Patent Publication Number: US-2021188456-A1

Title: Fuel tank inerting system using cabin outflow air

Description:
BACKGROUND OF THE INVENTION 
     This invention generally relates to the aircraft on-board systems, and more particularly, to a temperature control system for on-board fuel tank inerting systems. 
     Fuel tank inerting systems are used to introduce an inert gas, such as nitrogen, into the fuel tanks of a vehicle, such as an aircraft. The inert gas displaces potentially dangerous fuel and air mixtures, thereby reducing the risk of explosion or fire. Typically, on-board fuel inerting systems process air from an air source, such as bleed air taken from the engines of an aircraft. The bleed air is provided to a hollow fiber membrane where it is separated into nitrogen and oxygen. The separating efficiency of the membrane is directly dependent on the temperature of the air. However, there is a maximum allowable temperature of the bleed air to maintain the safety of the components downstream of the bleed air, such as filter, valves, and sensors, as well as safety relative to the fuel tank. Bleed air leaving the engines is extremely hot and therefore must be cooled before being processed. However, existing systems for cooling the bleed air to a safe temperature for inerting requires some consumption of the aircraft&#39;s limited heat sink and leaving the environmental control system of the aircraft with less heat sink and thereby causing a negative impact on its performance. 
     BRIEF DESCRIPTION OF THE INVENTION 
     According to one embodiment, a fuel tank inerting system of an aircraft includes a first air flow provided from a first source having a first temperature and a second air flow including cabin outflow air having a second temperature. The first temperature is greater than the second temperature. A fuel tank inerting heat exchanger is arranged in fluid communication with both the first air flow and the second air flow. At least one air separating module is configured to separate an inert gas from the first air flow output from the fuel tank inerting heat exchanger. 
     In addition to one or more of the features described above, or as an alternative, in further embodiments a configuration of the fuel tank inerting heat exchanger is selected to achieve a desired temperature of the first air flow associated with operation of the at least one air separating module. 
     In addition to one or more of the features described above, or as an alternative, in further embodiments the desired temperature of the first air flow is between about 150° F. and about 250° F. 
     In addition to one or more of the features described above, or as an alternative, in further embodiments the fuel tank inerting heat exchanger is located remotely from a ram air circuit. 
     In addition to one or more of the features described above, or as an alternative, in further embodiments the fuel tank inerting heat exchanger is operably coupled with a cabin pressure control system. 
     In addition to one or more of the features described above, or as an alternative, in further embodiments the cabin pressure control system includes: a conduit for receiving the cabin outflow air from a cabin; and an outflow valve movable to control a flow of the cabin outflow air exhausted from the conduit. 
     In addition to one or more of the features described above, or as an alternative, in further embodiments the fuel tank inerting heat exchanger is located between the cabin and the outflow valve. 
     In addition to one or more of the features described above, or as an alternative, in further embodiments the second air flow output from the fuel tank inerting heat exchanger is exhausted overboard. 
     In addition to one or more of the features described above, or as an alternative, in further embodiments the fuel tank inerting heat exchanger is operably coupled with an air conditioning system. 
     In addition to one or more of the features described above, or as an alternative, in further embodiments the fuel tank inerting heat exchanger is arranged between a cabin and the air conditioning system relative to the cooling air flow. 
     In addition to one or more of the features described above, or as an alternative, in further embodiments the air conditioning system includes an air cycle machine and the second air flow output from the fuel tank inerting heat exchanger is provided to the air cycle machine. 
     In addition to one or more of the features described above, or as an alternative, in further embodiments energy is extracted from the second air flow provided to the air cycle machine. 
     In addition to one or more of the features described above, or as an alternative, in further embodiments the air cycle machine further comprises a turbine and the cooling air flow is provided to the turbine. 
     In addition to one or more of the features described above, or as an alternative, in further embodiments the fuel tank inerting system is part of an aircraft. 
     In addition to one or more of the features described above, or as an alternative, in further embodiments the first flow is bleed air drawn from at least one of an engine and an auxiliary power unit. 
     According to another embodiment, a method of inerting a gas tank of an aircraft includes providing a first air flow having a first temperature to a fuel tank inerting heat exchanger, providing a second air flow including cabin outflow air having a second temperature to the fuel tank inerting heat exchanger, the first temperature being greater than the second temperature, transferring heat from the first air flow to the second air flow within the fuel tank inerting heat exchanger, and separating an inert gas from the first air flow output from the fuel tank inerting heat exchanger. 
