Abstract:
A mixer having two or more concentric cylindrical flow channels is placed in a supply conduit for a combustion chamber to mix airborne particulate coal prior to entering a manifold supplying four parallel branch conduits. Vanes are mounted in the mixer channels for imparting spin to the coal/air flow. The spin direction in one channel is opposite the spin direction of the adjacent channels(s). A turbulence-producing transition section is located downstream of the mixer.

Description:
FIELD OF THE INVENTION  
         [0001]    This invention relates to systems for supplying airborne particulate coal to the combustion chamber of a coal-fired boiler of the type used to generate steam for turbines in an electric utility plant, and more particularly to a mixer device for reducing or eliminating non-uniform flow rates in parallel supply conduits located between a pulverizer and a combustion chamber.  
         BACKGROUND OF THE INVENTION  
         [0002]    It is well known to feed combustion chambers for turbine generator boilers with airborne particulate coal; structures for carrying out this function are commonly found in electric utility plants throughout the United States and Canada. It is common in these systems to use a main supply conduit to receive particulate coal from a pulverizer/classifier. It is also common to divide the main supply conduit into several parallel branches which are connected to spaced points around the combustion chamber.  
           [0003]    A problem which arises in systems of the type described above is ensuring that the branch conduits exhibit at least approximately equal coal flow rates so that the fireball in the combustion chamber is stabilized as to size and location within the combustion chamber. The flow of particulate coal through parallel branch conduits of different lengths and configurations tends to be unstable and inherently non-uniform. Many devices have been created to deal with this problem; see, for example, U.S. Pat. Nos. 5,873,156, 6,055,914, 6,186,079, 6,257,415 and 6,234,090.  
         SUMMARY OF THE INVENTION  
         [0004]    The present invention is, according to one aspect, a mixer for use in a coal-fired combustion chamber supply conduit, typically the main supply conduit downstream of a pulverizer, the effect of which is to promote uniformity in the rate of flow of airborne particulate coal from the main supply conduit to the various branches of a parallel branch feed system. In general, the invention comprises a mixer comprising a plurality of substantially concentric walls, typically but not necessarily cylindrical and made of a wear-resistant material such as steel or a steel alloy, defining at least two substantially concentric annular flow channels receiving airborne particulate coal from a source such as a pulverizer/classifier. The two channels may be referred to as “inner” and “outer” channels but it is to be understood that there may be three, four or more such concentric channels in a particular embodiment. In the case of three channels, they are referred to as “inner,” “intermediate” and “outer” channels. A first plurality of circumferentially spaced vanes are located in the outer flow channel and oriented to impart a clockwise spin to the airborne particulate coal flowing therethrough. A second plurality of circumferentially spaced vanes are located in the inner flow channel to impart a counterclockwise spin to the airborne particulate coal flowing therethrough.  
           [0005]    In the above description as well as throughout this document, the terms “clockwise” and “counterclockwise” are used only in a relative sense to make it clear that the flow in one of the annular flow channels spins or rotates around the axis of the supply conduit in a direction which is opposite to the spin or rotation of flow in the adjacent annular flow channel or channels.  
           [0006]    The mixer may be fabricated as an integral part of the supply conduit or made in the form of an insert which can be removed for servicing or replacement.  
           [0007]    In the preferred embodiment hereinafter described in detail, there are three or more annular and concentric flow channels defined by cylindrical walls and consisting of at least an outer flow channel, an intermediate flow channel and an inner flow channel. The cross-sectional areas of all of the flow channels are at least approximately the same. To achieve this, the radial spacing between the walls of the outermost flow channel is less than the radial spacing between the walls of the innermost flow channel. The vanes in these channels are located in an overlapping fashion so there is no straight path for coal particulates to follow through the mixer.  
           [0008]    The mixer may optionally be combined with other, downstream turbulence-causing features as hereinafter described.  
           [0009]    Other applications of the present invention will become apparent to those skilled in the art when the following description of the best mode contemplated for practicing the invention is read in conjunction with the accompanying drawings. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0010]    The description herein makes reference to the accompanying drawings wherein like reference numerals refer to like parts throughout the several views, and wherein:  
         [0011]    [0011]FIG. 1 is a diagram of a complete system for supplying pulverized and classified airborne particulate coal to a combustion chamber through a main supply conduit and a manifold section having four branches;  
         [0012]    [0012]FIG. 2 is a cross-section through the mixer portion of the system of FIG. 1;  
         [0013]    [0013]FIG. 3 is a plan view of the mixer showing the three concentric annular channels thereof; and  
         [0014]    [0014]FIG. 4 is a cutaway of a mixer showing the opposite sense of rotation between an inner channel and an outer channel in schematic fashion. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0015]    Referring first to FIG. 1, a conventional coal pulverizer/classifier  10  is shown to have a central vertical coal inlet supply conduit  12  for feeding lump coal into the pulverizer/classifier in controlled quantities. The pulverizer/classifier  10  comprises a main outlet supply conduit  14  which, in the illustrated embodiment, is concentric with the inlet supply conduit  12  but substantially larger in diameter. Some pulverizers have side feed features in which case the conduit  12  serves as a center outflow channel with or without vanes. Alternatively, it can be blocked off. A mixer insert  16  is located in the outlet supply conduit  14  as better shown in FIGS. 2-4. The outlet supply conduit  14  merges into a frustoconical transition section  18  which acts as a manifold to supply airborne particulate coal to four parallel branch conduits  20 ,  22 ,  24 ,  26  which are arranged in the fashion disclosed in my prior patents listed above to supply the four corners of a combustion chamber  28  which is associated with a boiler for supplying steam to the turbine of an electrical power generator. The transition section  18  may be straight-sided; i.e., substantially cylindrical.  
