Abstract:
A strainer wall structure that removes foreign substances from a fluid suctioned into a pipe and a re-circulation pump that is part of an emergency core cooling system (ECCS). The strainer wall structure has an inlet side and an outlet side through which cooling water is introduced and discharged, respectively, and includes a body having an opening in a direction of the inlet side, closed side surfaces, and an outlet port disposed at one of the closed side surfaces. The strainer includes a punched plate filter screen inserted into the opening. A modular cassette apparatus including grooved first filter plates is inserted into the body, and second filter plates having second grooves is inserted into the first grooves.

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates to a strainer wall structure (referred to as a passive filtration apparatus) for filtering foreign substances, settlings, etc., generated upon occurrence of failures or accidents of an apparatus requiring a water circulation system, and more particularly, to a strainer wall structure used to remove foreign substances from a fluid suctioned into a pipe and a re-circulation pump when the re-circulation pump goes through an operation of an emergency core cooling system (ECCS) when a pipe failure occurs in a nuclear power plant. 
       BACKGROUND ART 
       [0002]    A nuclear reactor of a nuclear power plant is surrounded by a safety vessel formed of concrete and steel, which is referred to as a containment, in which a coolant circulates to maintain a proper temperature. In addition, the nuclear reactor includes an ECCS for cooling the nuclear reactor upon occurrence of failures or accidents. 
         [0003]    The ECCS must be operated upon occurrence of accidents such as coolant leakage, etc., to cool the nuclear reactor for 30 days with no external interference. The ECCS is a system for collecting coolant discharged and water sprinkled upon a pipe failure into a sump disposed at the lowermost part in the containment, sprinkling the water from an upper part of the containment using the re-circulation pump to cool the containment, and circulating some of the water through a nuclear reactor cooling system to remove remaining heat of the nuclear reactor using a remaining heat removing pump. 
         [0004]    When coolant leakage occurs due to damage to a pipe, etc., in a primary system of the nuclear power plant, foreign substances such as lagging materials, coating materials, latent foreign substances, etc., are generated due to discharge of a coolant. In addition, the discharged coolant and water sprinkled from a sprinkler system of the containment move all foreign substances to a re-circulation sump disposed at a lower end of the containment of the nuclear reactor. Therefore, in order for the foreign substances not to decrease performance of the ECCS, a filtration apparatus is provided in front of an introduction part of a suction pipe guided to an emergency cooling pump. 
         [0005]    When a high temperature and high pressure pipe is broken, foreign substances such as fragments of lagging materials, coating materials, etc., are generated and moved toward the sump, and the filtration apparatus functions to filter the foreign substances moved to the sump and supply the filtered water into the re-circulation pump, without interfering with the operation of the re-circulation pump. 
         [0006]    The filtration apparatus ensures that the foreign substances generated due to accidents can be filtered and the water can appropriately pass therethrough. In this case, a pressure drop due to the foreign substances must be guaranteed not to exceed an allowable critical value. 
         [0007]    A conventional filter screen used in a pressurized water reactor type nuclear power plant has a small screen surface only, and the screen surface is mainly formed of flat grid segments. Thus, when the screen surface is contaminated with fiber settlings, a pressure drop at the screen may be largely increased to an unallowable level. 
         [0008]    However, the filtration apparatus having a single surface may be easily deformed by a high pressure, and a small effective filtration area per a unit volume may decrease filtration efficiency. In order to solve the problem, while the number of filtration apparatus may be increased, their installation cost is high, which causes economical problems. Therefore, a filtration apparatus capable of increasing a filtration area per unit volume is still needed. 
       Technical Problem 
       [0009]    In order to solve the foregoing and/or other problems, it is an aspect of the present invention to provide a strainer wall structure, a filtration method using the same, and a method of fabricating the same that are capable of providing a substantially larger effective filtration area in the same length and width, substantially reducing foreign substances covering a suction surface and a flow resistance of the foreign substances, and reducing a pressure drop at a cooling water pass corresponding thereto. 
         [0010]    It is another aspect of the present invention to provide a strainer wall structure, a filtration method using the same, and a method of fabricating the same in which maintenance and installation thereof can be easily performed, and manufacturing and installation costs can be reduced to solve economical problems in exchange and installation thereof. 
       Technical Solution 
       [0011]    The foregoing and/or other aspects of the present invention may be achieved by providing a strainer wall structure including an inlet side through which cooling water is introduced and an outlet side through which the cooling water is discharged, including: a body having an opening in a direction of the inlet side, closed side surfaces, and an outlet port disposed at one of the closed side surfaces; a filter screen inserted into the opening and formed of a punched plate having a plurality of filter holes; and a modular cassette apparatus including a plurality of first filter plates inserted into the body, each having a plurality of first grooves, and formed by bending the punched plate; and a plurality of second filter plates each having a plurality of second groves inserted into the first grooves, and formed by bending the punched plate, so that the plurality of first filter plates disposed in one direction are perpendicularly assembled to the plurality of second filter plates to form a grid structure. 
