Abstract:
The present invention relates to a tube for a heat exchanger, wherein at least a part of the tube has a variable cross-section in longitudinal direction, wherein a cross-sectional area decreases from a maximum value close to an outer end of the tube to a minimum value close to an opposite outer end thereof. 
     The invention further relates to a heat exchanger provided with at least one such tube and to a central heating installation and a tap water system comprising such a heat exchanger.

Description:
RELATED APPLICATION DATA 
       [0001]    This application is a National Stage Application under 35 U.S.C.  371  of co-pending PCT application PCT/NL2014/050674 designating the United States and filed Oct. 1, 2014; which claims the benefit of Dutch patent application number 2011539 and filed Oct. 2, 2013, each of which are hereby incorporated by reference in their entireties. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The invention relates to a tube for a heat exchanger, wherein at least a part of the tube has a variable cross-section in longitudinal direction. 
       BACKGROUND 
       [0003]    Such a heat exchanger tube is known, for instance from EP 1 429 085. Said document EP 1 429 085 describes a heat exchanger with a number of parallel tubes. The cross-section of each tube goes from being round close to a first outer end attached to a mounting plate to being elliptical in a central part. From there the cross-section changes again to a round shape at the second outer end, which is likewise attached to a mounting plate. The round cross-sectional shape at the ends is chosen here for a simple mounting in round openings in the plates. 
       SUMMARY 
       [0004]    The invention now has for its object to provide a tube for a heat exchanger, the cross-section of which varies in longitudinal direction of the tube such that an optimum heat transfer is possible from a medium flowing through the tube to a medium surrounding the tube. According to the invention this is achieved in the case of such a tube in that a cross-sectional area decreases from a maximum value close to an outer end of the tube to a minimum value close to an opposite outer end thereof. Decreasing the area of the tube achieves that the velocity of the medium flowing through the tube increases, whereby the heat transfer is optimized. 
         [0005]    The ratio of the cross-sectional area and the periphery varies along the length of the tube. Optimal flow conditions for the medium can thus be set in the tube. 
         [0006]    A ratio of the periphery and the area of the cross-section can advantageously increase here from a minimum value close to an outer end of the tube to a maximum value close to an opposite outer end thereof. This ratio determines the wall area available per unit of the tube for the heat-exchanging contact between the media in and round the tube. 
         [0007]    In a preferred embodiment of the tube the ratio of the periphery and the area of the cross-section increases in the direction of the tube in which the cross-sectional area decreases. The velocity and the turbulence of the medium thus increases, whereby the heat transfer is improved. 
         [0008]    Although it is possible to envisage the cross-section of the tube being substantially round close to the one outer end and having a flat shape close to the other outer end, a further advantageous embodiment is obtained when the peripheral shape close to the one outer end is substantially round and substantially star-shaped close to the other outer end. In the case of a round cross-section the ratio of the periphery and the area is minimal, while in the case of a star shape it is conversely relatively large. The star can have three or more points here. A circle has a maximum cross-sectional area relative to the periphery, whereby the heat transfer at the outer end where the tube has a round cross-section can deliberately be limited in order to prevent the tube being heated there to undesirably high temperatures. Conversely, a high heat transfer is on the other hand obtained close to the outer end where the tube has the star-shaped periphery. 
         [0009]    A structurally simple solution is obtained when the variation in the area and/or the peripheral shape of the cross-section is achieved by deforming at least a part of a wall of the tube. 
         [0010]    It is further recommended that, for each cross-section of the tube, a line is defined enveloping the cross-section, and the envelopes are substantially identical along the length of the tube. The external dimension of the tube thus remains constant along its length, whereby it is easy to place a number of tubes adjacently of each other in a heat exchanger. 
         [0011]    The variable peripheral shape can in this case be formed by at least one inward folded part of the tube wall. By folding the wall inward the cross-section remains within the constant envelope. 
         [0012]    In order to prevent disruption of the flow of the medium in the tube it is recommended that the variation in the area and/or the peripheral shape of the cross-section is substantially gradual. 
         [0013]    The tube can otherwise also have a part of constant cross-section. Where there is variation however, this variation preferably therefore has a gradual progression. 
         [0014]    The invention also relates to a heat exchanger provided with at least one tube for a first medium, which at least one tube is in heat-exchanging contact with a second medium flowing therealong. According to the invention the at least one tube is a tube of the above described type. 
         [0015]    For the purpose of a controlled heat transfer the at least one tube is preferably received in a housing in which the second medium flows. 
         [0016]    As stated, the variation in the cross-section provides the option of adapting this variation to the temperature gradient in the medium in the tube. This is particularly advantageous when the first medium is a heating medium and the at least one tube is connected to a heat source, while the second medium is a heat-absorbing medium. The temperature of the heating medium can after all be properly controlled by means of the heat source. 
         [0017]    The outer end of the at least one tube where the cross-sectional area is maximal and/or the ratio of the periphery and the area of the cross-section has a minimum value is preferably connected to the heat source. This achieves that the heating medium will first flow relatively slowly through the wide part of the tube so that there is sufficient time to transfer the large quantity of heat in the heating medium to the water-absorbing medium around the tube. Once the greater part of the heat has been transferred, the flow of the heating medium can then be accelerated by narrowing the tube. 
         [0018]    When the housing has an inflow opening for the second medium formed in or close to a first side and an outflow opening for the second medium formed in or close to a second side, a number of tubes are preferably arranged substantially parallel in the housing and enclose an angle with a line which mutually connects the inflow opening and the outflow opening. Thus formed is a cross-current or cross-flow heat exchanger which is structurally simple, compact and efficient. 
         [0019]    The housing with the inflow opening and outflow opening can form part of a circuit in a central heating installation, and the tubes can form part of a flue duct of a heating burner. The heat exchanger can thus be applied in a central heating installation. 
         [0020]    The housing with the inflow opening and outflow opening can also form part of a tap water conduit, while the tubes form part of a flue duct of a heating burner. The heat exchanger is then suitable for use in a tap water system. 
         [0021]    Finally, the invention further relates to a central heating installation and a tap water system in which a heat exchanger of the above described type is applied. The central heating installation here comprises a heating burner, a circuit which extends along one or more spaces and in which a medium circulates, and a heat exchanger according to the invention mutually connecting the burner and the circuit. In similar manner the tap water system comprises a heating burner, a water conduit extending from a water source to a draw-off point and a heat exchanger mutually connecting the burner and the water conduit. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0022]    The invention will now be elucidated on the basis of two embodiments wherein reference is made to the accompanying drawing in which corresponding parts are designated with the same reference numerals, and in which: 
           [0023]      FIG. 1  is a schematic view of a heat exchanger with tubes according to a first embodiment of the invention, 
           [0024]      FIG. 2  is a perspective view of a tube for application in the heat exchanger of  FIG. 1 , with the flow velocities of a medium in the tube, 
           [0025]      FIGS. 3 and 4  show cross-sections along the respective lines and IV-IV in  FIG. 2 , 
           [0026]      FIG. 5  is a schematic view of a second embodiment of the heat exchanger, 
           [0027]      FIG. 6  is a view corresponding to  FIG. 2  of the second variant of the tube, and 
           [0028]      FIGS. 7 and 8  show cross-sections along the respective lines VII-VII and VIII-VIII in  FIG. 6 . 
       
