Patent Publication Number: US-11644223-B2

Title: Falling film evaporator

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a U.S. National Stage Application of PCT Application No. PCT/CN2019/100330, entitled “FALLING FILM EVAPORATOR,” filed Aug. 13, 2019, which claims priority to and the benefit of Chinese Patent Application No. 201810923286.9, filed Aug. 14, 2018, and Chinese Utility Model Application No. 201821312966.9, filed Aug. 14, 2018, each of which is herein incorporated by reference in its entirety for all purposes. 
     TECHNICAL FIELD 
     The present application relates to the technical field of falling film evaporators. 
     BACKGROUND ART 
     Falling film evaporators generally use a refrigerant distributor to distribute a refrigerant to the surfaces of heat exchange tubes in a heat exchange tube bundle, so as to form a liquid film for evaporation; they exploit the mechanism of thin-film evaporation from heat exchange tube surfaces, have the advantages of high heat transfer efficiency and small refrigerant charge, and have been a focus of research in the refrigeration and air conditioning industries in recent years. However, the uniformity of distribution of refrigerant on the heat exchange tube bundle in the evaporator is a key factor limiting the evaporator&#39;s heat exchange performance. The state of refrigerant entering the refrigerant distributor is generally gas and liquid phases; if the two phases of refrigerant are not uniformly distributed onto the heat exchange tube bundle of the falling film evaporator, the result will be that the refrigerant distributor supplies too much refrigerant to a portion of the heat exchange tubes and too little refrigerant to another portion of the heat exchange tubes, and the phenomenon of “dry spots” will occur, leading to a drop in the overall heat exchange performance of the falling film evaporator. 
     SUMMARY OF THE INVENTION 
     An object of the present application is to provide an improved falling film evaporator, capable of distributing a refrigerant uniformly to heat exchange tubes. 
     To achieve the above object, the present application provides a falling film evaporator, comprising: a housing, a heat exchange tube, a perforated plate, a spray tube and a liquid entry tube. The housing has an accommodating cavity; the length direction of the heat exchange tube is the same as the length direction of the housing; the perforated plate is arranged above the heat exchange tube, and the perforated plate is provided with multiple distribution holes; the spray tube is arranged above the perforated plate, the spray tube having multiple spray ports, the spray ports being distributed at intervals in the length direction of the spray tube, and the spray ports being configured to be capable of spraying a refrigerant toward the perforated plate; and the liquid entry tube is in fluid communication with the spray tube, such that the refrigerant flowing through the liquid entry tube can flow into the spray tube; wherein the heat exchange tube, the perforated plate and the spray tube are all arranged in the accommodating cavity; and the length direction of the spray tube is substantially perpendicular to the length direction of the housing. 
     In the falling film evaporator described above, the length direction of the perforated plate is the same as the length direction of the housing, and the spray port is configured such that after the refrigerant has been sprayed toward the perforated plate, the refrigerant can flow in the length direction of the perforated plate. 
     In the falling film evaporator described above, the bottom of the spray tube has a circular-arc end face, the circular-arc end face protruding in the direction of the perforated plate, the spray port is in the form of a strip, and at least a part of the spray port is arranged on the circular-arc end face. 
     In the falling film evaporator described above, the spray tube has two extension parts extending in the length direction of the housing, an end of the extension part comprises an outwardly protruding circular-arc end face, the spray port is in the form of a strip, and at least a part of the spray port is arranged on the circular-arc end face. 
     In the falling film evaporator described above, a cross section of the spray tube has a flattened oval shape, the two extension parts are located at left and right ends of the spray tube respectively, the spray port is in the form of a strip, and the spray port extends toward the circular-arc end faces at the left and right ends of the spray tube respectively from the bottom of the spray tube. 
     In the falling film evaporator described above, a cross section of the spray tube has an inverted-“Y” shape, the two extension parts are separately located at the bottom of the spray tube and extend obliquely downward, the spray port is in the form of a strip, and at least a part of the spray port is arranged on the circular-arc end face. 
