Patent Publication Number: US-2013248157-A1

Title: Method of coating a part of a heat exchanger and heat exchanger

Description:
TECHNICAL FIELD 
     The present invention relates to a method of coating an internal surface of a heat exchanger and a heat exchanger comprising a surface having a nickel plating. 
     BACKGROUND 
     Heat exchangers may be used for heat exchange between two fluids. Typically, a heat exchanger has an inlet and an outlet for each of the two fluids. Inside the heat exchanger one flow passage is provided for each fluid. The flow passages are kept apart by one or more heat transfer elements, through which heat is transferred from one fluid to the other fluid. For instance, in plate heat exchangers the heat transfer elements are formed by heat transfer plates, and in spiral heat exchangers heat transfer elements are formed by spiral sheets. 
     Different kinds and types of fluids may pass through a heat exchanger. Some fluids are erosive, e.g. because of particles contained in the fluid. The heat transfer elements of a heat exchanger are thus subjected to wear during use with such fluids. Also, fluids may pass through a heat exchanger for various purposes. For instance, in some heat exchangers a fluid may be caused to boil. Thus, the heat transfer elements of heat exchangers have different requirements depending on the fluids flowing through a heat exchanger and the purpose of a heat exchanger. 
     Heat transfer elements may thus be manufactured from various different materials, the material being suitable for a particular heat exchanger application. Also, heat transfer elements may be coated with different kinds of materials, the coating material being suitable for a particular heat exchanger application. 
     U.S. Pat. No. 6,513,581 discloses plate and spiral heat exchangers wherein surfaces have been coated by means of electroless chemical deposition. A metal/phosphorus and metal/polymer layer is formed by dipping a workpiece comprising the surface to be coated into a metal electrolyte solution. 
     WO 92/16310 discloses a method of providing heat transfer plates of a plate heat exchanger with a layer of surface protecting plastic material. In an assembled heat exchanger a gaseous medium containing the plastic material is introduced into interspaces between the heat transfer plates. The plastic material may be introduced in the form of mist or in evaporated form. The plastic material is caused to deposit onto the heat transfer plates in the interspaces. 
     WO 96/06705 is concerned with brazed heat exchangers which are brazed with a copper brazing material. The copper brazing material is not able to withstand a heat exchange fluid containing ammonia. A method of protecting the brazing joints of a brazed heat exchanger is thus disclosed in WO 96/06705. According to the method a protective coating is diffused into the brazing joints of an assembled heat exchanger. According to the method the coating material, either fluid tin or a water solution of silver nitrate, is poured through the four connecting ports of a plate heat exchanger into the plate heat exchanger to completely fill the plate heat exchanger. The coating material is allowed to circulate in the plate heat exchanger and is then emptied out of the heat exchanger. The tin, or the silver, diffuses into the copper brazing joints. 
     SUMMARY 
     An object of embodiments is to provide a method of efficiently plating heat transfer elements of a heat exchanger. 
     According to an aspect of the invention, the object is achieved by a method of coating an internal surface of a heat exchanger. The heat exchanger comprises a first passage for a first heat exchange fluid, and a second passage for a second heat exchange fluid. The first and second passages are separated by at least one heat transfer element. The heat transfer element has a first surface facing the first passage and a second surface facing the second passage. The method comprises: pre-treating the first surface by circulating at least one pre-treatment liquid through the first passage of the heat exchanger and a pre-treatment liquid storage separate from the heat exchanger, and electroless nickel plating the first surface by circulating a solution comprising nickel ions through the first passage of the heat exchanger and a solution container separate from the heat exchanger. 
     Since the pre-treatment liquid and the electroless plating solution comprising nickel ions are circulated, each through a dedicated storage and container, respectively, and the first passage of the heat exchanger, the first surface is homogeneously nickel plated in a rational and easily controlled process. The circulation of the pre-treatment liquid through the dedicated storage and the first passage means that the pre-treatment liquid will be applied to relevant areas of the heat transfer element to prepare the first surface for electroless nickel plating. The circulation of the electroless plating solution through the first passage and the solution container means that the solution containing nickel ions flows along/over the first surface and solution from the solution container is constantly provided to the first surface. This achieves favourable conditions for the electroless nickel plating. Furthermore, an easily performed method in comparison with nickel plating by dipping of separate heat transfer elements into different baths is provided. Also, by performing the nickel plating on an assembled heat exchanger entails that the nickel plating may be performed as a later production step when manufacturing a heat exchanger. Thus, the nickel plating will not risk being damaged by production steps or handling of heat transfer elements between production steps. Furthermore, a used heat exchanger may be re-plated using to the present method. As a result, the above mentioned object is achieved. 
