Abstract:
Disclosed is an electron beam irradiating apparatus including an electron beam source; an accelerating unit for accelerating electrons emitted from the electron beam source; a deflecting unit for deflecting a highly energized electron beam generated by the accelerating unit in a scanning direction; a vacuum vessel accommodating the electron beam source, the accelerating unit, and the deflecting unit in a vacuum environment; a window foil for ejecting the electron beam from the vacuum environment into a gas environment; a crosspiece for adhering to and supporting the window foil; and a cooling block for shielding the crosspiece from the electron beam in areas that the electron beam intersects the crosspiece.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an electron beam irradiating apparatus for processing, for example, waste gas discharged from a thermal power plant. The present invention especially relates to a window foil for ejecting electrons from a vacuum vessel into a gas environment, and particularly to a crosspiece fixed to the vacuum vessel for supporting the window foil against atmospheric pressure. 
     2. Description of the Related Art 
     Some of today global problems, such as global warming and acid rain, are thought to be caused by air pollution and specifically by SOx, NOx, and other toxic components in waste combustion gases that are discharged from thermal power plants and the like. One method for removing such toxic components, as SOx and NOx, has been to conduct desulfurization and denitration by irradiating waste combustion gases with an electron beam. 
     FIG. 1 shows an example of an electron beam irradiating apparatus for use in such applications. An apparatus for processing waste combustion gas includes a power source  10  for generating a high DC voltage, an electron beam irradiating apparatus  11  for irradiating an electron beam onto waste combustion gas, a window foil  15  serving as an irradiating outlet for the electron beam of the electron beam irradiating apparatus  11 , and a channel  19  disposed along the window foil  15  through which the waste combustion gas flows. The window foil  15  is composed of a thin plate formed of titanium or the like. 
     The electron beam ejected externally via the window foil  15  irradiates such molecules in the waste combustion gas as oxygen (O 2 ) and water vapor (H 2 O) to form radicals that are extremely strong oxidizers, including OH, O, and HO 2 . These radicals oxidize the toxic components SOx and NOx to generate intermediate products of sulfuric acid and nitric acid. The intermediate products react with ammonium gas (NH 3 ) that has already been introduced to form ammonium sulfate and ammonium nitrate, which can be recovered for use as a fertilizer. Accordingly, this type of waste gas processing system can remove such toxic components as SOx and NOx from waste combustion gas and can recover useful by-products of ammonium sulfate and ammonium nitrate for use as a fertilizer. 
     In this example, the electron beam irradiating apparatus  11  includes as main components a thermoelectron generator  12  such as a thermionic filament; an accelerating tube  13  for accelerating electrons emitted from the thermoelectron generator  12 ; a focusing electromagnet  16  for controlling a diameter of an electron beam by applying a magnetic field to a highly energized electron beam formed by the accelerating tube  13 ; and a scanning electromagnet  17  for deflecting the electron beam by applying a magnetic field to the electron beam after it has been focused to a specific diameter. These components are accommodated in vacuum vessels  18   a  and  18   b , which maintain a high vacuum atmosphere of approximately 10 −6  Pa. By applying a magnetic field using the scanning electromagnet  17 , the highly energized electron beam is deflected in a scanning motion through the window foil  15  and ejected onto the waste combustion gas within a prescribed range of the channel  19 . 
     As described above, this type of electron beam irradiating apparatus must eject an electron beam into the atmosphere after electrons have been accelerated in a vacuum environment. The window foil used in this electron beam irradiating apparatus is generally a film formed of pure titanium or a titanium alloy having a thickness of several tens of micrometers (for example, 40 μm) in order to attain a high electron transmission efficiency for ejecting the electron beam. The window foil  15  is mounted on an end of the vacuum vessel  18   a  via a mounting flange (not shown). The size of the window foil  15  is as large as 3 m×0.6 m. Here, an atmospheric pressure of approximately 1000 hPa is applied to a surface of the window foil, against an internal pressure of approximately 10 −6  Pa in the vacuum vessel. Accordingly, a large force is applied to the window foil via a relationship of area and pressure differential. Therefore, a crosspiece is affixed to a portion of the window foil surface, thereby dividing the window foil into a plurality of sections. 
