Abstract:
A recuperator movable between a first position and a second position in or relative to a power generating system. When the recuperator is in the first position, a gas side inlet of the recuperator is coupled to a turbine exhaust outlet of the turbomachine. When the recuperator is in the second position, a gas side outlet of the recuperator is coupled to the turbine exhaust outlet, whereby the direction of gas flow inside the recuperator is reversed. Reversing the gas flow direction extends the life of the recuperator by reducing the total amount of time that the gas inlet face is exposed to high temperatures. Reversing the gas flow direction also allows for the removal of condensation of exhaust gas byproducts on cooler passage surfaces of the recuperator gas side. It is emphasized that this abstract is provided to comply with the rules requiring an abstract that will allow a searcher or other reader to ascertain quickly the subject matter of the technical disclosure. The abstract is submitted with the understanding that it will not be used to interpret or limit the scope or meaning of the claims. 37 C.F.R. 1.72(b).

Description:
BACKGROUND OF THE INVENTION  
         [0001]    This invention relates generally to heat exchangers for power generating systems. A specific embodiment relates to recuperators for turbomachinery. The present invention also relates to microturbine power generating systems, which are small, multi-fuel, modular distributed generation units.  
           [0002]    Although the recuperator of the present invention can be used with stationary microturbines, with other turbomachinery (such as turbomachinery used for automotive and air transportation), and with other power generating systems such as fuel cells, the recuperator is described here for convenience primarily in connection with microturbines. Microturbine power generating systems generally includes a combustor, a turbine stage, a compressor stage and an electrical generator. A microturbine power generating system may also include a recuperator for transferring heat from hot exhaust gas leaving the turbine stage to compressed air entering the combustor. Transferring the heat raises the temperature of the air entering the combustor and cools the exhaust gas leaving the turbine stage. Raising the temperature of the compressed air enhances combustion and increases efficiency of the system.  
           [0003]    There are potential problems associated with the recuperator. One potential problem arises from thermal stresses in the recuperator. The turbine exhaust gas entering the recuperator is hotter than the exhaust gas leaving the recuperator. Consequently, the front face of the recuperator is hotter than the exit face. The resulting thermal stresses can reduce the operating life of the recuperator.  
           [0004]    Another potential problem is associated with the buildup of combustion products in the recuperator. As the exhaust gas is passing through the recuperator, combustion products in the exhaust gas can condense and build up on cooler heat transfer surfaces of the recuperator. The buildup can decrease heat transfer efficiency. The buildup can also restrict the flow of exhaust gas and thereby reduce system efficiency. The recuperator may be cleaned periodically, but the periodic cleaning would increase the cost of maintaining the microturbine power generating system.  
         SUMMARY OF THE INVENTION  
         [0005]    The present invention may be regarded as a recuperator movable between a first position and a second position in a power generating system such as a microturbine. When the recuperator is in the first position, a gas side inlet of the recuperator is coupled to a turbine exhaust outlet of the microturbine. When the recuperator is in the second position, a gas side outlet of the recuperator is coupled to the turbine exhaust outlet, whereby the direction of gas flow inside the recuperator is reversed. Reversing the gas flow direction reduces the total amount of time that the hotter sections of the recuperator are exposed to higher temperatures, thereby extending the life of the recuperator. Reversing the gas flow direction also allows deposited combustion products to be removed from the heat transfer surfaces of the recuperator, resulting in a self-cleaning feature that reduces maintenance of the power generating system. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0006]    [0006]FIG. 1 is a block diagram of a power generating system according to the present invention, the power generating system including a recuperator;  
         [0007]    [0007]FIG. 2 is an illustration of a core of the recuperator;  
         [0008]    [0008]FIGS. 3 a  and  3   b  are illustrations of the recuperator in first and second positions; and  
         [0009]    [0009]FIG. 4 is a flowchart of a method of using the recuperator. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0010]    [0010]FIG. 1 shows a power generating system  10  including a compressor  12 , a turbine  14  and an electrical generator  16  cantilevered from the compressor  12 . The compressor  12 , the turbine  14  and the electrical generator  16  are rotated by a single common shaft  18 . Although the compressor  12 , turbine  14  and electrical generator  16  may be mounted to separate shafts, the use of the single common shaft  18  adds to the compactness and reliability of the power generating system  10 .  