     In the falling film evaporator described above, multiple said spray tubes are arranged in the falling film evaporator, and top ends of the multiple spray tubes are in communication with each other, so that the multiple spray tubes are in fluid communication with each other. 
     In the falling film evaporator described above, the number of the spray tubes is an even number, and the multiple spray tubes are distributed symmetrically relative to the liquid entry tube. 
     The falling film evaporator described above further comprises a liquid entry box, the liquid entry box being arranged between the liquid entry tube and the spray tube, such that the liquid entry tube and the spray tube can be in fluid communication with each other by means of the liquid entry box. 
     The falling film evaporator described above further comprises a cover plate, the cover plate being arranged at an upper part of the spray tube, and two side edges of the cover plate extend toward the perforated plate and are directly or indirectly connected to two side edges of the perforated plate in a sealed fashion. 
     In the falling film evaporator of the present application, the length direction of the spray tube is configured to be substantially perpendicular to the length direction of the evaporator housing; this configuration enables refrigerant sprayed out of the spray ports to move substantially in the length direction of the housing, thus extending the flow path of the refrigerant sprayed out of the spray ports, and avoiding the problem of sprayed refrigerant being sprayed unevenly over the surface of the heat exchange tube due to the flow thereof being hindered. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    is a schematic diagram of the three-dimensional structure of a falling film evaporator  100  in an embodiment of the present application. 
         FIG.  2    is a structural schematic diagram of some of the components located inside a housing  101  of the falling film evaporator  100  shown in  FIG.  1   . 
         FIG.  3    is a radial sectional view, at the position of a liquid entry tube  102 , of the falling film evaporator  100  shown in  FIG.  1   . 
         FIG.  4    is a partial enlarged drawing, in a region of a spray tube  202 , of the falling film evaporator  100  shown in  FIG.  3   . 
         FIG.  5    is a schematic diagram of the three-dimensional structure of the spray tube  202  in  FIG.  2   . 
         FIG.  6    shows a cross section, at the position of a spray port  301 , of the spray tube  202  shown in  FIG.  5   . 
         FIG.  7    shows movement paths of refrigerant after being sprayed out of the spray tube  202  with the positional arrangement shown in  FIG.  4   . 
         FIG.  8 A  shows a first embodiment of the cross-sectional shape, at the position of the spray port  301 , of the spray tube  202 . 
         FIG.  8 B  shows a second embodiment of the cross-sectional shape, at the position of the spray port  301 , of the spray tube  202 . 
         FIG.  9    is an axial sectional view, at the position of the liquid entry tube  102 , of a falling film evaporator having two spray tubes  202 . 
         FIG.  10 A  shows a first embodiment of a structure of two spray tubes in the falling film evaporator. 
         FIG.  10 B  shows a second embodiment of a structure of two spray tubes in the falling film evaporator. 
         FIG.  10 C  shows a third embodiment of a structure of two spray tubes in the falling film evaporator. 
         FIG.  10 D  shows a fourth embodiment of a structure of two spray tubes in the falling film evaporator. 
         FIG.  11    shows a comparative embodiment of a positional arrangement of a spray tube inside a falling film evaporator. 
         FIG.  12    shows an axial sectional view, at the position of the liquid entry tube, of the falling film evaporator having the positional arrangement of the spray tube shown in  FIG.  11   . 
         FIG.  13    shows a radial sectional view, at the position of the liquid entry tube, of the falling film evaporator having the positional arrangement of the spray tube shown in  FIG.  11   . 
         FIG.  14    shows movement paths of refrigerant after being sprayed out of the spray tube shown in  FIG.  13   . 
         FIG.  15    shows flow rates of refrigerant flowing through different positions in the width direction of the perforated plate shown in  FIG.  14   . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Various specific embodiments of the present application will be described below with reference to the drawings which form a part of this Specification. It should be understood that although terms indicating direction such as “front”, “rear”, “upper”, “lower”, “left”, “right”, “top”, “bottom”, etc. are used in the present application to describe various demonstrative structural parts and elements of the present application, these terms are used herein for convenience of description only, determined on the basis of the demonstrative orientations shown in the figures. Since the embodiments disclosed herein may be arranged in different orientations, these terms indicating direction are merely illustrative and should not be regarded as limiting. 
