Abstract:
A fuel heating system for a gas turbine engine comprises a first heat exchanger, a second heat exchanger, a fuel pump and a valve. The first heat exchanger produces a heated air flow. The second heat exchanger receives the heated air flow from the first heat exchanger. The fuel pump provides a fuel flow. The valve is coupled to the fuel pump to intermittently include the second heat exchanger in the fuel flow based on a temperature of the fuel flow. A method of heating fuel in a gas turbine engine comprises providing fuel to a gas turbine engine with a fuel pump to sustain a combustion process, heating a flow of air with exhaust gas from the combustion process, and heating fuel from the fuel pump en route to the gas turbine engine with the flow of air based on a temperature of the fuel.

Description:
BACKGROUND 
     The present invention is directed generally to fluid control systems for gas turbine engines and more particularly to fuel heating systems. 
     Gas turbine engines operate during varied environmental conditions, including at temperatures below the freezing point of water. Additionally, it is possible for the fuel to absorb water under various conditions. Ice crystals therefore have a tendency to form in the fuel under certain conditions, particularly at high altitudes or before the engine is operating. 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, last-chance screens are often provided just before the fuel pump to remove any ice crystals. The screens, however, must be periodically cleared to prevent blockage of fuel flow. It is, therefore, more desirable to eliminate ice crystals from the fuel system altogether. Typical ice removal systems comprise a heat exchanger that imparts heat to the fuel from engine oil used to cool bearings in the engine. However, such systems require time for the engine oil to heat up, thereby delaying the melting of any ice crystals. Further, at high altitude conditions the heat exchanger may not be able to extradite adequate heat from the engine oil to melt the ice. There is, therefore, a need for improved fuel heating systems. 
     SUMMARY 
     The present invention is directed to a fuel heating system for a gas turbine engine. The fuel heating system comprises a first heat exchanger, a second heat exchanger, a fuel pump and a valve. The first heat exchanger produces a heated air flow. The second heat exchanger receives the heated air flow from the first heat exchanger. The fuel pump provides a fuel flow to the second heat exchanger. The valve is coupled to the fuel pump to intermittently include the second heat exchanger in the fuel flow based on a temperature of the fuel flow. In another embodiment, a gas turbine engine has a combustor that receives the fuel flow and that produces exhaust gas in thermal communication with the first heat exchanger, and the valve is configured to bring the second heat exchanger into the fuel flow between the fuel pump and the combustor. 
     The present invention is also directed to a method of heating fuel in a gas turbine engine. The method comprises providing fuel to a gas turbine engine with a fuel pump to sustain a combustion process, heating a flow of air with exhaust gas from the combustion process, and selectively heating fuel from the fuel pump en route to the gas turbine engine with the flow of air based on a temperature of the fuel. In another embodiment, the method comprises passing the flow of air through a coil disposed in the exhaust gas, and then passing the flow of air through a heat exchanger in thermal communication with the fuel. In yet another embodiment, the method comprises sensing a temperature of the fuel before entering the fuel pump, and modulating flow of fuel to the heat exchanger using a valve responsive to a sensed temperature of the fuel. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The sole FIGURE shows a schematic of a gas turbine engine having a fuel heating system of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     The sole FIGURE shows a schematic of gas turbine engine  10  having fuel heating system  12  including fuel heater  14  of the present invention. Gas turbine engine  10  includes compressor  16 , turbine  18 , shaft  20  and combustor  22 . Fuel heating system  12  further includes fuel pump  24 , heat exchanger  26 , heater valve  28  and temperature sensor  29 . Gas turbine engine  10  is also interconnected with lubrication system  30 , which includes oil cooler  32  and fan  34 . In the embodiment discussed, gas turbine engine  10  comprises an auxiliary power unit (APU) configured to drive electrical generator  36 . Gas turbine engine  10  and heater  14  of the present invention may, however be implemented in other types of gas turbine engine, such as those used for propulsive force in aircraft and those used in the industrial gas turbine field. Combustor  22 , fan  34 , valve  28  and fuel pump  24  are in electronic communication with engine controller  37 . 
