Abstract:
A heat exchanger bypass system comprises a liquid circuit, a first fluid circuit, a second fluid circuit, a first heat exchanger, a second heat exchanger, a liquid bypass line and a heated bypass valve. The first heat exchanger thermally couples the first fluid circuit to the liquid circuit. The second heat exchanger thermally couples the second fluid circuit to the liquid circuit. The liquid bypass line circumvents the first heat exchanger. The heated bypass valve controls flow through the liquid bypass line.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims priority to U.S. Provisional Application No. 61/865,843, filed on Aug. 14, 2013, and entitled “Heated Bypass Valve for Heat Exchanger,” the disclosure of which is incorporated by reference in its entirety. 
    
    
     BACKGROUND 
     The present disclosure relates generally to fuel and oil systems for aircraft systems and more particularly to bypass valves for heat exchangers. 
     Gas turbine engines operate during varied environmental conditions, including at temperatures below the freezing point of water. Additionally, it is possible for fuel to absorb water under various conditions. Thus, under certain conditions it is possible for ice to form within the fuel system. For example, intercontinental flights at high altitudes, such as from Beijing to London, frequently produce conditions for icing. Ice crystals may also form under certain conditions before the engine is operating, such as when in hangars or on airstrips. The ice crystals can plug fuel lines and orifices in the fuel system, which may degrade performance of the gas turbine engine or even cause an engine stall. As such, gas turbine engines are equipped with systems for eliminating or removing ice particles from fuel lines. For example, heat exchangers are often provided just before the fuel pump to eliminate any ice crystals. Heat exchangers are desirable because the ice is removed from the system altogether and does not require periodic clearing or cleaning. 
     Typical ice removal systems comprise a heat exchanger that imparts heat to the fuel from engine oil used to cool various components of the engine. However, such systems require time for the engine oil to heat up, thereby delaying the melting of any ice crystals. Furthermore, at high altitude conditions the heat exchanger may not be able to extract adequate heat from the heat source, such as the electric generator oil or engine oil, to melt the ice. Thus, in the event the heat exchanger becomes clogged with ice to the point where free flow of fuel is inhibited, a bypass valve opens causing the fuel to circumvent the heat exchanger. The bypass flow of fuel keeps the engine running until such time the problem can be rectified by other means or the aircraft can be landed. However, the bypass valve itself produces a potential bottleneck that can become clogged with ice. There is, therefore, a need for improved systems for preventing ice blockage within fuel systems. 
     SUMMARY 
     The present invention is directed to a heat exchanger bypass system, such as may be used in a fuel and oil system for a gas turbine engine. The heat exchanger bypass system comprises a liquid circuit, a first fluid circuit, a second fluid circuit, a first heat exchanger, a second heat exchanger, a liquid bypass line and a heated bypass valve. The first heat exchanger thermally couples the first fluid circuit to the liquid circuit. The second heat exchanger thermally couples the second fluid circuit to the liquid circuit. The liquid bypass line circumvents the first heat exchanger. The heated bypass valve controls flow through the liquid bypass line. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1A  is a schematic for a fuel and oil system of a gas turbine engine showing heat exchangers connected with a heated bypass valve in a closed state. 
         FIG. 1B  shows the schematic for a fuel and oil system of a gas turbine engine of  FIG. 1A  with the heated bypass valve in an open state. 
         FIG. 2  shows a schematic view of a heat exchanger connected to the heated bypass valve of  FIGS. 1A and 1B . 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1A  shows a portion of a fuel and oil system  10  for gas turbine engine  12  with heated bypass valve  14 . Gas turbine engine  12  includes compressor  16 , turbine  18 , combustor  20 , turbine shaft  22 , generator shaft  24  and generator  26 . Fuel and oil system  10  includes boost pump  28 , generator heat exchanger  30 , engine heat exchanger  32 , fuel pump  34 , fuel metering unit (FMU)  36 , first oil pump  38  and second oil pump  40 . 
     In the configuration of  FIG. 1A , heated bypass valve  14  is closed such that direct flow between boost pump  28  and engine heat exchanger  32  through bypass line  41  is prevented (as indicated by a dashed line in  FIG. 1A ). As discussed with reference to  FIG. 1B , heated bypass valve  14  can be opened to bypass generator heat exchanger  30  in the event fuel within generator heat exchanger becomes blocked with ice and flow of fuel through line  48  is prevented (as indicated by a dashed line in  FIG. 1B ). Engine heat exchanger  32  heats bypass valve  14  to prevent formation of ice within bypass valve  14 . 