       FIG.  1    shows a three-dimensional structure of a falling film evaporator  100  in an embodiment of the present application. As shown in  FIG.  1   , the falling film evaporator  100  comprises a housing  101 , a liquid entry tube  102 , a gas suction tube  104  and tube plates  103 . The housing  101  is substantially cylindrical, and the tube plates  103  are arranged at two ends in the length direction of the housing  101  respectively. The liquid entry tube  102  is arranged at an upper part of the housing  101 , and configured to guide refrigerant into the interior of the housing  101 . The gas suction tube  104  is also arranged at an upper part of the housing  101 , and configured to discharge gaseous refrigerant from the housing  101 . 
       FIG.  2    is a structural schematic diagram of some of the components located inside the housing  101  of the falling film evaporator  100  shown in  FIG.  1   , wherein for convenience of illustration, the liquid entry tube  102  located outside the housing  101  is retained in  FIG.  2   . As shown in  FIG.  2   , the falling film evaporator  100  further comprises a spray tube  202 , a perforated plate  205  and a heat exchange tube bundle  201  which are arranged in an accommodating cavity of the housing  101  (shown in  FIG.  3   ). The spray tube  202  is arranged below the liquid entry tube  102 , the perforated plate  205  is arranged below the spray tube  202 , and the heat exchange tube bundle  201  is arranged below the perforated plate  205 . The spray tube  202  is substantially in the form of a tube with two ends closed. An inlet  206 , configured to be in fluid communication with the liquid entry tube  102 , is provided at the top of the spray tube  202 . Multiple spray ports  301  are provided at the bottom of the spray tube  202 , and are configured to spray refrigerant, which has entered the spray tube  202 , onto the perforated plate  205  below the spray tube  202 . The perforated plate  205  is substantially in the form of a long strip, and the length direction thereof is the same as the length direction of the housing  101 . The perforated plate  205  is provided with multiple distribution holes  305 , which are configured to redistribute refrigerant sprayed onto the perforated plate  205 , so that the refrigerant can be uniformly distributed onto the heat exchange tube bundle  201  below the perforated plate  205 . Side stop plates  204  are also provided at left and right opposite sides of the perforated plate  205 ; the two side stop plates  204  extend downward perpendicular to the perforated plate  205 , such that the two side stop plates  204  and the perforated plate  205  together form an accommodating space that opens downward. In the embodiment shown in  FIG.  2   , all of the distribution holes  305  in the perforated plate  205  are round. In other embodiments, the distribution holes  305  may also be of another shape, e.g. oval, square or rhombus, etc. Moreover, the length direction of the spray tube  202  is substantially perpendicular to the length direction of the perforated plate  205 . That is to say, the length direction of the spray tube  202  is the same as the width direction of the perforated plate  205 . Generally, the length direction of the spray tube  202  is perpendicular to the length direction of the perforated plate  205 , but deviation in the positional relationship of the two components within a certain range is not restricted. The spray tube  202  is arranged in a middle position in the length direction of the perforated plate  205 , so that refrigerant sprayed out from the spray tube  202  can be uniformly sprayed from the middle position in the length direction of the perforated plate  205  to two sides in the length direction of the perforated plate  205 . 
     The falling film evaporator  100  further comprises a liquid entry box  203  arranged between the spray tube  202  and the liquid entry tube  102 , and a cover plate  302  arranged at an upper part of the spray tube  202 . The liquid entry box  203  extends in the length direction of the spray tube  202 , and is configured to establish fluid communication between the liquid entry tube  102  and the inlet  206  of the spray tube  202 , in order to enable preliminary distribution of refrigerant in the length direction of the spray tube  202 . The cover plate  302  extends in the length direction of the perforated plate  205 , and two side edges of the cover plate  302  extend downward, such that the cover plate  302  appears as an inverted-“U”-shaped structure. The cover plate  302  is located between the liquid entry box  203  and the spray tube  202 , and is provided with an opening between the liquid entry box  203  and the spray tube  202 , so as to ensure communication between the liquid entry box  203  and the spray tube  202 . The spray ports  301  on the spray tube  202  are located in a cavity between the cover plate  302  and the perforated plate  205 , thereby ensuring that refrigerant sprayed out of the spray ports  301  can be guided by the cover plate  302  so as to flow toward the perforated plate  205 . 