         [0012]    The grid structure and the filter screen may define suction pockets. 
         [0013]    Each of the first filter plates and the second filter plates may have a dual wall structure to form a discharge cap disposed therein. 
         [0014]    In the modular cassette apparatus, the width of the first grooves may be equal to the thickness of the second filter plates, and the width of the second grooves may be equal to the thickness of the first filter plates, so that the first filter plates are press-fitted into the second filter plates. 
         [0015]    When the cooling water is suctioned into the suction pockets, the cooling water may be surrounded by five surfaces constituted by the first filter plates, the second filter plates and the filter screens. 
         [0016]    The filter holes may have a diameter of 1 to 3 mm. 
         [0017]    The strainer wall structure may further include a fixing frame coupled to the opening by connection members to fix the modular cassette apparatus into the body. 
         [0018]    At least two surfaces of the outer surfaces of the body may have openings, the filter screens may be inserted into the openings, respectively, the modular cassette apparatus may be installed at the filter screens, respectively, and the fixing frames may be coupled to the outer peripheries of the openings, respectively. 
         [0019]    The connection members may be pins, screws, rivets, or bolts. 
         [0020]    The body may further include L-shaped steel at corners thereof. 
         [0021]    The strainer wall structure may further include fixing plates installed in the body to fix the modular cassette apparatus in a direction of the outlet side. 
         [0022]    Another aspect of the present invention may be achieved by providing a filtration method using a strainer wall structure including: installing a body in a passage through which cooling water flows; inserting a filter screen formed of a punched plate into an opening of the body; forming a modular cassette apparatus including a plurality of first filter plates inserted into the body, each having a dual layer structure and a plurality of first grooves, and formed by bending the punched plate; and a plurality of second filter plates each having a dual layer structure and a plurality of second groves inserted into the first grooves, and formed by bending the punched plate, so that the plurality of first filter plates disposed in one direction are perpendicularly assembled to the plurality of second filter plates to form a plurality of suction pockets in a grid structure; inserting the modular cassette apparatus into the filter screen; coupling fixing frames to an outer periphery of the opening of the body to fix the modular cassette apparatus into the body; introducing the cooling water into an inlet side to be suctioned into the plurality of suction pockets; and passing the cooling water suctioned into the suction pockets through the dual wall, which forms the suction pockets, or the filter screen, and discharging the filtered cooling water through an outlet port. 
         [0023]    The body may include two openings, in inserting the filter screen, the filter screens may be inserted into the openings, respectively, in inserting the modular cassette apparatus, the modular cassette apparatus may be installed in the filter screens, respectively, and in fixing the modular cassette apparatus, the fixing frames may be fixed to corners of the openings, respectively. 
         [0024]    In suctioning and discharging the cooling water, the cooling water suctioned into the suction pockets may pass through the filter screen to be discharged to the outlet side, or pass through the dual wall to be introduced into the discharge cap and then pass through the filter screen to be discharged to the outlet side. 
         [0025]    Still another aspect of the present invention may be achieved by providing a method of fabricating a strainer wall structure including: forming a punched plate with a large area having a plurality of filter holes, and cutting the punched plate to form a plurality of rectangular holes in a matrix; cutting the punched plate with a large area along a line between the adjacent rectangular holes arranged in a longitudinal direction thereof to fabricate a first base plate, and bending the first base plate twice with respect to a first centerline thereof to form a first filter plate having a dual wall structure and a plurality of first grooves; cutting the punched plate with a large area to cut the rectangular holes arranged in a longitudinal direction thereof to divide them into halves to form a second base plate, and bending the second base plate twice with respect to a second centerline thereof to form a second filter plate having a dual wall structure and a plurality of second grooves; inserting the first grooves and the second grooves into each other to perpendicularly assemble the first filter plates to the second filter plates to form a modular cassette apparatus having a grid structure; inserting the modular cassette apparatus into the body; and coupling fixing frames to an opening of the body using connection members to fix the modular cassette apparatus into the body. 
         [0026]    The method may further include, before inserting the modular cassette apparatus, inserting filter screens formed of the punched plates into the body. 
         [0027]    The method punched plate may be cut by a laser to form the rectangular holes. 
         [0028]    According to a strainer wall structure of the present invention, it is possible to provide a substantially larger effective filtration area in the same length and width. Therefore, a flow resistance of settlings and foreign substances covering a suction surface can be substantially reduced. In addition, a pressure drop generated along the strainer wall structure can be reduced depending on reduction in flow resistance. 
         [0029]    Further, since the strainer wall structure of the present invention is fabricated by assembling a filter screen having a punched plate, a first filter plate and a second filter plate, without welding, it is possible to easily perform maintenance and installation thereof. Furthermore, since a plurality of first filter plates and second filter plates are vertically arranged, a load pressure can be distributed to increase structural integrity. 
     