     The heat exchanger may be for instance a spiral heat exchanger or a plate heat exchanger. The heat exchanger may be an assembled heat exchanger. By assembled heat exchanger it is to be understood that the heat exchanger may comprise a number of heat transfer elements, which elements are placed in relation to each other such that the first and second passages are formed and heat transfer between the two fluids may be performed. That is, parts of the heat exchanger which do not have a heat transferring function or not a function of limiting the first and second passages, such as frame parts, support arrangements, etc. may be attached after the electroless nickel plating has been performed. The heat transfer elements may be permanently assembled, e.g. by means of brazing or welding. The pre-treating and the electroless nickel plating may be seen as separate steps of the method. The pre-treating is performed before the electroless nickel plating. Pre-treating may include cleaning the first passage and/or rinsing the first passage and/or activating the first surface. Activating may be performed to further prepare the first surface for the electroless nickel plating. The pre-treatment liquid storage may comprise several containers suitably one for each pre-treatment liquid. The pre-treatment liquid storage may have the function of an intermediate storage for different pre-treatment liquids in the several containers. In case water is used as a pre-treating liquid, the water may be supplied from a water container or from a water source, such as a water tap. The electroless nickel plating will form a nickel plating on the first surface. The nickel plating may be non-diffusing into the first surface of the heat transfer element, i.e. the nickel plating being on top of the first surface. The nickel plating on the first surface may be one of for instance; a nickel/phosphorous plating, a nickel/polymer plating, a nickel/polytetrafluoroethylene (PTFE) plating, nickel/diamond plating, a nickel/Boron plating, a nickel/silver plating, a nickel/gold plating or combinations thereof. 
     The features; liquid storage separate from the heat exchanger and solution container separate from the heat exchanger are to be understood as; the liquid storage and the container being physically located separate from the heat exchanger. That is, the features circulating the at least one pre-treatment liquid through the first passage of the heat exchanger and circulating the solution comprising nickel ions through the first passage of the heat exchanger do not encompass the flow of pre-treatment liquid, or solution, occurring in the first passage when the heat exchanger is submerged in the pre-treatment liquid, or solution. In other words, circulating takes place without the heat exchanger being submerged in a pre-treatment liquid or solution. At least one pump is required to achieve the circulation. A control system may control the at least one pump and valves to achieve the circulation of pre-treatment liquid and solution through the first passage of the heat exchanger and a relevant storage and container, at one or more suitable flow rates. 
     According to embodiments the circulating of the at least one pre-treatment liquid and the circulating of the solution may be effected by at least one pump forming part of a conduit system that is configured to convey the at least one pre-treatment liquid respectively the solution to the heat exchanger. 
     According to embodiments the pre-treating may comprise: Circulating one of a pre-treatment liquid in the form of water, a solvent, an acid, or a liquid comprising solid particles through the first passage. Water may be circulated through the first passage, inter alia between other liquids/solutions are circulated in the first passage. The water will thus rinse previously used liquids from the first passage. The solvent may be a solvent which dissolves fat or grease. An acid may clean or active the first surface. The solid particles in a liquid comprising solid particles will form an abrasive, which may be useful for preparing the first surface for the electroless nickel plating. 
     According to embodiments the pre-treating may comprise: Circulating water through the first passage and a water container, or by directing water from a water source through the first passage, and cleaning the first surface by circulating a solvent, or a liquid which comprises solid particles, through the first passage and a container for the solvent, or a container for the liquid which contains solid particles. In this manner the rinsing with water may clean the first passage and the first surface at least to some extent, and thereafter the solvent or the liquid which contains solid particles may clean the first surface to a further degree. As mentioned above, rinsing with water may be performed again after the cleaning with the solvent or with the liquid comprising solid particles. The rinsing and the cleaning may be seen as steps of the method. 
     According to embodiments the pre-treating may comprise: A surface activating step for activating the first surface before the electroless nickel plating by circulating an activating liquid through the first passage and a container for the activating liquid. In this manner the first surface may easily be activated before the electroless nickel plating. 
     According to embodiments circulating the pre-treatment liquid and circulating the solution may be performed by one or more pumps forming part of a conduit system, the conduit system further comprising a releasable connection to the heat exchanger, the pre-treatment liquid storage, the solution container, and a valve arrangement for directing either the pre-treatment liquid, or the solution, through the pump and the heat exchanger. In this manner pre-treatment liquid may first be circulated through the first passage and the pre-treatment liquid storage by means of the pump and the valve arrangement set in a first position. Thereafter, the valve arrangement may be set in a different position to circulate the electroless plating solution through the first passage and the solution container. Again, circulation is performed by the pump. When one heat exchanger has been nickel plated it is removed from the releasable connection and a further heat exchanger to be electroless nickel plated is connected to the releasable connection and the circulation of the pre-treatment liquid and the electroless nickel plating solution is repeated. Thus, an efficient and easily administered method for nickel plating surfaces of heat exchangers is achieved. 
     According to embodiments the solution may be an aqueous solution comprising nickel ions, a chemical reducing agent, and a catalyst. The solution may comprise at least one of phosphorous ions, boron ions, polytetrafluoroethylene (PTFE) particles, or diamond particles. The solution may comprise further additives, e.g. for stabilizing the solution or regulating the pH of the solution. 
     According to embodiments the method may comprise heating the solution in the solution container by means of a heating element. In this manner the electroless nickel plating solution may be kept at a temperature, or within a temperature interval, at which the electroless nickel plating process is suitably performed. 
