure is shown in  FIG. 1  and is designated generally by reference character  100 . Other embodiments of air cycle machines, air cycle machine systems, and methods of controlling flow within air cycle machines, are provided in  FIGS. 2-5 , as will be described. The systems and methods described herein can be used to provide conditioned air to environmentally controlled spaces on vehicles, such as crew and passenger spaces on aircraft, though the present disclosure is not limited to any particular type of environmentally controlled space or to aircraft in general. 
     Referring to  FIG. 1 , a vehicle  10 , e.g., an aircraft, is shown. The vehicle  10  includes an air cycle machine system  200  including the air cycle machine  100 , a compressed air source  12 , and a heat load  14 . The vehicle  10  also includes a cabin  16 , an overboard air duct  18 , and a fan or ram air duct  20 . 
     The compressed air source  12  is configured to generate a compressed air flow  24  and is fluidly coupled to the air cycle machine  100 . In certain examples the compressed air source  12  is a compressor section of a gas turbine engine and the compressed air flow  24  is a bleed air flow. In accordance with certain examples the compressed air source  12  can be a main engine or an auxiliary power unit carried by an aircraft. It is also contemplated that the compressed air source can be ground cart. 
     The heat load  14  is configured to generate heat  22  and is in thermal communication with the air cycle machine  100 . It is contemplated that the heat load  14  communicate a portion of the heat  22  to the compressed air flow  24  as the compressed air flow traverses the air cycle machine  100 . In certain examples the heat load includes an electrical device, such as a motor or an electronics cabinet by way of non-limiting example. 
     The cabin  16  is in fluid communication with the compressed air source  12  through the air cycle machine  100 , is configured to receive the compressed air flow  24  from the air cycle machine  100  as a conditioned compressed air flow  24 , and in certain examples fluidly couples the air cycle machine  100  to the overboard air duct  18 . It is contemplated that, in certain examples, the cabin  16  can be a crew cabin located within an aircraft. For example, in certain examples, the cabin  16  is a passenger cabin located within an aircraft. It is also contemplated that cabin  16  can be cargo space located within an aircraft. 
     The fan or ram air duct  20  is configured to communicate an ambient air flow  28  to the air cycle machine  100  for removing heat from the air cycle machine  100 . In this respect the fan or ram air duct  20  fluidly couples the external environment  32  to the air cycle machine  100 . 
     As will be appreciated by those of skill in the art, air cycle machines are generally sized to provide suitable performance throughout a range of operating conditions. For example, air cycle machines are typically sized to provide sufficient conditioned air flow within a range of compressed air pressures and mass flow rates. Air cycle machines are also generally sized to provide sufficient conditioned air flow over a range of heat load and/or ambient air flows. As will also be appreciated by those of skill in the art, the size of the operating condition range within which the air cycle machine provides acceptable performance can be limited by the arrangement of the air cycle machine, e.g., by the amount of hydraulic resistance provided by the air cycle machine. To increase the range of operating conditions over which the conditioned air flow  26  can be provided to the cabin  16  the air cycle machine  100  is provided. 
     With reference to  FIG. 2 , the air cycle machine system  200  is shown. The air cycle machine  100  generally includes a compressor  102 , an output conduit  104 , a first turbine  106 , a second turbine  108 , and a valve  110 . The compressor  102  is in fluid communication with the output conduit  104 . The first turbine  106  is operably connected to the compressor  102  and fluidly couples the compressor  102  to the output conduit  104 . The second turbine  108  is operably connected to the compressor  102  and fluidly couples the compressor  102  to the output conduit  104 . The valve  110  couples the compressor  102  with the output conduit  104  and has a valve open position  112  (shown in  FIG. 3 ) and a valve closed position  114  (shown in  FIG. 4 ). When the valve  110  is in the valve open position  112  the first turbine  106  and the second turbine  108  are fluidly connected in parallel between the compressor  102  and the output conduit  104 . When the valve  110  is in the valve closed position  114  the first turbine  106  is fluidly connected in series with the second turbine  108  between the compressor  102  and the output conduit  104 . 