     Gas turbine engine  10  operates in a conventional manner by combusting fuel from fuel pump  24  and compressed air from compressor  16  in combustor  22  to produce high energy gases for driving turbine  18 . Fuel pump  24  includes inlet line  38 , which receives fuel from a fuel tank (not shown), and outlet line  40 , which delivers fuel to combustor  22 . Outlet line  40  may include other components, such as a metering valve, connected to controller  37  to precisely regulate fuel flow to combustor  22 . Engine controller  37  determines the rate of fuel flow to combustor  22  based on the demands placed on engine  10 , as is known in the art. For example, engine controller  37  may comprise a Full Authority Digital Engine Controller (FADEC). Compressor  16  draws in ambient air A A , compresses it and provides it to combustor  22 . Combustor  16  includes conventional 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  20 , which rotates compressor  16  and electrical generator  36 . Electrical generator  36  is shown schematically being driven by tower shaft  41 , which is coupled to shaft  20  through a gearbox, as is known in the art. Fuel pump  24  also includes bypass line  42  and return line  44 , which are regulated by valve  28 . Valve  28  controls flow of fuel to heat exchanger  26  to melt ice before entering fuel line  40  where clogging of orifices within the fuel metering valve or the combustor may occur. 
     Aside from exhaust gas G E , operation of gas turbine engine  10  produces heat, particularly in bearings used to support shaft  20 . As such, lubrication system  30  provides a continuously circulated flow of oil between the bearings, oil sumps and an oil tank by an oil pump (not shown) coupled to oil lines  46 A and  46 B. Oil cooler  32  is coupled into oil lines  46 A and  46 B, such as to be provided with heated oil from the oil sumps and to provide cooled oil to the oil pump. In the embodiment discussed, oil cooler  32  comprises an air cooled heat exchanger that receives cooling air A C  from fan  34 . Engine controller  37  is in communication with sensors (not shown) that determine the temperature of the oil and can adjust the speed of fan  34  to provide increased or decreased cooling to oil cooler  32 . Oil cooler  32  and fan  34  are shown positioned upstream of compressor  16 , but need not be in other configurations. 
     Fan  34  includes mechanically rotated fan blades to push a flow of cooling air A C  across cooling fins in oil cooler  32 . Fan  34  can be mechanically driven by shaft  20 , a gear train coupled to shaft  20 , or an electric motor powered by electrical generator  36  or some other such electrical power supply. Fan  34  is fluidly connected to heat exchanger  26  through air line  48 A, heater  14  and air line  48 B. In the embodiment shown, the fluid comprises compressed air that is siphoned from fan  34  and provided to heat exchanger  26 . The compressed air from fan  34  is sufficiently pressurized by the fan blades to produce flow through air line  48 A, heater  14 , air line  48 B, heat exchanger  26  and air line  48 C. For example, the speed of fan  34  and the resulting pressure of the air can be increased by engine controller  37  as needed. Compressed air from heat exchanger  26  is expelled from fuel heating system  12  through air line  48 C. In other embodiments, a dedicated coolant can be continuously cycled between heater  14  and heat exchanger  26  via a pump. 
     Heat exchanger  26  receives a motive flow of a heated fluid from heater  14  and a motive flow of cold fuel from pump  24 . In one embodiment, heat exchanger  26  comprises a dual-fluid plate-fin heat exchanger. After passing through heat exchanger  26 , the cooled motive fluid is dumped from system  12 . Cold fuel from fuel pump  24  enters heat exchanger  26  through bypass line  42  and the heated fuel is restored to fuel pump  24  through return line  44 . Valve  28  is responsive to input from temperature sensor  29  to periodically connect heat exchanger  26  in series between pump  24  and combustor  22  based on temperatures sensed by sensor  29 . 
     Engine controller  37  is in electronic communication with sensor  29  and fuel pump  24  to modulate circulation of fuel through heat exchanger  26 , depending on atmospheric conditions such as temperature and barometric pressure. Fuel pump  24  is configured to provide motive flow of fuel to combustor  22  under default operating conditions. Under adverse atmospheric conditions, valve  28  is actuated by engine controller  37  to circulate fuel through heat exchanger  26  before allowing the fuel to continue to combustor  22 . 
