Abstract:
A compensations duct assembly [ 100]  is described that provides a gas tight connection between a backpass [ 9]  of a furnace [ 3]  and an air preheater pipe [ 10]  having one end fixed relative to a floor. The compensation duct assembly [ 100]  has an air preheater opening [ 133]  that is connected to an air preheater duct [ 10].  A liquid seal [ 120]  is employed to compensate for the movement due to thermal expansion between the backpass [ 9]  and the compensation duct assembly [ 100].  The liquid seal [ 120]  provides a gas tight connection under various conditions of thermal expansion.

Description:
BACKGROUND 
       [0001]    The present disclosure generally relates to a furnace flue gas connection that adjusts to movement caused by expansion. 
         [0002]    Various furnaces are used in industry to create power, incinerate waste or perform chemical processes. The flue gases are conveyed by interconnected flue gas ducts. These must maintain a gas-tight seal to prevent leakage of flue gases. 
         [0003]      FIG. 1  shows flue gas ducts  1  from a conventional furnace  3  (partially shown). The flue gas ducts  1  include a top pass  7  starting above a nose  5  of the furnace  3  connecting to a backpass  9 . 
         [0004]    The backpass  9  extends downward to an economizer gas outlet duct  11 . A backpass outlet manifold  12  attached to the economizer gas outlet duct  11 . The backpass outlet manifold  12  includes an ash hopper  45  adapted to collect fly ash carried by the flue gases. 
         [0005]    Support structures  37  extend over the furnace and flue gas ducts  1 . The furnace  3  and flue gas ducts  1  typically are hung from the support structures  37  with hanger rods  39 . The furnace  3  is typically fixed at its top left. When the backpass  9  and the flue gas ducts  1  expand, they move downward as shown by arrow “A” and to the right, as indicated by arrow “B”. 
         [0006]    An air preheater duct  10  extends vertically from the air preheater (not shown) that is secured to a floor. When air preheater duct  10  heats up, it expands upward in the direction shown by arrow “C”. An elbow duct  19  is attached to the air preheater duct  10 , which also moves upward as air preheater duct  10  expands. The motion due to thermal expansion causes the backpass outlet manifold  12  to become misaligned with the elbow duct  19 . 
         [0007]    Therefore a toggle section  13  is typically employed to adjust for the misalignment and movement due to thermal expansion. The toggle section  13  is designed to make a gas tight connection between the backpass outlet manifold  12  and the elbow duct  19 . Since the expansions cause motion in the three directions indicated by arrows “A”, “B” and “C”, there should be some flexibility built into the toggle section  13 . 
         [0008]    Toggle section  13  includes expansion joints  15  on either side of a toggle duct  17 . These are designed to adjust to the various movements of the ducts. 
         [0009]    With the increase in size and power of furnaces and boilers, there is increased expansion requiring the toggle section  13  employed to become larger and heavier. These larger toggle sections  13  require additional support structures and make the system more expensive. 
         [0010]    Reheater vertical tubes  41  support the reheaters  29  and economizer vertical tubes  43  support the economizers  33  in the backpass  9 . 
         [0011]    The fly ash and hopper  45  add considerable weight that must supported by the vertical tubes  41 ,  43 , support structures  37  and hanger rods  39 . 
         [0012]    Since the toggle sections are getting larger with larger boilers, they require extra room, which results in a larger ‘footprint’. This larger footprint also incurs increased costs. 
         [0013]    Currently, there need for a device to provide a lighter, smaller, simpler, and less expensive gas-tight connection to connect misaligned flue gas ducts. 