    
     
       DESCRIPTION OF DRAWINGS 
         [0030]    The above and other aspects and advantages of the present invention will become apparent and more readily appreciated from the following description of exemplary embodiments, taken in conjunction with the accompanying drawings of which: 
           [0031]      FIG. 1  is a perspective view of a strainer wall structure in accordance with an exemplary embodiment of the present invention; 
           [0032]      FIG. 2  is a front view of a strainer wall structure in accordance with an exemplary embodiment of the present invention; 
           [0033]      FIG. 3  is a side view of a strainer wall structure in accordance with an exemplary embodiment of the present invention; 
           [0034]      FIG. 4  is an exploded perspective view of a strainer wall structure in accordance with an exemplary embodiment of the present invention; 
           [0035]      FIG. 5  is a perspective view of a body in accordance with an exemplary embodiment of the present invention; 
           [0036]      FIG. 6  is a front view of a body in accordance with an exemplary embodiment of the present invention when seen from an open side; 
           [0037]      FIG. 7  is a perspective view of a filter screen in accordance with an exemplary embodiment of the present invention; 
           [0038]      FIG. 8  is a perspective view of a first filter plate in accordance with an exemplary embodiment of the present invention when seen from an inlet side; 
           [0039]      FIG. 9  is a perspective view of the first filter plate in accordance with an exemplary embodiment of the present invention when seen from an outlet side; 
           [0040]      FIG. 10  is a plan view of  FIG. 9 ; 
           [0041]      FIG. 11  is a perspective view of a second filter plate in accordance with an exemplary embodiment of the present invention when seen from an inlet side; 
           [0042]      FIG. 12  is a perspective view of the second filter plate in accordance with an exemplary embodiment of the present invention when seen from an outlet side; 
           [0043]      FIG. 13  is a side view of  FIG. 12 ; 
           [0044]      FIG. 14  is a perspective view of a modular cassette apparatus in accordance with an exemplary embodiment of the present invention; 
           [0045]      FIG. 15  is a perspective view of a filter screen into which the modular cassette apparatus in accordance with an exemplary embodiment of the present invention is inserted; 
           [0046]      FIG. 16  is an enlarged view of a suction pocket in accordance with an exemplary embodiment of the present invention; 
           [0047]      FIG. 17  is a cross-sectional view taken along line A-A′ of  FIG. 16 ; 
           [0048]      FIG. 18  is a cross-sectional view taken along line B-B′ of  FIG. 16 ; 
           [0049]      FIG. 19  is a cross-sectional view taken along line C-C′ of  FIG. 16 ; 
           [0050]      FIG. 20  is a flowchart of a filtration method using a strainer wall structure in accordance with an exemplary embodiment of the present invention; 
           [0051]      FIG. 21  is a flowchart of a method of fabricating a strainer wall structure in accordance with an exemplary embodiment of the present invention; 
           [0052]      FIG. 22  is a cross-sectional view showing a method of fabricating a first filter plate and a second filter plate in accordance with an exemplary embodiment of the present invention; and 
           [0053]      FIG. 23  is a perspective view of another embodiment of the present invention including a plurality of strainer wall structures. 
       
    
    
       [0054]      
         [0000]    
       
         
               
             
               
               
             
           
               
                   
               
               
                 *Description of Major Reference Numerals* 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                  10: Strainer wall structure 
                  20: Filter hole 
               
               
                  30: Inlet side 
                  40: Outlet side 
               
               
                  50: Large punched plate 
                  60: Rectangular hole 
               
               
                  70: First base plate 
                  71: First centerline 
               
               
                  80: Second base plate 
                  81: Second centerline 
               
               
                 100: Body 
                 110: Opening 
               
               
                 120: Closed surface 
                 130: L-shaped steel 
               
               
                 140: Outlet port 
                 150: Fixed plate 
               
               
                 200: Filter screen 
                 210: Filter screen plate 
               
               
                 220: Outer periphery plate 
                 300: First filter plate 
               
               
                 310: First groove 
                 320: Dual wall of first filter plate 
               
               
                 330: Discharge cap of first 
                 400: Second filter plate 
               
               
                 filter plate 
               
               
                 410: Second groove 
                 420: Dual wall of second filter plate 
               
               
                 430: Discharge cap of second 
                 500: Fixed frame 
               
               
                 filter plate 
               
               
                 510: Connection member 
                 520: First coupling member 
               
               
                 521: Second coupling member 
                 600: Modular cassette apparatus 
               
               
                 610: Suction pocket 
                 700: Discharge space 
               
               
                 D: Diameter of filter hole 
                 L F1 : Length of first filter plate 
               
               
                 H F1 : Height of first filter plate 
                 t f1 : Thickness of first filter plate 
               
               
                 I g1 : Interval of first grooves 
                 W g1 : Width of first groove 
               
               
                 H g1 : Depth of first groove 
                 L f2 : Length of second filter plate 
               
               
                 H f2 : Height of second filter plate 
                 t f2 : Thickness of second filter plate 
               
               
                 I g2 : Interval of second grooves 
                 W g2 : Width of second groove 
               
               
                 H g2 : Depth of second groove 
               
               
                   
               
             
          
         
       
     
