Abstract:
A combined cycle power plant includes at least one gas turbine, at least one steam turbine and a heat recovery boiler in combination to produce electricity and/or process steam. The heat recovery boiler has a duct for receiving and confining gas turbine exhaust gas from the gas turbine. Heat transfer tubes for heating water and steam for use in the bottoming steam cycle (steam turbine and/or process steam) are disposed within the heat recovery boiler and have exterior surfaces in fluid communication with the gas turbine exhaust gas and interior surfaces in circulatory fluid communication with water and/or steam. A cellular material is attached to the exterior surfaces of the heat transfer tubes and operates to enhance heat transfer from the gas turbine exhaust gas to the water and/or steam.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    The subject matter disclosed herein relates to a combined cycle power plant and, more particularly, to the heat recovery boiler with enhanced heat transfer surface area for improved combined cycle performance and economics. The subject matter applies to a single shaft, combined cycle power plant, a multi-shaft, combined cycle power plant and a cogeneration power plant. In addition the subject matter may be applicable to natural circulation, forced circulation and once through heat recovery boilers configured with horizontal and/or vertical heat transfer tubes with gas turbine exhaust flow in either a horizontal and/or vertical direction. The heat recovery boiler tube arrangement may be in-line or staggered and the heat recovery boiler may be unfired or supplementary fired. 
         [0002]    Combined cycle power plants utilize at least one gas turbine, at least one steam turbine and heat recovery boilers in combination to produce electricity and/or to process steam. The power plant is arranged such that the gas turbine is thermally connected to the steam turbine and/or process steam system through a heat recovery boiler such as a Heat Recovery Steam Generator (“HRSG”). The HRSG is essentially a large duct with water/steam filled heat exchanger tube bundles interposed therein. Feed water enters an economizer and circulates through the tube bundles such that the water is converted to steam as the gas turbine exhaust gas passes through the duct and over the tube bundles. The combined cycle power plant derives its thermal efficiency from the use of the heat rejected from the gas turbine as a supply of energy for the steam bottoming cycle (steam turbine and/or process steam). The performance and economics of the HRSG is directly related to the efficiency of heat transfer between the gas turbine exhaust gas (hot side) and the water/steam in the tube bundles (cold side). HRSGs typically utilize finned type tubes (solid and serrated) to enhance the rate of heat transfer from the gas turbine exhaust gas to the water/steam in the tube bundles however, overall heat transfer is limited by the heat transfer surface area which is located within the duct of the HRSG and the need to maintain reasonable flow characteristics and pressure drop of the gas turbine exhaust gas. 
         [0003]    It is therefore desired to provide a combined cycle power plant that is configured to have increased performance and economics through the improvement of heat transfer in the HRSG. 
       BRIEF DESCRIPTION OF THE INVENTION 
       [0004]    A combined cycle power plant includes a gas turbine and a steam turbine that are thermally connected by a heat recovery steam generator. The heat recovery steam generator includes a duct for receiving and confining gas turbine exhaust gas from the gas turbine. Heat transfer tubes are disposed within the duct and have exterior surfaces in fluid communication with the gas turbine exhaust gas and interior surfaces in circulatory fluid communication with water and steam. A cellular material is attached to the exterior surfaces of the heat transfer tubes and operates to enhance heat transfer from the gas turbine exhaust gas to the water and/or steam. 
         [0005]    A method of configuring a combined cycle power plant heat recovery steam generator may comprise the steps of configuring a duct to receive and confine gas turbine exhaust gas. Disposing a series of heat transfer tubes in the duct that are configured to receive water and/or steam and applying a cellular material to the exterior surfaces of the heat transfer tubes. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0006]    The invention, in accordance with preferred and exemplary embodiments, together with further advantages thereof, is more particularly described in the following detailed description taken in conjunction with the accompanying drawings in which: 
           [0007]      FIG. 1  is a schematic view of a single shaft, combined cycle power plant embodying the present invention; 
           [0008]      FIG. 2  is an illustration of a portion of heat transfer tubes of the combined cycle power plant HRSG of  FIG. 1 ; 
           [0009]      FIG. 3  is an enlarged view of one of the heat transfer tubes illustrated in  FIG. 2 ; 
           [0010]      FIG. 4  is an enlarged view of a metal foam segments illustrated in  FIG. 2 ; 
           [0011]      FIG. 5  illustrates the porosity versus back pressure of the metal foam segments of  FIG. 4 ; and 
           [0012]      FIG. 6  is a sectional view taken along line  6 - 6  in  FIG. 3  that illustrates the flow performance of the metal foam segments. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0013]    Referring now to the drawings, in which like numerals indicate like elements throughout the views,  FIG. 1  shows an exemplary embodiment of a single shaft, combined cycle power plant  10 . The combined cycle power plant includes a gas turbine system  12 , a steam turbine system  14  and a generator  16  that is driven by the gas and steam turbines to generate electricity. The gas turbine system includes, in serial flow communication, a multistage axial compressor  18 , a combustor  20 , and a multi-stage turbine  22 . The gas turbine system  12  and the steam turbine system  14  are thermally connected through HRSG  24 . The HRSG  24  is a heat exchanger configured to include a duct  28  for receipt and confinement of gas turbine exhaust gas  30  exiting the multi-stage turbine  22 . Bundles of heat transfer tubes  32  are disposed within the duct  28  of the HRSG  24  and are adapted to receive feed water  34  (condensate from steam turbine condenser  15 ) that is circulated through the tubes by the feed water pump  36 . As the feed water  34  passes through the heat transfer tubes  32 , heat from the gas turbine exhaust gas  30  passing through the HRSG duct  28  is transferred to the water, creating steam. The steam is supplied to, and drives, the steam turbine  14 , through the main steam piping  38 . In addition, the steam from the HRSG may also be supplied to a process steam system (not shown). 