     A lubricant, such as oil, is stored within fuel and oil system  10 , such as in oil tank  43 , and is provided to generator  26  and shaft  22 . Using pump  38 , oil from generator heat exchanger  30  is pumped to generator  26  through line  42 A and subsequently pumped to generator heat exchanger  30  through line  42 B. Likewise, oil from engine heat exchanger  32  is provided to shaft  22  through oil line  44 A, and oil is returned to engine heat exchanger  32  through line  44 B using pump  40 . In the disclosed embodiment, oil tank  43  is disposed within line  42 A and, although not shown, line  44 A may also be connected to the same or a different oil tank. Likewise, in the disclosed embodiment, pumps  38  and  40  are shown being placed in lines  42 A and  44 B, respectively, but may be located in other locations. 
     Boost pump  28  receives fuel from fuel tank  45 , and delivers fuel to fuel line  46 , which routes fuel to generator heat exchanger  30 . Fuel line  48  connects generator heat exchanger  30  and engine heat exchanger  32 . Outlet line  50  routes fuel to FMU  36 , which provides fuel to combustor  20  through fuel line  52 . 
     Gas turbine engine  12  operates in a conventional manner by combusting fuel from FMU  36  and compressed air from compressor  16  in combustor  20  to produce high energy gases for driving turbine  18 . Compressor  16  draws in ambient air A A , compresses it and provides it to combustor  20 . Boost pump  28  pushes fuel through generator heat exchanger  30  and engine heat exchanger  32  to fuel pump  34 . Fuel pump  34  provides pressurized fuel to FMU  36 , which is electronically controlled, such as through a Full Authority Digital Engine Controller (FADEC), to deliver precise amounts of fuel to combustor  20  based on performance needs of gas turbine engine  12 . Fuel not needed by combustor  20  is returned to the fuel system via appropriate fuel lines (not shown). 
     Combustor  20  includes fuel injectors and igniters for burning a mixture of fuel and air to provide exhaust gas G E  that turns turbine  18 . Rotation of turbine  18  drives shaft  22 , which rotates compressor  16 . In addition to driving operation gas turbine engine  12 , rotation of engine shaft  22  causes generator shaft  26  to rotate and provide a mechanical input to electrical generator  26 . Electrical generator  26  is shown schematically being driven by tower shaft  24 , which is coupled to shaft  22  through a gearbox, as is known in the art. 
     Aside from exhaust gas G E , operation of gas turbine engine  12  produces heat, particularly in bearings used to support shaft  22  and within generator  26  itself. Thus, generator  26  and the bearings for shaft  22  typically require lubrication to remove heat. Generator heat exchanger  30  and engine heat exchanger  32  are interconnected with fuel lines and oil lines to transfer heat generated by generator  26  and shaft  22  to the fuel. The oil is thereby cooled and the heated fuel improves operating efficiency of gas turbine engine  12  and prevents formation of ice within the fuel lines. 
     Heat exchangers  30  and  32  each receive a motive flow of heated oil and a motive flow of relatively cooler fuel. Pump  38  circulates a continuous flow of heated oil from generator  26  to generator heat exchanger  30  through line  42 B. Pump  40  circulates a continuous flow of heated oil from the bearings for shaft  22  (or oil sumps within engine  12  that collect oil from the bearings) to engine heat exchanger  32  through line  44 B. Colder fuel from boost pump  28  flows through heat exchangers  30  and  32 . 
     Heat exchangers  30  and  32  transfer heat from the oil to the fuel. Oil cooled in engine heat exchanger  32  is returned to shaft  22  via line  44 A. Oil cooled in generator heat exchanger  30  is returned to generator  26  via line  42 A. Heated fuel is consumed within combustor  20 . As such, heat from fuel and oil system  10  is continuously removed from the oil by the fuel and removed from engine  12  by burning of the fuel. 