       FIG.  3    is a radial sectional view, at the position of the liquid entry tube  102 , of the falling film evaporator  100  shown in  FIG.  1   . As shown in  FIG.  3   , the housing  101  contains two heat exchange tube bundles  201 , wherein one heat exchange tube bundle  201  is arranged in the accommodating space formed by the perforated plate  205  and the two side stop plates  204 , and the other heat exchange tube bundle  201  is arranged at the bottom of the accommodating cavity of the housing  101 , each heat exchange tube bundle  201  comprising multiple heat exchange tubes  304 . 
       FIG.  4    is a partial enlarged drawing, in a region of the spray tube  202 , of the falling film evaporator  100  shown in  FIG.  3   . As shown in  FIG.  4   , the multiple spray ports  301  are arranged at the bottom of the spray tube  202 , spaced apart in the length direction of the spray tube  202 . The cover plate  302  is connected to the side stop plates  204  in a sealed fashion, so as to ensure that all of the refrigerant sprayed out of the spray ports  301  flows toward the perforated plate  205 , and is distributed, via the distribution holes  305  in the perforated plate, onto the heat exchange tube bundle  201  to undergo heat exchange. In other embodiments, the cover plate  302  may also be directly connected in a sealed fashion to two side edges in the width direction of the perforated plate  205 ; such a configuration can likewise ensure that all of the refrigerant sprayed out of the spray ports  301  flows toward the perforated plate  205 . 
       FIG.  5    shows the three-dimensional structure of the spray tube  202  shown in  FIG.  2   . As shown in  FIG.  5   , the multiple spray ports  301  are provided at the bottom of the spray tube  202 . Each spray port  301  is in the form of a strip, and extends from the bottom of the spray tube  202  toward two side walls; due to the direction of extension of the openings of the spray ports  301 , the plane in which each spray port  301  lies is perpendicular to the length direction of the spray tube  202 . The multiple spray ports  301  are parallel to each other and arranged spaced apart in the length direction of the spray tube  202 . 
       FIG.  6    shows a cross section, at the position of the spray port  301 , of the spray tube  202  shown in  FIG.  5   . As shown in  FIG.  6   , an upper part of the cross section of the spray tube  202  is substantially rectangular, while a lower part is substantially a semicircular arc; the spray port  301  is located at the position of the semicircular arc at the bottom of the spray tube  202 , as shown by the blank part at the lower part of the spray tube in  FIG.  6   . When refrigerant is sprayed out of the spray port  301  of the spray tube  202 , the refrigerant is spread outward uniformly in the opening direction of the spray port  301 . It can be seen from  FIG.  5    that the refrigerant sprayed out of the spray ports  301  has a certain flow speed, and due to the fact that the spray ports  301  take the form of long, narrow strips, there is almost no spreading of the sprayed refrigerant in the length direction of the spray tube  202 ; most of the refrigerant is sprayed out only in the width direction of the spray tube  202 . 