     FIG. 2 shows an example of a construction for dividing the window foil  15  with a crosspiece, and a scanning path for an electron beam. As described above, the window foil  15  is relatively large, i.e. 3 m by 0.6 m. A central crosspiece  21  is disposed lengthwisely across a center of the window foil  15  and adhered thereon for supporting the same. Accordingly, the crosspiece  21  supports a central portion of the window foil  15  and divides the surface of the window foil  15  into two sections. This configuration prevents the window foil  15  from deforming, even while incurring a large pressure from an atmospheric side toward an evacuated side of the window foil. 
     Since the electron beam is emitted over a large range through the window foil  15 , which has a relatively large area, it is possible to avoid heat damage to the window foil  15 . Therefore, the electron beam scans the window foil  15  in lengthwise direction thereof along a path P in the direction shown by the arrow in FIG. 2 in order to avoid heat damage to the window foil  15 . At this time, the electron  20  beam scans along a path that does not traverse the crosspiece in the lengthwise direction. The electron beam is accelerated by a voltage of approximately 800 kV. Even with a large electric current of approximately 500 mA, energy loss occurs when the electron beam passes through the window foil, thereby generating a large amount of heat therein. For this reason, a cooling pipe is provided in the crosspiece  21  and cooling water is flowed through the pipe to prevent damage to the window foil  15  due to excess heat. 
     However, in order to scan the window foil  15  along the path P shown in FIG. 2, the electron beam inevitably must cross end portions  21   a  and  21   b . When the electron beam E crosses these portions and irradiates the end portions  21   a  and  21   b , a large amount of heat is generated on these portions of the crosspiece  21 . Over extended use, this excess heat can cause thermal fatigue in portions of the cooling water pipe provided in the crosspiece  21 , leading to such accidents as water leaking into the evacuated section. If the vacuum vessel becomes contaminated, it is necessary to disassemble the electron beam irradiating apparatus to repair the damage and then reassemble the apparatus. This process requires much time and labor. Further, once the damage has been repaired, more time is required to evacuate the vacuum vessel again, thereby requiring that operations be suspended for a considerably long time, in addition to difficult repair work. 
     SUMMARY OF THE INVENTION 
     In view of the foregoing, it is an object of the present invention to provide an electron beam irradiating apparatus for scanning an electron beam across an entire surface of a window foil, wherein the window foil is reinforced by a crosspiece capable of preventing excess heat from being generated due to electron beam irradiation, thereby enabling the apparatus to perform stable operations for an extended time. It is another object of the present invention to provide such an electron beam irradiating apparatus that is easy to maintain. 
     These objects and others will be attained by an electron beam irradiating apparatus comprising an electron beam source for emitting electrons; an accelerating unit for accelerating electrons emitted from the electron beam source to generate a highly energized electron beam; a deflecting unit for deflecting the highly energized electron beam generated by the accelerating unit in a scanning direction; a vacuum vessel accommodating the electron beam source, the accelerating unit, and the deflecting unit in a vacuum environment; a window foil for ejecting the electron beam from the vacuum environment into a gas environment; a crosspiece for adhering to and supporting the window foil; and a cooling block for shielding the crosspiece from the electron beam in areas that the electron beam intersects the crosspiece. 
     With this construction, the cooling block can prevent excess heat from being generated in portions of the crosspiece over which the electron beam passes by receiving irradiation of the electron beam at these portions. This construction allows stable operation of the electron beam irradiating apparatus. 
     According to another aspect of the present invention, the cooling block can be detachably mounted. With this construction, the cooling block can be easily replaced after incurring fatigue by irradiation of an electron beam over a long period of time. Therefore, the present invention can prevent such serious accidents as water leakage and the like through regular replacement of the cooling block. 
     According to another aspect of the present invention, the cooling block includes a cooling pipe formed of a metal material and has a fluid therein. With this construction, the cooling block can be easily manufactured, and a portion of the cooling block irradiated by an electron beam can be easily cooled. 
     According to another aspect of the present invention, a cross-section of the cooling block is gently curved on a side facing the electron beam source. With this construction, an electron beam evenly irradiates a surface of the cooling block, thereby preventing excess heat from energy of the electron beam from being concentrated on a portion of the block. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is an explanatory diagram showing an overall construction of an electron beam irradiating apparatus; 
     FIG. 2 is an explanatory diagram showing a position of a crosspiece in the electron beam irradiating apparatus of FIG. 1 and a scanning path of an electron beam; 
     FIG. 3 is a cross-sectional view showing a construction of a cooling block in an electron beam irradiating apparatus of a first embodiment; 
     FIGS. 4A-4D include a front view (FIG.  4 A), a view (FIG. 4B) indicated by arrow B in FIG. 4A, a view (FIG. 4C) indicated by arrow C in FIG. 4A, and a view (FIG. 4D) indicated by arrow D in FIG. 4A, showing a detailed construction of the cooling block of FIG. 3; and 
     FIG. 5 is a cross-sectional view showing a construction of a cooling block in an electron beam irradiating apparatus of a second embodiment. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     An electron beam irradiating apparatus according to preferred embodiments of the present invention will be described with reference to FIGS. 3 and 4. 