     The valve  110  is a first valve  110  and the air cycle machine  100  also includes a second valve  116 , a primary heat exchanger  118 , and a secondary heat exchanger  120 . The air cycle machine  100  additionally includes a bleed air conduit  122 , a manifold  124 , and a bypass conduit  126 . The air cycle machine  100  further includes a check valve  128 , a first load cooling heat exchanger  130 , and a second load cooling heat exchanger  132 . 
     The bleed air conduit  122  fluidly couples the compressed air source  12  to the primary heat exchanger  118 . The primary heat exchanger  118  is in thermal communication with the fan or ram air duct  20  and fluidly couples the bleed air conduit  122  to the compressor  102 . The compressor  102  fluidly couples the primary heat exchanger  118  to the secondary heat exchanger  120 . The secondary heat exchanger  120  is in thermal communication with the fan or ram air duct  20  and fluidly couples the compressor  102  to the manifold  124 . 
     The manifold  124  fluidly couples the compressor to the first turbine  106 . The first turbine  106  is operably connected to the compressor  102 , e.g., through a first turbine compressor drive shaft  134 , and fluidly couples the manifold to the first load cooling heat exchanger  130 . The first load cooling heat exchanger  130  is in thermal communication with the heat load  14  (shown in  FIG. 1 ) and fluidly couples the first turbine  106  with the first valve  110 . The first valve  110  connects the first turbine  106  to the output conduit  104  and has the first valve open position  112  (shown in  FIG. 3 ) and the first valve closed position  114  (shown in  FIG. 4 ). In the first valve open position  112  the first valve  110  fluidly couples the first load cooling heat exchanger  130 , which is in thermal communication with the heat load  14  (shown in  FIG. 1 ), with the output conduit  104 . In the first valve closed position the  114  the first valve  110  fluidly separates the first load cooling heat exchanger from the output conduit  104 . 
     The manifold  124  also fluidly couples the compressor  102  to the second valve  116 . The second valve  116  connects the manifold  124  to the second turbine  108  and has a second valve open position  134  (shown in  FIG. 3 ) and a second valve closed position  136  (shown in  FIG. 4 ). In the second valve open position  134  the second valve  116  fluidly couples the manifold to the second turbine  108 . In the closed position the second valve  116  fluidly separates the manifold  124  from the second turbine  108 . The second turbine  108  is operably connected to the compressor  102 , e.g., via a second turbine compressor drive shaft  140 , and fluidly couples the second valve  116  to the second load cooling heat exchanger  132 . The second load cooling heat exchanger  132  is in thermal communication with the heat load  14  (shown in  FIG. 1 ) and fluidly couples the second turbine  108  to the output conduit  104 . In certain examples the first turbine compressor drive shaft  138  and the second turbine compressor drive shaft  140  are portions of c common compressor drive shaft  134 / 140 . 
     The bypass conduit  126  connects the first load cooling heat exchanger  130  to the second turbine  108 . More specifically, the bypass conduit  126  connects a first union  142  to a second union  144 . The first union  142  fluidly couples the first load cooling heat exchanger  130  to the first valve  110  and the second union  144  fluidly couples the second valve  116  to the second turbine  108 . The check valve  128  is arranged along the bypass conduit  126 , fluidly couples the first union  142  to the second union  144  and has a closed position  146  (shown in  FIG. 3 ) and an open position  148  (shown in  FIG. 4 ). In the closed position  146  the check valve  128  fluidly separates the first union  142  from the second union  144  through the bypass conduit  126 . In the open position  148  the check valve  128  fluidly couples the first union  142  to the second union  144 . It is contemplated that the check valve allows one-way fluid communication through the bypass conduit  126 , i.e., from the first union  142  to the second union  144  only. 