     According to embodiments the method may comprise heating the pre-treatment liquid in the pre-treatment liquid storage by means of a heating element. In this manner the pre-treatment liquid may be kept at a temperature, or within a temperature interval, at which the pre-treating is suitably performed. 
     According to embodiments the method may comprise stirring the solution in the solution container by means of a stirring element. In this manner the electroless nickel plating solution may be kept at an even temperature and/or at an even concentration in the solution container. 
     According to embodiments the method may comprise stirring the pre-treatment liquid in the pre-treatment liquid storage by means of a stirring element. In this manner the pre-treatment liquid may be kept at an even temperature and/or at an even concentration in the pre-treatment liquid storage. 
     According to embodiments the method may comprise: Removing an old nickel plating layer from the first surface by circulating a removing liquid through the first passage of the heat exchanger and a container for the removing liquid, before the pre-treating is performed. In this manner the method may be used to re-plate a used heat exchanger with electroless nickel plating. 
     According to embodiments the heat exchanger may comprise at least two permanently joined heat transfer elements, the first and second passages being separated by at least a first heat transfer element of the at least two permanently joined heat transfer elements. The method may suitably be performed on assembled heat exchangers with permanently join heat transfer elements. 
     According to embodiments the heat exchanger may be a spiral heat exchanger and the at least one heat transfer element may comprise a first spiral shaped sheet metal piece extending in a spiral in a first plane, the first plane extending perpendicularly to the first spiral shaped sheet metal piece. 
     According to embodiments the said circulating the pre-treatment liquid may comprise the pre-treatment liquid flowing through the first passage at least partially in a main direction and the said circulating the solution may comprise the solution flowing through the first passage at least partially in the main direction, the main direction extending substantially perpendicularly to the first plane. In this manner the liquids and solution used in the method may flow across the first spiral shaped sheet metal piece instead of along the spiral formed by the first spiral shaped sheet metal piece. The flow distance for the liquids and solution through the spiral heat exchanger during coating thus, may be considerably shorter than a flow distance along the spiral formed by the first spiral shaped sheet metal piece. The flow in the main direction may permit a more even concentration of the contents of the liquids and the solution during flow through the spiral heat exchanger than if the flow is along the spiral formed by the first spiral shaped sheet metal piece. An even coating thickness along the first spiral shaped sheet metal piece thus may be ensured. The spiral heat exchanger, or portions thereof, may have to be specially adapted to permit access to the first passage allowing flow in the main direction. 
     According to embodiments the first passage may be closed by means of at least one closing portion at one side of the first spiral shaped sheet metal piece, in a plane substantially parallel to the first plane. The closing portion may be provided with a number of openings, which openings are flowed through by the pre-treatment liquid during circulating the pre-treatment liquid and which openings are flowed through by the solution during circulating the solution. One kind of spiral heat exchangers may comprise such closing portions. Thanks to the provision of the openings also this kind of spiral heat exchanger may be flowed through at least partially in the main direction, i.e. substantially across the first spiral shaped sheet metal piece. 
     According to embodiments the method may comprise sealing the openings after coating the internal surface. In this manner, in use, the performance of the spiral heat exchanger may remain unaffected. 
     According to embodiments the method may comprise arranging the spiral heat exchanger with the first plane extending in a substantially horizontal direction and the main direction extending in a substantially vertical direction during circulating the pre-treatment liquid and during circulating the solution. In this manner buoyancy may be utilized for letting gas produced during coating, e.g. hydrogen, easily escape from the first surface. 
     According to embodiments the method may comprise circulating the pre-treatment liquid and circulating the solution through the first passage having a width of between 5-40 mm. Thus, a nickel plated first passage of a spiral heat exchanger may be provided, which first passage is suitable for being flowed through by a heat exchange liquid which has abrasive properties. 
     According to embodiments the first spiral shaped sheet metal piece may have a thickness of between 2-4 mm. The first spiral shaped sheet metal piece may be made from carbon steel. 
     According to embodiments the heat exchanger may comprise a second heat transfer element compring a second spiral shaped sheet metal piece extending in a spiral in the first plane substantially concentrically with the first spiral shaped sheet metal piece, and wherein studs extend in the first passage between the first and second spiral shaped sheet metal pieces the studs being arranged at a density of 280-780 studs per square metre. 
     According to embodiments the method may provide a Nickel/Boron coating or a Nickel/Diamond coating. In this manner a spiral heat exchanger with a first passage suitable to be flowed through by a heat exchange liquid with abrasive properties may be provided. 
     An object of the embodiments is to provide a heat exchanger comprising a first passage for a first heat exchange fluid, and a second passage for a second heat exchange fluid, the first and second passages being separated by at least one heat transfer element, the heat transfer element having a first surface facing the first passage, the first surface having a nickel plating applied in accordance with above mentioned method aspects and embodiments. 
     According to embodiments the heat transfer element of the heat exchanger is welded to a further heat transfer element having a first surface facing the first passage, at least part of the first passage being formed between the heat transfer element and the further heat transfer element. 
