Abstract:
A heat exchange unit for a heat exchanger assembly is provided for use in a cooling system of a CT imaging system having a gantry frame with an annular interior region. The heat exchange unit is formed of a plurality of tubes bent at discrete locations to divide the tubes into straight leg sections to fit within an arcuate portion of the gantry frame&#39;s annular region. The bent tubes are maintained between a pair of tube sheets in an array with the straight leg sections parallel and the bends generally radially aligned. The tubes are orthogonally joined to the tube sheets, and the faying surfaces are flat and parallel, leading to a dependable joint. The heat exchanger assembly includes mounting brackets with mounting fixtures axially aligned with the gantry frame, to reduce stress on the fasteners and the mounting brackets.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    Computed tomography (CT) imaging systems are used to provide three-dimensional or 360° views of a patient. These systems typically have an annular gantry frame that spins about a patient or object to be imaged. An X-ray tube is mounted to the gantry frame. A cooling system is also provided within the gantry frame to dissipate heat generated by the X-ray tube. 
       SUMMARY OF THE INVENTION 
       [0002]    A heat exchange unit in a heat exchanger assembly is provided for use in a cooling system of a CT imaging system having a gantry frame with an annular interior region. The heat exchange unit is formed of a plurality of tubes bent at discrete locations to provide several straight leg sections so that the heat exchange unit can fit within an arcuate portion of the annular interior region of the gantry frame. 
         [0003]    In one embodiment, a heat exchange unit includes a plurality of tubes having open ends and flat exterior upper and lower surfaces. Flow passages for a first heat exchange fluid extend through the interior of the tubes. The tubes are attached to a pair of tube sheets, which maintain the tubes in an array spaced apart a distance to allow fluid flow of a second heat exchange fluid between the flat exterior surfaces of the tubes. 
         [0004]    Each of the tubes has at least one bend formed at discrete locations to divide each of the tubes into a plurality of straight leg sections. The tube sheets maintain the tubes in an array with each of the straight leg sections of adjacent tubes generally parallel and the bends generally radially aligned with respect to the gantry frame. In one embodiment, the bends in the tubes are located symmetrically about a centerline through an arc tangent to the tubes at the centerline and where the tubes join the tube sheets. 
         [0005]    The tubes are joined to the tube sheets at end portions extending perpendicular to the faces of the tube sheets. The faying surfaces of the tubes and the slots of the tube sheets are flat and parallel, leading to a dependable joint formed by, for example, brazing or welding. 
         [0006]    In another aspect, the heat exchange unit is part of a heat exchanger assembly including mounting brackets for mounting to the gantry frame. The mounting brackets are attached to opposite sides of the heat exchange unit adjacent the tube sheets and include mounting fixtures that extend in the axial direction of the gantry frame. This reduces the stress on the mounting fixtures and the mounting brackets, compared to prior art heat exchanger assemblies that use radially oriented mounting fixtures. 
     
    
     
       DESCRIPTION OF THE DRAWINGS 
         [0007]    The invention will be more fully understood from the following detailed description taken in conjunction with the accompanying drawings in which: 
           [0008]      FIG. 1  is an isometric view of an embodiment of a heat exchange unit according to the present invention; 
           [0009]      FIG. 2  is an isometric view of the heat exchanger assembly of  FIG. 1  installed in an arcuate portion of a gantry frame; 
           [0010]      FIG. 3A  is a front view of the heat exchange unit of  FIG. 1 ; 
           [0011]      FIG. 3B  is an enlarged view of a portion of  FIG. 3A ; 
           [0012]      FIG. 3C  is a side view of a tube sheet employed in the heat exchange unit of  FIG. 3A ; 
           [0013]      FIG. 4  is a front view of the heat exchange unit of  FIG. 1  illustrating various geometric relationships; 
           [0014]      FIG. 5  is a front view of an embodiment of a heat exchange unit illustrating three bends; 
           [0015]      FIG. 6  is a front view of an embodiment of a heat exchange unit illustrating four bends; 
           [0016]      FIG. 7A  is an illustration of a rectangular heat exchange unit overlaid on an arcuate portion of a gantry frame; and 
           [0017]      FIG. 7B  is an illustration of the heat exchange unit of  FIG. 1  overlaid on an arcuate portion of a gantry frame for comparison with  FIG. 7A . 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0018]    One embodiment of a heat exchanger assembly that fits within an annular or arc-shaped interior region of a gantry frame of a CT system to provide cooling for an X-ray tube is shown in  FIG. 1 . The heat exchanger assembly  10  includes a heat exchange unit  12  formed from a plurality of tubes  14  arranged in parallel and spaced a distance apart. When the heat exchanger assembly is installed in a gantry frame  16 , a first heat exchange fluid flows through the interior of the tubes  14 , for heat transfer with a second heat exchange fluid that flows in the regions  18  between the tubes and past the exterior surfaces of the tubes. 