     In addition to one or more of the features described above, or as an alternative, in further embodiments the temperature of the first air flow output from the fuel tank inerting heat exchanger is between about 150° F. and about 250° F. 
     In addition to one or more of the features described above, or as an alternative, in further embodiments comprising extracting energy from the second air flow output from the fuel tank inerting heat exchanger. 
     In addition to one or more of the features described above, or as an alternative, in further embodiments the energy is extracted from the second air flow at an air cycle machine of an air conditioning system. 
     In addition to one or more of the features described above, or as an alternative, in further embodiments the first air flow is s bleed air drawn from at least one of an engine and an auxiliary power unit. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The subject matter, which is regarded as the invention, is particularly pointed out and distinctly claimed in the claims at the conclusion of the specification. The foregoing and other features, and advantages of the invention are apparent from the following detailed description taken in conjunction with the accompanying drawings in which: 
         FIG. 1  is a schematic diagram of a portion of a fuel tank inerting system; 
         FIG. 2  is a schematic diagram of an existing fluid flow path for the air to be provided to the fuel tank inerting system; 
         FIG. 3  is a schematic diagram of a fluid flow path of the air to be provided to the fuel tank inerting system according to an embodiment; and 
         FIG. 4  is a schematic diagram of a fluid flow path of the air to be provided to the fuel tank inerting system according to another embodiment. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring now to  FIG. 1 , an example of a basic fuel tank inerting system (FTIS)  20  for controlling a supply of inerting gas to a fuel tank  22 , such as commonly used in an aircraft for example, is illustrated. The FTIS  20  uses an on-board supply of air A 1  to generate the inerting gas. In the illustrated, non-limiting embodiment, the air flow A 1  provided to the FTIS  20  is bled from one or more engines or auxiliary power units  26  of an aircraft. Accordingly, the air flow A 1  is a hot, high pressure bleed air. However, it should be understood that other suitable mediums, such as outside air, or air discharged from another system of the aircraft are also within the scope of the disclosure. The air A 1  flows through a filter  28  before being provided to one or more air separating modules (ASM)  30 . The ASMs  30  typically include a permeable membrane  32  having two sides. The oxygen rich bleed air passes across a first side of the membrane  32  and a secondary fluid flow passes over a second, opposite side of the membrane  32  to create a pressure differential across the membrane  32 . The pressure differential causes oxygen to diffuse from the bleed air to the secondary fluid stream, and a magnitude of the pressure differential may therefore be used to control how much oxygen is diffused from the stream of bleed air. The secondary fluid flow may be provided from any suitable system having a low pressure airflow. Further, what remains of the treated bleed air to be provided to fuel tank  22  is an inert gas, nitrogen. 
     To maintain safety and a desired level of efficiency of the membrane  32  of the ASM  30  by controlling the temperature thereof, the air A 1  provided to the FTIS  20  is cooled prior to passing through the ASM  30 . In an embodiment the air provided to the membrane  32  is between about 150° F. and about 250° F., and more specifically between about 150° F. and about 215° F. In existing systems, best shown in  FIG. 2 , air flow A 1 , for example drawn from a bleed air system is cooled by a cooling air flow having a temperature less than the temperature of the air flow A 1  to achieve a desired temperature. In the illustrated, non-limiting embodiment, the air A 1  is provided to a fuel tank inerting heat exchanger  34  located within a ram air circuit  42  of an air conditioning system (ACS)  40  of an aircraft environmental control system. To achieve the necessary cooling of the air A 1 , the fuel tank inerting heat exchanger  34  is arranged upstream from the one or more ram heat exchangers  44  relative to a flow of ram air. 
     Inclusion of the fuel tank inerting heat exchanger  34  within the ram air circuit  42  reduces the overall cooling capacity of the ACS  40  because the temperature of the ram air provided to the ram heat exchangers  44  is increased via the heat transfer with the air A 1  thus reducing overall ACS heat sink. Accordingly, other sources of a cooling air flow within the aircraft may be used to cool the temperature of the air A 1  to be provided to the ASM  30 . In an embodiment, the air A 1  is cooled through a heat exchanger located outside of the ram air circuit  42  and where the cool fluid source is a media other than ram air (see  FIGS. 3 and 4 ) 
     Existing aircraft include not only the ACS  40 , but also a separate cabin pressure control system (CPCS)  50  operable to maintain the pressure within the cabin  52  of the aircraft. As best shown in  FIG. 3 , the CPCS  50  is a separate subsystem apart from the ACS  40 ; though they both belong as part of the aircraft ECS. The CPCS  50  includes an outflow valve  54  arranged in fluid communication with the cabin  52  via a conduit  56 . The valve is transformable between a closed position and an open position to exhaust pressurized air from the cabin, such as to overboard or outside of the aircraft. 