     According to example embodiments the heat transfer element of the heat exchanger is brazed to a further heat transfer element having a first surface facing the first passage, at least part of the first passage being formed between the heat transfer element and the further heat transfer element. 
     According to embodiments the heat exchanger may be a spiral heat exchanger and the at least one heat transfer element comprises a first spiral shaped sheet metal piece extending in a spiral in a first plane, the first plane extending perpendicularly to the first spiral shaped sheet metal piece. The first passage may be closed by means of at least one closing portion at one side of the first spiral shaped sheet metal piece, in a plane substantially parallel to the first plane. The closing portion may be provided with a number of openings, which openings are sealed. 
     Further features of, and advantages of, embodiments will become apparent when studying the appended claims and the following detailed description. Those skilled in the art will realize that different features of the present invention may be combined to create embodiments other than those described in the following, without departing from the scope of the present invention, as defined by the appended claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The various aspects of embodiments, including its particular features and advantages, will be readily understood from the following detailed description and the accompanying drawings, in which: 
         FIG. 1  illustrates a spiral heat exchanger according to embodiments, 
         FIG. 2  illustrates a cross section of a plate heat exchanger according to embodiments, 
         FIG. 3  illustrates embodiments of a system for electroless nickel plating an assembled heat exchanger, 
         FIGS. 4 and 5  illustrate embodiments of methods of coating an internal surface of an assembled heat exchanger, 
         FIG. 6  illustrates a container and two valves, and 
         FIGS. 7 and 8  illustrate cross sections through portions of spiral heat exchangers according to embodiments. 
     
    
    
     DETAILED DESCRIPTION 
     The present invention will now be described more fully with reference to the accompanying drawings, in which example embodiments are shown. However, this invention should not be construed as limited to the embodiments set forth herein. Disclosed features of example embodiments may be combined as readily understood by one of ordinary skill in the art to which this invention belongs. Like numbers refer to like elements throughout. 
     Well-known functions or constructions will not necessarily be described in detail for brevity and/or clarity. 
       FIG. 1  illustrates a spiral heat exchanger  20  according to embodiments. The spiral heat exchanger  20  comprises heat transfer elements in the form of two spiral shaped sheet metal pieces  22 ,  24 , which are welded together. A first passage  8  for a first heat transfer fluid and a second passage (not shown) for a second heat transfer fluid are provided between the spiral shaped sheet metal pieces  22 ,  24 . Each sheet metal piece  22 ,  24  has a first surface  12  facing the first passage  8  and a second surface (not shown) facing the second passage. The first surface  12  of each heat transfer element is provided with a nickel plating, which has been applied to the first surface  12  after the heat exchanger  20  has been assembled. 
     The heat exchanger  20  is provided with inlet and outlet pipe sections  26  (two out of four pipe sections are illustrated). In use two heat exchange fluids are conducted to and from the first and second passages through the pipe sections  26 . 
     The pipe sections  26  are provided on a housing  25  of the spiral heat exchanger  20 . The housing  25  comprises a tubular centre section  27  and two lids  28  (one of which is shown in  FIG. 1 ). The two spiral shaped sheet metal pieces  22 ,  24  are arranged inside the tubular centre section and in use of the spiral heat exchanger  20  the lids  28  are attached to the tubular centre section  27 . 
       FIG. 7  illustrates a cross section through a portion  21  of a spiral heat exchanger according to embodiments. The portion  21  may constitute an assembled heat exchanger in accordance with the definition above. The spiral heat exchanger comprises a first heat transfer elements in the form of a first spiral shaped sheet metal piece  22 , and a second heat transfer element in the form of a second spiral shaped sheet metal piece  24 . The first and second spiral shaped sheet metal pieces  22 ,  24  each extend in a spiral in a first plane  100 . The first plane  100  extends perpendicularly to the first and second spiral shaped sheet metal piece  22 ,  24 . A first passage  8  for a first heat exchange fluid and a second passage  10  for a second heat exchange fluid are formed between the first and second sheet metal pieces  22 ,  24 . A main direction  102  extends substantially perpendicularly to the first plane  100 . 
     Each spiral shaped sheet metal piece  22 ,  24  has a first surface  12  facing the first passage  8  and a second surface  14  facing the second passage. The first surface  12  of each heat transfer element is provided with a nickel plating, which has been applied to the first surface  12  after the spiral shaped sheet metal pieces  22 ,  24  have been assembled. The nickel plating may be e.g. a Nickel/Boron coating or a Nickel/Diamond coating. In this manner the first surfaces  12  of the spiral heat exchanger may withstand abrasive heat exchange fluids such as slurry originating from mining applications. 
     The first passage  8  is open in the main direction  102  at both sides of the first spiral shaped sheet metal piece  22  parallel with the first plane. The second passage  10  is closed at both sides of the first spiral shaped sheet metal piece  22  parallel with the first plane. 