         [0014]    Within the duct  28  of HRSG  24  the exterior of the heat transfer tubes  32  represent the “hot side” surface area while the interior, or wet side, of the heat transfer tubes represent the “cold side” surface area. Enhanced transfer of heat between the hot and cold side surface areas of the HRSG will operate to increase the performance and economics of the combined cycle power plant  10 . Referring to  FIG. 2 , a portion of the heat transfer tubes  32  is shown. In an exemplary embodiment of the invention a cellular material, such as metal foam extended surface  40 , surrounds the heat transfer tubes  32 . The metal foam extended surface is comprised of a relatively new class of materials having low density, high surface area and good strength. The metal foam extended surface may be formed from various powder metals and, with varying porosity. 
         [0015]    Controlling the material properties and configuration of the metal foam extended surface  40  is an important aspect of the invention in that the pressure drop of the gas turbine exhaust gas  30  passing through the HRSG duct  28  is a consideration in the design of the HRSG. High pressure drops across the bundles of heat transfer tubes  32  will reduce the power output of the gas turbine system  12 . As indicated in the plot shown, studies show that metal foam with appropriate porosity and density, which relates directly to pressure drop, can achieve lower pressure drop. Additionally, due to the unique structural characteristics of the metal foam, it has sound and vibration absorptive characteristics that are useful in relieving acoustical and vibration issues in the HRSG  24  and may allow for reduction or elimination of costly silencers and acoustic baffles used to modify the acoustic characteristics of the HRSG. 
         [0016]    Referring to  FIGS. 2 ,  3  and  4 , in an exemplary embodiment the metal foam extended surface  40  is defined by a series of metal foam segments  42  having a relatively disc-shaped configuration with an axially extending opening  45  and interior and exterior surfaces  44  and  46 , respectively. The axially extending openings have a diameter “d” configured to receive a heat transfer tube  32  therein. The diameter “d” closely matches the outside diameter of the heat transfer tubes  32  such that when a metal foam segment  42  is installed onto a tube, surface area contact between the interior surface  44  of the segment and the outer surface  52  of the heat transfer tube  32  is high. Once mounted on the heat transfer tubes, the metal foam segments  42  are attached using appropriate metallurgical joining techniques such as brazing, welding, diffusion bonding or other suitable methods. 
         [0017]    The cylindrical, metal foam segments  42  of the metal foam extended surface  40  operates to increase the rate of heat transfer from the gas turbine exhaust gas passing through the HRSG duct  28  to the water/steam  35  circulating in the heat transfer tubes  32 . At the same time, exhaust gas backpressure experienced by the gas turbine can be maintained at acceptable levels. For example, each metal foam segment  42  has an axial height “h” and is preferably spaced from adjacent segments by an axial distance “g”. Backpressure within the HRSG duct  28  has been found to fall within a desirable range when the ratio of the metal foam segment height “h” to the segment spacing “g” is within a range of greater than 0 to about 50 (i.e. 0&lt;h/g&lt;50). In addition, heat transfer reaches satisfactory levels when the ratio of the metal foam segment outer diameter “D” to the segment inner diameter “d” is within a range of about 1.2 to about 10 (i.e. 1.2&lt;D/d&lt;10). 
         [0018]    As indicated, due to the high surface area of metal foam, the overall heat transfer from the gas turbine exhaust gas  30  to the water/steam  35  is enhanced significantly. Additionally, due to the non-smooth surface characteristics of the metal foam segments  42 ,  FIG. 6 , the gas turbine exhaust gas flowing through the HRSG duct  28  will produce a smaller wake area  54 , as compared with a conventionally finned tube, due to delayed flow separation. The smaller wake area  54  is a result of a turbulent boundary layer directly adjacent the exterior surface  46  and it operates to reduce backpressure through the HRSG  24 . It has been determined that the benefit of delayed flow separation can be achieved when the heat transfer tubes  32  having metal foam extended surfaces are installed in the duct  28  of HRSG  24  such that they have a ratio of frontal spacing “T” (transverse pitch) to metal foam segment outside diameter “D” within the range of about 1.05 to about 2 (i.e. 1.05&lt;T/D&lt;2), and a ratio of axial spacing “L” (longitudinal pitch) to metal foam segment outside diameter “D” within the range of about 0.2 to about 1.5 (i.e. 0.2&lt;L/D&lt;1.5). 
         [0019]    While the described metal foam segments  42  have been generally described as cylindrical in configuration, the present invention contemplates segments in a variety of shapes and sizes. It is contemplated that the configuration and spacing of the metal foam segments  42  will be selected to efficiently transfer heat to the heat transfer tubes  32  while lowering the gas turbine exhaust backpressure within the duct  28  of the HRSG  24 . In addition, while the combined cycle power plant  10  has been described having a single shaft configuration with a single gas turbine, steam turbine and generator, any number of application driven variations are also contemplated. For example it is contemplated that the invention has similar applicability to multiple shaft, combined cycle power plants and cogeneration power plants. The invention may be used in natural circulation, forced circulation and once through HRSGs configured with horizontal and/or vertical heat transfer tubes with gas turbine exhaust gas flow in horizontal and/or vertical directions. The HRSG tube arrangement may be either in-line or staggered and the HRSG may be either unfired or supplementary fired. 
         [0020]    The written description uses examples to disclose the invention, including the best mode, and also to enable any person skilled in the art to practice the invention, including making and using any devices or systems and performing any incorporated methods. The patentable scope of the invention is defined by the claims, and may include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims if they have structural elements that do not differ from the literal language of the claims, or if they include equivalent structural elements with insubstantial differences from the literal language of the claims.