         [0011]    The shaft  18  may be supported by self-pressurized air bearings such as foil bearings. Foil bearings eliminate the need for a separate bearing lubrication system and reduce the occurrence of maintenance servicing.  
         [0012]    Air is compressed by the compressor  12 , and the compressed air is circulated through air side passages of a recuperator  22 . Compressed air leaving the air side passages of the recuperator  22  is supplied to a combustor  24 .  
         [0013]    Fuel is also supplied to the combustor  24 . Either gaseous or liquid fuel may be used. Choices of fuel include diesel, flare gas, wellhead natural gas, waste hydrocarbon fuel streams, gasoline, naphtha, propane, JP-8, methane, natural gas and other man-made gases.  
         [0014]    The flow of fuel to the combustor  24  is controlled by a flow control valve  26 . The fuel is injected into the combustor  24  by an injection nozzle  28 .  
         [0015]    Inside the combustor  24  the fuel and compressed air are mixed and ignited by an igniter  27  in an exothermic reaction. Hot, expanding gases resulting from combustion in the combustor  24  are directed to an inlet nozzle  30  of the turbine  14 . The inlet nozzle  30  may have a fixed geometry. The hot, expanding gases resulting from the combustion are expanded through the turbine  14 , thereby creating turbine power. The turbine power, in turn, drives the compressor  12  and the electrical generator  16 . For transportation applications, the generator may be reduced in size or eliminated, and the excess resulting power supplied to a drive train.  
         [0016]    Turbine exhaust gas is passed through gas side passages of the recuperator  22 . Inside the recuperator  22 , heat from the turbine exhaust gas in the gas side passages is transferred to the compressed air in the air side passages. In this manner, some heat of combustion is recuperated and used to raise the temperature of the compressed air en route to the combustor  24 . After surrendering part of its heat, the turbine exhaust gas exits the recuperator  22 . Additional heat recovery stages may be added onto the power generating system  10 . A muffler  32  reduces the noise created by the turbine exhaust gas leaving the recuperator  22 .  
         [0017]    The generator  16  may be a ring-wound, two-pole toothless (TPTL) brushless permanent magnet machine having a permanent magnet rotor  34  and stator windings  36 . The rotor  34  is attached to the shaft  18 . When the rotor  34  is rotated by the turbine  14 , an alternating current is induced in the stator windings  36 . Speed of the turbine  34  can be varied in accordance with external energy demands placed on the system  10 . Variations in the turbine speed will produce a variation in the frequency of the alternating current generated by the electrical generator  16 . Regardless of the frequency of the ac power generated by the electrical generator  16 , the ac power can be rectified to dc power by a rectifier  38 , and then chopped by a solid-state electronic inverter  40  to produce ac power having a fixed frequency. Accordingly, when less power is required, the turbine speed can be reduced without affecting the frequency of the ac output.  
         [0018]    Use of the rectifier  38  and the inverter  40  allows for wide flexibility in determining the electric utility service to be provided by the power generating system  10  of the present invention. Because any inverter  40  can be selected, frequency of the ac power can be selected by the consumer. If there is a direct use for ac power at wild frequencies, the rectifier  38  and inverter  40  can be eliminated.  
         [0019]    A controller  42  controls the turbine speed by controlling the amount of fuel flowing to the combustor  24 . The controller  42  uses sensor signals generated by a sensor group  44  to determine the external demands upon the power generating system  10  and then controls the fuel valve  26  accordingly. The sensor group  44  could include sensors such as position sensors, turbine speed sensors and various temperature and pressure sensors for measuring operating temperatures and pressures in the system  10 . Using the aforementioned sensors, the controller  42  can control both startup and optimal performance during steady state operation.  
         [0020]    Reference is now made to FIG. 2. The recuperator  22  includes a heat exchanger core  50  having a standard construction. Air and gas side passages  52  and  54  are formed within the heat exchanger core  50 . The heat exchanger core  50  may be made of a stack of plates that form the air and gas side passages  52  and  54 .  