       FIG.  7    shows an axial sectional view, at the position of the liquid entry tube  102 , of the housing  101  of the falling film evaporator  100 , wherein the arrows indicate the movement paths of refrigerant after being sprayed out of the spray tube  202 . As shown in  FIG.  7   , the cover plate  302 , perforated plate  205  and side stop plates  204  have the same length direction as the housing  101 , and are all of substantially the same length, with all of the ends thereof extending to the tube plates  103 . Influenced by the shape of the openings of the spray ports  301  and the pressure difference between the inside and outside of the spray tube  202 , the refrigerant sprayed out of the spray tube  202  is sprayed into a region below the spray ports  301  until it reaches the perforated plate  205 . Since the refrigerant has a high initial speed when sprayed out of the spray ports  301 , the refrigerant still retains a high speed after being sprayed to the perforated plate  205 , thus the refrigerant will flow toward the two ends of the perforated plate  205  in the length direction of the perforated plate  205 . The perforated plate  205  is of adequate length, so the speed of the refrigerant will fall as it flows continuously, and when the refrigerant has moved to positions close to the tube plates  103  at the two sides, the speed of the refrigerant is already very low, so eddies will not form at the tube plates  103  at the two sides, and uniform distribution of refrigerant over the surface of the perforated plate is thereby achieved. As the refrigerant moves in the length direction of the perforated plate  205 , the refrigerant can flow from the distribution holes  305  in the perforated plate  205  toward the heat exchange tube bundle  201  below the perforated plate  205 , such that the refrigerant is uniformly distributed onto the multiple heat exchange tubes  304  in the heat exchange tube bundle  201 . 
       FIGS.  8 A and  8 B  show cross sections, at the position of the spray port, of two other embodiments of the spray tube  202  respectively. In these two embodiments, the cross-sectional shape of the spray tube  202  is different from the cross-sectional shape of the spray tube  202  shown in  FIG.  6   . The cross section of the spray tube  202  shown in  FIG.  6    is longer in the vertical direction overall, with a narrower transverse width; the specific manifestation of this is that the upper part is rectangular while the lower part is a semicircular arc. However, when the transverse width of the cross section of the spray tube  202  is narrower, the movement distance of refrigerant in the length direction of the perforated plate  205  will be restricted; thus, in order to enable the refrigerant sprayed out of the spray tube  202  to move to positions close to the tube plates  103  successfully, in some embodiments of the present application, the spray tube  202  is extended in the width direction of the spray tube  202  (i.e. the length direction of the housing  101 ) to form two extension parts  801 , and the spray ports are at least partially arranged on the extension parts, thus helping to increase the spraying distance of refrigerant in the length direction of the housing  101 . 
     As shown in  FIG.  8 A , the cross section of the spray tube  202  has a flattened oval shape, with flat and straight edges at upper and lower sides, the two extension parts  801  being located at left and right sides of the spray tube  202  respectively, and an end of each extension part  801  having an outwardly protruding circular-arc end face  501 ; due to the structure just described, the cross section of the spray tube  202  has a longer transverse span.  FIG.  8 A  shows the position of the spray port  301  at the blank part of the cross section of the spray tube; the spray port  301  is in the form of a strip, located in the lower half of the spray tube  202 , and extends from the bottom of the spray tube  202  toward the circular-arc end faces  501  at the two sides. 
     The cross section of the spray tube  202  shown in  FIG.  8 B  has an inverted-“Y” shape; the two extension parts  801  are arranged at two sides at the bottom of the spray tube  202  respectively and extend obliquely downward, such that a certain angle A is formed between the two extension parts  801 . In some embodiments, the angle A is greater than or equal to 60°, so that the transverse width of the spray tube  202  is extended to a greater extent. It can be seen from  FIG.  8 B  that an end of each extension part  801  has an outwardly protruding circular-arc end face  501 , with spray ports  301  being substantially located on the two circular-arc end faces  501 .  FIG.  8 B  shows two spray ports  301  located on the same cross section of the spray tube  202 . In the length direction of the spray tube  202 , a row of spray ports  301  is arranged spaced apart on the circular-arc end face  501  at each side; thus, two rows of spray ports  301  are arranged on the single spray tube  202  shown in  FIG.  8 B , greatly increasing the spraying distance of refrigerant in the length direction of the perforated plate  205 . 