       DETAILED DESCRIPTION 
       [0055]    Reference will now be made in detail to the embodiments of the present invention, examples of which are illustrated in the accompanying drawings. 
         [0056]      FIG. 1  is a perspective view of a strainer wall structure in accordance with an exemplary embodiment of the present invention,  FIG. 2  is a front view of a strainer wall structure in accordance with an exemplary embodiment of the present invention, and  FIG. 3  is a side view of a strainer wall structure in accordance with an exemplary embodiment of the present invention. 
         [0057]    As shown in  FIG. 1 , a strainer wall structure  10  of the present invention is fixedly installed at a passage through which cooling water flows. The cooling water flows into an inlet side  30  of the strainer wall structure  10  to be discharged through an outlet port  140 . While two inlet sides  30  are provided as shown in  FIG. 1 , one or more inlet sides may be provided. 
         [0058]    A filter screen  200  is inserted into a body  100  having openings  110  formed at the inlet sides  30  and closed side surfaces  120 . 
         [0059]    In addition, a modular cassette apparatus  600 , in which a first filter plate  300  and a second filter plate  400  having a dual wall ( 320 ,  320 ) structure formed of punched plates are assembled in a grid structure, is inserted into the filter screen  200 . 
         [0060]    As shown in  FIG. 2 , the modular cassette apparatus  600  having a grid structure and the filter screen  200  (see  FIG. 4 ) are installed in the body  100 . In addition, the first filter plate  300 , the second filter plate  400  and the filter screen  200  constituting the modular cassette apparatus  600  are formed of punched plates each having a plurality of filter holes  20 . 
         [0061]    The diameter D of the filter hole  20  is designed to be 2.5 mm in this embodiment, and is preferably 1 to 3 mm in consideration of an installation position of the filtration apparatus or the size of foreign substances. 
         [0062]    As shown in  FIG. 3 , the outlet port  140  is installed at a lower end of one side surface of the body  100  to discharge cooling water. While not seen from the exterior, referring a partially cut view of  FIG. 3 , it will be appreciated that two modular cassette apparatus  600  in which the first filter plate  300  and the second filter plate  400  are assembled are inserted in the body  200  together with the filter screens  200 , respectively. In addition, a discharge space  700  is provided between the two filter screens  200 . 
         [0063]      FIG. 4  is an exploded perspective view of a strainer wall structure in accordance with an exemplary embodiment of the present invention. 
         [0064]    As shown in  FIG. 4 , a fixing frame  500  is coupled to an outer periphery of an opening  110  of the body  100  by a connection member  510  to fix the modular cassette apparatus  600 . 
         [0065]    The strainer wall structure  10  is fabricated by assembling the respective components, without welding the components, to each other. In particular, the modular cassette apparatus  600  having a grid structure formed of a plurality of suction pockets  610  is assembled by inserting the first filter plate  300  and the second filter plate  400  into first grooves  310  and second grooves  410 , respectively, without welding or separate coupling means. 
         [0066]    Side surfaces of the body  100  are formed of closed surfaces  120 , and the outlet port  140  is installed at one side surface. The opening  110  is formed in a direction of the inlet side  30  to introduce the cooling water into the opening  110 . In this embodiment, two inlet sides  30  are installed to form the body  110  having openings  110  at both sides thereof. 
         [0067]    Two filter screens  200  are inserted into the body  100  through the inlet sides  30 , respectively. While  FIG. 4  shows one side only, the filter screen  200 , the modular cassette apparatus  600  and the fixed frame  500  are assembled through the opening  110  of the body  100  at the other inlet side  30 . 
         [0068]    In this embodiment, the filter screen  200  is formed of stainless steel, and a filter screen plate  210  and the outer periphery plate  220  are formed of punched plates having a plurality of filter holes  20 . In addition, the filter screen  200  is surrounded by four outer periphery plates  200  having lengths corresponding to the heights H f1  and H f2  of the first filter plate  300  and the second filter plate  400 . 
         [0069]    Each modular cassette apparatus  600  is inserted into each filter screen  200 . Since they are also assembled by insertion thereof, there is no need for welding or coupling members. First, the plurality of first filter plates  300  are disposed at the filter screen  200  at predetermined intervals. In this embodiment, seven first filter plates  300  are disposed. Each of the first filter plates  300  has first grooves  310  formed at predetermined intervals. As shown in  FIG. 4 , the first grooves  310  are formed in a direction of the inlet side  30 . The interval of the first grooves  310  is designed within a range of substantially 80 to 150 mm, and 110 mm in this embodiment. 
         [0070]    In addition, the second filter plates  400  cooperate with and are perpendicularly assembled to the first filter plates  300  to form a grid structure. The second filter plates  400  are also formed of punched plates having a plurality of filter holes  20 , each of which is formed of a dual wall  420 . The second grooves  410  of the second filter plate  400  are coupled and assembled to the first grooves  310  of the first filter plate  300 . Meanwhile, the second grooves  410  of the second filter plate  400  are formed in a direction of an outlet side. An interval Ig 2  of the first grooves  310  is designed within a range of substantially 80 to 150 mm, and 100 mm in this embodiment. 