     Temperature sensor  29  is in thermal communication with fuel in line  38 . When temperature sensor  29  detects temperatures above a threshold level, a signal is sent to engine controller  37  to maintain valve  28  in a closed state. With valve  28  closed, fuel is permitted to flow uninterruptedly from the fuel tank, through inlet line  38 , pump  24  and outlet line  40 , while fuel is prevented from entering line  42 . Check valve  52  prevents backflow of fuel into line  44 . The threshold level may be the freezing point of water (0° C. or 32° F.), some threshold temperature above the freezing point of water to provide a safety factor, or a temperature above the freezing point of water at which the water may freeze due to elevated altitude, which can be sensed by engine controller  37 . Default operation is desirable and acceptable, and indicates that the presence of ice in the fuel lines is absent and not possible. 
     Temperatures at or below the threshold level produce conditions at which ice crystals may form in fuel lines  38  and  40 , fuel pump  24 , the fuel metering valve or injectors within combustor  22 , which may adversely impact the operation of engine  10 . Fuel lines  38  and  40  are provided with screens to filter the crystals from the system. For example, screen  50  is positioned upstream of fuel pump  24  to remove ice crystals from fuel heating system  12 . The screens can further be provided with means for removing or melting the crystals in the screen to prevent blockage of fuel flow, as is known in the art. However, it is desirable to altogether prevent the formation of the crystals to avoid the need for their removal and disposal. For example, at start-up of engine  10  ice crystals may have formed in fuel already present in line  40  that cannot be caught by filter  50 . Thus, temperatures that are at or below the threshold indicate to engine controller  37  a need to heat the fuel. When engine controller  37  detects a temperature from temperature sensor  29  at or below the threshold level, a signal is sent to open valve  28 . 
     Temperature sensor  29  is positioned upstream of fuel pump  24  so that engine control  37  can act to prevent ice crystals from reaching combustor  22  where injectors having fine orifices are located. Using lines  42  and  44 , valve  28  routes fuel to heat exchanger  26  to melt ice crystals and prevent blocking of the injector orifices before permitting the fuel to continue on to combustor  22 . Heater  14  is positioned so as to be in thermal communication with exhaust gas G E  so that the compressed air from fan  34  is heated. Heater  14  can be placed directly in the flow of exhaust gas G E  or adjacent the flow of exhaust gas G E . 
     Heater  14  comprises a heat exchanger that transfers heat from exhaust gas G E  to compressed air from fan  34 . As such, heater  14  may comprise a dual-fluid plate-fin heat exchanger that is coupled to tubing comprising lines  48 A and  48 B. In the embodiment shown, heater  14  comprises a tube that is coiled in a helical fashion and disposed within the outer diametrical limits of the flow of exhaust gas G E . Thus, heater  14  has approximately the same diameter as the downstream exit of turbine  18 . In other embodiments, heater  14  may comprise a tube coiled in a serpentine fashion so as to shape a planar body that can be placed perpendicular to the flow of exhaust gas G E . Coiled embodiments of heater  14  may also include heat transfer-enhancing features such as fins in other embodiments. In another embodiment, heat exchanger  26  can be omitted and heater  14  comprises a tube wrapped directly around a fuel line connecting lines  42  and  44 . 
     Heat from exhaust gas G E  increases the temperature of the compressed air within heater  14  to temperatures sufficiently high so as to be able to increase the temperature of the fuel within heat exchanger  26  to melt ice crystals within the fuel and to prevent reforming of ice crystals within lines  44  and  40 . Engine controller  37  can increase the speed of fan  34  to increase the flow of compressed air to heater  14 , thereby increasing the heating of the fuel, based on temperatures detected by sensor  29 . Thus, the risk of ice crystals clogging fuel line  40  and small orifices within fuel pump  24  and combustor  22  is eliminated, thereby increasing the operating efficiency and safety of gas turbine engine  10 . Heat from exhaust gas G E  is immediately available at the start-up of engine  10 . For example, as soon as engine  10  is operating, combustion is occurring within combustor  22  at temperatures exceeding 1000° F. (538° C.), sufficient enough to heat the fuel and melt any ice crystals within moments of ignition. Thus, any wait time needed for the heating of the fuel is eliminated, as was required in prior art fuel heating systems using heat generated by sustained circulation of the engine oil. 
     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.