     In the illustrated example the air cycle machine  100  also includes a controller  150 , a link  152 , and a sensor  154 . The controller  150  includes a processor  156  and a device interface  158 . The controller  150  also includes a user interface  160  and a memory  162 . The link  152  connects the controller  150  with the first valve  110 , the controller  150  thereby operably connected to the first valve  110  for moving the first valve  110  between the first valve open position  112  (shown in  FIG. 3 ) and the first valve closed position  114  (shown in  FIG. 4 ). The link  152  connects the controller  150  with the second valve  116 , the controller  150  thereby operably connected to the second valve  116  for moving the second valve  116  between the second valve open position  134  (shown in  FIG. 3 ) and the second valve closed position  136  (shown in  FIG. 4 ). It is also contemplated that the line  152  connect the controller  150  with the sensor  154 , the controller  150  thereby receiving a signal  34  indicative of one or more of temperature, pressure, and/or mass flow rate of the compressed air flow  24  within the bleed air conduit  122 , e.g., a bleed air pressure measurement. 
     The processor  156  is disposed in communication with the device interface  158 , the user interface  160 , and the memory  162 . Communication with the device interface  158  enables the processor  156  to operate the first valve  110  and the second valve  116 , and to receive the signal  34 . Communication with the user interface  160  allows the processor  156  to receive user input and/or provide output to a user. Communication with memory  162  allows the processor  156  to read instructions, recorded in a plurality of program modules  164  recorded on the memory  162 , that cause the processor  156  to execute certain operations. Among those operations are operations of a method  200  (shown in  FIG. 5 ) of controlling flow through an air cycle machine, e.g., the air cycle machine  100 , as will be described. It is contemplated that the memory  162  include a non-transitory machine-readable medium. It is also contemplated that the controller  150  be implemented with circuitry, software, or a combination of circuitry and software. 
     With reference to  FIG. 3 , the air cycle machine  100  is shown in the first mode A. In the first mode A the first turbine  106  and the second turbine  108  are fluidly connected in parallel with one another between the manifold  124  and the output conduit  104  of the air cycle machine  100 . Parallel connection is accomplished by (a) moving the first valve  110  to the first valve open position  112 , and (b) moving the second valve  116  to in the second valve open position  134 . Opening of the first valve  110  causes the first valve  110  to fluidly couple the first load cooling heat exchanger  130  with the output conduit  104 . Opening of the second valve  116  causes the second valve  116  to fluidly couple the manifold  124  to the second turbine  108 . 
     In certain examples opening is accomplished coincidently, e.g., by moving both the first valve  110  to the first valve open position  112  and the second valve  116  to the second valve open position  134  at the same time. In accordance with certain example coincident opening is accomplished by moving the first valve  110  to the first valve open position  112  from the first valve closed position  114  (shown in  FIG. 4 ) and by moving the second valve  116  to the second valve open position  134  from the second valve closed position  136  (shown in  FIG. 4 ). This can be done, for example, by issuing an opening signal  40  to an actuator (or actuators) operably connected to the first valve  110  and the second valve  116  from the controller  150  (shown in  FIG. 2 ). The opening signal  42  can be generated, for example, by comparing a measurement of compressed air flow  24  provided by the sensor  154  (shown in  FIG. 2 ) to a predetermined pressure value, e.g., a predetermined bleed air pressure value. 
     During operation in the second mode B the compressed air flow  24  traverses the primary heat exchanger  118 . The primary heat exchanger  118  cools the compressed air flow  24  by transferring heat from the compressed air flow  24  to air from the ambient environment traversing the fan or ram air duct  20  and communicates the compressed air flow  24  to the compressor  102 . The compressor  102  in turn further compresses the compressed air flow  24  using rotation  36  received from the first turbine  106  through the first turbine compressor drive shaft  138  and rotation  38  received from the second turbine  108  through the second turbine compressor drive shaft  140 , the compressor  102  communicating the further compressed air flow  24  to the secondary heat exchanger  120 . The secondary heat exchanger  120  further cools the compressed air flow  24  to the air from traversing the fan or ram air duct  20  and thereafter communicates the cooled compressed air flow  24  to the manifold  124 . 