       FIG.  9    is an axial sectional view, at the position of the liquid entry tube  102 , of a falling film evaporator having two spray tubes  202 . As shown in  FIG.  9   , in order to adapt to a longer length of the housing  101 , and increase the spraying distance of the spray tube  202  in the length direction of the housing  101 , the embodiment shown in  FIG.  9    uses two spray tubes  202  arranged side by side in the interior of the evaporator housing  101 . The cross sections of the spray tubes  202  may be any of the shapes in  FIGS.  6 ,  8 A and  8 B , and one liquid entry box  203  is provided above each spray tube  202 , such that refrigerant can undergo preliminary distribution in the length direction of the spray tube  202  before entering the spray tube  202 . To facilitate uniform distribution of refrigerant, the liquid entry tube  102  is arranged in a middle position in the axial direction of the housing  101 , and the two spray tubes  202  are arranged in parallel at the same height above the perforated plate  205 , and arranged symmetrically at left and right sides of the liquid entry tube  102 . As shown in  FIG.  9   , the gap between a center axis of either one of the two spray tubes  202  in the vertical direction and a center axis of the liquid entry tube  102  is L, and the distance between the center axis of either one of the two spray tubes  202  in the vertical direction and the tube plate  103  at the side corresponding thereto is also L. The symmetric structural arrangement of the spray tube  202  that has just been described helps to spray refrigerant to the surface of the perforated plate  205  uniformly. 
     To achieve the abovementioned arrangement of the spray tubes  202 , the liquid entry tube  102  in the embodiment shown in  FIG.  9    is arranged as follows: one end, close to a refrigerant inlet, of the liquid entry tube  102  is extended vertically; before extending into the housing  101 , the liquid entry tube  102  is bifurcated into two branch tubes, which extend horizontally toward two sides in the length direction of the housing  101  respectively; above the positions of the two spray tubes  202 , the two branch tubes are each formed into perpendicular corners and thereby extend vertically downward, until they enter the interior of the housing  101 , so as to be respectively connected to the two liquid entry boxes  203  arranged above the two spray tubes  202 . By means of the arrangement just described, refrigerant is bifurcated into two paths after entering the liquid entry tube  102 , and flows into the two different spray tubes  202  respectively. 
     In some embodiments, the number of spray tubes  202  may be set to be an even number greater than two, in order to adapt to a falling film evaporator having a housing of greater length. Setting the number of spray tubes  202  to be an even number facilitates the uniform distribution thereof at the two sides of the liquid entry tube  102 , so that the refrigerant flowing through the liquid entry tube  102  is uniformly distributed to the spray tubes  202 . 
       FIGS.  10 A- 10 D  show other embodiments in which two spray tubes  202  are simultaneously arranged in the falling film evaporator. 
     As shown in  FIG.  10 A , the two spray tubes  202  are arranged side by side at the same height, and share one liquid entry box  203 . The liquid entry box  203  has a wide cross section, so that two side parts in the width direction of the liquid entry box  203  can be connected to top ends of the two spray tubes  202  respectively, and be in communication with the top ends of the two spray tubes  202 . In the arrangement just described, fluid communication with the two spray tubes  202  can be achieved simultaneously by means of the shared liquid entry box  203 , using one straight-through liquid entry tube  102 ; thus, the housing  101  need only be provided with one opening for the liquid entry tube  102  to pass through, thereby simplifying the structure of the liquid entry tube  102  and housing  101 . 
       FIG.  10 B  shows another embodiment of a double-spray-tube structure. As shown in  FIG.  10 B , the two spray tubes  202  are arranged in parallel at the same height, and the cross section of each spray tube  202  is substantially round; the round cross section design facilitates uniform scattering of refrigerant in the direction of the spray ports. 
       FIGS.  10 C and  10 D  show structures in which the two spray tubes  202  are arranged at a certain angle in the falling film evaporator. As shown in  FIGS.  10 C and  10 D , in the same cross section of the falling film evaporator, the center axes of the two spray tubes  202  form a certain angle B, the angle B being greater than or equal to 60°. This setting of angle B helps to increase the spraying distance of the spray tubes  202  in the falling film evaporator housing in the length direction. In order for the center axes of the two spray tubes  202  in the same cross section of the falling film evaporator to be configured at a certain angle B, the liquid entry tube  102  is configured such that one end thereof is extended vertically downward, and bifurcated into two branch tubes before coming into communication with the spray tubes  202 , such that the two branch tubes extend horizontally in opposite directions; above the two spray tubes  202 , the two branch tubes form corners, which are obtuse angles, so that the two branch tubes extend obliquely downward away from each other, until they lead into the two spray tubes  202  respectively. 