     Heat exchangers  30  and  32  are connected in series and cold fuel is heated incrementally at each of heat exchangers  30  and  32 . Generator heat exchanger  30  is positioned upstream (relative to the flow direction of the fuel) of engine heat exchanger  32 . Series placement of generator heat exchanger  30  and engine heat exchanger  32  is desirable because it maintains the flow velocity of the fuel and maximizes heat transfer, as opposed to parallel flow heat exchangers where flow velocity is reduced. In series connected heat exchangers, oil used to cool the bearings for shaft  22  reaches higher temperatures than the oil used to cool generator  26 . Configured as such, the coldest fuel cools generator  26  in order to reduce overheating of generator  26  and loss of electrical power to gas turbine engine  12 . It is, thus, highly desirable to keep fuel running through system  10  under all conditions to, among other things, prevent overheating of generator  26 . 
     As shown in  FIG. 1A , fuel is allowed to flow from generator heat exchanger  30  to engine heat exchanger  32  through fuel line  48 . With bypass valve  14  closed, fuel flows uninterruptedly from fuel tank  45 , through boost pump  28 , inlet line  46 , generator heat exchanger  30 , line  48 , engine heat exchanger  32 , line  50  and pump  34  to FMU  36 . Thus, engine  12  operates under normal conditions. 
     If atmospheric conditions become sufficient, water within the fuel will become frozen into ice particles, even though generator heat exchanger  30  operates to heat the fuel. Small amounts of ice within system  10  may be tolerated. It is, however, desirable to prevent formation of ice within system  10  altogether. Thus, under normal operating conditions operation of heat exchangers  30  and  32  is sufficient to prevent the formation of ice. 
     Under more extreme atmospheric temperatures and altitudes, ice may still form in the fuel. Enough ice may form to cause blockage of heat exchanger  30  and prevent fuel from being delivered to combustor  20 , which is highly undesirable due to the potential to stall operation of engine  12 . Bypass valve  14  is operable to allow fuel to circumvent generator heat exchanger  30  and flow directly from boost pump  28  to engine heat exchanger  32 . 
       FIG. 1B  shows fuel and oil system  10  for gas turbine engine  12  of  FIG. 1A  with heated bypass valve  14  in an open state to allow fuel flow through bypass line  41 .  FIG. 1B  additionally shows oil bypass valves  56  and  58  and fuel bypass valve  60 . As indicated by a dashed line in  FIG. 1B , fuel is prevented from flowing through fuel line  48  by clogging of ice within generator heat exchanger  30 . 
     Bypass valve  14  is responsive to pressure differentials across generator heat exchanger  30 . Specifically, when the pressure in bypass line  41  becomes greater than the pressure in line  50  beyond a threshold pressure, bypass valve  14  will open. Pressure in bypass line  41  increases as ice within generator heat exchanger  30  reduces flow through heat exchanger  30  and increases the backpressure in line  46 . The location of bypass valve  14  in close proximity to heat generated by engine heat exchanger  32  inhibits ice from forming within bypass valve  14 . 
     Bypass valve  14  will open to allow fuel to flow through bypass line  41 . Heat from oil within engine heat exchanger  32  is used to heat bypass valve  14  to prevent ice particles within the fuel from clogging bypass valve  14 . The heat emitted from heat exchanger  32  increases the temperature of the fuel within valve  14  to temperatures sufficiently high so as to be able to melt ice crystals within the fuel and to prevent ice crystals from clogging heat exchanger  32 . Thus, the risk of ice crystals clogging fuel lines  50  and  52  and small orifices within fuel pump  34  and combustor  20  is mitigated, thereby increasing the operating efficiency and safety of gas turbine engine  12 . As will be discussed in greater detail with reference to  FIG. 2 , bypass valve  14  may be positioned anywhere to allow the heat from engine heat exchanger  32  to impart heat into valve  14  sufficient to melt ice. In the event ice crystals do form within heat exchanger  32  sufficient to cause blockage, bypass valve  60  can be opened to allow fuel to bypass heat exchanger  32 . 
     Continuous flow of heated oil from generator  26  will flow into generator heat exchanger  30  through line  42 B to melt the ice forming the blockage. After enough ice has melted to allow flow through heat exchanger  30  and the pressure within bypass line  41  to drop, bypass valve  14  will close and fuel flow through line  48  will be restored. Flow of oil through generator heat exchanger  30  may be bypassed by valve  56 . Similarly, flow of oil through engine heat exchanger  32  may be bypassed by valve  58 . Operation of valves  56  and  58  may be manually closed for service or may be automatically closed by the FADEC based on engine conditions. 
