Abstract:
A silencer for a gas turbine includes a first duct portion; a second duct portion connected to the first duct portion, the first and second duct portions forming an elbow region at the connection; and a plurality of elbow shaped vanes provided in the elbow region. The plurality of elbow shaped vanes have equal lengths and are spaced equally apart.

Description:
The invention relates generally to noise suppression techniques, and more particularly to systems and methods that provide improved sound attenuaton properties. More particularly, the invention relates to silencers for ducts, including inlet and exhaust ducts, of a gas turbine. 
     BACKGROUND OF THE INVENTION 
     The environmental noise caused by airplanes, automobiles, and other modern machinery can often be an annoyance. To maintain noise below acceptable levels, noise suppression techniques are often employed. Noise suppression has, therefore, become a technology with a wide variety of industrial and residential applications. Noise suppression devices are often applied in heating, ventilation and air conditioning (HVAC) systems, industrial machinery and complexes, transportation vehicles, and any machinery that may tend to produce unacceptably high levels of noise. 
     In gas turbines, parallel baffle silencers are mounted in the inlet and exhaust ducts to achieve required acoustic performance. A certain duct length for inlet and exhaust is required to accommodate these silencers. The overall pressure loss in this arrangement from the face of silencer to an end of an elbow is the sum of frictional, entry and exit losses of the silencer and turning losses in the elbow. Two stages of silencers may be used in the elbow. A first stage may be used to attenuate low and mid range frequencies. The second stage may be provided to attenuate high frequencies. Alternatively, only one stage may be used in the exhaust duct to attenuate some frequencies. The use of parallel baffle silencers results in longer inlet and exhaust ducts. The overall pressure in the inlet and exhaust ducts tends to be high. 
     BRIEF DESCRIPTION OF THE INVENTION 
     According to one embodiment of the invention, a silencer for a gas turbine comprises a first duct portion; a second duct portion connected to the first duct portion, the first and second duct portions forming an elbow region at the connection; and a plurality of elbow shaped vanes provided in the elbow region, wherein the plurality of elbow shaped vanes have equal lengths and are spaced equally apart. 
     According to another embodiment of the invention, a silencer for a gas turbine comprises a first duct portion; a second duct portion connected to the first duct portion, the first and second duct portions forming an elbow region at the connection; and a plurality of parallel L-shaped shaped baffles provided in the elbow region, wherein the plurality of L-shaped baffles have equal lengths and are spaced equally apart. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  schematically depicts a plan view of an exhaust duct for a gas turbine according to the prior art; 
         FIG. 2  schematically depicts a side view of the exhaust duct of  FIG. 1 ; 
         FIG. 3  schematically depicts a perspective view of an exhaust duct according to an embodiment of the invention; 
         FIG. 4  schematically depicts a side view of the exhaust duct of  FIG. 3 ; 
         FIG. 5  schematically depicts a plan view of the exhaust duct of  FIG. 3 ; 
         FIG. 6  schematically depicts a perspective view of an exhaust duct according to another embodiment of the invention; 
         FIG. 7  schematically depicts a plan view of an exhaust duct according to the prior art; 
         FIG. 8  schematically depicts a side view of an exhaust duct according to another embodiment of the invention; 
         FIG. 9  schematically depicts a plan view of an exhaust duct according to another embodiment of the invention; 
         FIG. 10  schematically depicts a side view of the exhaust duct of  FIG. 9 ; 
         FIG. 11  schematically depicts silencer baffles in a duct according to the prior art; 
         FIG. 12  schematically depicts a perspective view of a duct elbow including silencer vanes according to another embodiment of the invention; 
         FIG. 13  schematically depicts a perspective view of a duct elbow including silencer vanes according to another embodiment of the invention; and 
         FIG. 14  schematically depicts a perspective view of a duct elbow including L-type silencer baffles according to another embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to  FIG. 1 , an exhaust duct  2  according to the prior art includes an exhaust duct inlet  4  and an exhaust chimney  10 . A transition  6  is provided between the exhaust duct inlet  4  and the exhaust chimney  10 . Parallel silencer baffles  8  are provided after the transition  6  and prior to the exhaust chimney  10 . 