     The manifold  124 , fluidly coupled to both the first turbine  106  and the second turbine  108 , divides the compressed air flow  24  into a first compressed air flow portion  24   A  for communication to the first turbine  106  and a second compressed air flow portion  24   B  for communication to the second turbine  108 . The first turbine  106  extracts work from the first compressed air flow portion  24   A  and communicatees the extracted work to the compressor  102  via the first turbine compressor drive shaft  138 . As the first compressed air flow portion  24   A  traverses the first turbine  106  the air forming the first compressed air flow portion  24   A  drops in both pressure and temperature and is thereafter communicated to the first load cooling heat exchanger  130 . The first load cooling heat exchanger  130  transfers heat from a heat load, e.g., the heat load  14  (shown in  FIG. 1 ), increasing temperature and pressure of the first compressed air flow portion  24   A , and thereafter communicates the first compressed air flow portion  24   A  to the first union  142 . 
     The first union  142 , fluidly separated from the bypass conduit  126  bypass conduit  126  by the check valve  128 , communicates the first compressed air flow portion  24   A  to the output conduit  104  of the air cycle machine  100 . It is contemplated that the first compressed air flow portion  24   A  be of pressure insufficient to move the check valve  128  from the check valve closed position  146  (shown in  FIG. 4 ) to the check valve open position  148 , the bypass conduit  126  thereby being fluidly separated from the second union  44 . The first compressed air flow portion  24   A  therefore traverses the first turbine  106  and thereafter flows through the first load cooling heat exchanger  130 , wherein the first compressed air flow portion  24   A  receives heat from the heat load  14  (shown in  FIG. 1 ), and thereafter joins the second compressed air flow portion  24   B  for communication therewith to the cabin or overboard air duct  18  via the output conduit  104  of the air cycle machine  100 . 
     The second compressed air flow portion  24   B  flows from the manifold  124  to the second valve  116 . The second valve  116  communicates the second compressed air flow portion  24   B  to the second union  144 , and the therethrough to the second turbine  108 . The second turbine  108  extracts work from the second compressed air flow portion  24   B  and communicates the extracted work to the compressor  102  via the second turbine compressor drive shaft  140  as the second turbine mechanical rotation  38 . The second compressed air flow portion  24   B  thereafter flows through the second load cooling heat exchanger  132 . The second load cooling heat exchanger  132  in turn communicates heat to the second compressed air flow portion  24   B , heating the second compressed air flow portion  24   B  as the second compressed air flow portion  24   B  as traverses the second load cooling heat exchanger  132 . The second compressed air flow portion  24   B  thereafter joins the first compressed air flow portion  24   A  in the output conduit  104  and flows to the cabin or overboard air duct  18 . 
     With reference to  FIG. 4 , the air cycle machine  100  is shown in the second mode B. In the second mode B the second turbine  108  is fluidly coupled in series with the first turbine  106  between the manifold  124  and the output conduit  104  of the air cycle machine  100 . The serial connection accomplished by moving the first valve  110  to the first valve closed position  114  and moving the second valve  116  to in the second valve closed position  136 . Closure of the first valve  110  causes the first valve  110  to fluidly separate the first load cooling heat exchanger  130  from the output conduit  104 . Closure of the second valve  116  causes the second valve  116  to fluidly separate the bypass conduit  126  from the second turbine  108 . 
     In certain examples closure is accomplished coincidently, e.g., by moving both the first valve  110  to the first valve closed position  114  and the second valve  116  to the second valve closed position  136  at the same time. In accordance with certain example coincident closure is accomplished by moving the first valve  110  to the to the first valve closed position  114  from the first valve open position  112  (shown in  FIG. 3 ) and moving the second valve  116  from the second valve closed position  136  from the second valve open position  134  (shown in  FIG. 3 ). This can be done, for example, by issuing a closure signal  40  to an actuator (or actuators) operably connected to the first valve  110  and the second valve  116  from the controller  150  (shown in  FIG. 2 ). The closure signal  40  can be generated, for example, by comparing a measurement of compressed air flow  24  provided by the sensor  154  (shown in  FIG. 2 ) to a predetermined pressure value. 