     FIG. 3 is a cross-sectional view showing a construction of a window foil and a surrounding area in an electron beam irradiating apparatus. FIG. 4 shows a detailed construction of a cooling block. Parts similar to those described above are referred to using the same part number. 
     Window foil  15  formed of titanium material is fixed to a bottom surface of vacuum vessel  18   a  via a flange  30 . Crosspiece  21  is disposed at a central bottom portion of the vacuum vessel  18   a . A bottom surface of the crosspiece  21  is affixed to the window foil  15  to prevent the window foil  15  from deforming due to atmospheric pressure when the vacuum vessel  18   a  is maintained in a high vacuum state. The crosspiece  21  is formed of a stainless steel material having a width of approximately 3 centimeters and a height of approximately 40 centimeters. The crosspiece  21  is disposed across a central portion of the window foil  15 . The window foil  15  has a surface area of 3×0.6 meters, as shown in FIG.  2 . Both lengthwise ends of the crosspiece  21  are joined to the vacuum vessel  18   a  by portions  21   c . With this construction the window foil  15  is reinforced against strong pressure applied from an atmospheric side of the window foil. Cooling pipes  22   a  and  22   b  are provided inside the crosspiece  21 . Cooling water flowed through the cooling pipes  22   a  and  22   b  absorb heat generated by electron beams penetrating the window foil  15  because of good thermal conductivity of the window foil material, and also absorb heat generated from the electron beam directly ejected on a top surface of the crosspiece  21 . 
     In the electron beam irradiating apparatus of the present invention, a cooling block  25  is disposed over end positions  21   a  and  21   b , over which portions of the crosspiece  21  an electron beam crosses when scanning between lengthwise directions of the crosspiece. In other words, the cooling block  25  serves to protect the crosspiece  21  by shielding the same from irradiation by the electron beam. The cooling block  25  has a width of 36 millimeters, which is slightly larger than the width of the crosspiece  21 , and a length of approximately 35 centimeters. The cooling block  25  is sufficiently large to block an entire scanning diameter of the electron beam. The cooling block  25  is formed of aluminum, copper, silver, or the like, and has a thermal conductivity of 200 W*m −1 *k −1  or greater. A water channel  26  is formed inside the cooling block  25 . Cooling water flowed through the water channel  26  absorbs heat generated by irradiation from the electron beam. A stainless steel pipe  27  is connected to the water channel  26 . The stainless steel pipe  27  is configured to penetrate a plate member  33  for fixing the stainless steel pipe  27 , and an end of the stainless steel pipe  27  protrudes outside of the vacuum vessel  18   a . The plate member  33  is detachably fixed by a bolt or the like to a flange  29  of a bracket  28 . A piping  31  connects the stainless steel pipe  27  to the cooling pipes  22   a  and  22   b . Cooling water is supplied through a supply terminal  23   a . The cooling water circulates through the water channel  26  on a left side of the cooling block  25 , then flows into the cooling pipe  22   b  on top of the crosspiece  21 . From here, the cooling water flows through the cooling block  25  on the right side of the cooling block and then returns to the cooling pipe  22   a  on the bottom of the crosspiece  21 . The cooling water is discharged from a discharge terminal  23   d.    
     FIGS. 4A-4D show a more detailed construction of the cooling block  25 . As shown in FIG. 4A, the water channel  26  is formed in the cooling block  25 , which in this case is copper. As shown in FIG. 4C, a cross-section of the cooling block  25  is substantially rectangular in shape. A top end of the cooling block  25  curves gently as a semicircle. The stainless steel pipe  27  is fixed to both open ends of the water channel  26  by silver brazing. The stainless steel pipe  27  is welded to the plate member  33  at a portion thereof through which the stainless steel pipe  27  penetrates. A terminal is fixed to an external end of the stainless steel pipe  27  for connecting the stainless steel pipe  27  to piping for supplying cooling water. FIG. 4B shows a view indicated by arrow B in FIG.  4 A. As shown, a plurality of generally rectangular bolt holes  34  are formed in the plate member  33  for detachably bolting the plate member  33  to the flange  29 . FIG. 4D shows a view indicated by arrow D in FIG.  4 A. As shown, the stainless steel pipe  27  is connected to the cooling block  25  at an angle. Another end of the stainless steel pipe  27  is fixed to the plate member  33 . 