     As shown in  FIG. 2 , the exhaust duct  2  has the general shape of an elbow. The exhaust duct  2  includes a generally horizontal portion and a generally vertical portion. The generally horizontal portion includes the inlet  4 , the transition  6 , and the parallel silencer baffles  8 . The vertical portion comprises the exhaust chimney  10 . 
     The overall pressure loss in the exhaust duct  2  of the prior art from the exhaust duct inlet  4  to the outlet of the exhaust chimney  10  is the sum of frictional, entry and exit losses of the parallel silencer baffles  8  and turning losses in the elbow. 
     Referring to  FIGS. 3-5 , an exhaust duct  12  in accordance with an embodiment of the invention includes an exhaust duct inlet  14  followed by an inlet transition  16 . A plurality of silencer guide vanes  18  are provided in the elbow of the exhaust duct  12 . The silencer vanes  18  are followed by an outlet transition  20  and an exhaust chimney  22 . The exhaust duct  12  includes side walls  24 . The side walls  24  may have slots  19  through which the silencer guide vanes  18  are inserted into the exhaust duct  12 . 
     Referring to  FIG. 4 , the exhaust duct  12  according to this embodiment of the invention has a reduced duct surface area and duct length (i.e. footprint) from the point A at the end of the end of the inlet transition  16  to the point B at the end of the outlet transition  20  than the duct surface area and the duct length from the point A to the point B of the exhaust duct  2  shown according to the prior art in  FIG. 2 . 
     As shown in  FIGS. 3-5 , the exhaust duct  12  includes fewer silencer guide vanes  18  in contrast to the number of parallel silencer baffles  8  of the prior art exhaust duct  2  of  FIG. 1 . For example, the exhaust duct  12  according to this embodiment may have six silencer guide vanes  18  whereas the prior art exhaust duct  2  may have seven parallel silencer baffles  8 . The silencer guides vanes  18  of  FIGS. 3-5  provide an open area and a blockage which is equal to the open area and blockage of the exhaust duct  2  of the prior art shown in  FIGS. 1 and 2 . However, the exhaust duct  12  of the embodiment of  FIGS. 3-5  provides the open area and blockage with a duct surface area and duct length that is less than the prior art, as discussed above. 
     As shown in detail in  FIG. 3 , each silencer vane  18  comprises sheet material  25 ,  29  that encloses sound attenuating material  31 . The sheet material  25 ,  29  may be, for example, sheet metal. The sound attenuating material  31  may be, for example, fiberglass or foam. The surface of the sheet  25  in contact with the flow in the duct  12  may comprise perforations  27 . Although two sheets are shown, it should be appreciated that each silencer vane may be formed of a single sheet. 
     Referring to  FIG. 4 , each silencer vane  18  is generally elbow or L-shaped. The silencer vanes  18  each have equal lengths, i.e. the distance from the front edge of the vane in the horizontal duct to the back edge of the vane in the vertical duct. The silencer vanes  18  are spaced so as to provide an equal flow gap between the silencer vanes  18 . The silencer vanes  18  may also be placed about a line of symmetry  23  such that each silencer vane  18  has equal extension in the horizontal and vertical ducts. It should be appreciated however that the silencer vanes  18  may be configured to have unequal extension into the horizontal and vertical ducts. 
     The provision of equal length and equally spaced silencer vanes provides uniform attenuation of sound compared to vanes of unequal lengths. The equal length and equally spaced silencer vanes also reduce turning losses, i.e. pressure drops, in the elbow compared to vanes of unequal length. Providing equal length and equally spaced silencer vanes reduces the amount of sound attenuating material compared to vanes of unequal length. 
     Referring to  FIG. 6 , according to another embodiment of the invention, an exhaust duct  12  may comprise an inlet  14  and an exhaust chimney  22  that are configured as circular ducts. 
     Referring to  FIG. 7 , an exhaust duct  26  according to the prior art comprises an exhaust duct inlet  28  and an inlet transition  30  following the inlet  28 . A plurality of first parallel silencer baffles  32  are provided after the inlet transition to dampen or silence, low frequencies in the exhaust duct. A plurality of second parallel silencer baffles  34  are provided after the first silencer baffles  32  to dampen or silence high frequencies in the exhaust duct  26 . The second silencer baffles  34  are followed by an exhaust chimney  36 . 