     During operation in the second mode B the compressed air flow  24  traverses the primary heat exchanger  118 . The primary heat exchanger  118  cools the compressed air flow  24  by transferring heat from the compressed air flow  24  to air from the ambient environment traversing the fan or ram air duct  20  and communicates the compressed air flow  24  to the compressor  102 . The compressor  102  in turn further compresses the compressed air flow  24  using rotation  36  received from the first turbine  106  through the first turbine compressor drive shaft  138  and rotation  38  received from the second turbine  108  through the second turbine compressor drive shaft  140  and communicates the compressed air flow  24  to the secondary heat exchanger  120 . The secondary heat exchanger  120  further cools the compressed air flow  24  to the air from traversing the fan or ram air duct  20  and thereafter communicates the compressed air flow  24  to the manifold  124 . 
     The manifold  124 , fluidly separated from the second turbine  108  by the second valve  116 , communicates the entirety of the compressed air flow  24  to the first turbine  106 . The first turbine  106  extracts work from the compressed air flow  24  and communicatees the extracted work to the compressor  102  via the first turbine compressor drive shaft  138 . As compressed air flow  24  traverses the first turbine  106  the air forming the compressed air flow  24  drops in pressure and temperature and is thereafter communicated to the first load cooling heat exchanger  130 . The first load cooling heat exchanger  130  transfers heat from a heat load, e.g., the heat load  14  (shown in  FIG. 1 ), increasing temperature and pressure of the compressed air flow  24 , and thereafter communicates the compressed air flow  24  to the first union  142 . 
     The first union  142 , fluidly separated from the output conduit  104  by the first valve  110 , communicates the compressed air flow  24  to the bypass conduit  126 . It is contemplated that the compressed air flow  24  be of pressure sufficient to move the check valve  128  from the check valve closed position  146  (shown in  FIG. 4 ) to the check valve open position  148 , the bypass conduit  126  thereby fluidly coupling the first union  142  to the second union  44  and communicating the compressed air flow  24  to the second union  144 . The second union  144 , fluidly separated from the manifold  124  by the second valve  116 , communicates the entirety of compressed air flow to the second turbine  108 . 
     The second turbine  108  extracts work from the compressed air flow  24  and communicatees the extracted work to the compressor  102  via the second turbine compressor drive shaft  140 . As compressed air flow  24  traverses the second turbine  108  the compressed air flow  24  drops in both pressure and temperature and is thereafter communicated to the second load cooling heat exchanger  132 . The second load cooling heat exchanger  132  transfers heat from a heat load, e.g., the heat load  14  (shown in  FIG. 1 ), increasing temperature and pressure of the compressed air flow  24  and thereafter communicates the compressed air flow  24  to the output conduit  104 . Notably, the entirety of the compressed air flow  24  serially traverses the first turbine  106  and the second turbine  108 . 
     With reference to  FIG. 5 , the method  300  of controlling flow in air cycle machine, e.g., the air cycle machine  100  (shown in  FIG. 1 ), is shown. The method  300  includes receiving a bleed air flow measurement, e.g., the bleed pressure measurement signal  50  (shown in  FIG. 2 ), as shown with box  310 . The bleed pressure measurement is compared to a predetermined bleed pressure measurement, e.g., a predetermined bleed pressure measurement record in the program module  162  (shown in  FIG. 2 ), as shown with box  320 . 
