Patent Publication Number: US-11022376-B2

Title: Heat exchanger

Description:
TECHNICAL FIELD 
     The present invention relates to a heat exchanger, and more particularly, it relates to a heat exchanger that performs heat exchange between a first fluid and a second fluid. 
     BACKGROUND ART 
     Conventionally, a heat exchanger that performs heat exchange between a first fluid and a second fluid is known. Such a heat exchanger is disclosed in Japanese Patent Laid-Open No. 2010-101617, for example. 
     Japanese Patent Laid-Open No. 2010-101617 discloses a plate-fin heat exchanger including a layer through which no fluid flows between heat exchange passage packages in which first passages through which a first fluid flows and second passages through which a second fluid flows are alternately disposed. In the heat exchange between the first fluid and the second fluid, the thermal stress increases as the temperature gradient increases. Therefore, in Japanese Patent Laid-Open No. 2010-101617, the layer through which no fluid flows is disposed between the heat exchange passage packages such that the temperature gradient is significantly reduced, and the thermal stress is reduced. The heat exchanger disclosed in Japanese Patent Laid-Open No. 2010-101617 is particularly used for applications such as liquefaction or vaporization of a natural gas having a large temperature difference with a fluid. 
     PRIOR ART 
     Patent Document 
     Patent Document 1: Japanese Patent Laid-Open No. 2010-101617 
     SUMMARY OF THE INVENTION 
     Problems to be Solved by the Invention 
     When the low-temperature first fluid is a cryogenic liquefied gas and the high-temperature second fluid is water or antifreeze, for example, there is a possibility that the passages are clogged by solidifying (freezing). 
     In the heat exchanger disclosed in Japanese Patent Laid-Open No. 2010-101617, although it is possible to reduce the thermal stress by providing the layer through which no fluid flows and significantly reducing or preventing excessive heat transfer between the flow paths, no consideration is given to the risk of occurrence of freezing in the flow paths, and there is a problem that the flow paths may be clogged by occurrence of freezing. In addition, simply providing the layer through which no fluid flows between the flow paths reduces the heat exchange performance, and thus there is a problem that the size of the heat exchanger is increased due to an increase in flow path length, for example. 
     The present invention has been proposed in order to solve the aforementioned problems, and one object of the present invention is to provide a heat exchanger in which an increase in its size can be significantly reduced or prevented while fluid freezing is significantly reduced or prevented even when heat exchange is performed between fluids having a large temperature difference. 
     Means for Solving the Problems 
     In order to attain the aforementioned object, a heat exchanger according to the present invention includes a first flow path through which a first fluid flows, a second flow path through which a second fluid flows, and an adjustment layer disposed between the first flow path and the second flow path adjacent to each other and that adjusts an amount of heat exchange between the first flow path and the second flow path, and the adjustment layer includes a first portion and a second portion having a heat transfer performance lower than that of the first portion, and has a heat transfer performance varied depending on a position in the adjustment layer. 
     As described above, the heat exchanger according to the present invention includes the adjustment layer disposed between the first flow path and the second flow path adjacent to each other and that adjusts the amount of heat exchange between the first flow path and the second flow path. Accordingly, the adjustment layer between the first flow path and the second flow path can significantly reduce or prevent excessive heat transfer between the first flow path and the second flow path. Consequently, fluid freezing can be significantly reduced or prevented even when heat exchange is performed between fluids having a large temperature difference. Furthermore, the adjustment layer includes the first portion and the second portion having a heat transfer performance lower than that of the first portion, and has a heat transfer performance varied depending on the position in the adjustment layer  30 . Accordingly, the second portion is disposed in a portion in which freezing is likely to occur in the flow path to sufficiently decrease the heat transfer performance while the first portion is disposed in a portion in which freezing is unlikely to occur to relatively increase the heat transfer performance such that the high heat exchange performance can be ensured. Accordingly, an increase in a flow path length required to realize a desired amount of heat exchange can be significantly reduced or prevented. Thus, an increase in the size of the heat exchanger can be significantly reduced or prevented while fluid freezing is significantly reduced or prevented even when heat exchange is performed between fluids having a large temperature difference. 
     According to the present invention including the aforementioned configuration, even when there is a possibility of fluid boiling due to heat exchange, the fluid boiling can be significantly reduced or prevented. Occurrence of unintentional boiling in the flow path may increase the load related to the strength of the heat exchanger, and may not be acceptable due to the specification of the heat exchanger. According to the present invention, the second portion is disposed in a portion in which boiling is likely to occur in the flow path such that the heat transfer performance can be sufficiently decreased while the first portion is disposed in a portion in which boiling is unlikely to occur such that the heat transfer performance can be relatively increased. Accordingly, an increase in a flow path length required to realize a desired amount of heat exchange can be significantly reduced or prevented. Thus, an increase in the size of the heat exchanger can be significantly reduced or prevented while unintentional fluid boiling is significantly reduced or prevented. 
     In the aforementioned heat exchanger according to the present invention, in the adjustment layer, the second portion is preferably provided within a predetermined range including a portion that overlaps a vicinity of an inlet or a vicinity of an outlet of the second fluid. According to this configuration, when the temperature of the second fluid monotonously decreases along the second flow path, for example, the second portion includes the portion that overlaps the vicinity of the outlet of the second fluid, which is highly likely to freeze such that occurrence of freezing can be effectively and significantly reduced or prevented. When the temperature of the first fluid becomes cryogenic in the vicinity of the inlet of the second fluid in a parallel-flow heat exchanger and the inner surface temperature of the second flow path is close to the freezing temperature, for example, the second portion includes the portion that overlaps the vicinity of the inlet of the second fluid, which is highly likely to freeze such that occurrence of freezing can be effectively and significantly reduced or prevented. 
     In the aforementioned heat exchanger according to the present invention, the second flow path preferably includes a risk area in which an inner surface temperature of the second flow path is closest to a temperature of the first fluid, and in the adjustment layer, the second portion is preferably disposed within a predetermined range including a portion that overlaps the risk area of the second flow path. According to this configuration, the second portion overlaps the risk area such that occurrence of freezing can be more reliably and significantly reduced or prevented. The risk area can be set as an area in which the inner surface temperature of the second flow path obtained by calculating the temperature distribution of the inner surface of the second flow path when the adjustment layer is not provided (when the first flow path and the second flow path are directly adjacent to each other), for example, is closest to the temperature of the first fluid. 
     In the aforementioned heat exchanger according to the present invention, the adjustment layer preferably includes heat conduction portions that make a connection between the first flow path and the second flow path adjacent to each other, and the first portion and the second portion preferably include the heat conduction portions having different heat transfer performances. According to this configuration, the shape and dimensions of the adjustment layer itself are not adjusted, but the number, size, material, etc. of the heat conduction portions are changed such that the distribution of the heat transfer performances in the first portion and the second portion can be easily adjusted. Consequently, the appropriate distribution of the heat transfer performances according to the risk of occurrence of fluid freezing in the adjustment layer can be easily realized. 
     In this case, a density per unit area of the heat conduction portions in the adjustment layer is preferably varied such that the heat conduction portions have the different heat transfer performances. According to this configuration, unlike the case in which a plurality of types of heat conduction portions made of different materials are provided, for example, the number of heat conduction portions per unit area is changed or a plurality of heat conduction portions having different sizes are arranged at an equal pitch, for example, such that the heat transfer performances of the heat conduction portions can be easily varied. 