         [0071]    As shown in  FIG. 4 , the seven first filter plates  300  and the eight second filter plates  400  corresponding thereto are assembled to each other by perpendicularly fitting the first grooves  310  to the second grooves  410  to form the modular cassette apparatus  600  having a grid structure. Specifically, the thickness t f1  of the first filter plate  300  is equal to the width W g2  of the second groove  410  (designed as 30 mm in this embodiment, t f1 =W g2 ), and the thickness t f2  of the second filter plate  400  is equal to the width W g1  of the first groove  310  (designed as 30 mm in this embodiment, t f2 =W g1 ). In addition, the depth H g1  of the first groove  310  is equal to a distance of the height H f2  of the second filter plate  400  minus the depth H g2  of the second groove  410  (H g1 =H f2 −H g2 ). Further, the depth H g2  of the second groove  410  is equal to a distance of the height H f1  of the first filter plate  300  minus the depth H g1  of the first groove  310  (H g2 =H f1 −H g1 ). Therefore, the first grooves and the second grooves are perpendicularly engaged to form a grid structure. 
         [0072]    As described in this embodiment, the seven first filter plates  300  and the eight second filter plates  400  are assembled to form the modular cassette apparatus  600  having a grid structure. The modular cassette apparatus  600  includes a plurality of suction pockets  610 , and in this embodiment, 72 suction pockets. The suction pockets  610  are opened in a direction of the inlet side  30 , surrounded by the filter screen  200  at the outlet side  40 , and surrounded by the dual wall  320  of the first filter plate  300  and the dual wall  420  of the second filter plate  400  at the remaining four surfaces. Therefore, the cooling water introduced into the suction pocket  610  is filtered through the filter holes  20  formed at the five surfaces to be introduced into the discharge space  700 . 
         [0073]    After the modular cassette apparatus  600  are assembled to the filter screens  200  inserted into the two openings  110  of the body, respectively, the fixed frame  500  is coupled to an outer periphery of the opening  110  by connection members  510  (fixing pins, in this embodiment) to fix the modular cassette apparatus  600  (while  FIG. 4  shows an insertion operation through one opening, the other opening is also assembled in the same manner). The connection members  520  may be pins, screws, rivets, bolts, etc. 
         [0074]      FIG. 5  is a perspective view of a body in accordance with an exemplary embodiment of the present invention, and  FIG. 6  is a front view of a body in accordance with an exemplary embodiment of the present invention when seen from an open side. 
         [0075]    As shown in  FIG. 5 , side surfaces of the body  100  are provided as closed surfaces, and the inlet side  30  is provided as an opening  110 . Fixed plates  150  are installed in the body to fix the filter screen  200  in a direction of the outlet side  40 . 
         [0076]    Eight fixed plates  150  are installed at inner corners in the body. As shown in  FIG. 6 , when the cooling water is introduced into the suction pocket  610 , since the body  100  receives a high pressure, an L-shaped steel  130  may be welded to the outer periphery to reinforce the body. In this embodiment, the L-shaped steel  130  is coupled to a center part of the side surface to maintain the shape of the body even under a high pressure. 
         [0077]    It will be appreciated that the hollow outlet port  140  is installed at a lower end of one side surface of the body and is reinforced by the L-shaped steel  130  at the corners and the center part of the closed surface  120 . In addition, as shown in a partially cut view, the fixed plates  150  are installed in the body. When the two filter screens  200  are inserted through the opening  110 , the discharge space  700  is formed between the two filter screens  200  (i.e., between the two fixed plates  150 ). 
         [0078]      FIG. 7  is a perspective view of a filter screen in accordance with an exemplary embodiment of the present invention. 
         [0079]    The filter screen plate  210  formed of a punched plate and the outer periphery thereof are surrounded by outer periphery plates  220  having a predetermined height. The diameter D of the filter holes  20  of the punched plate is 1 to 3 mm, which is designed as 2.5 mm in this embodiment. The diameter D of the filter holes  20  is designed in consideration of the size, etc., of foreign substances generated in the containment and arriving at the filtration apparatus upon accidents. The height of the outer periphery plates  220  is equal to the height of the first filter plate  300  and the second filter plate  400  (145 mm in this embodiment). 
         [0080]      FIG. 8  is a perspective view of a first filter plate in accordance with an exemplary embodiment of the present invention when seen from an inlet side,  FIG. 9  is a perspective view of the first filter plate in accordance with an exemplary embodiment of the present invention when seen from an outlet side, and  FIG. 10  is a plan view of  FIG. 9 . 
         [0081]    As shown in  FIG. 8 , the first filter plate  300  is formed of a punched plate and includes a plurality of first grooves  310 . The first grooves  310  are formed in a direction of the inlet side  30 . In this embodiment, the first filter plate  300  has eight first grooves  310 . A dual wall  320  formed of a punched plate forms an outer surface of the suction pocket  610 . In addition, the cooling water passes through the dual wall  320  to be introduced into a discharge cap  330 . A flow direction of the cooling water is shown by arrows. 
         [0082]    As shown in  FIG. 9 , the first filter plate  300  is formed of a dual wall  320  structure, and includes the discharge cap  330 . The cooling water is filtered through the dual wall  320  formed of a punched plate and introduced into the discharge cap  330  to be discharged through the filter screen plate  210 . A flow direction of the cooling water is shown by arrows. 