     When the bleed flow measurement is below the predetermined pressure measurement a first turbine of the air cycle machine is connected with a second turbine in parallel between a manifold and an output conduit of the air cycle machine, e.g., the first turbine  106  (shown in  FIG. 2 ) is connected in parallel with the second turbine  108  (shown in  FIG. 2 ) between the manifold  126  (shown in  FIG. 2 ) and the bleed air conduit  104  (shown in  FIG. 2 ), as shown with box  342 . As shown with box  330 , parallel connection is accomplished by opening a first valve and a second valve, e.g., the first valve  110  (shown in  FIG. 2 ) and the second valve  116  (shown in  FIG. 2 ). In certain examples connecting the first turbine in parallel with the second turbine reduced hydraulic resistance of the air cycle machine, as shown with box  350 . In accordance with certain examples connecting the first turbine in parallel with the second turbine can include moving the either (or both) the first valve and the second valve from closed positions to the open positions, e.g., without throttling flow through either valve with the first valve and/or the second valve, as shown with box  342 . It is contemplated that the first turbine and the second turbine remain connected in parallel with one another while the bleed pressure is below the predetermined pressure, as shown with arrow  352 . 
     When the bleed flow measurement is above the predetermined pressure measurement the first turbine of the air cycle machine is connected in series with the second turbine between the manifold and the output conduit of the air cycle machine, as shown with box  370 . As shown with box  360 , serial connection is accomplished by closing the first valve and the second valve. In certain examples connecting the first turbine in series with the second turbine increases hydraulic resistance of the air cycle machine, as shown with box  380 . In accordance with certain examples, connecting the first turbine in series with the second turbine can include moving the either (or both) the first valve and the second valve from open positions to closed positions, e.g., without throttling flow through either valve with the first valve and/or the second valve, as shown with box  372 . It is contemplated that the first turbine and the second turbine remain connected to one another in series while the bleed pressure is above the predetermined pressure, as shown with arrow  382 . 
     Air cycle machines can be required to operate over large envelopes and a wide range of conditions. While air cycle machines are generally sized to operate at or near peak performance at a particular design/sizing condition, the drop off on performance as conditions deviate from the design condition must be managed. 
     In examples described herein air cycle machines configured for bootstrap cycle operation include a manifold connecting a secondary heat exchanger to a first turbine and a second turbine. The first turbine and the second turbine are in turn operably connected to a compressor, which is fluidly coupled to the manifold by the secondary heat exchanger. It is contemplated that the air cycle machine have a first mode, wherein the first valve and the second valve are open and connect the second turbine fluidly in parallel with the first turbine between the manifold the output conduit of the air cycle machine, and a second mode, wherein the first valve and the second valve are closed such that the second turbine is connected fluidly in series with the first turbine between the manifold and the output conduit of the air cycle machine. 
     Advantageously, the first mode and the second mode provide air cycle machines described herein the flexibility to switch between parallel turbine operation and serial turbine operation, increasing and/or improving performance of the air cycle machine at the extremes of the air cycle machine operating envelope and/or expanding the range of operating conditions within which the air cycle machine can operation. For example, when engine bleed pressure is high, air cycle machines described herein can arrange the first turbine and the second turbine in series with one another—providing better performance than when high engine bleed pressure is provided to turbines arranged in parallel with one another. Oppositely, when engine bleed pressure is low, air cycle machines described herein can arrange the second turbine in parallel with the first turbine, reducing hydraulic resistance with respect serial arrangement and providing better performance than serially arranged turbines under low bleed pressure conditions. 
     In certain examples a common drive shaft operably connects both the first turbine and the second turbine to the compressor, providing the above described operational flexibility within a single air cycle machine. In accordance with certain examples a first load cooling heat exchanger is arranged fluidly downstream of the first turbine and a second load cooling heat exchanger is arranged fluidly downstream of the second turbine to absorb heat from a heat load and flow air from the air cycle machine to either the aircraft cabin or an overboard duct. 
     The term “about” is intended to include the degree of error associated with measurement of the particular quantity based upon the equipment available at the time of filing the application. 
     The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the present disclosure. As used herein, the singular forms “a”, “an” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms “comprises” and/or “comprising,” when used in this specification, specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, element components, and/or groups thereof. 
     While the present disclosure has been described with reference to an exemplary embodiment or embodiments, 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 present disclosure. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the present disclosure without departing from the essential scope thereof. Therefore, it is intended that the present disclosure not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this present disclosure, but that the present disclosure will include all embodiments falling within the scope of the claims.