       FIG. 2  shows a schematic view of engine heat exchanger  32  connected to heated bypass valve  14  of  FIGS. 1A and 1B . Engine heat exchanger  32  includes bypass valve  14 , bypass line  41 , oil input line  44 A, oil output line  44 B, fuel input line  48 , fuel output line  50 , bypass line  61 , housing  62  and heat exchange mechanism  64 . 
     Fuel input line  48  delivers cool fuel to heat exchange mechanism  64  while oil input line  44 A delivers hot oil to heat exchange mechanism  64 . Heat exchange mechanism  64  transfers heat from the oil to the fuel. Heat exchange mechanism  64  may comprise any suitable heat transfer mechanism as is known in the art. For example, heat exchangers  30  and  32  may comprise dual-fluid plate-fin or shell-and-tube heat exchangers. 
     Heat exchange mechanism  64  is disposed within housing  62 , which also provides a framework for mounting the components of heat exchanger  32 . For example, fuel line  48 , bypass line  41 , oil lines  44 A and  44 B and bypass line  61  may pass through housing  62  to join with heat exchange mechanism  64 . As mentioned above, bypass valve  14  may be positioned anywhere near heat exchanger  32  where there is sufficient heat to melt ice within bypass valve  14 . As explained earlier, bearings for engine shaft  22  generate much higher heat than does generator  26 . Thus, heat exchanger  32  may be able to melt ice that heat exchanger  30  was unable to melt. Additionally, due to the extremely elevated temperatures of the oil used to cool bearings for shaft  22 , engine heat exchanger  32  produces heat zone  66 . As such, bypass valve  14  may be positioned outside of heat exchanger  32  anywhere within heating zone  66  where there is sufficient heat to melt ice. In the embodiment depicted, bypass valve is attached to the outside of housing  62  within heat zone  66  so as to be in thermal communication with heat from the oil. In other embodiments, bypass valve  14  may be attached to the inside of housing  62  or anywhere within bypass line  41  between housing  62  and input line  48  in thermal communication with heat from the oil. In yet other embodiments, bypass valve  14  can be heated more directly from the heat of oil used to cool and lubricate the bearings of shaft  22 . For example, bypass valve  14  could be thermally coupled to line  44 A, line  44 B, pump  40 , an oil sump or another component having heated bearing oil, rather than being coupled to heat exchanger  32 . 
     Engine heat exchanger  32  also includes bypass valve  60  which is disposed within bypass line  61 . Bypass line  61  forms a secondary fuel bypass circuit around heat exchange mechanism  64  in the event heat from oil within oil lines  44 A and  44 B is insufficient to melt ice within the fuel. Bypass line  61  extends from fuel line  48  downstream of bypass line  41  and bypass valve  14  to fuel outlet line  50  downstream of heat exchange mechanism  64 . Thus, fuel including frozen ice particles may travel from boost pump  28  ( FIG. 1A ) to fuel pump  34  ( FIG. 1A ) without passing through heat exchangers  30  or  32 . Bypass valve  60  may be sized accordingly to allow large ice crystals to pass through without clogging the valve mechanism. Such a condition is undesirable, but may provide fuel to engine  12  for a time sufficient for heat exchangers  30  and  32  to melt ice blocking the heat exchangers. 
     By using the heat of oil used to cool and lubricate bearings within engine  12 , which is typically much higher than the heat of generator heat exchanger  30 , blockage of generator heat exchanger  30  can be mitigated. For example, the presence of an unheated bypass valve around the heat exchange mechanism of heat exchanger  30  is eliminated. This eliminates a potential choke point for ice particles within the system. Thus, flow of fuel can never be completely choked off at generator heat exchanger  30 . The elevated heat from engine heat exchanger  32  will always be available within the system to melt ice, whether it is blocking generator heat exchanger  30  or engine heat exchanger  32 . Heat from oil engine bearings is conveniently accessed at engine heat exchanger  32 , which is typically positioned in close proximity to heat exchanger  30  and input line  48 . 
     While the invention has been described with reference to an exemplary embodiment(s), 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 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(s) disclosed, but that the invention will include all embodiments falling within the scope of the appended claims.