         [0083]    The second filter plates  400  are perpendicularly inserted into the first grooves  310  to assemble the first filter plates  300  to the second filter plates  400 . 
         [0084]    As shown in  FIG. 10 , in a specific embodiment, eight first grooves  310  are disposed at an interval I g1  of 110 mm. The width W g1  of the first grooves  310  is 30 mm, the depth H g1  is 72.5 mm, and the thickness t f1  is 30 mm. In addition, the diameter D of the filter holes  20  may be 1 to 3 mm. A flow direction of the cooling water is shown by arrows. 
         [0085]    However, it will be appreciated that limitations to these specific numbers are described for the illustrative purpose only, and thus, should not affect determination of the scope of the present invention due to the specific numbers while maintaining the technical sprit and equivalency of the present invention. 
         [0086]      FIG. 11  is a perspective view of a second filter plate in accordance with an exemplary embodiment of the present invention when seen from an inlet side,  FIG. 12  is a perspective view of the second filter plate in accordance with an exemplary embodiment of the present invention when seen from an outlet side, and  FIG. 13  is a side view of  FIG. 12 . 
         [0087]    As shown in  FIGS. 11 and 12 , the second filter plate  400  is formed of a punched plate having a plurality of filter holes  20 . In addition, the second filter plate  400  has a dual wall  420  structure and includes the discharge cap  430  therein. It will be appreciated that the seven second grooves  410  are formed in a direction of the outlet side  40 . The seven first filter plates  300  are inserted into the second grooves  410 , respectively, so that the seven first filter plates  300  are perpendicularly assembled to the eight filter plates  400  to form the modular cassette apparatus  600  having a grid structure. The thickness t f2  of the second filter plates  400  is designed as 30 mm equal to that of the first filter plates  300 , which must be equal to the width W g1  of the first grooves  310  of the first filter plates  300  (t f2 =W g1 ). A flow direction of the cooling water is shown by arrows. 
         [0088]    As shown in  FIG. 13 , since the interval I g2  of the second grooves  410  of the second filter plate  400  is designed as 130 mm and the width W g2  of the second grooves  410  is equal to the thickness t f1  of the first filter plate  300  (30 mm in this embodiment), the first filter plates  300  can be inserted into the second grooves  410 , respectively. In addition, two modular cassette apparatus  600 , in which the seven first filter plates  300  are perpendicularly assembled to the eight second filter plates  400 , respectively, are inserted into the filter screen  200 . A flow direction of the cooling water is shown by arrows. The cooling water passes through the dual wall  420  to be introduced into the discharge cap  430 , and the introduced cooling water passes through the filter screen plate  210  to be discharged to the outlet side  40 . Further, the fixed frames  500  are installed at corners of the opening  110  by the connection members  510  to fix the modular cassette apparatus  600 . 
         [0089]      FIG. 14  is a perspective view of a modular cassette apparatus in accordance with an exemplary embodiment of the present invention, and  FIG. 15  is a perspective view of a filter screen into which the modular cassette apparatus in accordance with an exemplary embodiment of the present invention is inserted. 
         [0090]    As shown in  FIG. 14 , since the thickness t f1  of the first filter plates  300  is equal to the width W g2  of the second grooves  410 , the first filter plates  300  are inserted into the seven second grooves  410 , respectively. In addition, since the thickness t f2  of the second filter plates  400  is equal to the width W g1  of the first grooves  310 , the second filter plates  400  are inserted into the eight first grooves  310  to be assembled thereto. The modular cassette apparatus  600  includes a plurality of suction pockets  610  (72 suction pockets  610  in this embodiment). The cooling water in the inlet side  30  is suctioned into the suction pockets  610 . A flow direction of the cooling water is shown by arrows. The cooling water in the inlet side  30  is suctioned into the suction pockets  610  and then filtered to be discharged to the outlet side  40 . 
         [0091]    As shown in  FIG. 15 , the cooling water is suctioned into the suction pockets  610  from the inlet side  30  to be filtered by the first filter plates  300 , the second filter plates  400  and the filter screen plate  210  and then discharged to the outlet side  40 . The filter screens  200  into which the modular cassette apparatus  600  are inserted are inserted into the openings  110  of the body  100 , respectively. 
         [0092]      FIG. 16  is an enlarged view of a suction pocket in accordance with an exemplary embodiment of the present invention,  FIG. 17  is a cross-sectional view taken along line A-A′ of  FIG. 16 ,  FIG. 18  is a cross-sectional view taken along line B-B′ of  FIG. 16 , and  FIG. 19  is a cross-sectional view taken along line C-C′ of  FIG. 16 . 
         [0093]    As shown in  FIG. 16 , the cooling water is introduced through the inlet side  30  to be suctioned into the suction pockets  610 . Side surfaces of each suction pocket  610  are constituted by the dual walls  320  and  420  of the first filter plate  300  and the second filter plate  400 , and the filter screen plate  210  is installed at the outlet side  40 . Therefore, the cooling water introduced into the suction pockets  610  is surrounded by five surfaces. All of the five surfaces are formed of punched plates. Therefore, a filtration area per unit volume can be increased. 