         [0019]    The tubes  14  are bent at bends  22  to form several straight leg sections  24  to accommodate the annular configuration of the gantry frame  16 . See  FIG. 2 . The tubes are typically flat tubes and may contain microchannels or baffles, as known in the art. Mounting brackets  32  (described further below) are attached at each side of the heat exchange unit  12  for attachment to the gantry frame  16 . The mounting brackets also enclose manifolds (not shown) for delivering and discharging the first heat exchange fluid to and from the interior of the tubes of the heat exchange unit. In the embodiment shown, inlet and outlet fittings  34  are connected to the heat exchange unit through the mounting brackets  32 . Alternatively, inlet and outlet fittings may be connected at opposite sides of the heat exchanger assembly through both mounting brackets if desired for a particular application. The inlet and outlet fittings connect via suitable piping to a fluid tank or reservoir and fluid pump (not shown) for the heat exchange fluid. 
         [0020]    In operation, the first heat exchange fluid flows on a circulatory flow path through appropriate piping past an X-ray tube (not shown), where the fluid is heated by heat transfer with the X-ray tube. The heated fluid then flows to the heat exchange unit  12 , where it flows through flow passages within the tubes  14  and is cooled through heat transfer with the second heat exchange fluid, typically air, flowing in the regions  18  and across the exterior surfaces of the tubes  14 . The air flows in an axial direction of the gantry frame (indicated by axis  44 , shown in  FIG. 2 ) and generally perpendicular to the elongated direction the tubes. Additional heat exchange elements, such as fins made of corrugated aluminum (not shown), may be disposed within the regions  18  to assist in the heat transfer between the fluids. A fan or other air moving device (not shown) may be provided to force the air flow through the heat exchange unit. 
         [0021]    Referring to  FIGS. 1 ,  3 A,  3 B, and  3 C, the tubes  14  of the heat exchange unit  12  are supported at their opposite ends by tube sheets  52 . Both tube sheets are identical, which simplifies their manufacture. Each tube sheet may be formed from a plate with opposed flat surfaces  54 ,  56  having a plurality of elongated, parallel slots  58 . An end portion  62  of each tube  14  is received in an associated slot  58 , as shown more particularly in  FIG. 3B . The end portions  62  of the tubes  14  may be fastened to the tube sheets  52  in any suitable manner, such as by brazing or welding. The end portions  62  of the tubes are perpendicular to the flat surfaces  54 ,  56  of the tube sheets  52  where they enter the slots  58 . The faying surfaces  64 ,  66  of the end portions and the slots are flat and parallel to each other. Thus, clearances between the faying surfaces are minimized, allowing the joint to wet throughout during the jointing process, for example, vacuum brazing, which aids in ensuring a strong, dependable joint. The tubes may also be supported by upper and lower plates  67 ,  69 . 
         [0022]    In one embodiment, the discrete bends  22  in the tubes  14  are located symmetrically about a centerline  70  of the heat exchange unit  12 , as shown in  FIG. 4 . The centerline is coincident with a radial direction of the gantry frame when the unit is installed in the gantry frame. In this embodiment, three legs  24  are shown, defined by two discrete bends  22 . For each tube, each of the end legs  24 A has the same length A, and the center leg  24 B has a length B. The tube centerline  70  is perpendicular to a tangent to an arc C that extends from the end portions  62  of the tubes where they enter the tube sheets  52  on the end legs  24 A and is tangent to the center leg  24 B at the center line. For a two-bend unit, the bend angle D is equal to angle E/ 2 . 