     In the aforementioned configuration in which the adjustment layer includes the heat conduction portions, each of the first flow path, the second flow path, and the adjustment layer preferably includes a planar flow path layer, and includes a heat transfer fin inside the planar flow path layer, the heat conduction portions are preferably constituted by the heat transfer fin disposed in the adjustment layer, and at least one of intervals between fin sections of the heat transfer fin and thicknesses of the fin sections are preferably different from each other such that the heat conduction portions have the different heat transfer performances. According to this configuration, the first flow path, the second flow path, and the adjustment layer can share a similar basic structure, and thus each of the first flow path, the second flow path, and the adjustment layer can be each of the flow path layers of the so-called plate-fin heat exchanger. Consequently, unlike the case in which a special structure is used for the adjustment layer, the heat exchanger can be easily constructed even when the adjustment layer is provided. In addition, the heat transfer performance of the adjustment layer can be varied by a simple configuration in which the intervals between the fin sections or the thicknesses of the fin sections are simply different from each other. 
     In the aforementioned heat exchanger according to the present invention, the adjustment layer preferably has a hollow flow path structure disposed between the first flow path and the second flow path and through which a fluid can flow except during the heat exchange. According to this configuration, the hollow structure can easily decrease the heat transfer performance of the adjustment layer, and thus occurrence of freezing can be effectively and significantly reduced or prevented. In addition, the adjustment layer has a hollow flow path structure through which a fluid can flow except during the heat exchange such that as a measure against occurrence of fluid freezing, a heat medium having a temperature higher than the freezing temperature can flow through the adjustment layer except during the heat exchange between the first fluid and the second fluid so as to quickly eliminate freezing. 
     In the aforementioned heat exchanger according to the present invention, the first fluid is preferably a low-temperature liquefied gas evaporated in the first flow path, and the second fluid is preferably a liquid heat medium cooled by the liquefied gas. In such a configuration, there is a possibility of freezing on the second fluid side by heat exchange between the cryogenic first fluid and the second fluid. Even in this case, the first portion and the second portion are provided to vary the heat transfer performance of the adjustment layer such that the heat transfer efficiency can be increased as much as possible within a range in which freezing of the second fluid can be significantly reduced or prevented, and thus an increase in the size of the heat exchanger can be effectively and significantly reduced or prevented. 
     In this case, in the adjustment layer, the first portion is preferably disposed within a range that overlaps a vapor phase region of the first fluid that flows through the first flow path, and in the adjustment layer, the second portion is preferably disposed within a range that overlaps a vapor-liquid mixed phase region of the first fluid that flows through the first flow path. According to this configuration, in the vapor-liquid mixed phase region in which the heat transfer coefficient of the first fluid is high, freezing of the second fluid is significantly reduced or prevented by the second portion having a low heat transfer performance, and in the vapor phase region in which the heat transfer coefficient of the first fluid is low, heat exchange can be efficiently performed by the first portion having a high heat transfer performance. Consequently, the heat exchanger can be made as compact as possible while freezing of the second fluid is significantly reduced or prevented. 
     In the aforementioned structure in which the adjustment layer has a hollow flow path structure through which a fluid can flow except during the heat exchange, when freezing of the second fluid occurs in the second flow path, a heat medium is preferably supplied to the adjustment layer except during the heat exchange so as to eliminate the freezing of the second fluid. According to this configuration, even when freezing occurs in the second flow path, the heat medium for eliminating freezing is supplied to the adjustment layer after the heat exchange (supply of the first fluid and the second fluid) is stopped such that freezing can be easily and quickly eliminated. 
     Effect of the Invention 
     According to the present invention, as described above, the heat exchanger in which an increase in its size can be significantly reduced or prevented while fluid freezing is significantly reduced or prevented even when heat exchange is performed between fluids having a large temperature difference can be provided. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  A perspective view showing a heat exchanger according to the present embodiment. 
         FIG. 2  A schematic longitudinal section view of the heat exchanger showing a first flow path, a second flow path, and an adjustment layer. 
         FIG. 3  A schematic horizontal sectional view showing the structure of the first flow path. 
         FIG. 4  A schematic horizontal sectional view showing the structure of the second flow path. 
         FIG. 5  A schematic horizontal sectional view showing the structure of the adjustment layer. 
         FIG. 6  A schematic sectional view (A) showing the structure of a first portion of the adjustment layer and a schematic sectional view (B) showing the structure of a second portion of the adjustment layer. 
         FIG. 7  A diagram showing simulation results of changes in the temperatures of fluids in the heat exchanger according to the present embodiment. 
         FIG. 8  A diagram showing simulation results of changes in the temperature of the fluids in a heat exchanger according to Comparative Example 1. 
         FIG. 9  A diagram showing simulation results of changes in the temperature of the fluids in a heat exchanger according to Comparative Example 2. 
         FIG. 10  A diagram showing simulation results of changes in the temperature of the fluids in a heat exchanger according to Comparative Example 3. 
         FIG. 11  A schematic view (A) showing a modified example of the heat exchanger according to the present embodiment, a sectional view (B) on the upstream side of the heat exchanger according to the modified example, and a sectional view (C) on the downstream side of the heat exchanger according to the modified example. 
         FIG. 12  A schematic horizontal sectional view showing a modified example of the adjustment layer according to the present embodiment. 
         FIG. 13  A schematic longitudinal sectional view of the heat exchanger illustrating the modified example of the adjustment layer. 
         FIG. 14  A schematic view showing a configuration example of an adjustment layer in a cross-flow heat exchanger. 
         FIG. 15  A diagram showing a first example (low-temperature first fluid) when the first fluid does not undergo a phase change. 
         FIG. 16  A diagram showing a second example (high-temperature first fluid) when the first fluid does not undergo a phase change. 
     
    
    
     MODES FOR CARRYING OUT THE INVENTION 
     An embodiment of the present invention is hereinafter described on the basis of the drawings. 
     The configuration of a heat exchanger  100  according to the present embodiment is now described with reference to  FIGS. 1 to 6 . 
     Overall Configuration of Heat Exchanger 
     The heat exchanger  100  shown in  FIG. 1  is an apparatus (heat exchanger) that performs heat exchange between a low-temperature liquefied gas and a heat medium to cool the heat medium utilizing the cold heat of the liquefied gas. 
     The liquefied gas is hydrogen, oxygen, nitrogen or a natural gas, for example. The heat medium used for a liquefied gas evaporator is varied, but from the viewpoint of availability (low cost) etc., a liquid such as water, seawater, or antifreeze, air, or the like is used. These liquids and air (moisture in the air) have the property of freezing at a temperature higher than the supply temperature of the liquefied gas. 
     In the first embodiment, the heat exchanger  100  includes a plate-fin core  1 . The plate-fin core  1  is a heat exchanging portion having a stacked structure in which a plurality of planar flow path layers  2  are stacked. In the following description, for convenience, the stacking direction of the flow path layers  2  is defined as a Z direction (or an upward-downward direction), a longitudinal direction along one side of the core  1  in a horizontal plane orthogonal to the Z direction is defined as an X direction, and a short-side direction along another side of the core  1  in the horizontal plane orthogonal to the Z direction is defined as a Y direction. 