         [0094]    Specifically, the introduced cooling water may pass through the dual wall  320  or  420  of the first filter plate  300  or the second filter plate  400  constituting the side surfaces of the suction pockets  610 , or pass through the filter screen plate  210 . The cooling water passing through the first filter plates  300  or the second filter plates  400  is introduced into the discharge caps  330  and  430 , and the cooling water introduced into the discharge caps  330  and  430  passes through the filtered screen plates  210  to be introduced into the discharge space  700  and then discharged to the outlet port  140 . In addition, the cooling water passing through the filter screen plate  210  is introduced into the discharge space  700  to be discharged to the outlet port  140 . 
         [0095]    As show in  FIG. 17 , the cooling water is suctioned into the suction pockets  610  from the inlet side  30 . Then, the suctioned cooling water may be directly discharged to the outlet side  40  through the filter screen plate  210 . In addition, the cooling water passes through the dual wall  420  of the second filter plate  400  to be introduced into the discharge cap  430 . The cooling water introduced into the discharge cap  430  passes through the filter screen plate  210  to be discharged to the outlet side  40 . 
         [0096]    In this embodiment, since the two modular cassette apparatus  600  are symmetrically provided, the cooling water discharged to the outlet side is in the discharge space  700 . 
         [0097]    As shown in  FIG. 18 , the cooling water is suctioned into the suction pockets  610 . Then, the suctioned cooling water may be immediately discharged to the outlet side  40  through the filter screen plate  210 . In addition, the cooling water passes through the dual walls  320  of the first filter plates  300  to be introduced into the discharge caps  330 . The cooling water introduced into the discharge cap  330  passes through the filter screen plate  210  to be discharged to the outlet side  40 . 
         [0098]    In  FIG. 19 , {circumflex over (x)} means a direction that the cooling water flows through the figure, and a flow direction of the cooling water is shown by arrows. 
         [0099]    As shown in  FIG. 19 , the cooling water introduced into the suction pockets  620  is introduced into the discharge caps  330  and  430  through the dual walls  320  and  420  of the first filter plates  300  and the second filter plates, which are formed of punched plates. The cooling water introduced into the discharge caps  330  and  430  is filtered again by the filter screen plate  210  to be discharged to the outlet side  40 . The cooling water discharged to the outlet side  40  is introduced into the discharge space between the two filter screens  200  to be discharged through the outlet port  140  installed at the body  100 . 
         [0100]    &lt;Filtration Method Using Strainer Wall Structure&gt; 
         [0101]    Hereinafter, a filtration method using a strainer wall structure  10  of the present invention will be described.  FIG. 20  is a flowchart of a filtration method using a strainer wall structure in accordance with an exemplary embodiment of the present invention. 
         [0102]    First, a body  100  is fixed to a passage through which cooling water flows (S 10 ). As described above, the body  100  includes openings  110  in a direction of an inlet side  30 , closed side surfaces  120 , and an outlet port  140  at one of the closed side surfaces  120 . 
         [0103]    Then, filter screens  200  are inserted into the openings  110  of the body  110  (S 200 ). In this embodiment, two openings  110  are provided. Therefore, the filter screens  200  are inserted into the openings  110 , respectively. Each of the filter screens  200  includes a filter screen plate  210  formed of a punched plate, and an outer periphery plate  220  formed of a punched plate similar to the filter screen plate  210  and surrounding an outer periphery of the filter screen plate  210 . The filter screen  200  is fixed by fixing plates  150  in the body  100  at an outlet side  40 , and a discharge space  700  is formed in the body  100  between the two filter screens  200 . 
         [0104]    Next, a modular cassette apparatus  600  including a plurality of suction pockets  610  formed by perpendicularly assembling seven first filter plates  300  each having eight first grooves  310  and eight second filter plates  400  each having seven second grooves  410  is provided (S 30 ). Each of the first filter plates  300  is formed of a punched plate having a plurality of filter holes  20  and has a dual wall  320  structure to form a discharge cap  330  therein. The first grooves  310  are formed in a direction of the inlet side  30 . 
         [0105]    Each of the second filter plates  400  is also formed of a punched plate having a plurality of filter holes  20  and has a dual wall  420  structure to form a discharge cap  430  therein. The second grooves  410  are formed in a direction of the outlet side  40 . Therefore, the first filter plates  300  and the second filter plates  400  are perpendicularly assembled by the first grooves  310  and the second grooves  410  to form a grid structure of modular cassette apparatus  600  including a plurality of suction pockets  610 . Two modular cassette apparatus  600  are installed in the two filter screens  200 . 
         [0106]    Next, the modular cassette apparatus  600  are inserted and assembled into the filter screens  200  (S 40 ). In this embodiment, since two inlet sides  30  are provided, the modular cassette apparatus  600  are assembled to the filter screens  200  inserted into the inlet sides  30 . 
         [0107]    Fixing frames  500  are coupled to an outer periphery of the opening  110  by connection members  510  to fix the modular cassette apparatus  600  (S 50 ). The connection members  510  may be pins, screws, rivets, or bolts. The fixing frames  500  are installed at corners of the opening  110  of the body  100 . Therefore, the first filter plates  300 , the second filter plates  400  and the filter screens  200  form the strainer wall structure  10  by being assembled each other without welding. 
