Abstract:
The invention relates to a fluidized bed heat exchanger. The heat exchanger includes a plurality of tubes forming a tube bundle. Each tube follows a vertical boustrophedon path. The tubes are supported by at least two vertical supporting devices placed on both sides of the tube bundle. Every vertical supporting device includes at least two vertical supports for fixing the tubes.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority to European Application 12194042.3 filed Nov. 23, 2012, the contents of which are hereby incorporated in it entirety. 
       FIELD 
       [0002]    The invention is related to a boiler, typically a circulating fluidized bed boiler (CFB), which comprises a fluidized bed heat exchanger. Circulating fluidized bed boilers include a reactor where the solid particles are fluidized and where chemical reactions and/or combustion reactions can take place. The circulating fluidized regime enhances the mixing of particles along with potential exothermic or endothermic chemical reactions. 
         [0003]    The operating principle of a fluidized bed heat exchanger is relatively simple: hot solids are brought into a chamber, in which they are fluidized with air or re-circulated flue gas as a slowly bubbling fluidized bed. Heat exchanger elements, often in the form of tube coils following a boustrophedon path, are located inside this fluidized bed and are fed with a coolant fluid. 
         [0004]    The tube coils have to be maintained by supports. It is known to use a sliding connection with supports allowing differential movements between the tubes and the supports. The drawback of this sliding connection is that gaps and wearing associated therewith may occur. 
         [0005]    It is also known to use shims. However, this solution requires high construction skills to install all the different parts of the fluidized bed heat exchanger without gaps along with the difficulty to assess the good execution of the work afterwards. 
       SUMMARY 
       [0006]    Thus, an object of the present invention is to provide a boiler having a fluidized bed heat exchanger as to solve the above-described problems. 
         [0007]    The object mentioned above is accomplished by a boiler comprising a fluidized bed heat exchanger, the heat exchanger including a plurality of tubes forming a tube bundle, each tube following a vertical boustrophedon path, the tubes being supported by at least two vertical supporting devices, typically at least two separate vertical supporting devices, placed on both sides of the tube bundle. 
         [0008]    In the boiler of the invention, every vertical supporting device comprises at least two vertical supports for fixing the tubes, typically at least two separate vertical supports. 
         [0009]    Thus, the fixation of the tubes on the vertical supports prevents local vibrations and possible abrasions resulting therefrom. Using at least two supports by device makes it possible to split the amount of efforts. 
         [0010]    The boustrophedon path of each tube can comprise horizontal portions, two consecutive horizontal portions being linked together by a vertical portion thus forming a bend between the horizontal portions. 
         [0011]    One supporting device can be located on the left side of the tube bundle and one supporting device can be located on the right side of the tube bundle. 
         [0012]    The vertical supports can be vertical tubes. 
         [0013]    The tubes forming the vertical supports can be cooled or uncooled. 
         [0014]    For at least one supporting device, at least one horizontal portion—preferably each horizontal portion—of each tube of the heat exchanger is preferably fixed to only one vertical support of the supporting device. 
         [0015]    For each supporting device, at least one horizontal portion—preferably each horizontal portion—of each tube of the heat exchanger is preferably fixed to only one vertical support of the supporting device. 
         [0016]    For at least one supporting device, two consecutive horizontal portions of at least one tube—preferably each tube—of the heat exchanger are preferably not fixed to the same vertical support of the supporting device. 
         [0017]    For each supporting device, two consecutive horizontal portions of at least one tube—preferably each tube—of the heat exchanger are preferably not fixed to the same vertical support of the supporting device. 
         [0018]    For each supporting device, each tube of the heat exchanger is preferably fixed to only one vertical support of the supporting device and, for each supporting device, two consecutive horizontal portions of each tube of the heat exchanger are preferably not fixed to the same vertical support of the supporting device. 
         [0019]    The boiler can comprise a furnace in which combustion of solid particles is sustained and the fluidized bed heat exchanger can be placed outside the furnace. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0020]    Other features and advantages of the invention will become apparent from the following description of an embodiment of the invention given by way of a non-limiting example only, and with reference to the accompanying drawings, in which: 
           [0021]      FIG. 1  is a diagram of a boiler according to the invention with a furnace, a separator member and an external dense fluidized bed, 
           [0022]      FIGS. 2 through 4  are perspective views of a heat exchanger of the external dense fluidized bed, 
           [0023]      FIGS. 5 and 6  show fixations of the tubes of the heat exchanger, 
           [0024]      FIGS. 7 to 10  show different fixation embodiments of the tubes of the heat exchanger. 
       
    
    
     DETAILED DESCRIPTION 
       [0025]      FIG. 1  illustrates a boiler that includes a furnace  1  in which combustion of solid particles is sustained. The top of the furnace  1  is connected to a separator member  2  via an extraction duct  12  which conveys the flue gases and the recycled particles. 
         [0026]    The separator member  2 , e.g. a cyclone, delivers the gas to a flue duct  20  leading off from its top, and it delivers the particles to a recycling duct  23  which leads into an external dense fluidized bed  3  that is placed outside the furnace  1 . The recycling duct  23  is generally provided with a siphon and with lagging. The external bed  3  is provided with a first heat exchanger  3 A, and which, in this example, is in the form of a nest of tubes that zigzag in vertical planes, following a boustrophedon path, so that the long tube segments are preferably horizontal. The first heat exchanger  3 A is fed with a coolant fluid, e.g. water, via an inlet  39 . 
         [0027]    The outlet  30  of the heat exchanger  3 A can be connected to a second heat exchanger  3 B which can be located in the furnace  1 . 
         [0028]    The remainder of the description is dedicated to the support of the heat exchanger  3 A. It is to be noted that the invention is not limited to external fluidized bed heat exchangers but can also be used with other fluidized bed heat exchangers. 