     The flow path layers  2  of the core  1  each have a planar (flat plate) structure including a heat transfer fin  3  and side bars  4  that constitute the outer peripheral wall of the heat transfer fin  3 . In addition, each flow path layer  2  is divided by tube plates  5 , which are partition walls on the stacking direction side. The heat transfer fin  3  is a corrugated fin having a corrugated shape, and contacts the upper and lower tube plates  5  at the peak portions of the corrugated portions. The corrugated heat transfer fin  3  divides the inside of the flow path layer  2  to create a plurality of flow paths (channels). The tube plates  5  and the heat transfer fin  3  function as heat transfer surfaces that transmit heat in the core  1 . In the core  1 , a stacked body of the stacked flow path layers  2  is sandwiched by a pair of side plates  6  and is bonded by brazing or the like such that the core  1  has a rectangular box shape (rectangular parallelepiped shape) as a whole. The core  1  is made of a material such as stainless steel, for example. 
     The core  1  includes first flow paths  10  through which a first fluid  7  flows and second flow paths  20  through which a second fluid  8  flows. In the present embodiment, the first fluid  7  is a low-temperature fluid, and the second fluid  8  is a high-temperature fluid. That is, the first fluid  7  is a low-temperature liquefied gas evaporated in the first flow paths  10 , and the second fluid  8  is a liquid heat medium cooled by the liquefied gas. It is assumed that the first fluid  7  and the second fluid  8  are fluids, one of which may be frozen by heat exchange with the other. In the present embodiment, among the first fluid  7  and the second fluid  8 , the second fluid  8  is a fluid having a risk of occurrence of freezing in the flow path. As an example in the present embodiment, the liquefied gas is liquid hydrogen, for example, and the heat medium is antifreeze, for example. The antifreeze is a liquid that mainly contains water and a freezing point depressant (such as ethylene glycol). The first fluid  7  is an example of a “liquefied gas” in the claims. The second fluid  8  is an example of a “heat medium” in the claims. 
     In the present embodiment, the core  1  further includes an adjustment layer  30  disposed between the first flow path  10  and the second flow path  20  adjacent to each other and that adjusts the amount of heat exchange between the first flow path  10  and the second flow path  20 . The adjustment layer  30  is disposed between all the first flow paths  10  and the second flow paths  20 . That is, in the core  1 , the flow path layers are stacked in the order of the first flow path  10 , the adjustment layer  30 , the second flow path  20 , the adjustment layer  30 , . . . . Therefore, in the present embodiment, the first flow path  10  and the second flow path  20  are not directly adjacent to each other (with the tube plate  5  interposed therebetween). 
     As shown in  FIG. 2 , in the core  1 , heat exchange is performed between the low-temperature first fluid  7  that flows through the first flow path  10  and the high-temperature second fluid  8  that flows through the second flow path  20  via the adjustment layer  30 . In the first embodiment, the core  1  cools the second fluid  8  (antifreeze) that flows through the second flow path  20  by heat exchange with the first fluid  7  (liquid hydrogen) that flows through the first flow path  10 . As a result of the heat exchange, the heat exchanger  100  cools the liquid second fluid  8  to a predetermined temperature and supplies (discharges) the same, which remains in a liquid phase, to the outside. As a result of the heat exchange, the heat exchanger  100  evaporates the first fluid  7  in the liquid phase to convert the same into a gas  7   a  in a vapor state, and supplies (discharges) the gas  7   a  to the outside. 
     Structure of Flow Path Layer 
     The structure of each of the flow path layers  2  (the first flow path  10 , the second flow path  20 , and the adjustment layer  30 ) is now described with reference to  FIGS. 3 to 5 . A plurality of first flow paths  10  have the same shape, a plurality of second flow paths  20  have the same shape, and a plurality of adjustment layers  30  have the same shape. As can be seen from  FIG. 1 , in the first flow paths  10 , the second flow paths  20 , and the adjustment layers  30  (the respective flow path layers  2 ), only the positions of inlets and outlets of the fluids are different, and the first flow paths  10 , the second flow paths  20 , and the adjustment layers  30  have substantially the same planar shape (a shape in the X and Y directions). All of the first flow paths  10 , the second flow paths  20 , and the adjustment layers  30  have a width W 1  and a length L 1  (see  FIGS. 3 to 5 ). On the other hand, as shown in  FIG. 2 , the height H 1  of the first flow path  10 , the height H 2  of the second flow path  20 , and the height H 3  of the adjustment layer  30  may be equal to each other or may be different from each other. As described above, each of the first flow path  10 , the second flow path  20 , and the adjustment layer  30  includes the planar flow path layer  2 , and includes the heat transfer fin  3  (a heat transfer fin  13 ,  23 , or  34  described below) inside the planar flow path layer  2 . 
     &lt;First Flow Path&gt; 
     As shown in  FIG. 3 , the first flow path  10  includes an inlet (opening)  11  provided in an X 2 -side end face and an outlet (opening)  12  provided in an X 1 -side end face, and is a linear flow path that extends in the X direction. In a configuration example shown in  FIG. 3 , the first fluid  7  flows in an X 1  direction from the inlet  11  toward the outlet  12 . 
     The heat transfer fin  3  provided in the first flow path  10  is hereinafter referred to as the heat transfer fin  13 . The heat transfer fin  13  of the first flow path  10  extends from the inlet  11  to the outlet  12  of the first flow path  10 . In  FIG. 3 , the heat transfer fin  13  is illustrated only in a central portion of the first flow path  10  for convenience, and illustration of the heat transfer fin  13  in the remaining portions is omitted. The heat transfer fin  13  has a predetermined pitch P 1  over the entire first flow path  10 . The pitch is an interval between longitudinal plates (see  FIG. 6 ) of the heat transfer fin  13  (heat transfer fin  3 ). 
     Header tanks or the like (not shown) are attached to the inlet  11  and the outlet  12 , respectively. The first fluid  7  in the liquid phase is supplied from the outside to the inlet  11  via the header tank, and the first fluid  7  (gas  7   a ) after heat exchange (after vaporization) is discharged from the outlet  12  via the header tank. Therefore, the first flow path  10  includes a liquid phase region (L), a vapor-liquid mixed phase region (L+V), and a vapor phase region (V) from the inlet  11  side toward the outlet  12  side based on phase changes in the first fluid  7  that flows through the first flow path  10 . 
     &lt;Second Flow Path&gt; 
     As shown in  FIG. 4 , the second flow path  20  includes an inlet (opening)  21  provided at an X 1 -side end of a Y 2 -side end face and an outlet (opening)  22  provided at an X 2 -side end of a Y 1 -side end face, and is a linear flow path that extends in the X direction. In a configuration example shown in  FIG. 4 , the second fluid  8  flows in an X 2  direction from the inlet  21  toward the outlet  22 . Therefore, the heat exchanger  100  according to the present embodiment is a counter-flow heat exchanger in which the flowing direction (X 1  direction) of the first fluid  7  and the flowing direction (X 2  direction) of the second fluid  8  are opposite to each other. 
     The heat transfer fin  3  provided in the second flow path  20  is hereinafter referred to as the heat transfer fin  23 . The heat transfer fin  23  of the second flow path  20  extends from the inlet  21  to the outlet  22  of the second flow path  20 . In  FIG. 4 , the heat transfer fin  23  is illustrated only in a central portion of the second flow path  20  for convenience, and illustration of the heat transfer fin  23  in the remaining portions is omitted. The heat transfer fin  23  has a predetermined pitch P 2  over the entire linear portion  25  excluding distributors  24  provided at the inlet  21  and the outlet  22 . In the present embodiment, the pitch P 2  is smaller than the pitch P 1 . That is, the number of longitudinal plates per unit width is larger in the heat transfer fin  23  than in the heat transfer fin  13 , and the density of the longitudinal plates per unit area is higher in the heat transfer fin  23  than in the heat transfer fin  13 . In each of the distributors  24 , the second fluid  8  is distributed (or aggregated) between the linear portion  25  and the inlet  21  or the outlet  22 , and thus the pitch is different from that in the linear portion  25 . The distributors  24  and the linear portion  25  may have the same pitch. 