         [0108]    Next, the cooling water is introduced into the suction pockets  610  (S 60 ). The introduced cooling water passes through the filter holes  20  of the first filter plates  300 , the second filter plates  400  and the filter screen plates  210 , which are formed of punched plates, to be filtered and introduced into the discharge space  700 . Then, the filtered cooling water is discharged through the outlet port  140  (S 70 ). 
         [0109]    Method of Fabricating Strainer Wall Structure 
         [0110]    Hereinafter, a method of fabricating a strainer wall surface  10  of the present invention will be described.  FIG. 21  is a flowchart of a method of fabricating a strainer wall structure in accordance with an exemplary embodiment of the present invention, and  FIG. 22  is a cross-sectional view showing a method of fabricating a first filter plate and a second filter plate in accordance with an exemplary embodiment of the present invention. 
         [0111]    First, a plate (stainless steel in this embodiment) having a certain thickness (preferably, about 2 mm) is punched to form a plurality of filter holes  20  (having a diameter of 1 to 3 mm in this embodiment) (S 100 ). Then, the punched plate  50  having a large area is cut to form a plurality of rectangular holes, which will be formed as first grooves  310  or second grooves  410  of first filter plates  300  and second filter plates  400  (S 200 ). 
         [0112]    As shown in  FIG. 22 , the rectangular holes  60  are formed to be aligned in a matrix. The rectangular holes  60  may be formed by cutting the punched plate  50  using a laser beam. Then, the punched plate  50  having the plurality of rectangular holes  60  is cut. Two kinds of cutting methods are provided. The first filter plates  300  may be fabricated or the second filter plates  400  may be fabricated depending on the cutting methods. 
         [0113]    Specifically, in order to fabricate the first filter plates  300 , as shown in an upper part of  FIG. 22 , the punched plate  50  having the plurality of rectangular holes  60  is cut along a line between adjacent rectangular holes  60  arranged in a longitudinal direction thereof. Therefore, the punched plate  50  is cut to form first base plates  70  each having a row of rectangular holes  60 . Next, each of the first base plates  70  is bent twice with respect to a first centerline  71  of the rectangular holes  60  to fabricate first filter plate  300  (S 300 ). 
         [0114]    In addition, in order to fabricate the second filter plates  400 , as shown in a lower part of  FIG. 22 , the punched plate  50  having the plurality of rectangular holes  60  is cut to cross the rectangular holes arranged in a longitudinal direction thereof to divide them into halves. Therefore, second base plates  80  having grooves symmetrically disposed at both sides are fabricated. Then, each of the second base plates  80  is bent twice with respect to a second centerline  81  to form the second filter plate  400  (S 400 ). That is, the first filter plate  300  and the second filter plate  400  are fabricated by forming the rectangular holes  60  in the punched plate  50  with a large area using a laser beam, cutting the punched plate  50  through the above two methods, and bending the cut first base plate  70  or the second base plate  80  twice with respect to the first centerline  71  or the second centerline  81 , with no welding. 
         [0115]    As described above, the plurality of first grooves  310  and second grooves  410  are formed in the first filter plates  300  and the second filter plates  400 . The first and second filter plates  300  and  400  are perpendicularly assembled to form the modular cassette apparatus  600  having a grid structure (S 500 ). 
         [0116]    Next, the filter screen  200  is inserted into the body  100 , the modular cassette apparatus  600  is inserted into the filter screen  200 , and then, the fixing frame  500  is coupled to the body  100 , fabricating the strainer wall structure  10  (S 600 ). 
         [0117]    Another Embodiment of Strainer Wall Structure 
         [0118]    Hereinafter, another embodiment of the present invention will be described.  FIG. 23  is a perspective view of another embodiment of the present invention including a plurality of strainer wall structures. 
         [0119]    As shown in  FIG. 23 , two strainer wall structures  10  are coupled by coupling members  520  and  521 . In this embodiment, while the two strainer wall structures  10  are coupled, three or more strainer wall structures may be coupled according to embodiments. 
         [0120]    The strainer wall structure  10  has the same constitution as described above. That is, the structure  10  includes a body  100  having fixing plates  150  disposed therein, two filter screens  210  inserted into the body  100 , modular cassette apparatus  600  in which first and second filter plates  300  and  400  are assembled, and fixing frames  500 . As shown in  FIG. 23 , the fixing frames  500  of the strainer wall structure  10  are coupled by first coupling members  520 , and the first coupling members  520  are coupled by second coupling members  521  to couple the two strainer wall structures  10 . 
         [0121]    The above coupling method has been for illustrative purposes only, and other methods of coupling a plurality of strainer wall structures may fall into the scope of the present invention, not limited to the above embodiment. 
         [0122]    The foregoing description concerns an exemplary embodiment of the invention, is intended to be illustrative, and should not be construed as limiting the invention. The present teachings can be readily applied to other types of devices and apparatus. Many alternatives, modifications, and variations within the scope and spirit of the present invention will be apparent to those skilled in the art.