         [0016]    Referring to FIGS. 2-4, the mixer insert  16  is mounted in the main outlet supply conduit  14  by means of flanges and other mechanical assemblies not shown in detail and comprises a pair of radially spaced cylindrical walls  29  and  30  to define a first annular channel  32  for the upbound flow of particulate airborne coal. A third cylindrical wall  34  concentric with the wall  30  and the conduit  12  is mounted in coplanar and surrounding relationship to the wall  30  to define a second annular channel  36 . A fourth cylindrical wall  38  is mounted concentric and coplanar with the walls  29 ,  30  and  34  to define a third annular channel  40  for airborne particulate coal. In the arrangement shown in FIG. 2, channel  32  is referred to as the “inner” channel, channel  36  is referred to as the “intermediate” channel, and channel  40  is referred to as the “outer” channel. Obviously, these names can be interchanged or varied according to how many concentric channels there are in a particular embodiment of the invention. To fit the mixer  16  around the conduit  12 , it may be necessary to make the mixer  16  in two mirror-image sections and bolt them together around conduit  12 . The walls  29 ,  30 ,  34  and  38  may be slightly frustoconical if desired.  
         [0017]    As better shown in FIGS. 3 and 4, vanes  42  are welded between walls  29  and  30  to impart a clockwise rotation or spin to the airborne particulate coal flowing through the upbound channel  32 . Vanes  44  are mounted such as by welding between the walls  30  and  34  to impart a counterclockwise spin to the upbound airborne particulate coal flowing through channel  36 . Vanes  46  are mounted such as by welding between the walls  34  and  38  to impart a clockwise spin to the upbound airborne particulate coal flowing through channel  40 . The vanes as shown in FIGS. 3 and 4 are all angled relative to a longitudinal axis sufficiently to overlap in plan or projected view so as to eliminate any straight-through paths for the airborne particulate coal flowing through the channels  32 ,  36  and  40 . Annular kicker plates  52  are mounted on the tops of the walls  30 ,  34  and  38  to deflect the airborne particulate coal back inwardly toward the center of the assembly. The plates  52  are optional.  
         [0018]    As best shown in FIGS. 2 and 3, radial spacing between walls  29  and  30  is greater than the radial spacing between the walls  30  and  34  and the radial spacing between walls  30  and  34  is greater than the spacing between walls  34  and  38 . The spacing is arranged in such a fashion that the cross-sectional areas of the channels  32 ,  36  and  40  are approximately the same. It will be understood that the terms “clockwise” and “counterclockwise” are used in a relative sense.  
         [0019]    As best shown in FIG. 2, dentillated steel plates  47 ,  48  and  50  are mounted in the transition section  18  to create turbulence and additional mixing in the airborne particulate coal which emerges from the mixer insert  16 . The plates  47  are mounted essentially in parallel to the flow axis. Plates  50  are mounted around the outside wall of the transition section  18 . Radial plates  48 , also of a dentillated design, are disposed on plates  53  running radially outwardly from the plates  47 . These plates may be arranged in various fashions as is more completely described in my previously issued patents as set forth above. The plates  47 ,  48 ,  50  and  53  are an optional feature of the illustrated embodiment; i.e., the mixer  16  can be used with or without the additional turbulence-causing plates in the transition section  18 .  
         [0020]    In operation, lump coal is gravity fed through the inlet supply conduit  12  to the pulverizer/classifier  10  which operates in a conventional fashion. Pulverized coal is carried upwardly in an air stream through the main supply conduit  14  into the mixer insert  16  where the opposite sense spins are imparted to the three divided concentric annular flow quantities by the vanes  42 ,  44  and  46  disposed in the channels  32 ,  36  and  40 . The spinning airborne particulate coal then encounters the transition section and the various means  47 ,  48  and  50  therein where it is turbulently intermixed before entering the four parallel branch conduits  20 ,  22 ,  24  and  26 . Those conduits supply the four corners of the combustion chamber or “firebox”  28  of the turbine boiler.  
         [0021]    While the invention has been described in connection with what is presently considered to be the most practical and preferred embodiment, 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, which scope is to be accorded the broadest interpretation so as to encompass all such modifications and equivalent structures as is permitted under the law.