     Header tanks or the like (not shown) are attached to the inlet  21  and the outlet  22 , respectively. The second fluid  8  is supplied from the outside to the inlet  21  via the header tank, and the second fluid  8  after heat exchange is discharged from the outlet  22  via the header tank. 
     &lt;Adjustment Layer&gt; 
     As shown in  FIG. 5 , the adjustment layer  30  according to the present embodiment is a flow path layer  2  having a shape that matches with those of the first flow path  10  and the second flow path  20  in a plan view. On the other hand, the adjustment layer  30  according to the present embodiment is a layer through which no fluid flows. That is, the adjustment layer  30  in  FIG. 5  is surrounded by the side bars  4  on the entire circumference, and no inlet or outlet is provided. The adjustment layer  30  has a hollow structure. Although in  FIG. 5 , the inside of the adjustment layer  30  is illustrated as if it is completely closed, the adjustment layer  30  may be hermetically sealed in a vacuum state (low pressure state) or in a state filled with a predetermined gas, or may partially communicate with the outside such that the inside and outside of the adjustment layer  30  are in the same atmosphere. As shown in  FIG. 2 , the adjustment layer  30  is provided such that as compared with the case in which the first flow path  10  and the second flow path  20  are simply divided by the tube plate  5 , the performance of heat transfer between the first flow path  10  and the second flow path  10  decreases. That is, the adjustment layer  30  has an adjustment function so as to reduce the amount of heat exchange (as compared with the case in which the first flow path  10  and the second flow path  20  are directly adjacent to each other) between the first flow path  10  and the second flow path  20 . 
     Returning to  FIG. 5 , in the present embodiment, the adjustment layer  30  includes a first portion  31  and a second portion  32  having a heat transfer performance lower than that of the first portion  31 , and has a heat transfer performance varied depending on a position in the adjustment layer  30 . That is, the adjustment layer  30  includes a portion (first portion  31 ) having a high heat transfer performance and a portion (second portion  32 ) having a low heat transfer performance in a plane parallel to the first flow path  10  and the second flow path  20 , and the adjustment layer  30  has a distribution of high and low heat transfer performances. 
     In this specification, the heat transfer performance of the adjustment layer  30  indicates the ease of heat transmission when heat is transmitted between the first flow path  10  and the second flow path  20  via the adjustment layer  30 . The heat transfer performance can be considered as total performance including heat transmission due to each of heat conduction, heat transfer (convection heat transfer), and heat radiation. 
     In a configuration example shown in  FIG. 5 , the adjustment layer  30  includes one first portion  31  and one second portion  32 . In the adjustment layer  30 , the second portion  32  is provided within a predetermined range including a portion that overlaps the vicinity of the inlet  21  or the vicinity of the outlet  22  of the second flow path  20 . In the present embodiment, the second portion  32  is provided in a portion adjacent to (overlapping) a region in the vicinity of the outlet  22  of the second flow path  20 . The first portion  31  is provided in a region of the adjustment layer  30  other than the predetermined range in which the second portion  32  is provided. Consequently, in the adjustment layer  30 , the heat transfer performance on the downstream side of the second flow path  20  is lower than the heat transfer performance on the upstream side of the second flow path  20 . 
     In the present embodiment, in the adjustment layer  30 , the second portion  32  is disposed within the predetermined range including a portion that overlaps a risk area RA of the second flow path  20 . The risk area RA is an area of the second flow path  20  in which the inner surface temperature is closest to the temperature of the first fluid  7 . The inner surface temperature of the second flow path  20  is the surface temperatures of the tube plates  5  that define the second flow path  20 . The inner surface temperature of the second flow path  20  is influenced by the temperature of the low-temperature first fluid  7  and the heat transfer performance on the first flow path  10  side, and thus the positions and ranges of the first portion  31  and the second portion  32  are set by the relationship between the first fluid  7  that flows through the first flow path  10  and the second fluid  8  that flows through the second flow path  20 . 
     Specifically, referring to  FIGS. 3 and 5 , in the adjustment layer  30 , the first portion  31  is disposed within a range that overlaps the vapor phase region (V) of the first fluid  7  that flows through the first flow path  10 , and in the adjustment layer  30 , the second portion  32  is disposed within a range that overlaps the vapor-liquid mixed phase region (L+V) of the first fluid  7  that flows through the first flow path  10 . Furthermore, in the present embodiment, the second portion  32  is also provided in a range that overlaps the liquid phase region (L) in addition to the vapor-liquid mixed phase region (L+V). 
     The heat transfer performance in the first flow path  10  varies with phase changes in the liquefied gas that flows through the first flow path  10 . The vapor-liquid mixed phase region (L+V) is a region in which the heat transfer coefficient of the first fluid  7  becomes the highest and the inner surface temperature of the second flow path  20  becomes closest to the temperature of the first fluid  7  with heat exchange. That is, the risk area RA in which the risk of occurrence of freezing of the second fluid  8  in the second flow path  20  is the highest is an area that overlaps the vapor-liquid mixed phase region (L+V) of the first flow path  10 . Furthermore, in the second flow path  20 , a region that overlaps the liquid phase region (L) of the first flow path  10  is on the downstream side (outlet  22  side) of the risk area RA, and thus in the region, the risk of occurrence of freezing is the second highest next to that in the vapor-liquid mixed phase region (L+V). On the other hand, the vapor phase region (V) is a region in which the temperature of the first fluid  7  increases in the first flow path  10 , and in the region, the heat transfer coefficient of the first fluid  7  is the lowest. In addition, as compared with the remaining regions, the inner surface temperature of the second flow path  20  is not decreased. Therefore, a region that overlaps the vapor phase region (V) is a region in which the first portion  31  with a low risk of occurrence of freezing and a high heat transfer performance can be placed. 
     The liquid phase region (L), the vapor-liquid mixed phase region (L+V), and the vapor phase region (V) in the first flow path  10  can be analytically determined based on the type of fluid, the flow rate, the inlet temperature and outlet temperature, the working pressure, and design information about the structure of each flow path, for example. 
     In the configuration examples shown in  FIGS. 3 to 5 , the liquid phase region (L) and the vapor-liquid mixed phase region (L+V) are ranges up to a distance D 1  (position S) from the inlet  11  of the first flow path  10 . Therefore, the second portion  32  of the adjustment layer  30  is set in the range of the distance D 1  from the X 2 -side end. The vapor phase region (V) is a range of a distance D 2  from a position S to the downstream side (outlet  12  side) in the first flow path  10 . The first portion  31  of the adjustment layer  30  is set in the range of the distance D 2  on the downstream side from the position S. 
     In the present embodiment, the adjustment layer  30  includes heat conduction portions  33  that make a connection between the first flow path  10  and the second flow path  20  adjacent to each other. The heat conduction portions  33  contact the tube plate  5  (see  FIG. 2 ) that divides the adjustment layer  30  from the first flow path  10 , contact the tube plate  5  that divides the adjustment layer  30  from the second flow path  20 , and transmit heat mainly by internal heat conduction. 