     A distance between the first and second spiral shaped sheet metal pieces  22 ,  24  may be between 5-40 mm, i.e. a width of the first passage  8  may be 5-40 mm. The first and/or second spiral shaped sheet metal pieces  22 ,  24  may have a thickness of between 2-6 mm. Studs  103  may extend in the first passage  8  between the at least one heat transfer element and the second heat transfer element, i.e. between the between the first and second spiral shaped sheet metal pieces  22 ,  24 . The studs  103  may be arranged at a density of 280-780 studs per square metre. Studs  103  may also extend in the second passage  10  between the first and second spiral shaped sheet metal pieces  22 ,  24 . In  FIG. 7  the studs  103  have only been partly illustrated and in  FIG. 8 , the studs have been omitted. In practice however, studs  103  may be arranged throughout the first and second passages  8 ,  10 . 
     The portion  21  is provided with a releasable first flow distribution connector  104  and a releasable second flow distribution connector  106 , the use of which will be discussed below in connection with aspects of methods of coating an internal surface of a spiral heat exchanger. In use of the spiral heat exchanger, the portion  21  may be arranged in a housing  25  as illustrated in connection with  FIG. 1 . 
       FIG. 8  illustrates a portion  23  of a spiral heat exchanger according to embodiments. The portion  23  may constitute part of an assembled heat exchanger in accordance with the definition above. The spiral heat exchanger comprises a first and a second spiral shaped sheet metal piece  22 ,  24  extending in a spiral in a first plane  100 . The first plane  100  extends perpendicularly to the first and second spiral shaped sheet metal pieces  22 ,  24 . A first passage  8  for a first heat exchange fluid and a second passage  20  for a second heat exchange fluid are separated by the first and second spiral shaped sheet metal pieces  22 ,  24 . The first and a second spiral shaped sheet metal pieces  22 ,  24  have a first surface  12  facing the first passage  8 . The first surface  12  is provided with a nickel plating. The first passage  8  is closed by means of at least one closing portion  108  at one side of the first spiral shaped sheet metal piece, in a plane substantially parallel to the first plane  100 . 
     The closing portion  108  is provided with a number of openings  110 . The openings are arranged to be sealed, e.g. by bolts  112 . 
       FIG. 2  illustrates a cross section of a plate heat exchanger  2  according to embodiments. Heat transfer elements in the form of heat transfer plates  4  are arranged in a stack  6 . A first passage  8  for a first heat transfer fluid and a second passage  10  for a second heat transfer fluid are provided in the stack  6 . A passage  8 ,  10  is in this embodiment formed by several plate interspaces. Except for the outer plates of the stack  6 , each heat transfer plate  4  has a first surface  12  facing the first passage  8  and a second surface  14  facing the second passage  10 . The first surface  12  of each heat transfer element is provided with a nickel plating, which has been applied to the first surface  12  after at least the heat transfer plates  4  of the plate heat exchanger  2  have been assembled. The heat transfer plates  4  of the heat exchanger  2  have been permanently joined by means of brazing. The heat transfer plates  4  may alternatively have been joined by means of welding. 
     Four port channels  16 , two of which are shown, extend through the stack  6  and communicate with the first and second passages  8 ,  10 . Inlet and outlet pipe sections  18  provide means for directing the first and second heat transfer fluids into the plate heat exchanger  2 . Each of the first and second passages  8 ,  10  communicates with two port channels. Of the two port channels communicating with one passage  8 ,  10 , in use, one conducts a heat exchange fluid to the passage and the other conducts it from the passage. 
       FIG. 3  illustrates schematically embodiments of a system  30  for electroless nickel plating an assembled heat exchanger. An assembled spiral plate heat exchanger  20  is illustrated in  FIG. 3  but an assembled plate heat exchanger or other type of assembled heat exchanger may equally well be electroless nickel plated in the system  30 . The system  30  comprises a conduit system with conduits  32  (schematically illustrated). The conduit system further comprises a pump  34 , a releasable connection  36  for connecting an assembled heat exchanger  20  to the conduit system, a pre-treatment liquid storage  38 , a solution container  40  for a solution S containing nickel ions and to be used for the electroless nickel plating. The conduit system further comprises a valve arrangement  42  comprising several valves. The system  30  may be utilized for a method of coating an internal surface of an assembled heat exchanger according to embodiments. 
     The spiral heat exchanger  20  comprises first and second spiral shaped sheet metal pieces extending about a centre axis of a tubular centre section  27  of a housing  25  of the spiral heat exchanger  20 . The spiral heat exchanger  20  illustrated in  FIG. 3  is arranged with the centre axis extending in a substantially horizontal direction. Alternatively, the spiral heat exchanger or a portion  21 ,  23  of a spiral heat exchanger may be arranged with the centre axis extending in a substantially vertical direction, i.e. with the first plane  100  in a substantially horizontal plane as illustrated in  FIGS. 7 and 8 . 
       FIG. 4  illustrates embodiments of a method of coating an internal surface of an assembled heat exchanger. Reference is made in the following to  FIGS. 3 and 4 . 