       BRIEF SUMMARY 
       [0014]    An embodiment of the present invention may be described as a compensation duct assembly [ 100 ] adapted to convey combustion flue gases from a backpass outlet [ 111 ] at a lower end of a backpass wall [ 129 ] to an air preheater duct [ 10 ] while compensating for thermal expansion. The device includes a connection duct [ 110 ] that is a flue gas conduit with a top side [ 151 ], a bottom side [ 153 ], a backpass end [ 155 ], an air preheater end [ 157 ], a connection duct upper opening [ 115 ] on a top side [ 151 ] of its backpass end [ 155 ], sized and shaped substantially the same as the backpass outlet [ 111 ] with an air preheater opening [ 133 ] at its air preheater end [ 157 ] on its bottom side [ 153 ] connected to the air preheater duct [ 10 ]. The compensation duct assembly [ 100 ] also employs a liquid seal [ 120 ] between the connection duct upper opening [ 115 ] of the connection duct [ 110 ] and the lower end of the backpass wall [ 129 ] acting to create a gas-tight seal between them. 
         [0015]    The present invention may also be described as a method for creating a gas tight seal during thermal expansion between a backpass [ 9 ] of a furnace [ 3 ] and a relatively fixed duct  10 , comprising the steps of: 
         [0016]    providing a connection duct [ 110 ] being a flue gas conduit having an air preheater end, a backpass end a top side and a bottom side; 
         [0017]    attaching the air preheater end of the connection duct [ 110 ] to said air preheater duct [ 10 ]; 
         [0018]    providing a liquid seal [ 120 ] at the topside of the backpass end of a connection duct [ 110 ]; and 
         [0019]    inserting a lower portion of the backpass [ 9 ] into the liquid seal [ 120 ] to result in a gas seal that compensates for movement due to thermal expansion. 
         [0020]    The disclosure may be understood more readily by reference to the following detailed description of the various features of the disclosure and the examples included therein. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0021]    Referring now to the figures wherein the like elements are numbered alike: 
           [0022]      FIG. 1  is a side elevational view of conventional steam generator with backpass; 
           [0023]      FIG. 2  is a side elevational view of a steam generator retrofitted with a flue gas connection according to one embodiment of the present invention. 
           [0024]      FIG. 3  is an enlarged side elevational view of a portion of the water seal according to an embodiment of the present invention. 
           [0025]      FIG. 4  is a plan view of the connection duct  110 , from the top. 
           [0026]      FIG. 5  is a plan view of the connection duct  110 , from the bottom. 
           [0027]      FIG. 6  is a plan view of the backpass, viewed from below. 
       
    
    
     DETAILED DESCRIPTION 
       [0028]    Referring now to  FIG. 1 , hot flue gases from the furnace  3  pass over super heaters  21  located in the top of the furnace and in a top pass  7 . They receive steam and superheat the steam. The superheated steam is collected at the superheat headers  23 . Similarly, steam enters the reheaters  25  in the top pass  7 . There are also reheaters  29  and reheater headers  31  in a backpass  9 . The backpass is lined with water tubes, but for clarity, are not shown here. 
         [0029]    The lower backpass  9  has economizers  33  that receive feedwater, heat the feedwater. The heated feedwater is collected at a backpass lower ring header  35  and provide the heated feedwater to waterwalls of the furnace  3 . 
         [0030]    Typically, the top left of a furnace  3  is fixed to a stationary structure. When operating, the furnace  3  and flue gas ducts  1  expand to the right as indicated by arrow “B” and downward as indicated by arrow “C”. 
         [0031]    An elbow duct  19  is connected to the air preheater duct  10  that is connected to the air preheater that is fixed to the ground. Therefore, it can only expand upward, as shown by arrow “C”. 
         [0032]    The expansion, measured at the backpass lower ring header  35  can be typically 30.5 cm. (12 inches) down, 12.7 cm. (5 inches) rearward, and 6.4 cm. (2½ inches) out to the sides. 
         [0033]    The toggle section  13  must be able to adjust for the relative movement between these ducts. Expansion joints  15  are on each side of toggle duct  17 . These allow for the required adjustments. 
         [0034]    The toggle section  13 , as well as the ash hopper (with ash), economizer gas outlet duct  11 , backpass  9 , top pass  7  and furnace  3  are all suspended from hangar rods  39  which hang down from the support structure  37 . 