     The adjustment layer  30  has a hollow structure through which no fluid flows, and thus most of heat transmission is due to heat conduction through the heat conduction portions  33  while heat transmission due to heat transfer (convection heat transfer) and heat radiation is slight as compared with heat conduction. Therefore, in the adjustment layer  30 , it is possible to vary the heat transfer performance depending on the structure, arrangement, and number of the heat conduction portions  33 . 
     The heat conduction portions  33  are not particularly restricted as long as the same each have a structure that makes a connection between the first flow path  10  and the second flow path  20  (between the tube plates  5 ). The heat conduction portions  33  may be columnar or block-shaped members, or may be plate-shaped or lattice-shaped members, for example. In the present embodiment, the heat conduction portions  33  are constituted by the heat transfer fin  34  (heat transfer fin  3 ) disposed in the adjustment layer  30 . The heat transfer fin  34  is a corrugated fin similar to the heat transfer fins  13  and  23  of the other flow path layers  2 . In this case, as shown in  FIG. 6 , the heat conduction portions  33  are constituted by the longitudinal plates  35  of the heat transfer fin  34 , which make a connection between the tube plates  5 . Therefore, as shown in  FIG. 5 , the heat conduction portions  33  extend along the flowing direction (X direction) of the first fluid  7  and are disposed at an interval with a predetermined pitch. 
     In the present embodiment, the first portion  31  and the second portion  32  include the heat conduction portions  33  having different heat transfer performances. Specifically, the density per unit area of the heat conduction portions  33  in the adjustment layer  30  is varied such that the heat conduction portions  33  have different heat transfer performances. In the present embodiment in which the heat conduction portions  33  are constituted by the heat transfer fin  34 , intervals between the longitudinal plates  35  of the heat transfer fin  34  are different from each other such that the heat conduction portions  33  have different heat transfer performances. That is, the pitches of the heat conduction portions  33  (the longitudinal plates  35  of the heat transfer fin  34 ) are different between the first portion  31  and the second portion  32 . The longitudinal plates  35  are examples of a “fin section” in the claims. 
     That is, as shown in  FIG. 6(B) , a heat transfer fin  34   a  having a pitch P 3  is provided in the second portion  32  of the adjustment layer  30 , and as shown in  FIG. 6(A) , a heat transfer fin  34   b  having a pitch P 4  is provided in the first portion  31  of the adjustment layer  30 . The pitch P 3  is larger than the pitch P 4  (P 3 &gt;P 4 ). In other words, the number of heat conduction portions  33  (the longitudinal plates  35  of the heat transfer fin) in the unit width is smaller in the second portion  32  than in the first portion  31 . Therefore, the density of the heat conduction portions  33  per unit area becomes relatively sparse (low density) in the second portion  32  along the flowing direction (X direction) of the first fluid  7 , and becomes relatively dense (high density) in the first portion  31 . The pitch P 3  and the pitch P 4  are examples of an “interval between the fin sections” in the claims. 
     For example, a configuration example in  FIGS. 6(A) and 6(B)  shows that the heat transfer fin  34   a  having a pitch P 3  includes ten longitudinal plates  35  (heat conduction portions  33 ) per unit width (1 inch), and the heat transfer fin  34   b  having a pitch P 4  includes fourteen longitudinal plates  35  (heat conduction portions  33 ) per unit width. 
     The thickness of each of the longitudinal plates  35  may be different between the first portion  31  and the second portion  32 . That is, the thickness t 1  in the heat transfer fin  34   a  of the second portion  32  and the thickness t 2  in the heat transfer fin  34   b  of the first portion  31  may be different from each other such that the heat conduction portions  33  may have different heat transfer performances. Both the pitch and the thickness of the longitudinal plates  35  may be different between the first portion  31  and the second portion  32 . In this case, the density of the longitudinal plates  35  per unit area may be relatively low in the second portion  32  and may be relatively high in the first portion  31 . 
     With such a configuration, the heat transfer performance of the second portion  32  of the adjustment layer  30  is relatively low. Consequently, the second portion  32  significantly reduces or prevents freezing of the second fluid  8  of the second flow path  20  even when the cryogenic first fluid  7  flows in through the inlet  11  of the first flow path  10 . 
     On the other hand, the heat transfer performance of the first portion  31  of the adjustment layer  30  is relatively high. Consequently, the first portion  31  promotes heat exchange between the first flow path  10  and the second flow path  20  as compared with the second portion  32 . 
     Effects of Present Embodiment 
     According to the present embodiment, the following effects are achieved. 
     According to the present embodiment, as described above, the adjustment layer  30  disposed between the first flow path  10  and the second flow path  20  adjacent to each other and that adjusts the amount of heat exchange between the first flow path  10  and the second flow path  20  is provided. Accordingly, the adjustment layer  30  between the first flow path  10  and the second flow path  20  can significantly reduce or prevent excessive heat transfer between the first flow path  10  and the second flow path  20 . Consequently, fluid freezing can be significantly reduced or prevented even when heat exchange is performed between fluids having a large temperature difference. Furthermore, the adjustment layer  30  includes the first portion  31  and the second portion  32  having a heat transfer performance lower than that of the first portion  31 , and has a heat transfer performance varied depending on the position in the adjustment layer  30 . Accordingly, the second portion  32  is disposed in a portion in which freezing is likely to occur in the flow path to sufficiently decrease the heat transfer performance while the first portion  31  is disposed in a portion in which freezing is unlikely to occur to relatively increase the heat transfer performance such that the high heat exchange performance can be ensured. Accordingly, an increase in a flow path length required to realize a desired amount of heat exchange can be significantly reduced or prevented. Thus, an increase in the size of the heat exchanger  100  can be significantly reduced or prevented while fluid freezing is significantly reduced or prevented even when heat exchange is performed between fluids having a large temperature difference. 
     According to the present embodiment, as described above, in the adjustment layer  30 , the second portion  32  is provided within the predetermined range (the range of the distance D 1 ) including the portion that overlaps the vicinity of the inlet  21  or the vicinity of the outlet  22  of the second fluid  8 . Accordingly, when the temperature of the second fluid  8  monotonously decreases along the second flow path  20 , for example, the second portion  32  includes the portion that overlaps the vicinity of the outlet  22  of the second fluid  8 , which is highly likely to freeze such that occurrence of freezing can be effectively and significantly reduced or prevented. 
     According to the present embodiment, as described above, in the adjustment layer  30 , the second portion  32  is disposed within the predetermined range (the range of the distance D 1 ) including the portion that overlaps the risk area RA (the area in which the inner surface temperature of the second flow path  20  is closest to the temperature of the first fluid  7 ) of the second flow path  20 . Accordingly, the second portion  32  overlaps the risk area RA such that occurrence of freezing can be more reliably and significantly reduced or prevented. 
     According to the present embodiment, as described above, the adjustment layer  30  includes the heat conduction portions  33  that make a connection between the first flow path  10  and the second flow path  20  adjacent to each other, and the first portion  31  and the second portion  32  include the heat conduction portions  33  having different heat transfer performances. Accordingly, the shape and dimensions of the adjustment layer  30  itself are not adjusted, but the number, size, material, etc. of the heat conduction portions  33  are changed such that the distribution of the heat transfer performances in the first portion  31  and the second portion  32  can be easily adjusted. Consequently, the appropriate distribution of the heat transfer performances according to the risk of occurrence of fluid freezing in the adjustment layer  30  can be easily realized. 