     The pump  34  circulates pre-treatment liquid A, SV, W from the pre-treatment liquid storage  38  through a first passage of the heat exchanger  20  and back to the pre-treatment liquid storage  38 . Also, the pump  34  circulates the solution S from solution container  40  through the first passage of the heat exchanger  20  and back the solution container  40 . The valve arrangement  42  is used for connecting either the pre-treatment liquid storage  38  or the solution container  40  to the pump  34 , and the heat exchanger  20 . Accordingly, pre-treating  410  a first surface of a heat exchange element of the heat exchanger  20  is performed by circulating the pre-treatment liquid A, SV, W through a first passage of the heat exchanger  20  and the pre-treatment liquid storage  38 , and electroless nickel plating  420  the first surface in the heat exchanger  20  is performed by circulating the solution S through the first passage and the solution container  40 . Known solutions S containing Ni ions may be used for the electroless nickel plating, such as e.g. disclosed in US2006/0024514, U.S. Pat. No. 6,066,406, US2009/123777, and U.S. Pat. No. 5,019,163. Pre-treatment liquids as such are known, such as e.g. discussed in US 2009/123777 and U.S. Pat. No. 5,019,163. 
       FIG. 5  illustrates embodiments of a method of coating an internal surface of an assembled heat exchanger. Reference is made in the following to  FIGS. 3 and 5 . 
     The pre-treatment liquid storage  38  according to embodiments comprises three containers  44 ,  46 ,  48 . A water container  44  is connected to the conduits  32  by means of two valves  50 ,  52 . Pre-treatment liquid in the form of water W may thus be circulated in the system  30  by means of the pump  34  when the valves  50 ,  52  are open, as represented by the circulating water step  510  in  FIG. 5 . A container  46  containing a solvent SV is connected to the conduits  32  by means of two valves  54 ,  56 . The solvent SV may be water with an added detergent, a hydrocarbon based solvent, or a different solvent. Pre-treatment liquid in the form of solvent SV may thus be circulated in the system  30  by means of the pump  34  when the valves  54 ,  56  are open, as represented by the circulating solvent step  520  in  FIG. 5 . A container  48  for activating liquid A, such as an acid, is connected to the conduits  32  by means of two valves  58 ,  60 . Pre-treatment liquid in the form of activating liquid A may thus be circulated in the system  30  by means of the pump  34  when the valves  58 ,  60  are open, as represented by the surface activating step  530  in  FIG. 5 . Activating liquids A as such are known, such as e.g. discussed in US 2009/123777. 
     The solution container  40  is connected to the conduits  32  by means of two valves  62 ,  64 . The solution S comprising nickel ions may thus be circulated in the system  30  by means of the pump  34  when the valves  62 ,  64  are open, as represented by the electroless nickel plating step  540  in  FIG. 5 . 
     The step  510  may be repeated after one or more of the steps circulating solvent step  520 , circulating activating liquid step  530 , and circulating solution comprising nickel ions of the electroless nickel plating step  540 . In this manner the heat exchanger  20  may be rinsed with water to remove a previously used liquid or solution. 
     Alternatively, the circulating water step  510  may be replaced or complement with a directing water step  550 , in which water from a water source, such a water tap, is directed through the first passage of the heat exchanger  20 . 
     The surface activating step  530  and the nickel plating step  540  may be repeated one or more times. Accordingly, after a nickel plating step  540  the first surface may be activated again by circulating activating liquid A in the system  30  and through the first passage of the heat exchanger. Thereafter electroless nickel plating of the first surface is performed again by circulating the solution S comprising nickel ions in the system  30  and the first passage. Between any surface activating step  530  and any subsequent nickel plating step  540 , a circulating water step  510  and/or a directing water step  550  may be performed. 
     The method may include a preceding step of connecting  560  a heat exchanger  20  to the releasable connection  36  such that the liquids and solution may be directed through the first passage of the heat exchanger  20 . In case the heat exchanger  20  comprises a nickel plating on the first surface, for instance if the heat exchanger  20  is a used heat exchanger which is to be re-plated with a nickel plating, the method may include a removing step  570 , in which the nickel plating is removed by means of a removing liquid being circulated through the first passage and a container for removing liquid by means of the pump  34 . Removing liquids as such are known, such as e.g. discussed in U.S. Pat. No. 4,554,049. A removing liquid may also be known as a stripping solution/liquid. The removing step  570  may not be required in some embodiments, wherein the heat exchanger instead is subjected only to one or more of the pre-treatment steps  510 - 530  before the electroless nickel plating step  540 .  FIG. 6  illustrates a container  70  for removing liquid RL and two valves  72 ,  74  connected via conduits to the container  70  for removing liquid. This container  70  and these valves  72 ,  74  may be connected to the conduits  32  of the system  30  illustrated in  FIG. 3  to permit the removing liquid RL to be circulated by the pump  34  through the heat exchanger  20  and the container  70  for removing liquid. 