     It can be seen from  FIGS.  2 - 10 D  that in the present application, the length direction of the spray tube  202  is arranged to be perpendicular to the length direction of the housing  101  of the falling film evaporator  100 , and the spray ports  301  are in the form of strips, such that the refrigerant sprayed out of the spray tube  202  can flow substantially in the length direction of the housing  101 , thereby increasing the movement space of the refrigerant, such that the refrigerant can be uniformly sprayed onto the surface of the perforated plate  205 . If the manner of arranging the spray tube  202  in the present application is not employed, the movement path of refrigerant after being sprayed out of the spray tube  202  is highly likely to be restricted due to insufficient radial width of the housing  101 , with the result that refrigerant cannot be uniformly sprayed onto the heat exchange tube bundle  201 . 
       FIG.  11    shows a comparative example of a positional arrangement of a spray tube  1202  inside a falling film evaporator. Unlike the embodiments of the present application, in which the length direction of the spray tube  202  is arranged to be perpendicular to the length direction of the perforated plate  205 , in the comparative example shown in  FIG.  11    the length direction of the spray tube  1202  is arranged to be the same as the length direction of the perforated plate  1205 . As shown in  FIG.  11   , the length of the spray tube  1202  is substantially the same as the lengths of a cover plate  1302 , a perforated plate  1205  and side stop plates  1204 ; the spray tube  1202  is arranged above the perforated plate  1205 , and a liquid entry tube  1102  and a liquid entry box  1203  are both located above the spray tube  1202 . Refrigerant can enter the liquid entry box  1203  from the liquid entry tube  1102 , and then be sprayed onto the surface of the perforated plate  1205  by means of the spray tube  1202 . The configuration of spray ports  1301  on the spray tube  1202  in the comparative example is the same as the configuration of spray ports  301  shown in  FIG.  5    in an embodiment of the present application: multiple spray ports  1301  are parallel to each other and arranged spaced apart at equal distances in the length direction of the spray tube  1202 . The difference is that, because the length direction of the spray tube  1202  is arranged to lie in the length direction of the perforated plate  1205  in the comparative example, the abovementioned configuration of the spray ports  1301  causes refrigerant to move substantially in the width direction of the perforated plate  1205  after being sprayed out of the spray tube  1202 . 
       FIG.  12    shows an axial sectional view, at the position of the liquid entry tube  1102 , of the falling film evaporator having the positional arrangement of the spray tube  1202  shown in  FIG.  11   . As shown in  FIG.  12   , the liquid entry box  1203 , spray tube  1202 , cover plate  1302 , perforated plate  1205  and side stop plates  1204  are all arranged in the interior of a housing  1101  of the falling film evaporator, and the lengths of the spray tube  1202 , cover plate  1302 , perforated plate  1205  and side stop plates  1204  are substantially the same as the length of the housing  1101 . 
       FIG.  13    shows a radial sectional view, at the position of the liquid entry tube  1102 , of the falling film evaporator having the positional arrangement of the spray tube  1202  shown in  FIG.  11   . As shown in  FIG.  13   , the left and right sides of the falling film evaporator are arranged symmetrically, wherein the spray tube  1202  is located in a middle position in the width direction of the perforated plate  1205 , and two heat exchange tube bundles  1201  are provided below the perforated plate  1205 , wherein one heat exchange tube bundle  1201  is accommodated in an accommodating space formed by the perforated plate  1205  and the side stop plates  1204  at two sides thereof, and the other heat exchange tube bundle  1201  is arranged in a bottom space of the housing  1101 , with the length direction of each heat exchange tube in the two heat exchange tube bundles  1201  being arranged to lie in the length direction of the housing  1101 . 