     According to the present embodiment, as described above, the density per unit area of the heat conduction portions  33  (the pitch of the longitudinal plates  35 ) in the adjustment layer  30  is varied such that the heat conduction portions  33  have different heat transfer performances. Accordingly, the heat transfer performances of the heat conduction portions  33  can be easily varied depending on their positions in the flowing direction, unlike the case in which a plurality of types of heat conduction portions  33  made of different materials are provided, for example. 
     According to the present embodiment, as described above, the first flow path  10 , the second flow path  20 , and the adjustment layer  30  each include the planar flow path layer  2 . Furthermore, the heat conduction portions  33  are constituted by the heat transfer fin  34  (heat transfer fin  3 ) disposed in the adjustment layer  30 , and at least one of the pitches (P 3 , P 4 ) between the longitudinal plates  35  of the heat transfer fin  34  ( 34   a ,  34   b ) and the thicknesses (t 1 , t 2 ) of the longitudinal plates  35  are different from each other such that the heat conduction portions  33  have different heat transfer performances. Accordingly, the first flow path  10 , the second flow path  20 , and the adjustment layer  30  can share a similar basic structure, and thus each of the first flow path  10 , the second flow path  20 , and the adjustment layer  30  can be each of the flow path layers  2  of the plate-fin heat exchanger  100 . Consequently, unlike the case in which a special structure is used for the adjustment layer  30 , the heat exchanger  100  can be easily constructed even when the adjustment layer  30  is provided. In addition, the heat transfer performance of the adjustment layer  30  can be varied by a simple configuration in which the pitches between the longitudinal plates  35  or the thicknesses of the longitudinal plates  35  are simply different from each other. 
     According to the present embodiment, as described above, the first fluid  7  is a low-temperature liquefied gas evaporated in the first flow path  10 , and the second fluid  8  is a liquid heat medium cooled by the liquefied gas. In such a configuration, there is a possibility of freezing on the second fluid  8  side by heat exchange between the cryogenic first fluid  7  and the second fluid  8 . Even in this case, the first portion  31  and the second portion  32  are provided to vary the heat transfer performance of the adjustment layer  30  such that the heat transfer efficiency can be increased as much as possible within a range in which freezing of the second fluid  8  can be significantly reduced or prevented, and thus an increase in the size of the heat exchanger  100  can be effectively and significantly reduced or prevented. 
     According to the present embodiment, as described above, in the adjustment layer  30 , the first portion  31  is disposed within the range that overlaps the vapor phase region (V) of the first fluid  7  that flows through the first flow path  10 , and in the adjustment layer  30 , the second portion  32  is disposed within the range that overlaps the vapor-liquid mixed phase region (L+V) of the first fluid  7  that flows through the first flow path  10 . Accordingly, in the vapor-liquid mixed phase region (L+V) in which the heat transfer coefficient of the first fluid  7  is high, freezing of the second fluid  8  is significantly reduced or prevented by the second portion  32  having a low heat transfer performance, and in the vapor phase region (V) in which the heat transfer coefficient of the first fluid  7  is low, heat exchange can be efficiently performed by the first portion  31  having a high heat transfer performance. Consequently, the heat exchanger  100  can be made as compact as possible while freezing of the second fluid  8  is significantly reduced or prevented. 
     Description of Simulation Results 
     The effects of the heat exchanger  100  according to the present embodiment are now described using simulation results with reference to  FIGS. 7 to 10 . In the simulation, temperature changes in the first fluid  7  and the second fluid  8  during passing through the heat exchanger were calculated, and the flow path lengths required for the fluids to reach predetermined target temperatures (required to obtain a predetermined amount of heat exchange) were obtained. 
     The simulation was performed on Comparative Example 1 in which the adjustment layer  30  was not provided (in which the first flow path  10  and the second flow path  20  are divided by the tube plate  5 ), Comparative Example 2 in which only the low-density heat transfer fin  34   a  was provided over the entire adjustment layer  30  (in which the heat transfer performance of the entire adjustment layer  30  corresponded to the heat transfer performance of the second portion  32 ), and Comparative example 3 in which only the high-density heat transfer fin  34   b  was provided over the entire adjustment layer  30  (in which the heat transfer performance of the entire adjustment layer  30  corresponded to the heat transfer performance of the first portion  31 ) in addition to the heat exchanger  100  according to the present embodiment described above. 
     In the simulation, hydrogen (liquid hydrogen) was used as the first fluid  7 , antifreeze was used as the second fluid  8 , and a calculation was performed with the same conditions such as the flow rate and the pressure. As the simulation conditions, the inlet temperature of the liquid hydrogen was −253° C., the boiling point thereof was −242.5° C., and the outlet temperature thereof was −50° C. The freezing point of the antifreeze was −50° C., the inlet temperature thereof was −39° C., and the outlet temperature (target temperature) thereof after cooling with hydrogen was −43° C. In the simulation, the average of the surface temperature (the surface temperature on the second flow path  20  side; see  FIG. 2 ) of the tube plate  5  between the second flow path  20  and the adjustment layer  30  was calculated. When the surface temperature reaches −50° C., it is believed that freezing of the second fluid  8  occurs in the second flow path  20 . 
       FIG. 7  shows the simulation results of the heat exchanger  100  according to the present embodiment,  FIG. 8  shows the simulation results of Comparative Example 1,  FIG. 9  shows the simulation results of Comparative Example 2, and  FIG. 10  shows the simulation results of Comparative Example 3. In  FIGS. 7 to 10 , the vertical axis represents the temperature [° C.], and the horizontal axis represents the amount of heat exchange [kcal/h]. In all the simulation results, the total amounts of heat exchange are the same, but the flow path lengths required to reach the outlet temperatures are different. In the simulation, the flow path lengths of First Comparative Example to Third Comparative Example were calculated from a value of the ratio with the flow path length of the heat exchanger  100  according to the present embodiment taken as 1 (reference). 
     &lt;Risk of Occurrence of Freezing&gt; 
     As a common trend in  FIGS. 7 to 10 , the liquid hydrogen flows into the inlet in the liquid phase and then becomes a vapor-liquid mixed phase at the boiling point (−242.5° C.), and after the temperature constant state continues until an amount corresponding to the latent heat, the temperature increases again in a vapor state. The surface temperature of the tube plate  5  became the lowest when the hydrogen was in a vapor-liquid mixed phase state. In other words, the risk of occurrence of freezing of the antifreeze (second fluid  8 ) is maximized in a portion of the second flow path  20  that overlaps the vapor-liquid mixed phase region (L+V). 
     In the heat exchanger  100  (see  FIG. 7 ) according to the present embodiment, the surface temperature of the tube plate  5  (the inner surface temperature of the second flow path  20 ) became the lowest in the vapor-liquid mixed phase region (L+V), which was −49.8° C. In Comparative Example 1 (see  FIG. 8 ), the lowest surface temperature of the tube plate  5  was −57.3° C. In Comparative Example 2 (see  FIG. 9 ), the lowest surface temperature of the tube plate  5  was −49.8° C. In Comparative Example 3 (see  FIG. 10 ), the lowest surface temperature of the tube plate  5  was −50.9° C. 
     In the heat exchanger  100  according to the present embodiment and Comparative Example 2, it has been found that the surface temperature is −50° C. or higher, and thus freezing of the antifreeze hardly occurs. On the other hand, in Comparative Example 1 and Comparative Example 3, it has been found that the surface temperature is lower than −50° C., and thus freezing of the antifreeze occurs. 