     The system  30  illustrated in  FIG. 3  may be used for electroless nickel plating of spiral heat exchangers as well as plate heat exchangers. In embodiments where the heat exchanger is a spiral heat exchanger, specific arrangements may be made to perform at least some steps of the methods described in connection with  FIGS. 4 and 5  with the spiral heat exchanger with its spiral shaped sheet metal pieces arranged as illustrated in  FIGS. 7 and 8 , i.e. with the first plane  100  in a substantially horizontal plane. In the following reference is made to  FIGS. 3 ,  5 ,  7 , and  8 . 
     When the first and second spiral shaped sheet metal pieces  22 ,  24  are arranged with the first plane  100  in substantially a horizontal direction, buoyancy may be utilized for letting gas produced during coating, e.g. hydrogen, easily escape from the first surface  12 . In this manner gas will not impede the treatment and coating of the first surface  12 . Accordingly, the method may comprise a step of arranging  600  the spiral heat exchanger  20  with the first plane  100  extending in a substantially horizontal direction and the main direction  102  extending in a substantially vertical direction during circulating  510 ,  520 ,  530  the pre-treatment liquid and during circulating the solution S. 
     In  FIGS. 7 and 8  the flow of the pre-treatment liquids A, SV, W and the solution S through the first passages  8  of the portions  21 ,  23  is indicated by small arrows. Alternatively, the flow may be in the opposite direction. During use of spiral heat exchangers comprising the portions  21 ,  23 , lids seal against both sides of the first and second spiral shaped sheet metal pieces  22 ,  24  in planes parallel to the first plane  100 . Accordingly, in use of a spiral heat exchanger the heat transfer fluids flow through the first and second passages  8 ,  10  along the spiral and not in the direction of the small arrows. 
     During circulation of the pre-treatment liquids A, SV, W and the solution S, the pre-treatment liquids A, SV, W and the solution S have to be directed to and from the first passage  8 . The system illustrated in  FIG. 3  is therefore provided with a releasable connection  36 . In order to provide a flow of the pre-treatment liquids A, SV, W and the solution S along the main direction  102  in embodiments of  FIGS. 7 and 8 , the releasable connection may comprises the first flow distribution connector  104  and the second flow distribution connector  106  illustrated in  FIG. 7 . 
     In embodiments of  FIG. 7 , the first passage  8  is open at both its ends parallel with the first plane  100 . The pre-treatment liquids A, SV, W and the solution S may thus flow through the first passage in the main direction  102 . In embodiments of  FIG. 8 , the first passage  8  is open at one end parallel with the first plane  100  and is provided with at least one closing portion  108 . The closing portion  108  is provided with a number of openings  110  to permit a flow of the pre-treatment liquids A, SV, W and the solution S through the first passage  8 . In the  FIG. 8  embodiments, near the openings  110  the liquids and the solution may flow in a different direction than along the main direction  102  to reach an opening  110 . Accordingly, the circulating  510 ,  520 ,  530  the pre-treatment liquid may comprises the pre-treatment liquid flowing through the first passage  8  at least partially in the main direction  102 , and circulating the solution S may comprise the solution S flowing through the first passage  8  at least partially in the main direction  102 . 
     In embodiments comprising openings  110 , the method may comprise a step of sealing  602  the openings  110  after coating the internal surface  12 . For example, sealing may be done by screwing bolts  112  into the openings  110 , or by pressing resilient plugs into the openings  112 . 
     The valves  50 - 64  of the valve system  42  and the pump  34  may be manually operated or automatically controlled by a schematically disclosed control system  66 . As schematically illustrated, the control system  66  may be connected to the pump  34  and all of the valves  50 - 64 . To perform a method of electroless nickel plating of a first surface of a heat exchanger according to embodiments illustrated in  FIGS. 4 and 5 , the control system  66  may manipulate pump  34  and the valve arrangement  42  such that the valves  50 - 64  are opened two at a time to allow a relevant liquid or solution to be circulated by the pump  34  through the first passage of the heat exchanger  20  and a relevant container  40 ,  44 - 48  for a certain period of time. 
     Some or all of the containers  40 ,  44 - 48  may be provided with heating elements  80 - 86  for heating a respective liquid or solution contained therein. Also one or more of the containers  40 ,  44 - 48  may be provided with stirring elements  90 - 94  for stirring a respective liquid or solution contained therein. The heating elements  80 - 86  and the stirring elements  90 - 94  may be controlled by the control system  66 . The temperature of a liquid or solution may for instance be kept at a temperature of 1-50 degrees Celsius below a boiling temperature of the relevant liquid or solution by means of a relevant heating element controlled by the control system  66 . Further suitable temperatures for the solution S comprising nickel ions are known, e.g. from previously mentioned prior art documents. A temperature sensor (not shown) is suitably arranged in each of the containers  40 ,  44 - 48 . Each temperature sensor is connected to the control system  66  to permit controlling of the respective heating elements  80 - 86 . Each stirring element  90 - 94  may be controlled to stir a relevant liquid or solution at least while the liquid or solution is circulated through the first passage and the relevant container  40 ,  44 - 48 . 