         [0021]    Compressed air is supplied to an air inlet manifold  56 , which distributes the compressed air to the air side passages  52  in the heat exchanger core  50 . Air leaving the air side passages  52  is collected by an air outlet manifold  58 . The air manifolds  56  and  58  may be formed integrally with the heat exchanger core  50 . For example, the air manifolds  56  and  58  may be formed by the plates.  
         [0022]    The turbine exhaust gas stream enters a first face  60  of the heat exchanger core  50 , flows through the gas passages  54  in the core  50 , and exits from a second face  62  of the heat exchanger core  50 . As the air flows across the core  50 , heat is transferred from the exhaust gas to the compressed air. However, as the turbine exhaust gas is passing through the gas side passages  54 , combustion products in the turbine exhaust gas can condense and build up on cooler sections of the gas side passages  54 . This buildup can decrease heat transfer efficiency. The buildup can also restrict the flow of turbine exhaust gas and thereby reduce system efficiency.  
         [0023]    The heat exchanger core  50  can be rotated by 180 degrees about a pivot point A, whereby the positions of the inlet and outlet manifolds  56  and  58  are reversed. A direction of rotation is indicated by the arrow R. When the core  50  is rotated by 180 degrees, the air and gas flow directions are reversed. Air flows into the air outlet manifold  58  and out of the air inlet manifold  56 . Turbine exhaust gas enters the second face  62  of the heat exchanger core  50  and exits from the first face  60 . Reversing the gas flow direction allows deposited combustion products to be removed from the heat transfer surfaces of the recuperator  22 . Reversing the gas flow direction also reduces the total amount of time that the hotter sections of the recuperator  22  are exposed to higher temperatures, thereby extending the life of the recuperator  22 .  
         [0024]    Reference is now made to FIGS. 3A and 3B. The recuperator  22  further includes a casing  64  for the heat exchanger core  50 . The casing  64  has external insulation (not shown) and mounting brackets  66 .  
         [0025]    The recuperator  22  is mounted on a mounting stand  68 . The stand  68  includes mounting pins  70  that are pivotally attached to the mounting brackets  66 . The mounting stand  68  allows the recuperator  22  to be rotated about the axis A, which extends through the mounting pins  70 . The recuperator  22  can be rotated between a first position (shown in FIG. 3 a ) and a second position (shown in FIG. 3B). Rotating the recuperator  22  from the first position to the second position (or vice versa) causes the air and gas flow directions inside the recuperator  22  to be reversed.  
         [0026]    The casing  64  also provides a ducting interface for the recuperator  22 . The ducting interface includes a gas side inlet flange  72 , a gas side outlet flange  74 , an air inlet flange  76 , and an air outlet flange  78 .  
         [0027]    When the recuperator  22  is in the first position, the ducting interface flanges are attached as follows. The air inlet flange  76  is connected to a flange  80  on a first duct  82 , which places the air inlet manifold  56  in fluid communication with an outlet of the compressor  12 . The air outlet flange  78  is connected to a flange  84  on a second duct  86 , which places the air outlet manifold  58  in fluid communication with an air inlet of the combustor  24 . The gas inlet flange  72  is connected to a flange  88  on a third duct  90 , which places the bottom face  60  of the heat exchanger core  50  in fluid communication with an exhaust outlet of the turbine  14 . The gas outlet flange  74  is connected to a flange  92  on a fourth duct  94 , which places the top face  62  of the heat exchanger core  50  in fluid communication with an inlet of the muffler  32 .  
         [0028]    Additional reference is now made to FIG. 4. After the recuperator  22  has been used over a period of time, the flanges  72 ,  74 ,  76  and  78  of the ducting interface are disconnected (step  102 ), and the recuperator  22  is rotated from the first position to the second position (step  104 ). This step may be accomplished in one of several ways. The recuperator  22  can be shaped so as to be rotatable in-situ on mounting pins  70 , without requiring any movement of flanges  88  or  92  relative to one another or to the mounting stand  68 . Or, mounting pins  70  can be slideably attached to mounting stand  68  or mounting brackets  66 , thereby allowing recuperator  22  to be slid out from between flanges  88  and  92 , rotated on mounting pins  70 , and re-inserted in reverse-flow position between flanges  88  and  92 . Alternatively, the fourth duct  94  and flange  92  could be removed from the system  10 ; the recuperator  22  detached from the other ducts of the system  10 , lifted, rotated, replaced on flange  88  in reverse-flow position; and the fourth duct  94  and flange  92  remounted in the system  10 . This latter approach, of course, would allow the use of a mounting mechanism that does not require mounting pins  70  that are pivotally attached to mounting brackets  66 . Still other approaches can be used.  