         [0023]    The bent tube heat exchange unit  12  can be readily sized to fit gantry frames having a variety of circumferences and arc lengths. If the angle E of the arc becomes large, the number of bends can be increased to increase the resolution of the arc while maintaining symmetry about the centerline of the heat exchange unit. For example,  FIG. 5  shows a heat exchange unit  12 ′ with three bends  22 ′. One bend is along the centerline  70 ′. Two bends are offset from the centerline an equal distance in each direction along the arc C′.  FIG. 6  shows a heat exchange unit  12 ″ with four bends  22 ″. A first set of two bends are offset from the centerline  70 ″ a first distance in each direction along the arc C″, and a second set of two bends are offset from the centerline a further distance in each direction along the arc C″. 
         [0024]    By having a symmetrical arrangement with paired legs of equal length, the tube can be formed more simply. For example, a press brake can be set up with one angle for all the bends. For tubes with two bends, one back stop setting can be used for both end legs of the tube. Forming discrete bends with a press brake is faster and more efficient than bending a large arc gradually between rollers and comparing it to a template, such as with curved tubes used in some prior art heat exchange devices. Also, forming the discrete bends takes place away from the faying surfaces where the tube passes through the slot in the tube sheet. The tubes can thus be held perpendicular to the surfaces of the tube sheets, as noted above, leading to a dependable joint. In contrast, with an arced or curved tube, it is more difficult to be assured of that perpendicularity at the tube sheet. Bending the tube at discrete locations also results in less plastic deformation to the tube than forming an arc, which improves repeatability. A better joint can be made between the tubes and the tube sheets, because the parallelism of the tubes can be more readily controlled due to the long flat sections of the tubes and their undeformed condition. Tooling for the jointing process is also more simple to manufacture and change, because the pressure is applied with a flat surface in a linear direction, eliminating the need to form curved tooling. 
         [0025]    Depending on the angle E of the annular portion of the gantry frame to be covered, the present heat exchange unit presents a surface area of the tubes to the entering air that is comparable to the surface area of a curved-tube heat exchanger geometry. Also, the present angled or bent configuration of the heat exchange unit presents more surface area than a prior art square or rectangular heat exchanger geometry, resulting in a 25 to 50% increase in heat rejection.  FIGS. 7A and 7B  illustrate a comparison in which a rectangular heat exchange unit  101  is overlaid over an annular frame  16 . For example, the surface area of the tubes of the rectangular configuration is 719.6 cm 2 , compared to a surface area of 980.7 cm 2  for a bent tube heat exchange unit  12 . In this case, the present heat exchange unit results in a 36.3% increase in surface area. Also, the increased surface area presented by the tubes results in a lower pressure drop across the tubes for the same air flow rates compared to a square or rectangular geometry. 
         [0026]    In a further aspect, the mounting brackets  32  of the heat exchanger assembly  10  include circumferentially extending ear portions  84  that have apertures  86  that align with the axial direction  44  of the gantry frame  16  when the assembly is installed therein. Bolts or other fastener members pass through the axial apertures for attachment to the gantry frame. In this manner, the weight of the heat exchanger assembly is supported primarily by the surface-to-surface contact of the mounting brackets and the inside diameter of a drum of the gantry frame. This configuration is less dependent on the strength of the fasteners and the mounting bracket, in contrast to prior art designs, in which the heat exchanger assembly is mounted with bolts extending in the radial direction of the gantry frame. This prior art design puts a greater amount of tension force on the bolts in a spinning mechanism. 
         [0027]    It will be appreciated that the embodiments and aspects of the present invention may be combined with each other in various ways. For example, although the embodiments illustrated show symmetrically located bends, the bends do not have to be symmetrically located. The invention is not to be limited by what has been particularly shown and described, except as indicated by the appended claims.