     &lt;Flow Path Length&gt; 
     When the flow path length of the heat exchanger  100  according to the present embodiment was 1, the flow path length was 0.38 in Comparative Example 1, 1.18 in Comparative Example 2, and 0.99 in Comparative Example 3. That is, the flow path length required to move the same amount of heat is in the order of Comparative Example 1&lt;Comparative Example 3&lt;the present embodiment&lt;Comparative Example 2. 
     The simulation results together indicate that although the heat transfer performance is high and the flow path length can be reduced in Comparative Example 1 in which the adjustment layer  30  is not provided and Comparative Example 3 in which only the high-density heat transfer fin  34   b  is provided in the adjustment layer  30 , freezing occurs in the second flow path  20 , and thus there is a risk of clogging the flow path. On the other hand, the simulation results together indicate that although freezing in the second flow path  20  can be prevented in Comparative Example 2 in which only the low-density heat transfer fin  34   b  is provided in the adjustment layer  30 , the flow path length is 1.18 times that in the present embodiment, and the size of the heat exchanger is increased. 
     On the other hand, the simulation results together indicate that in the heat exchanger  100  according to the present embodiment, freezing in the second flow path  20  can be prevented similarly to Comparative Example 3, and the temperature of the liquid hydrogen can be increased to the target temperature with the same flow path length as that in Comparative Example 2. Therefore, in the heat exchanger  100  according to the present embodiment, it has been confirmed that an increase in its size can be significantly reduced or prevented while fluid freezing is significantly reduced or prevented. 
     In the heat exchanger  100 , the risk area RA and the position and range of the second portion  32  in the adjustment layer  30  can be set based on the temperature distribution in Comparative Example 1 (in which the adjustment layer  30  is not provided) shown in  FIG. 8 . That is, first, the structures of the first flow path  10  and the second flow path  20  are determined, and the temperature distribution in the case in which the adjustment layer  30  is not provided as in Comparative Example 1 is obtained. From the calculation results, it has been found that in an example shown in  FIG. 8 , the risk area RA exists in the vapor-liquid mixed phase region (L+V). Therefore, in the adjustment layer  30 , the second portion  32  is disposed in the risk region RA (vapor-liquid mixed phase region (L+V)) and the liquid phase region (L) on the downstream side to insure a margin of safety, and the first portion  31  having a high heat transfer performance is disposed in a region other than the second portion  32  such that the position and range of the second portion  32  can be set. 
     [Modified Examples] 
     The embodiment disclosed this time must be considered as illustrative in all points and not restrictive. The scope of the present invention is not shown by the above description of the embodiment but by the scope of claims for patent, and all modifications (modified examples) within the meaning and scope equivalent to the scope of claims for patent are further included. 
     For example, while the example in which the low-temperature liquefied gas is used as the first fluid  7  and the liquid heat medium for vaporizing the liquefied gas is used as the second fluid  8  has been shown in the aforementioned embodiment, the present invention is not restricted to this. According to the present invention, the first fluid  7  may be a high-temperature gas such as exhaust gas after combustion or after reaction, and the second fluid  8  may be a liquid refrigerant (such as water) for cooling the high-temperature gas. That is, the first flow path  10  may be a flow path on the high-temperature side, and the second flow path  20  may be a flow path on the low-temperature side. In this case, boiling of the second fluid  8  may occur in the second flow path  20  due to heat exchange. The occurrence of unintentional boiling in the flow path may increase the load related to the strength of the heat exchanger, and may not be acceptable due to the specification of the heat exchanger. In the present invention, even when there is a possibility of fluid boiling, boiling of the second fluid  8  in the second flow path  20  can be significantly reduced or prevented by the adjustment layer  30 . Furthermore, the adjustment layer  30  includes the first portion  31  and the second portion  32  having different heat transfer performances such that the high heat exchange performance can be ensured, and thus an increase in the size of the heat exchanger can be significantly reduced or prevented. 
     While the example in which the plate-fin heat exchanger  100  is provided has been shown in the aforementioned embodiment, the present invention is not restricted to this. According to the present invention, a heat exchanger other than the plate-fin heat exchanger may be used. 
     For example, the present invention may be applied to a multi-tube heat exchanger  200  as in a modified example shown in  FIGS. 11(A) to 11(C) . In the heat exchanger  200 , three cylindrical flow path layers  102  are concentrically disposed. For example, a first flow path  10  includes an innermost flow path layer  102 , and a second flow path  20  includes an outermost flow path layer  102 . An adjustment layer  30  includes an intermediate flow path layer  102  between the first flow path  10  and the second flow path. In this modified example, the heat transfer performance of the adjustment layer  30  is different at a position S 1  on the upstream side and a position S 2  on the downstream side, for example, in the flowing direction (X direction) of a first fluid  7  as in the aforementioned embodiment. Specifically, as shown in  FIG. 11(B)  showing the cross-section at the position S 1  and  FIG. 11(C)  showing the cross-section at the position S 2 , heat conduction portions  33  are disposed in the adjustment layer  30 , and the density (number) of the heat conduction portions  33  may be varied. 
     Besides this, the heat exchanger according to the present invention may be a plate heat exchanger in which corrugated metal plates including flow paths integrally formed on the front and back sides are stacked and bonded by seal, welding, or the like such that flow path layers are formed between the metal plates. Alternatively, the heat exchanger may be a diffusion-bonded heat exchanger in which metal plates including flow paths formed by grooving are stacked and integrated by diffusion-bonding, for example, such that flow path layers are provided between the metal plates. 
     While the example in which the flow path layers are alternately stacked one by one in the order of the first flow path  10 , the adjustment layer  30 , the second flow path  20 , the adjustment layer  30 , . . . has been shown in the aforementioned embodiment, the present invention is not restricted to this. According to the present invention, a plurality of same flow path layers may be successively stacked. That is, a plurality of first flow path layers  10  may be successively stacked in such a manner that the first flow path  10 , the first flow path  10 , the adjustment layer  30 , the second flow path  20 , the adjustment layer  30 , the first flow path  10 , the first flow path  10 , . . . are stacked. Alternatively, a plurality of adjustment layers  30  may be successively stacked in such a manner that the first flow path  10 , the adjustment layer  30 , the adjustment layer  30 , the second flow path  20 , the adjustment layer  30 , the adjustment layer  30 , . . . are stacked. 
     While the example in which the adjustment layer  30  is a layer through which no fluid flows has been shown in the aforementioned embodiment, the present invention is not restricted to this. For example, as shown in a modified example of  FIG. 12 , an adjustment layer  130  through which a fluid can flow may be provided. The adjustment layer  130  in  FIG. 12  has a hollow flow path structure disposed between a first flow path  10  and a second flow path  20  and through which a fluid can flow except during heat exchange. Specifically, the adjustment layer  130  includes an inlet (opening)  131  provided at an X 2 -side end of a Y 2 -side end face and an outlet (opening)  132  provided at an X 1 -side end of a Y 1 -side end face, and is formed as a linear flow path that extends in an X direction. A fluid is supplied from the outside to the inlet  131  via a header tank (not shown), and is discharged from the outlet  132  via a header tank. In this case, at the time of heat exchange between a first fluid  7  and a second fluid  8 , no fluid flows through the adjustment layer  130  but the adjustment layer  130  is filled with air such that the similar effects to those of the adjustment layer  30  according to the aforementioned embodiment can be obtained. 