     Referring to  FIG. 8 , an exhaust duct  38  according to another embodiment of the invention includes an exhaust duct inlet  40  followed by an inlet transition  42 . As shown in  FIGS. 7 and 8 , the inlet transition  42  according to this embodiment of the invention shown in  FIG. 8  may have the same length as the inlet transition  30  of the exhaust duct  26  of the prior art shown in  FIG. 7 . The inlet transition  42  according to this embodiment of the invention may be followed by a horizontal stage  44  which is followed by the plurality of silencer guide vanes  46 . The silencer guide vanes  46  are followed by an outlet transition  48  which is followed by an exhaust chimney  50 . As shown in  FIG. 8 , the exhaust duct  38  has a shorter overall duct length compared to the prior art duct  26  shown in  FIG. 7   
     The horizontal stage  44  may be provided to attenuate high frequency noise. The horizontal stage  44  may be provided in the horizontal duct with the vane type silencers  46  in the elbow in situations in which very stringent requirements on sound levels are required. The horizontal stage provides the benefit of relocating the first stage from the horizontal duct to the elbow, thereby providing reduction in pressure loss (gain in power output) and duct cost savings. 
     Referring to  FIGS. 9 and 10 , an exhaust duct  52  according to another embodiment of the invention includes an exhaust duct inlet  54  followed by an inlet transition  56 . A plurality of parallel elbow or L-shaped silencer baffles  58  follow the inlet transition  56 . An outlet transition  60  is provided after the silencer baffles  58  and followed by an exhaust chimney  62 . As shown in  FIG. 9 , the parallel elbow or L-shaped silencer baffles  58  extend in the horizontal duct of the elbow to a position below and beyond the exhaust chimney  62 . The configuration of parallel elbow or L-shaped silencer baffles  58  shown in  FIGS. 9 and 10  maintains the same gap-velocity across the silencer as the embodiment shown in  FIGS. 3-5 . As shown in  FIGS. 9 and 10 , the exhaust duct  52  according to this embodiment also provides a reduction in duct surface area and the duct length from the points A to B than in the prior art arrangement shown in  FIGS. 1 and 2 . 
     Referring to  FIG. 11 , an inlet or outlet duct  64  according to the prior art includes a plurality of parallel silencer baffles  66 . As shown in  FIG. 12 , an inlet duct  70  according to an embodiment of the invention includes an inlet duct elbow  68  having a plurality of silencer guide vanes  72  provided in the elbow  68 . The silencer guide vanes  72  may have the same length as the parallel silencer baffles  66  of the prior art duct  64  shown in  FIG. 11 . By providing the silencer guide vanes  72  in the elbow  68 , the duct length may be reduced. In addition, the silencer guide vanes  72  provide guided flow in the inlet duct elbow  68  and reduce the pressure loss due to the combination of silencer and elbow losses. The silencer vanes  72  also block the direct line of sight of sound waves and provide higher sound attenuation and insertion loss. 
     Referring to  FIG. 13 , an exhaust duct elbow  74  according to an embodiment of the invention comprises an exhaust duct  76  and a plurality of silencer guide vanes  78  provided in an elbow of the exhaust duct. 
     The inlet and exhaust ducts  70 ,  74  of  FIGS. 12 and 13  may be used in all up and forward inlet and exhaust ducts in the gas turbine cycle. 
     Referring to  FIG. 14 , a duct  82  includes a duct elbow  80  having a plurality of L-type inverted silencer baffles  84 . The baffles  84  have equal lengths and may extend an equal distance into the first, horizontal portion of the duct and the second, vertical portion of the duct. 
     The embodiments described herein provide a reduction in the pressure drop and provide improved acoustic performance. The embodiments described herein also provide cost savings in steel liner material of the ducts and savings on insulation of the ducts. Furthermore, the embodiments described herein reduce associated support structure, such as bolts, spacebars, and stiffeners. 
     The silencer vane configurations disclosed herein provide reduction in the length of inlet and exhaust ducts and associated support structure. The silencer configurations as disclosed herein reduce pressure losses in the inlet and exhaust ducts, thereby increasing plant output. Only one stage of silencers in the duct is sufficient to provide required acoustic performance for a wide range of frequencies. 
     While the invention has been described in connection with what is presently considered to be the most practical and preferred embodiments, it is to be understood that the invention is not to be limited to the disclosed embodiments, but on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims.