Patent Publication Number: US-11391525-B2

Title: Heat exchanger for steam generator and steam generator comprising same

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a divisional application of U.S. application Ser. No. 15/026,938 filed on Apr. 1, 2016, which is a National Stage Entry of Application No. of PCT/KR2014/009118 filed on Sep. 29, 2014, which claims the benefit of priority from K.R. Provisional Application No. 10-2013-0124182, filed on Oct. 17, 2013. 
    
    
     TECHNICAL FIELD 
     These embodiments relate to a technology for utilizing a printed circuit heat exchanger, a plate type heat exchanger or the like as a steam generator for stably producing steam, namely, relates to a printed circuit steam generator or a plate type steam generator. 
     BACKGROUND ART 
     A printed circuit heat exchanger has been developed by the Heatric Ltd. in UK, and very variously used in general industrial fields. The printed circuit heat exchanger is a heat exchanger having a structure in which welding between plates of the heat exchanger is avoided using a dense arrangement of channels by a photo-chemical etching technique and diffusion bonding. Accordingly, the printed circuit heat exchanger is applicable to high-temperature and high-pressure environments and has a high-density and excellent heat exchange performance. The advantages of the printed circuit heat exchanger, such as durability against the high-temperature and high-pressure environments, the high-density and the excellent heat exchange efficiency, extend an application range of the printed circuit heat exchanger to various fields, such as an evaporator, a condenser, a cooler, a radiator, a heat exchanger, a reactor, and the like, involved in an air conditioning, a fuel cell, a vehicle, a chemical process, a medical instrument, atomic energy, a nuclear power plant, a communication device, a very low temperature environment and the like. 
     The plate type heat exchanger is widely applied in industrial fields over one hundred years. The plate type heat exchanger is generally configured such that plates are pressed out to form channels and then coupled using gaskets or by typical molding or brazing. Accordingly, the plate type heat exchanger is similar to the printed circuit heat exchanger in view of an application field, but is more widely used under a low-pressure environment. Heat exchange efficiency of the plate type heat exchanger is lower than that of the printed circuit heat exchanger but higher than that of a shell and tube heat exchanger. Also, the plate type heat exchanger is manufactured through more simplified processes than the printed circuit heat exchanger. 
     However, in the applications involving two phase flow such as evaporators, the printed circuit and plate type heat exchangers have been used within limited operating conditions. The printed circuit heat exchanger or plate type heat exchanger has not been widely used as a steam generator, due to flow instabilities in channels, although it exhibits much higher heat transfer efficiency than other types of heat exchangers, such as the shell and tube type heat exchanger and the like. 
     Therefore, a heat exchanger which is capable of generating steam stably in various operation ranges as well as solving flow instabilities in flow channels may be taken into account. 
     DISCLOSURE OF THE INVENTION 
     Therefore, an aspect of the detailed description is to provide a heat exchanger capable of being used as a steam generator. 
     Another aspect of the detailed description is to provide a heat exchanger capable of generating steam more stably with an improved structure. 
     To achieve these and other advantages and in accordance with the purpose of the present invention, as embodied and broadly described herein, there is provided a heat exchanger for a steam generator, the heat exchanger including a plate, and channels formed on the plate, wherein each of the channels includes a primary heat transmission section including a bent or curved flow path to extend longer than a distance between one side and another side, and a flow resistance section formed having a smaller width than the width of the channels formed on the primary heat transmission section, and connected to one side of the primary heat transmission section in a manner of having a bent or curved flow path to extend longer than a distance between an inlet and an outlet. 
     In accordance with one embodiment of the present invention, the heat exchanger may further include a flow path expanding section formed between the flow resistance section and the primary heat transmission section in a manner of having a gradually increasing width. 
     In accordance with one embodiment of the present invention, the flow resistance section may further include a bent or curved flow path for an increased flow resistance of the flow resistance section. 
     In accordance with one embodiment of the present invention, the flow resistance section may include first parts extending in a first direction as a direction connecting the inlet and the outlet to each other, and second parts extending in a second direction intersecting with the first direction. The first and second parts may be formed in an alternating manner. 
     In accordance with one embodiment of the present invention, the flow resistance section may further include a flow path region of sudden expansion or sudden contraction for an increased flow resistance of the flow resistance section. 
     In accordance with one embodiment of the present invention, one of the first and second parts may be connected to an edge of the other. 
     In accordance with one embodiment of the present invention, one of the first and second parts may be connected to a portion between both ends of the other. 
     In accordance with one embodiment of the present invention, the flow resistance section may be configured such that a forward path coming from the inlet toward the outlet has smaller flow resistance than that of a backward path coming from the outlet toward the inlet. 
     In accordance with one embodiment of the present invention, the flow resistance section may include first and second tilt portions connecting the inlet and the outlet, and a bypass portion formed in a manner that the backward path has greater flow resistance. 
     In accordance with one embodiment of the present invention, the bypass portion may be configured to extend from one end of one of the tilt portions to a portion between both ends of the other tilt portion so as to be getting away from the outlet. 
     In accordance with one embodiment of the present invention, the primary heat transmission section may include a first area in which fluid in a liquid state exists, a second area in which fluid in liquid and gaseous states exists, and a third area in which fluid in a gaseous state exists. At least one of channels of the first to third areas may be connected in a communicating manner. 
     In accordance with one embodiment of the present invention, the heat exchanger may further include a common header connected to inlets of the flow resistance section. 
     A heat exchanger for a steam generator according to another embodiment of the present invention, to achieve these and other advantages may include first to third plates overlaid on one another, and channels formed on the plates, respectively, wherein each of the channels includes a primary heat transmission section having a bent or curved flow path to extend longer than a distance between one side and another side, wherein the second plate includes a flow resistance section that is formed having a smaller width than the width of the channels of the primary heat transmission section, and connected to one side of the primary heat transmission section in a manner of having a bent or curved flow path to extend longer than a distance between an inlet and an outlet. 
     In accordance with one embodiment of the present invention, a first fluid may be introduced and discharged through the channels of the first plate, and a second fluid may be introduced and discharged through the channels of the second and third plates. 
     In accordance with one embodiment of the present invention, in the overlaid state of the second and third plates, the primary heat transmission section of the third plate may form an upper portion of a second channel, the primary heat transmission section of the second plate may form a lower portion of the second channel, and the first plate may form a channel with at least one plate. 
     In accordance with one embodiment of the present invention, the second plate may further include a lower flow path expanding section formed between the flow resistance section and the primary heat transmission section in a manner of having a gradually increasing width. 
     In accordance with one embodiment of the present invention, the third plate may further include an upper flow path expanding section formed at a position corresponding to the lower flow path expanding section. 
     In accordance with one embodiment of the present invention, the flow resistance section may further include a bent or curved flow path for an increased flow resistance of the flow resistance section. 
     In accordance with one embodiment of the present invention, the flow resistance section may include first parts extending in a first direction as a direction connecting the inlet and the outlet to each other, and second parts extending in a second direction intersecting with the first direction. The first and second parts may be formed in an alternating manner. 
     In accordance with one embodiment of the present invention, the flow resistance section may further include a flow path region of sudden expansion or sudden contraction, in order to increase flow resistance of the flow resistance section. 
     In accordance with one embodiment of the present invention, one of the first and second parts may be connected to an edge of the other. 
     In accordance with one embodiment of the present invention, one of the first and second parts may be connected to a portion between both ends of the other. 
     In accordance with one embodiment of the present invention, the flow resistance section may be configured such that a forward path coming from the inlet toward the outlet has smaller flow resistance than that of a backward path coming from the outlet toward the inlet. 
     In accordance with one embodiment of the present invention, the flow resistance section may include first and second tilt portions connecting the inlet and the outlet, and a bypass portion formed in a manner that the backward path has greater flow resistance. 
     In accordance with one embodiment of the present invention, the bypass portion may be configured to extend from one end of one of the tilt portions to a portion between both ends of the other tilt portion so as to be getting away from the outlet. 
     Advantageous Effect 
     In accordance with the detailed description, a heat exchanger for a steam generator according to at least one embodiment of the present invention with the configuration can increase flow resistance in a flow resistance section, which may enable more stable production of steam and therefore expand a lifespan of the heat exchanger for the steam generator. 
     Also, a wider flow path area can be applied to the steam generator, which may result in reducing contamination of the flow path. 
     And, with the use of simply switching flow paths, the heat exchanger for the steam generator according to the present invention can be applied to the related art heat exchanger for the steam generator. Also, the heat exchanger for the steam generator can be fabricated into a more compact size, and welded portions can be removed from primary heat transmission section. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a conceptual view of channels formed on a second plate of the related art heat exchanger. 
         FIG. 2  is a conceptual view of channels formed on a first plate of the related art heat exchanger. 
         FIGS. 3 to 7  are conceptual views of channels formed on a second plate of a heat exchanger for a steam generator in accordance with embodiments of the present invention. 
         FIGS. 8 to 12  are conceptual views of channels formed on a second plate of a heat exchanger for a steam generator in accordance with embodiments of the present invention. 
         FIGS. 13A and 13B  are conceptual views of channels formed on a second plate of a heat exchanger for a steam generator in accordance with another embodiment of the present invention. 
         FIG. 14  is a conceptual view of channels formed on a third plate of a heat exchanger for a steam generator in accordance with another embodiment of the present invention. 
         FIG. 15  is a conceptual view of channels formed on a second plate of a heat exchanger for a steam generator in accordance with another embodiment of the present invention. 
         FIG. 16  is a conceptual view of channels formed on a first plate of a heat exchanger for a steam generator in accordance with another embodiment of the present invention. 
         FIG. 17  is a cross-sectional view, taken along the line IV-IV of  FIGS. 14 to 16 . 
         FIG. 18  is a cross-sectional view, taken along the line V-V of  FIGS. 14 to 16 . 
         FIGS. 19 and 20  are conceptual views illustrating a flow of fluid in a flow resistance section illustrated in  FIGS. 7 and 12 , respectively. 
     
    
    
     MODES FOR CARRYING OUT THE PREFERRED EMBODIMENTS 
     Description will now be given in detail of a heat exchanger for a steam generator according to exemplary embodiments disclosed herein, with reference to the accompanying drawings. A suffix “module” or “unit” used for constituent elements disclosed in the following description is merely intended for easy description of the specification, and the suffix itself does not give any special meaning or function. For the sake of brief description with reference to the drawings, the same or equivalent components will be provided with the same reference numbers, and description thereof will not be repeated. A singular representation may include a plural representation unless it represents a definitely different meaning from the context. 
     A steam generator turns (converts) secondary water into steam using heat of primary water, supplies the steam to a turbine, and rotates the turbine using the supplied steam to generate electric power. A plurality of heat exchangers is disposed in the steam generator. And, when a first fluid passes through a first plate of a heat exchanger, a second fluid passing through a second plate is converted into steam by heat transferred to the second plate which is disposed adjacent to the first plate. 
       FIG. 1  is a conceptual view of channels C formed on a second plate  120  of the related art heat exchanger, and  FIG. 2  is a conceptual view of channels C formed on a first plate of the related art heat exchanger. 
     As illustrated in  FIGS. 1 and 2 , when a first coolant flows through the channels C formed on the first plate  110 , heat is transferred to the second plate  120 . The transferred heat may heat a second coolant which flows along the second plate  120 , thereby producing steam. 
     In this process, generally in heat exchangers involving two phase flow within flow channels, flow instabilities may occur if the flow path (d 1  in  FIG. 1 ) is used, due to the pressure wave propagation, which stems from a rapid increase in volume and decrease in density by steam generation. Accordingly, pressure waves are propagated forward and backward in a flow path direction. The pressure drop difference which is initiated from a discrepancy of the phase change location causes an unstable flow, and this increases the flow instability. Especially, for a steam generator having a plurality of flow channels connected to a common header, the instability becomes more stronger by the feedback effect of the phase change mismatch between the multi-channels (parallel channel oscillation), and a function as a steam generator could be lost. This is specifically an important issue for the steam generators with a wide range operation mode such as a startup and a low level power operation. 
     To reduce such effects a shell and tube type steam generator with a wide general operation range applies an orifice with high flow resistance at the inlet of the secondary tube. 
     As illustrated in  FIG. 1 , the related art technology (d 2  to d 4 ) simply reducing a flow path area may cause problems, such as flow path fouling, clogging, blocking and the like, and thus may be restricted from being applied to applications requiring for a long-term lifespan, such as a nuclear power environment. In the present invention, contamination of a flow path refers to an effect that various types of impurities, which are accumulated due to a long-term use of the steam generator, reduce or block a cross section of a flow path. As a result, this affects a flow rate of water. This problem may be accelerated as an inlet flow path cross section is more reduced. 
     The first plate and the second plate may be installed at positions where inlets or outlets thereof do not overlap each other, and thus the present invention may not be limited to the configuration of the printed circuit flow path as illustrated in  FIG. 1 or 2 . 
     Hereinafter, a heat exchanger or a heat exchanger for a steam generator disclosed herein, unless especially mentioned, generally refers to the general plate type and printed circuit heat exchangers, and also even a case of employing a different processing or bonding method for plates. 
       FIGS. 3 to 7  are conceptual views of channels formed on a second plate of a heat exchanger for a steam generator in accordance with embodiments of the present invention. 
     While a second fluid flows through the second plate  220 ,  320 ,  420 ,  520 ,  620 , phase transition from liquid to gas occurs, thereby generating steam. The second plate  220 ,  320 ,  420 ,  520 ,  620  may include a plurality of channels C, which may have widths in the range of one meter (m) to several millimeters (mm). 
     Each of the channels C may divided into a primary heat transmission section  221 ,  321 ,  421 ,  521 ,  621  and a flow resistance section  222 ,  322 ,  422 ,  522 ,  622 . The channel C of the primary heat transmission section  221 ,  321 ,  421 ,  521 ,  621  may be bent so as to extend longer than a distance between one side  221   a ,  321   a ,  421   a ,  521   a ,  621   a  and another side  221   b ,  321   b ,  421   b ,  521   b ,  621   b  (a length at which one side  221   a ,  321   a ,  421   a ,  521   a ,  621   a  and another side  221   b ,  321   b ,  421   b ,  521   b ,  621   b  are connected in a straight line). This may extend the length of each channel C than the straightly-connected length, which may greatly increase a heat exchange area and improve heat exchanger efficiency accordingly. The embodiment disclosed herein merely illustrates the bent shape, but the present invention may not be necessarily limited to the flow path in the bent shape because a similar effect can be obtained even in case of using a curved flow path. 
     Each of the channels of the flow resistance section  222 ,  322 ,  422 ,  522 ,  622  may have a width smaller than the width of the channel formed on the primary heat transmission section  221 ,  321 ,  421 ,  521 ,  621 , and may be bent so as to extend longer than a distance between one side  222   a ,  322   a ,  422   a ,  522   a ,  622   a  and another side  222   b ,  322   b ,  422   b ,  522   b ,  622   b  (a length at which one side  222   a ,  322   a ,  422   a ,  522   a ,  622   a  and another side  222   b ,  322   b ,  422   b ,  522   b ,  622   b  are connected in a straight line). The flow resistance section  222 ,  322 ,  422 ,  522 ,  622  may be connected to one side corresponding to an inlet of the primary heat transmission section  221 ,  321 ,  421 ,  521 ,  621 . The flow resistance section  222 ,  322 ,  422 ,  522 ,  622  may be provided with longer and narrower channels at the inlet area, resulting in greater flow resistance and thus reduced flow instability in each channel within a wide operation range. Accordingly, the steam generator can operate in a stable state. The embodiment disclosed herein merely illustrates the bent shape, but the present invention may not be necessarily limited to the bent shape because a similar effect can be obtained even in case of using a curved flow path. 
     A flow path expanding section  223 ,  323 ,  423 ,  523 ,  623  may be formed between the flow resistance section  222 ,  322 ,  422 ,  522 ,  622  and the primary heat transmission section  221 ,  321 ,  421 ,  521 ,  621 . The flow path expanding section  223 ,  323 ,  423 ,  523 ,  623  may have a width which gradually increases, thereby preventing a drastic change in the coolant flow. 
       FIGS. 3 and 4  illustrate exemplary configurations according to the present invention which employ flow path structures reducing a flow path area and increasing a flow path length, respectively, in order to increase flow resistance of the flow resistance sections  222 ,  322 , but the present invention may not be necessarily limited to these configurations. 
     Referring to  FIG. 3 , the flow resistance section  222  includes first (primary) parts  222   c  and second (secondary) parts  222   d . The first parts  222   c  are portions extending in a first (primary) direction which is a direction connecting an inlet and an outlet, and the second parts  222   d  are portions extending in a second (secondary) direction which intersects with the first direction. The first parts  222   c  and the second parts  222   d  may be formed in an alternating manner. One of the first and second parts  222   c  and  222   d  may be connected to an edge of the other. 
     Referring to  FIG. 4 , the flow resistance section  322  includes first tilt portions  322   c  and second tilt portions  322   d . The first tilt portion  322   c  and the second tilt portion  322   d  may be connected with each other at one end. 
       FIGS. 5 and 6  illustrate exemplary configurations according to the present invention which employ different flow path structures from those illustrated in  FIGS. 3 and 4 , respectively, in order to increase flow resistance of the flow resistance sections  422  and  522 , but the present invention may not be necessarily limited to this configuration. 
     Referring to  FIG. 5 , the flow resistance section  422  includes first parts  422   c  and second parts  422   d . The first parts  422   c  are portions extending in a first direction which is a direction connecting an inlet and an outlet, and the second parts  422   d  are portions extending in a second direction that intersects with the first direction. The first parts  422   c  and the second parts  422   d  may be formed in an alternating manner. One of the first and second parts  422   c  and  422   d  may be connected to an edge of the other. The first parts  422   c  and the second parts  422   d  have different lengths and more bent portions, respectively, unlike those illustrated in  FIG. 3 . This may increase the flow resistance further. 
     Referring to  FIG. 6 , the flow resistance section  522  includes first parts  522   c  and second parts  522   d . The first parts  522   c  are portions extending in a first direction which is a direction connecting an inlet and an outlet, and the second parts  522   d  are portions extending in a second direction that intersects with the first direction. The first parts  522   c  and the second parts  522   d  may be formed in an alternating manner. One of the first and second parts  522   c  and  522   d  is connected to a portion between both ends of the other. The first and second parts  522   c  and  522   d , unlike those illustrated in  FIG. 3 , may have different lengths, respectively, and also include a flow path region of sudden expansion or sudden contraction, so as to have a shape causing greater flow resistance. This may result in an increased the flow resistance. 
       FIG. 7  illustrates an exemplary configuration according to the present invention in which different flow path structures are applied in a forward direction and a backward direction in order to increase backward flow resistance of the flow resistance section  622 , but the present invention may not be limited to this configuration. 
     Referring to  FIG. 7 , the flow resistance section  622  includes first tilt portions  622   c  and second tilt portions  622   d . Here, the flow resistance section  622  is configured such that a forward path coming from an inlet to an outlet has smaller flow resistance than a backward path coming from the outlet to the inlet. Accordingly, the backward flow resistance may become greater than the forward flow resistance. 
     To achieve this, a bypass portion  622   e  is provided in which the backward path has greater flow resistance. The bypass portion  622   e  connects an edge of one of the tilt portions to a portion between both ends of the other tilt portion so as to be getting away from an outlet. 
       FIGS. 8 to 12  are conceptual views of channels formed on a second plate of a heat exchanger for a steam generator in accordance in accordance with embodiments of the present invention. In such case, the channels may be formed on the first plate by switching a flowing direction to be opposite to the flowing direction of  FIG. 1  (d 1 ). 
     The second plate  1220 ,  1320 ,  1420 ,  1520 ,  1620  may include a plurality of channels C, which have widths in the range of 1 m to several millimeters (mm). 
     Each of the channels C formed on the second plate  1220 ,  1320 ,  1420 ,  1520 ,  1620  may be divided into a primary heat transmission section  1221 ,  1321 ,  1421 ,  1521 ,  1621  and a flow resistance section  1222 ,  1322 ,  1422 ,  1522 ,  1622 . Each of the channels C of the primary heat transmission sections  1221 ,  1321 ,  1421 ,  1521 ,  1621  may be bent so as to extend longer than a distance between one side  1221   a ,  1321   a ,  1421   a ,  1521   a ,  1621   a  and another side  1221   b ,  1321   b ,  1421   b ,  1521   b ,  1621   b  (a length at which one side  1221   a ,  1321   a ,  1421   a ,  1521   a ,  1621   a  and another side  1221   b ,  1321   b ,  1421   b ,  1521   b ,  1621   b  are connected in a straight line). This may extend channel length, which may increase the heat exchange area and improve heat exchanger efficiency accordingly. The embodiment disclosed herein merely illustrates the bent shape, but the present invention may not be necessarily limited to the flow path in the bent shape because a similar effect can be obtained even in case of using a curved flow path. 
     Each of the channels of the flow resistance section  1222 ,  1322 ,  1422 ,  1522 ,  1622  may have a width smaller than a channel formed on the primary heat transmission section  1221 ,  1321 ,  1421 ,  1521 ,  1621 , and may be bent so as to extend longer than a distance between one side  1222   a ,  1322   a ,  1422   a ,  1522   a ,  1622   a  and another side  1222   b ,  1322   b ,  1422   b ,  1522   b ,  1622   b  (a length at which one side  1222   a ,  1322   a ,  1422   a ,  1522   a ,  1622   a  and another side  1222   b ,  1322   b ,  1422   b ,  1522   b ,  1622   b  are connected in a straight line). The flow resistance section  1222 ,  1322 ,  1422 ,  1522 ,  1622  may be connected to one side corresponding to an inlet of the primary heat transmission section  1221 ,  1321 ,  1421 ,  1521 ,  1621 . The flow resistance section  1222 ,  1322 ,  1422 ,  1522 ,  1622  may form channels, which are longer in length and smaller in width, at the inlet area of the heat exchanger. This may result in greater flow resistance and thus reduced flow instability in each channel within a wide operation range. Accordingly, the steam generator can operate in a stable state. The embodiment disclosed herein merely illustrates the bent shape, but the present invention may not be necessarily limited to the bent shape because a similar effect can be obtained even in case of using a curved flow path. 
     A flow path expanding section  1223 ,  1323 ,  1423 ,  1523 ,  1623  may be formed between the flow resistance section  1222 ,  1322 ,  1422 ,  1522 ,  1622  and the primary heat transmission section  1221 ,  1321 ,  1421 ,  1521 ,  1621 . 
     The flow path expanding section  1223 ,  1323 ,  1423 ,  1523 ,  1623  may have a width which gradually increases, thereby preventing a drastic change in the coolant flow. 
     Also, a common header  1224 ,  1324 ,  1424 ,  1524 ,  1624  may be formed at an inlet of the flow resistance section  1222 ,  1322 ,  1422 ,  1522 ,  1622 . A second fluid supplied through the common header  1224 ,  1324 ,  1424 ,  1524 ,  1624  is distributed into the channels C of the second plate  1220 ,  1320 ,  1420 ,  1520 ,  1620 , respectively. 
       FIGS. 8 and 9  illustrate exemplary configurations according to the present invention employing flow path structures of reducing a flow path area and increasing a flow path length, in order to increase flow resistance of the flow resistance sections  1222 ,  1322 , but the present invention may not be necessarily limited to these configurations. 
     Referring to  FIG. 8 , the flow resistance section  1222  includes first parts  1222   c  and second parts  1222   d . The first parts  1222   c  are portions extending in a first direction which is a direction connecting an inlet and an outlet, and the second parts  1222   d  are portions extending in a second direction which intersects with the first direction. The first parts  1222   c  and the second parts  1222   d  may be formed in an alternating manner. One of the first and second parts  1222   c  and  1222   d  may be connected to an edge of the other. 
     Referring to  FIG. 9 , the flow resistance section  1322  includes first tilt portions  1322   c  and second tilt portions  1322   d . The first tilt portion  1322   c  and the second tilt portion  1322   d  may be connected with each other at one end. 
       FIGS. 10 and 11  illustrate exemplary configurations according to the present invention which employs different flow path structures from those illustrated in  FIGS. 8 and 9 , in order to increase flow resistance of the flow resistance sections  1422  and  1522 , but the present invention may not be necessarily limited to this configuration. 
     Referring to  FIG. 10 , the flow resistance section  1422  includes first parts  1422   c  and second parts  1422   d . The first parts  1422   c  are portions extending in a first direction which is a direction connecting an inlet and an outlet, and the second parts  1422   d  are portions extending in a second direction that intersects with the first direction. The first parts  1422   c  and the second parts  1422   d  may be formed in an alternating manner. One of the first and second parts  1422   c  and  1422   d  may be connected to an edge of the other. The first and second parts  1422   c  and  1422   d , unlike those illustrated in  FIG. 3 , have different shapes and more bent portions, respectively. This may increase the flow resistance further. 
     Referring to  FIG. 11 , the flow resistance section  1522  includes first parts  1522   c  and second parts  1522   d . The first parts  522   c  are portions extending in a first direction which is a direction connecting an inlet and an outlet, and the second parts  1522   d  are portions extending in a second direction that intersects with the first direction. The first parts  1522   c  and the second parts  1522   d  may be formed in an alternating manner. One of the first and second parts  1522   c  and  1522   d  is connected to a portion between both side ends of the other. The first and second parts  1522   c  and  1522   d , unlike those illustrated in  FIG. 3 , may have different lengths, respectively, and also include a flow path region of sudden expansion or sudden contraction, so as to have a shape causing greater flow resistance. This may result in an increased the flow resistance. 
       FIG. 12  illustrates an exemplary configuration according to the present invention which employs different flow path structures in a forward direction and a backward direction in order to increase backward flow resistance of the flow resistance section  1622 , but the present invention may not be limited to this configuration. 
     Referring to  FIG. 12 , the flow resistance section  622  includes first tilt portions  1622   c  and second tilt portions  1622   d . Here, the flow resistance section  1622  is configured such that a forward path coming from an inlet to an outlet has smaller flow resistance than a backward path coming from the outlet to the inlet. Accordingly, the backward flow resistance may be greater than the forward flow resistance. 
     To achieve this, a bypass portion  1622   e  is provided in which the backward path has greater flow resistance. The bypass portion  1622   e  connects one end of one of the tilt portions to a portion between both ends of the other tilt portion so as to be getting away from an outlet. 
       FIGS. 13A and 13B  are conceptual views of channels C formed on a second plate of a heat exchanger for a steam generator in accordance in another exemplary embodiment of the present invention. 
     Referring to  FIG. 13A , each of the channels C may be divided into a primary heat transmission section  221  and a flow resistance section  222 . Each of the channels C of the primary heat transmission section  221  may be bent so as to extend longer than a distance between one side  221   a  and another side  221   b  (a length at which one side  221   a  and another side  221   b  are connected in a straight line). This may extend the length of each channel C than the straightly-connected length, which may increase the heat exchange area and improve heat exchanger efficiency accordingly. 
     The embodiment disclosed herein merely illustrates the bent shape, but the present invention may not be necessarily limited to the flow path in the bent shape because a similar effect can be obtained even in case of using a curved flow path. 
     The primary heat transmission section  221  may be divided into a first area R 1  in which fluid in a liquid state exists, a second area R 2  in which fluid in liquid and gaseous states exists, and a third area R 3  in which fluid in a gaseous state exists. 
     The channels C of the second area R 2  or the third area R 3  may communicate with each other. In more detail, the channels C of the second area R 2  adjacent to the third area R 3  may communicate with each other. This may more facilitate the fluid in the gaseous state to flow along the channels C. 
     Each of the channels of the flow resistance section  222  may be configured to be narrower in width than the channel formed on the primary heat transmission section  221 , and configured into a bent form so as to extend longer than a distance between an inlet  222   a  and an outlet  222   b  (a length at which an inlet  222   a  and an outlet  222   b  are connected in a straight line). The flow resistance section  222  may be connected to one side corresponding to an inlet of the primary heat transmission section  221 . The flow resistance section  222  may form channels with a longer length and a smaller width at an inlet area of the heat exchanger, to generate great flow resistance, thereby reducing flow instability in each channel within a wide operation range. This may allow for a stable operation of the steam generator. This embodiment merely illustrates the bent shape, but the present invention may not be limited to the bent shape because a similar effect can be obtained even in case of using a curved flow path. 
     A flow path expanding section  223  may be formed between the flow resistance section  222  and the primary heat transmission section  221 . The flow path expanding section  223  may be formed to have a gradually increasing width, so as to prevent the drastic change in a flow of coolant. 
     Still referring to  FIG. 13A , the flow resistance section  222  includes first parts  212   c  and second parts  212   d . The first parts  212   c  are portions extending in the first direction which is a direction connecting an inlet and an outlet, and the second parts  212   d  are portions extending in a second direction which intersects with the first direction. The first parts  212   c  and the second parts  212   d  may be formed in an alternating manner. One of the first and second parts  212   c  and  212   d  may be connected to an edge of the other.  FIG. 13A  illustrates an exemplary configuration in which some flow paths communicate with each other, but the present invention may not be necessarily limited to such configurations. 
     Also, referring to  FIG. 13B , when most of the channels C of the primary heat transmission section  221  are configured to communicate with one another, the primary heat transmission section  221  may exhibit similar characteristics to an operation of a shell side of a shell and tube type heat exchanger. Therefore, the flow resistance section  222  serves as an economizer which enables a uniform distribution of a flow rate and improves heat transfer characteristics.  FIG. 13B  illustrates an exemplary configuration in which most channels of the primary heat transmission section  221  communicate with one another, but the present invention may not be necessarily limited to such configurations. 
       FIG. 14  is a conceptual view of channels C formed on a third plate of a heat exchanger for a steam generator in accordance with another embodiment of the present invention,  FIG. 15  is a conceptual view of channels C formed on a second plate of a heat exchanger for a steam generator in accordance with another embodiment of the present invention, and  FIG. 16  is a conceptual view of channels C formed on the first plate of a heat exchanger for a steam generator in accordance with another embodiment of the present invention. 
     And,  FIG. 17  is a cross-sectional view, taken along the line IV-IV of  FIGS. 14 to 16 , and  FIG. 18  is a cross-sectional view, taken along the line V-V of  FIGS. 14 to 16 . 
     As illustrated in  FIGS. 14 to 18 , the first to third plates  710 ,  720  and  730  are arranged in an overlaying manner. In more detail, the second plate  720  is disposed on the first plate  710 , and the third plate  730  may be disposed on the second plate  720 . Although not illustrated, at least one another plate may be disposed on the third plate  730 , and a second fluid may flow along the plate disposed on the third plate  730 . 
     While flowing along the first plate  710 , a first fluid transfers heat to a second fluid which flows along the second and third plates  720  and  730 . Phase transition of the second fluid from liquid to gas may occur due to the heat from the first fluid. 
     In this instance, the second and third plates  720  and  730  may form one channel at a predetermined section. That is, as illustrated in  FIG. 18 , when the second plate  720  forms a lower portion of a channel, the third plate may form an upper portion of the channel. Here, the predetermined section may correspond to the primary heat transmission sections  721  and  731  of the channels C formed on the second and third plates  720  and  730 , respectively. 
     Referring back to  FIG. 15 , each of the channels C of the second plate  720  may be divided into a primary heat transmission section  721  and a flow resistance section  722 . The channel C of the primary heat transmission section  721  may be configured into a bent form so as to extend longer than a distance between one side  721   a  and another side  721   a  (a length at which one side  721   a  and another side  721   a  are connected in a straight line). This may extend the length of the channel C than the straightly-connected length, which may increase the heat exchange area and improve heat exchanger efficiency accordingly. The embodiment disclosed here has illustrated the bent shape, but the present invention may not be necessarily limited to the flow path in the bent shape because a similar effect can be obtained even in case of using a curved flow path. 
     Each of the channels C of the flow resistance section  722  is configured to be narrower in width than the channel formed on the primary heat transmission section  721 , and configured into a bent form so as to extend longer than a distance between an inlet  722   a  and an outlet  722   b  (a length at which an inlet  722   a  and an outlet  722   b  are connected in a straight line). The flow resistance section  722  may be connected to one side corresponding to an inlet of the primary heat transmission section  721 . The flow resistance section  722  may form channels with a longer length and a smaller width at an inlet area of the heat exchanger, to generate great flow resistance, thereby reducing flow instability in each channel within a wide operation range. This may allow for a stable operation of the steam generator. This embodiment merely illustrates the bent shape, but the present invention may not be limited to the bent shape because a similar effect can be obtained even in case of using a curved flow path. 
     A flow path expanding section  723  may be formed between the flow resistance section  722  and the primary heat transmission section  721 . The flow path expanding section  723  may have a width which gradually increases, so as to prevent the drastic change in a flow of coolant. 
     Referring back to  FIG. 14 , each of the channels C of the third plate  730  may include only a primary heat transmission section  731  and a flow path expanding section  733 , without a flow resistance section. This results from that the second and third plates  720  and  730  form the lower and upper portions of the channel, respectively. The flow resistance section  722  of the second plate  720  may be connected to the flow path expanding sections  723  and  733  of the second and third plates  720  and  730 . 
     Referring back to  FIG. 16 , each of the channels formed on the first plate  710  includes the primary heat transmission section  711 . Each channel of the primary heat transmission section  711  may be bent to extend longer than a distance between one side  711   a  and another side  711   b  (a length at which one side  711   a  and another side  711   b  are connected in a straight line). This may extend the length of each channel C than the straightly-connected length, which may increase a heat exchange area and improve heat exchanger efficiency accordingly. The embodiment disclosed here has illustrated the bent shape, but the present invention may not be necessarily limited to a flow path in a bent shape because a similar effect can be obtained even in case of using a curved flow path. 
     The plates illustrated in  FIGS. 14 to 16  merely illustrate the embodiments constructing the plates of the heat exchanger. That is, as aforementioned with reference to  FIGS. 3 to 13 , a flow resistance section, a flow path expanding section or a common header may be formed on a plate according to design conditions of the heat exchanger. 
       FIGS. 19 and 20  are conceptual views illustrating fluid flows inside a flow resistance section illustrated in  FIGS. 7 and 12 , respectively. As illustrated, the flow resistance section  612 ,  622  includes first tilt portions  612   c ,  622   c  and second tilt portions  612   d ,  622   d . Here, the flow resistance section  612 ,  622  is configured such that a forward path coming from an inlet to an outlet exhibits smaller flow resistance than a backward path coming from the outlet to the inlet and a forward flow exhibits a smoother change than a backward flow. Accordingly, the backward flow resistance may be greater than the forward flow resistance. 
     To achieve this, bypass portion  612   e ,  622   e  with great flow resistances is provided, which results from extended backward path and interference between flowing directions intersecting with each. The bypass portion  612   e ,  622   e  is configured in a manner that connects one end of one of the tilt portions to a portion between both ends of the other tilt portion so as to be getting away from an outlet. 
     Fluid flows along the first tilt portion  612   c ,  622   c  and the second tilt portion  612   d ,  622   d  in the forward direction, whereas flowing along the first tilt portion  612   c ,  622   c  and then flowing toward a middle point of the second tilt portion  612   d ,  622   d  via the bypass portion  612   e ,  622   e  in the backward direction. Accordingly, the backward path may become longer than the forward path and flowing directions of the backward and forward paths may cross each other to cause interference therebetween. This may result in more increased backward flow resistance than forward flow resistance. 
     The aforementioned heat exchanger for the steam generator may not be necessarily limited to the configurations and methods of the foregoing embodiments, but a part or all of the embodiments can be selectively combined to derive many variations. 
     [Industrial Availability] 
     The heat exchanger for the steam generator according to the present invention may not be limited applied to the configurations and methods of the aforementioned embodiments, but a part or all of the embodiments can be selectively combined to derive various modifications.