         [0029]    The recuperator  22  may have any or all of the following design features: air inlet and outlet flanges  76  and  78  that are located symmetrically or near-symmetrically with respect to the axis of rotation A; gas inlet and outlet flanges  72  and  74  that are located symmetrically or near-symmetrically about the axis A of rotation; and symmetrically-opposed flanges that have the same bolt patterns. The system  10  may have any or all of the following design features: air inlet and outlet ducts  82  and  86  that are sized similarly; gas inlet and outlet ducts  90  and  92  that are sized similarly; and symmetrically-opposed flanges that have the same bolt patterns. Each of these design features reduces the amount of work needed to disconnect and reconnect the recuperator  22  in the system  10 .  
         [0030]    Following step  104 , the flanges  72 ,  74 ,  76  and  78  of the mounting interface are reconnected (step  106 ). The air inlet flange  76  is reconnected to the flange  84  on the second duct  86 , which places the air inlet manifold  56  in fluid communication with an air inlet of the combustor  24 . The air outlet flange  78  is connected to the flange  80  on the first duct  82 , which places the air outlet manifold  58  in fluid communication with the compressor outlet. The gas inlet flange  72  is connected to the flange  92  on the fourth duct  94 , which places the bottom face  60  of the heat exchanger core  50  in fluid communication with the muffler inlet. The gas outlet flange  74  is connected to the flange  88  on the third duct  90 , which places the top face  62  of the heat exchanger core  50  in fluid communication with the turbine exhaust outlet.  
         [0031]    The power generating system  10  is operated (Step  108 ). Previously hotter sections of the recuperator  22  now become subjected to cooler temperatures, thereby extending the useful life of the recuperator  22 . Additionally, combustion products that were deposited on the gas passages  54  near the colder gas outlet (prior to reversal) are now near the hotter gas inlet (after reversal). Further operation of the turbine  14  causes the deposited products near the gas inlet to be burned off and removed. Resulting is a self-cleaning feature of the recuperator  22 .  
         [0032]    After the recuperator  22  has been operated over an additional period of time, the recuperator  22  may be rotated back to the first position. The position of the recuperator  22  can be changed at any time. For example, the recuperator position could be changed halfway through the operating life, or the recuperator position could be changed whenever the microturbine power generating system  10  is overhauled.  
         [0033]    Thus disclosed is a recuperator that can be rotated so that gas side passages are reversed. Reversing the gas side passages allows deposited combustion products to be removed and thereby improves heat transfer efficiency and exhaust gas through-flow. Reversing the gas side passages also reduces overall thermal stresses, which allows creep criteria to be relaxed (hotter sections of the core are designed by creep criteria; the creep criteria accounts for steady-state and transient temperature stresses in the core), the recuperator life to be extended, or thinner materials to be used to produce a smaller, lighter, lower cost recuperator.  
         [0034]    The present invention is not limited to the specific embodiments disclosed above. For example the heat exchanger core could have a crossflow configuration instead of a counterflow configuration. An axis of rotation might be chosen such that the gas flow direction is reversed but the airflow direction is not reversed. The recuperator could be designed such that only the top and bottom faces of the heat exchanger core are rotated (and the inlet and outlet manifolds are not rotated). Configuration, geometry and dimensions of the recuperator will depend upon the intended application.  
         [0035]    The recuperator interfaces may or may not be connected to ducts. Instead, certain recuperator interfaces may be mounted directly to flanges on the combustor, muffler and turbine. The heat exchanger core of the recuperator may be a prime surface heat exchanger core or an extended surface (i.e., plate fin) heat exchanger core.  
         [0036]    In addition, the recuperator of the present invention could be used in a power generating system that does not use a turbomachine, such as a fuel cell power generating system.  
         [0037]    Therefore, the present invention is not limited to the specific embodiments disclosed above. Instead, the present invention is construed according to the claims that follow.