         [0035]    Therefore, the cost of the support structure  37  increases with the amount of weight it must support. 
         [0036]    It would be useful to compensate for flue gas duct expansion at the interface with the air preheater duct  10  which does not necessitate the use of a toggle section  13 . 
         [0037]    The toggle section  13  tends to leak once the expansion becomes larger. 
         [0038]    Referring now to both  FIGS. 1 and 2 , the present invention eliminates the backpass outlet manifold  12 , the toggle section  13  and the elbow duct  19  by replacing them with a compensation duct assembly  100 . The compensation duct assembly  100  is secured to the air preheater duct  10 . 
         [0039]    There is no longer a toggle section  13 , so the compensation duct assembly  100  does not float as did the backpass outlet manifold  12 . Now all of the expansion motions are centralized at the interface of the bottom of the backpass  9  and the compensation duct assembly  100 . 
         [0040]    Compensation duct assembly  100  employs a liquid seal  120  which allows for horizontal motions in two dimensions as well as vertical motion, while maintaining a gas-tight seal. 
         [0041]      FIG. 3  shows an enlarged view of a portion of the liquid seal  120 . 
         [0042]    The liquid seal  100  located below the backpass lower ring header  35 . The liquid seal  120  can compensate for the expansion differential in all three directions. Here the backpass wall  129  on one side is shown. The backpass wall  129  is immersed in a trough  121  filled with a liquid  131 , preferably water. The trough  121  has an inner wall  123 , and outer wall  127  and a bottom  125 . (It is assumed that the trough continues around the periphery defined by the backpass wall  129 .) As shown by the arrows, the backpass wall  129  may move relative to the trough  121 . However, if the trough is sufficiently long, wide and deep enough, there will be a gas-tight seal between the backpass  9  and the compensation duct assembly  100 , even during expansion. 
         [0043]      FIG. 4  is a plan view of the connection duct  110 , from the top. Here the connection duct upper opening  115  is visible on the top side  151  at the backpass end  155 . This is intended to connect, through the liquid seal, to the backpass ( 9  of  FIG. 1 ). 
         [0044]      FIG. 5  is a plan view of the connection duct  110 , from the bottom. This shows the bottom side  153  and the air preheater opening  133 . 
         [0045]      FIG. 6  is a plan view of a lower section of the backpass ( 9  of  FIG. 1 ), viewed from below. Here the backpass wall  129  and the back outlet  111  are visible. 
         [0046]    An advantage of using a liquid seal  120  is that all of the weight of the compensation duct assembly  100 , including the ash loading in the ash hopper  145  goes directly into the local duct supporting steel, rather than being transferred into the backpass pressure parts of the reheater  29 , the economizer  33 , especially the backpass lower ring header ( 35 ) and the backpass wall tubes (not numbered) and up to the support structure  37 , as it is done in the prior art design. This would require heavier and thicker pipes, adding cost. 
         [0047]    Another advantage of the present invention is that it would reduce the need for the toggle section  13  reducing the ‘footprint’ size, requiring less real estate resulting in a cost saving. 
         [0048]    Another advantage is that all movement compensation is performed in the liquid seal  120 . The other connections are now standard connections. Therefore, there is less probability of leaks and other irregularities. 
         [0049]    Unless otherwise specified, all ranges disclosed herein are inclusive and combinable at the end points and all intermediate points therein. The terms “first,” “second,” and the like, herein do not denote any order, quantity, or importance, but rather are used to distinguish one element from another. The terms “a” and “an” herein do not denote a limitation of quantity, but rather denote the presence of at least one of the referenced item. All numerals modified by “about” are inclusive of the precise numeric value unless otherwise specified. 
         [0050]    This written description uses examples to disclose the invention, including the best mode, and also to enable any person skilled in the art to make and use the invention. 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 languages of the claims.