     When the adjustment layer  130  having a hollow flow path structure through which a fluid can flow except during heat exchange is provided as described above, the hollow structure can easily decrease the heat transfer performance of the adjustment layer  130 , and thus occurrence of freezing and boiling can be effectively and significantly reduced or prevented. In addition, as a measure against occurrence of fluid freezing, a heat medium having a temperature higher than the freezing temperature can flow through the adjustment layer  130  except during heat exchange between the first fluid  7  and the second fluid  8  so as to quickly eliminate freezing. 
     That is, when freezing of the second fluid  8  occurs in the second flow path  20 , a heat medium is supplied to the adjustment layer  130  except during heat exchange so as to eliminate the freezing of the second fluid  8 . Accordingly, even when freezing occurs locally in the second flow path  20  after heat exchange, the heat medium for eliminating freezing is supplied to the adjustment layer  130  after the heat exchange (supply of the first fluid  7  and the second fluid  8 ) is stopped such that freezing can be easily and quickly eliminated. 
     While the example in which the adjustment layer  30  includes the same flow path layer  2  as those of the first flow path  10  and the second flow path  20  has been shown in the aforementioned embodiment, the present invention is not restricted to this. According to the present invention, the adjustment layer need not include the flow path layer, and may have a layer structure other than the flow path layer. For example, as in a modified example shown in  FIG. 13 , a plate member  230  including a heat insulator  231  may be provided as the adjustment layer  30 . The plate member  230  is a tube plate that divides a first flow path  10  and a second flow path  20  from each other. The heat transfer performance of the plate member  230  is decreased by the hollow heat insulator  231  provided therein, and the amount of heat exchange between the first flow path  10  and the second flow path  20  is adjusted. For example, a plurality of heat insulators  231  are provided in the plate member  230  and are divided by partition walls  232 . Heat conduction portions  33  that make a connection between the first flow path  10  and the second flow path  20  adjacent to each other are constituted by the partition walls  232 . In this modified example, the density of the partition walls  232  (i.e. the density of the heat insulators  231 ) is varied such that the heat transfer performance of the first portion  31  and the heat transfer performance of the second portion  32  can be different from each other. 
     While the counter-flow heat exchanger  100  in which the flowing direction of the first fluid  7  and the flowing direction of the second fluid  8  are opposite to each other has been shown as an example in the aforementioned embodiment, the present invention is not restricted to this. According to the present invention, the heat exchanger may be a parallel-flow heat exchanger other than the counter-flow heat exchanger. In the case of the parallel-flow heat exchanger, the inlet  11  of the first flow path  10  and the inlet  11  of the second flow path  20  are disposed on the same side. Therefore, when the risk of freezing the second fluid  8  is high, the temperature of the second fluid  8  can be increased in a region near the inlet at which the temperature of the first fluid  7  is the lowest, and thus the risk of freezing can be further significantly reduced or prevented. On the other hand, when the temperature difference between the first fluid  7  and the second fluid  8  is large near the outlet of the first flow path  10 , the counter-flow heat exchanger is preferable because the heat exchange efficiency is increased and the size thereof can be reduced. Alternatively, the heat exchanger may be a cross-flow heat exchanger in which the flowing direction of the first fluid  7  and the flowing direction of the second fluid  8  are orthogonal to each other. 
       FIG. 14  shows a configuration example (an arrangement example of a first portion  31  and a second portion  32 ) of an adjustment layer  30  in a cross-flow heat exchanger  300 .  FIG. 14  shows an example in which a first fluid  7 , which is a high-temperature fluid, flows in a Y 1  direction through a first flow path (not shown), and a second fluid  8 , which is a low-temperature fluid, flows in an X 1  direction through a second flow path (not shown). In this case, the second fluid  8  has a risk of occurrence of boiling, and a risk area RA is a portion in the vicinity of an outlet of the second flow path  20  and in the vicinity of an inlet of the first flow path  10 . Therefore,  FIG. 14  shows an example in which the second portion  32  of the adjustment layer  30  is set in a triangular range that overlaps a corner in the vicinity of the outlet of the second flow path  20  and in the vicinity of the inlet of the first flow path  10 , and the first portion  31  is set in the remaining region. 
     While the heat exchanger  100  including the plurality of first flow paths  10  and the plurality of second flow paths  20  has been shown as an example in the aforementioned embodiment, the present invention is not restricted to this. According to the present invention, the numbers of first flow paths and second flow paths are not particularly restricted. One first flow path and one second flow path may be provided, or two or more first flow paths and two or more second flow paths may be provided. 
     While the example in which the adjustment layer  30  is divided into two regions of the first portion  31  and the second portion  32 , and the first portion  31  and the second portion  32  have different heat transfer performances has been shown in the aforementioned embodiment, the present invention is not restricted to this. According to the present invention, the adjustment layer  30  may include three or more portions having different heat transfer performances. For example, in the adjustment layer, three portions of a portion adjacent to the liquid phase region (L) of the liquefied gas, a portion adjacent to the vapor-liquid mixed phase region (L+V), and a portion adjacent to the vapor phase region (V) may have different heat transfer performances. Alternatively, in the adjustment layer  30 , the heat transfer performance may continuously change, instead of including a plurality of regions having different heat transfer performances. For example, the density of the heat conduction portions  33  may be continuously increased from the upstream side to the downstream side in the flowing direction of the first fluid. 
     While the example in which the hollow adjustment layer  30  is provided has been shown in the aforementioned embodiment, the present invention is not restricted to this. According to the present invention, the inside of the adjustment layer  30  may be filled with a fluid or a solid such as a powder (particulate material) or a porous material. In this case, these fillers may function as heat conduction portions. The heat transfer performance can be varied by changing a material (thermal conductivity) of the filler, the particle diameter of the filler, the porosity of the filler, etc. 
     While the example in which the first fluid  7  in the first flow path  10  undergoes a phase change has been shown in the aforementioned embodiment, the present invention is not restricted to this. According to the present invention, as shown in  FIG. 15 , the low-temperature first fluid  7  may pass through the first flow path  10  in the liquid phase or the vapor phase without undergoing a phase change. When there is no phase change, the heat transfer performance on the first flow path  10  side may be considered to be substantially constant, and thus the risk area RA (the risk of occurrence of freezing) in the second flow path  20  is near the outlet of the second flow path  20 . Furthermore,  FIG. 16  shows an example in which the second fluid  8  is a low-temperature fluid, and the first fluid  7  is a high-temperature fluid. Also in this case, the risk area RA (the risk of occurrence of boiling) in the second flow path  20  is near the outlet of the second flow path  20 . Therefore, in the cases of  FIGS. 15 and 16 , the second portion  32  of the adjustment layer  30  may be set to include a portion that overlaps the vicinity of the outlet of the second fluid  8  and to correspond to the risk area RA near the outlet of the second flow path  20 . 
     DESCRIPTION OF REFERENCE NUMERALS 
       2 ,  102 : flow path layer 
       7 : first fluid (liquefied gas) 
       8 : second fluid (heat medium) 
       10 : first flow path 
       20 : second flow path 
       30 ,  130 : adjustment layer 
       31 : first portion 
       32 : second portion 
       33 : heat conduction portion 
     ( 34   a ,  34   b ): heat transfer fin 
       35 : longitudinal plate (fin section) 
       50 : risk area 
       100 ,  200 ,  300 : heat exchanger 
     P 3 , P 4 : pitch between the longitudinal plates (interval between the fin sections) 
     t 1 , t 2 : thickness of the longitudinal plate 
     X: flowing direction of the first fluid