         [0029]    As illustrated in  FIG. 2 , and in  FIG. 3  in a perspective view, the heat exchanger  3 A includes a plurality of tubes  7  (four tubes in this example) zigzagging in a vertical plane. The tubes  7  can be of a square or circular cross-section. The serpentine tubes  7  follow each a boustrophedon path. Boustrophedon is a kind of bi-directional text, mostly seen in ancient manuscripts and other inscriptions. Every other line of writing is flipped or reversed, with reversed letters. Rather than going left-to-right as in modern English, or right-to-left as in Arabic and Hebrew, alternate lines in boustrophedon must be read in opposite directions. The name “boustrophedon” is taken from the Greek language. Its etymology is from bous, “ox” and strephein, “to turn”, because the hand of the writer goes back and forth like an ox drawing a plough across a field and turning at the end of each row to return in the opposite direction (i.e., “as the ox ploughs”). In this way, the tubes path goes alternately from right to left and from left to right. 
         [0030]    Two or more distinct supporting devices  41 , 42  separated by an empty space realize the support of the fluidized bed heat exchangers bundle tubes. One device  41  is located on the left side of the exchanger and the other device  42  is located on the right side of the exchanger. Each device  41 , 42  comprises at least two vertical supports  411 , 412 ; 421 , 422  separated by an empty space and which are two close hanger tubes along with alternately welded interference clamps supports, which position can be adapted according to mechanical calculation results. The hanger tubes  411 , 412 ; 421 , 422  may be cooled, as is the case in the High Temperature Superheater (HTS) bundles. Alternatively, the hanger tubes  411 , 412 ; 421 , 422  may be uncooled, as is the case in the Intermediate Temperature Superheater (ITS) bundles. 
         [0031]      FIG. 4  illustrates the fixation points  6  between the tubes  7  and the vertical tubes  411 , 412 , 421 , 422 . 
         [0032]    As illustrated in  FIG. 5 , the fixations  6  of the tubes  7  being represented by circles, for each supporting device  41 , 42 , each horizontal portion of each exchanger tube  7  is preferably fixed to only one vertical tube of the supporting device  41 , 42 . This allows to decrease the bending stresses in the exchanger. Also for a decreasing of the bending stresses, as illustrated in  FIG. 6 , for each supporting device  41 , 42 , two consecutive horizontal portions of a tube  7  (i.e. an horizontal portion of the tube placed before one bend of the boustrophedon path and the horizontal portion placed after the bend) are preferably not fixed to the same vertical tube. 
         [0033]    A first fixation embodiment which is consistent with these preferred conditions is shown on  FIG. 7 . On the left side, the first supporting device  41  comprises two vertical supports  411 , 412 , i.e. one left support  411  and one right support  412 . In the same manner, on the right side, the second supporting device  42  comprises two vertical supports  421 , 422 , i.e. one left support  421  and one right support  422 . The tube bundle comprises five tubes  71  to  75 . The tubes  71 , 72 , 73 , 74 , 75  follow a boustrophedon path including four bends A,B,C,D. Naturally, the number of tubes and the number of bends are given by way of example, and a person skilled in the art can choose a different number of tubes and bends. 
         [0034]    If one considers tube  71  (i.e. a tube numbered N), this tube  71  is connected before the bend A to the right support  412  of the first supporting device  41  and after the bend A to the left support  411  of the first supporting device  41 . After the bend A, the tube  71  is then connected before the bend B to the left support  421  of the second supporting device  42  and after the bend B to the right support  422  of the second supporting device  42 . The same connection mode is then repeated: the tube  71  is connected before the bend C to the right support  412  of the first supporting device  41  and after the bend C to the left support  411  of the first supporting device  41 . The tube  71  is then connected before the bend D to the left support  421  of the second supporting device  42  and after the bend D to the right support  422  of the second supporting device  42 . 
         [0035]    The tubes  73  and  75  (i.e. tubes numbered N+ 2 ) are connected in the same way as for the tube  71  (a tube numbered N). 
         [0036]    According to this first embodiment, the tubes  72  and  74  (i.e. tubes numbered N+1) are connected in the opposite way, i.e. a connection of a tube N to the right support  412  of the first supporting device  41  is replaced for a tube N+1 by a connection to the left support  411  of the first supporting device  41  and vice versa. In the same manner, a connection of a tube N to the right support  422  of the second supporting device  42  is replaced for a tube N+1 by a connection to the left support  421  of the second supporting device  41  and vice versa. 
         [0037]    A second embodiment is shown on  FIG. 8 . According to this embodiment, all the tubes  71  to  75  are connected before the bend A to the right support  412  of the first supporting device  41 . Thus, all the tubes  71  to  75  are connected after the bend A to the left support  411  of the first supporting device  41 . In the same manner, all the tubes  71  to  75  are connected before the bend B and after the bend A to the right support  422  of the second supporting device  42 . Thus, all the tubes  71  to  75  are connected after the bend B to the left support  421  of the second supporting device  42 . 
         [0038]      FIGS. 9 and 10  show a third and a fourth embodiment, respectively. Thus, one skilled in the art is taught that various configurations can be carried out. These configurations are consistent with the two conditions illustrated in  FIGS. 5 and 6 , i.e. 1) for at least one supporting device, and preferably for both supporting devices, each horizontal portion of an exchanger tube is fixed to only one vertical tube of the supporting device, and 2) for each supporting device, and preferably for both supporting devices, two consecutive horizontal portions of an exchanger tube (i.e. an horizontal portion of the tube placed before one bend of the boustrophedon path and the horizontal portion placed after the bend) are fixed to a different vertical tube of the supporting device.