     In an embodiment, illustrated in  FIG. 3 , the fuel tank inerting system may be integrated with the CPCS  50  of the aircraft via a fuel tank inerting heat exchanger  34 . As shown, the fuel tank inerting heat exchanger  34  is arranged at an intermediate location between the cabin  52  and the outflow valve  54 . Within the fuel tank inerting heat exchanger  34 , heat is configured to transfer from the air A 1  to be provided to the ASM  30  to the pressurized air from the cabin, also referred to herein as cabin outflow air or cabin discharge air Ac. The fuel tank inerting heat exchanger  34  may be configured such that either of the fluid flows A 1 , Ac may make one or more passes there through to achieve a desired level of cooling. The cooled air A 1  output from fuel tank inerting heat exchanger  34  may be at a desired temperature selected for operation of the ASM  30 , and the heated cabin outflow air Ac output from the fuel tank inerting heat exchanger  34  may be exhausted overboard, or alternatively, provided to another system within the aircraft, such as a portion of the ACS  40  for example. By heating the outflow air Ac prior to overboarding it through the outflow valve, a higher temperature in the outflow valve and thus a higher sonic speed may be achieved, resulting in more thrust recovery from the outflow. 
     With reference now to  FIG. 4 , in another embodiment, the functionality of the CPCS has been integrated into the ACS  40 . More specifically, in such embodiments, the ACS  40  is configured to monitor the pressure within the cabin  52 , and control the amount of cabin outflow air Ac provided to the ACS  40  to meet the pressure demands of the cabin  52 , for example to maintain a minimum pressure within the cabin  52 . In such embodiments, the CPCS  50  separate from the ACS  40  (as shown in  FIG. 3 ) has been eliminated. Various examples of an environmental control system where the functionality of the CPCS is integrated into the ACS are described in more detail in U.S. patent application Ser. No. 15/545,686 filed on Aug. 20, 2019, the entire contents of which are incorporated herein by reference. 
     With continued reference to  FIG. 4 , the integration of the fuel tank inerting heat exchanger  34  is similar to that of  FIG. 3 . As shown, at least a portion of cabin outflow air Ac is provided from the cabin  52  to one or more components of the downstream ACS  40 . The fuel tank inerting heat exchanger  34  is positioned at some intermediate location between the cabin  52  and a downstream component of the ACS  40 , such as portion of an air cycle machine  46  for example. Within the fuel tank inerting heat exchanger  34 , heat is configured to transfer from the hot, high pressure air A 1  to be provided to the ASM  30  to the pressurized air Ac discharged from the cabin  52 . As previously noted, the fuel tank inerting heat exchanger  34  may be configured such that either of the fluid flows A 1 , Ac may make one or more passes there through to achieve a desired level of cooling. The cooled flow of air A 1  output from fuel tank inerting heat exchanger  34  may be at a desired temperature selected for operation of the ASM  30 , and the flow of heated cabin outflow air Ac output from the fuel tank inerting heat exchanger  34  is provided to one or more components of the ACS  40 . In an embodiment, the heated cabin outflow air Ac is provided to a turbine of an air cycle machine  46  of the ACS  40  for example. After energy is extracted from the cabin outflow air Ac by the turbine, the cabin outflow air Ac may be exhausted overboard or into the ram air circuit  42  upstream or downstream from the ram heat exchangers  44 . Alternatively, the cabin outflow air Ac may be provided to another component within the ACS  40  or to another system within the aircraft. By heating the air Ac provided to the turbine of the ACM  46 , additional power or energy may be extracted therefrom. 
     The FTIS  20  described herein is configured to operate at a temperature to optimize efficiency of the ASM membrane  32  while maintaining a desired level of safety at the fuel tank  22 . The cooling air source disclosed herein may be used to achieve the desired temperature with minimal impact to the ACS  40  of an aircraft. Accordingly, the FTIS  20  may be used in both new and retrofit applications. 
     While the invention has been described in detail in connection with only a limited number of embodiments, it should be readily understood that the invention is not limited to such disclosed embodiments. Rather, the invention can be modified to incorporate any number of variations, alterations, substitutions or equivalent arrangements not heretofore described, but which are commensurate with the spirit and scope of the invention. Additionally, while various embodiments of the invention have been described, it is to be understood that aspects of the invention may include only some of the described embodiments. Accordingly, the invention is not to be seen as limited by the foregoing description, but is only limited by the scope of the appended claims.