     Next, a function of the cooling block will be described. As described above, the cooling block  25  is integrally fixed to the stainless steel pipe  27  and plate member  33 . By fixing this assembly to through-holes in the bracket  28  of the vacuum vessel  18   a , the cooling block  25  can shield and protect the crosspiece  21  from the electron beam, as the electron beam passes over the end portions  21   a  and  21   b . The plate member  33  is fixed by bolts or the like to the flange  29  to form a vacuum seal. Next, piping for introducing cooling water is connected to terminals of the pipes shown in FIG.  3 . During operations of the electron beam irradiating apparatus after the vacuum vessel  18   a  has been evacuated, the electron beam scans a path shown in FIG. 2, crossing the end portions  21   a  and  21   b  of the crosspiece  21 . Although the electron beam irradiates the cooling block  25  at these portions, the cooling block  25  shields and protects the crosspiece  21  so that temperature of the crosspiece  21  does not rise at all. Accordingly, the cooling block  25  prevents occurrence of leakage or other accidents caused by fatigue of the crosspiece  21 . Heat is generated in the window foil  15  due to transmission of the electron beam. However, as described with regard to conventional apparatus, cooling water in the crosspiece  21  cools the window foil  15 . 
     The cooling block  25 , on the other hand, can rise in temperature when incurring direct contact from the electron beam. However, with cooling water circulating through the water channel  26  formed in the cooling block  25 , such temperature rise can be prevented. Still, it is a fact that fatigue or the like can occur in the cooling block  25  due to great energy loss caused by irradiation of the electron beam. For this reason, the cooling block  25  is configured to be detachable and should be replaced on a regular basis. If the cooling block  25  is replaced regularly, it is possible to prevent serious accidents, such as leakage. Although, water is ordinarily used as a cooling medium in the cooling block  25 , a substitute for water can also be selected according to the irradiating apparatus. Examples of substitutes include alcohol or other liquids and cooling gases such as He, N 2 , and Ar. 
     FIG. 5 shows an electron beam irradiating apparatus according to a second embodiment of the present invention. The construction of cooling block  25  is similar to that in the first embodiment. Stainless steel pipe  27  is connected to a water channel formed in the cooling block  25 . A connecting terminal  23  is provided at an end of the stainless steel pipe  27  for connecting the stainless steel pipe  27  to external piping  31 . A construction for flowing cooling water is the same as that described in the first embodiment. Further, a function of the cooling block  25  for shielding crosspiece  21  from the electron beam as the beam passes over end portions  21   a  and  21   b  is also the same as that described in the first embodiment. Accordingly, by shielding and protecting the crosspiece  21  from irradiation by the electron beam, the cooling block  25  prevents leakage or other accidents as described above. However, the stainless steel pipe  27  is fixed via a fixing member to vacuum vessel  18   a  using a brazing process or the like. As a result, the cooling block  25  cannot be easily detached. However, by converting existing electron beam irradiating apparatus to this construction, the cooling block  25  can shield and protect the crosspiece  21  from electron beam irradiation. Further, repair of the cooling block  25  is relatively easy in the event that metal fatigue occurs therein. 
     In the embodiments described above, the present invention employs a cooling block to shield portions of a crosspiece overwhich an electron beam crosses to prevent overheating of and damage to the crosspiece. As a result, the electron beam irradiating apparatus can be operated stably for an extended time, thereby improving reliability of the electron beam irradiating apparatus. Also, by making the cooling block detachable, it is possible to regularly replace the cooling block, thereby facilitating maintenance and preventing accidents due to damage to the cooling block itself. 
     While a cooling block for protecting a center crosspiece in an electron beam irradiating apparatus having an oblong window foil has been described in detail with reference to specific embodiments thereof, it will be apparent to those skilled in the art that many modifications and variations may be made therein without departing from the spirit of the invention, the scope of which is defined by the attached claims.