     Heating the solution S in the solution container  40  is performed by the heating element  86  in the solution container  40 , as represented by the solution heating step  580  in  FIG. 5 , and the solution heating step  440  in  FIG. 4 . 
     Heating the pre-treatment liquid A, SV, W in the pre-treatment liquid storage  38  is performed by a heating element  80 - 84  in the pre-treatment liquid storage  38 , as represented by the pre-treatment liquid heating step  450  in  FIG. 4 . The pre-treatment liquid heating step may be performed by one or more separate steps in which a respective of the pre-treatment liquids A, SV, W is heated. A water heating step  582  ( FIG. 5 ) may be performed by a water heating element  80  in the water container  44 . A solvent heating step  584  ( FIG. 5 ) may be performed by a solvent heating element  82  in the container  46  containing a solvent SV. An activating liquid heating step  586  ( FIG. 5 ) may be performed by an activating liquid heating element  84  in the container  48  for activating liquid A. 
     Stirring the solution S in the solution container  40  is performed by the stirring element  94  in the solution container  40 , as represented by the solution stirring step  590  in  FIG. 5 , and the solution stirring step  460  in  FIG. 4 . 
     Stirring the pre-treatment liquid in the pre-treatment liquid storage  38  is performed by a stirring element  90 ,  92  in the pre-treatment liquid storage  38 , as represented by the pre-treatment stirring step  470  in  FIG. 4 . The pre-treatment stirring step may be performed by one or more separate steps in which a respective of the pre-treatment liquids is stirred. A water stirring step  592  ( FIG. 5 ) may be performed by a water stirring element (not shown) in the water container  44 . A solvent stirring step  594  ( FIG. 5 ) may be performed by a solvent stirring element  90  in the container  46  containing a solvent SV. An activating liquid stirring step  596  ( FIG. 5 ) may be performed by an activating liquid stirring element  92  in the container  48  for activating liquid A. 
     It may be noted that: increasing the circulation time for the solution comprising nickel ions will yield a thicker coating (up to a certain thickness); a higher temperature may promote reaction—resulting in an increased coating speed; different substrate material, i.e. material of the heat transfer elements, will result in different coating speeds; different solutions for electroless electroless nickel plating, e.g. for Ni—B plating, Ni-diamond plating, etc will result in different coating speeds. These relationships are well known to a person skilled in the art. 
     Circulation of the pre-treatment liquids A, SV, W and the solution S may be performed at different flow rates. Moreover, circulation of at least one of the pre-treatment liquids A, SV, W and the solution S may be performed at varying flow rates. A slow flow rate may be utilized in a treating or coating step and a faster flow rate may be utilized for exchanging liquid or solution in the first passage. Circulation may thus take place at various different flow rates. A flow rate of 0 cubic metres for periods of time is encompassed in some embodiments. In such embodiments circulation still takes place in the sense that liquid or solution is circulated from the storage or container to the heat exchanger and back to the storage or container at time periods when the flow rate is &gt;0 cubic metres. Suitable flow rates may be empirically determined. 
     Example embodiments described above may be combined as understood by a person skilled in the art. It is also understood by those skilled in the art that nickel plating may be performed simultaneously in several heat exchangers connected in parallel or series to the system  30  illustrated in  FIG. 3 . Accordingly, the method may comprise nickel plating more than one heat exchanger at a time. 
     During performing the method of coating an internal surface of a spiral heat exchanger, a second passage of a spiral heat exchanger may be temporarily closed. The second passage may be masked during performing at least parts of the method to prevent pre-treatment liquids and solution from flowing into the second passage. After coating and before the spiral heat exchanger is put to use, the masking is removed. 
     A connection to a drain may be provided in the embodiment system  30  of  FIG. 3 . More than one pump  34  may be used in the system  30 . For example, one pump for each liquid/solution may be arranged in the conduit system of the system  30 . The concentration of substances in the liquids and solutions may be measured. The control system  66  may provide a warning if a relevant concentration value is over, or below, a threshold value. The concentration of substances in the liquids and solutions may be corrected by means of the exchanging a liquid or solution or by adding concentrates of a relevant substance. A heating element and a stirring element may be used in the container  70  for removing liquid. The releasable first and second flow distribution connectors  104 ,  106  may be connectable to a portion of a spiral heat exchanger as illustrated in  FIG. 7 . Alternatively, the releasable first and second flow distribution connectors  104 ,  106  may be connectable to a part of a housing of a spiral heat exchanger, such as the tubular centre section  27  illustrated in  FIG. 1 . 
     The second surface of a heat transfer element may also be nickel plated in accordance with the method. This may be performed at the same time as the first surface is nickel plated. Alternatively, it may be performed in a separate process. The nickel platings on the first and second surfaces may be of the same kind or of different kinds, e.g. nickel/boron plating on one surface and nickel/polymer plating on the other surface. 
     Therefore, it is to be understood that the foregoing is illustrative of various example embodiments and the invention is not to be limited to the specific embodiments disclosed and that modifications to the disclosed embodiments, combinations of features of disclosed embodiments as well as other embodiments are intended to be included within the scope of the appended claims.