       FIG.  14    shows movement paths of refrigerant after being sprayed out of the spray tube  1202  shown in  FIG.  13   . As shown in  FIG.  14   , the refrigerant sprayed out of the spray tube  1202  has a high initial speed, and when it advances to edge parts in the width direction of the perforated plate  1205 , the refrigerant still retains a certain transverse speed, but the movement path of the refrigerant is substantially in a radial direction of the housing  1101 , and the radial width of the housing  1101  is narrow, restricting the width of the perforated plate  1205 , thus the perforated plate  1205  does not have sufficient width for the refrigerant to advance further; the refrigerant having a certain transverse speed develops eddies at the edge parts of the perforated plate  1205  under the blocking action of the cover plate  1302 , with the result that a greater amount of refrigerant collects at two sides in the width direction of the perforated plate  1205  than at the middle position. 
       FIG.  15    shows flow rates of distributed refrigerant flowing through different positions in the width direction of the perforated plate  1205  shown in  FIG.  14   . The width of the perforated plate  1205  is limited because the radial width of the housing  1101  is narrow; thus, when the length direction of the spray tube  1202  is the same as the length direction of the housing  1101 , the refrigerant sprayed out of the spray tube  1202  still has a high transverse speed when it reaches the width edges of the perforated plate  1205 , and is therefore restricted in movement, resulting in uneven distribution of refrigerant in the width direction of the perforated plate  1205 . As shown in  FIG.  15   , since the various components in the falling film evaporator are arranged symmetrically at left and right sides in a radial direction thereof, the refrigerant flow rates are also symmetrical in the width direction of the perforated plate  1205  with respect to a center point position thereof. Specifically, the refrigerant flow rate is smallest at a middle position located directly below the spray tube  1202 , and as the position is moved toward the two sides in the width direction of the perforated plate  1205 , the refrigerant flow rate gradually increases, with the largest refrigerant flow rate at the positions of the two edge parts of the perforated plate  1205 . 
     As can be seen, in the falling film evaporator of the comparative example, the length direction of the spray tube  1202  is configured to lie in the length direction of the housing  1101 , such that refrigerant sprayed out of the spray tube  1202  moves substantially in a radial width direction of the housing  1101 , and because the radial width of the housing  1101  is narrow, the movement range of the refrigerant after being sprayed out of the spray tube  1202  is greatly restricted, with the result that the refrigerant cannot be uniformly sprayed onto the heat exchange tubes. In the falling film evaporator  100  in an embodiment of the present application, the length direction of the heat exchange tubes  202  is arranged to be perpendicular to the length direction of the housing  101 , such that refrigerant sprayed out of the spray tube  202  can move substantially in the length direction of the housing  101 , thus increasing the movement path of refrigerant, preventing uneven spraying of refrigerant onto the heat exchange tubes due to movement of the refrigerant being restricted, and thereby avoiding the phenomenon of “dry spots” on the heat exchange tubes caused by uneven spraying of refrigerant. Furthermore, the configuration of the present application described above increases the movement path of refrigerant in the width direction of the spray tube  202 , that is to say, the use of the configuration of the spray tube  202  in an embodiment of the present application greatly increases the area of coverage when the perforated plate  205  is sprayed by unit length of the spray tube  202 ; thus, in order to achieve a spraying effect for a perforated plate of the same area, the use of the configuration of the spray tube  202  in an embodiment of the present application greatly reduces the length of the spray tube  202 , and correspondingly, the abovementioned configuration also reduces the number of openings of the spray ports  301  on the spray tube  202 , thereby significantly reducing the difficulty and cost of manufacturing the spray tube. 
     Although the present application is described with reference to the particular embodiments shown in the drawings, it should be understood that the falling film evaporator of the present application can have many variations without departing from the spirit and scope and background of teaching of the present application. Those skilled in the art will also realize that there are different ways of changing structural details in the embodiments disclosed in the present application, all falling within the spirit and scope of this Description and the claims.