    
    
     DETAILED DESCRIPTION 
       [0029]    A central heating (CH) installation  1  ( FIG. 1 ) comprises a heating burner  2  and a circuit (not shown) for a medium M 2  which is guided along one or more spaces and there flows through radiators. The medium M 2  is heated indirectly by burner  2 . Placed for this purpose between the circuit and heating burner  2  is a heat exchanger  3  in which flows a medium M 1 . 
         [0030]    In the shown embodiment the medium M 1  is formed by the flue gases released when a combustible mixture C is combusted in burner  2 . This combustible mixture C is fed to burner  2  through a conduit  4 , while the flue gases leaving burner  2  are in the first instance collected in an outlet manifold  5 . From here the flue gases are distributed over a number of parallel tubes  6  arranged in a housing  7  of heat exchanger  3 . At an opposite side of housing  7  the tubes  6  debouch into an accumulation chamber  8 , from where the flue gases are discharged through an outlet  9 . 
         [0031]    Housing  7  is further provided with an inflow opening  10  in a side  11  and an outflow opening  12  in an opposite side  13 . Inflow opening  10  is connected here to a return conduit  14  of the circuit of CH installation  1 , while outflow opening  12  is connected to a feed conduit  15  of the circuit. After passing through the circuit the medium M 2 , once it has relinquished its heat to the spaces for heating, can thus flow through heat exchanger  3  and be brought there into heat-exchanging contact with the heating medium M 1  (the flue gases) flowing through tubes  6 . The heated medium M 2  can then pass through the circuit again. 
         [0032]    Because tubes  6  extend in the shown embodiment at substantially a right angle relative to a line mutually connecting inflow opening  10  and outflow opening  12 , the heat exchanger in the shown embodiment is a cross-current or cross-flow heat exchanger. 
         [0033]    According to the invention the tubes  6  have a variable cross-section, in any case along a part of their length. In the shown embodiment the variations are limited to the final part of tubes  6  as seen in the flow direction of medium M 1 . Tubes  6  here have a constant cross-section along the first half of their length L, but the area A and the peripheral shape P of the cross-section then change. 
         [0034]    The area A decreases here as seen in flow direction so that the outflow area is smaller than the inflow area: A out &lt;A in . The decrease in the area has the result that the flow velocity of medium M 1  in tube  6  will increase in order to maintain a constant mass flow: V out &gt;V in . Owing to the lower flow velocity in the first part of tube  6  close to burner  2 , the residence time of medium M 1  in this part of tube  6  is relatively long, whereby the then still very hot medium M 1  can transfer a greater amount of heat to medium M 2 . The residence time decreases as the flow velocity increases as a result of the narrowing of tube  6 , whereby less heat will also be transferred. 
         [0035]    This effect is compensated in that in the shown embodiment the peripheral shape of tube  6  also changes, this such that the ratio of the periphery P and the area A of the cross-section increases. An increasingly larger wall part  16  of tube  6  hereby becomes available per unit of cross-sectional area A of tube  6  for heat-exchanging contact between the two media M 1  and M 2  on either side of wall  16 . 
         [0036]    In the shown embodiment the outer dimension of tube  6  does not vary. The area A fits at any point of tube  6  within the same envelope  17 . Tubes  6  can hereby be accommodated in simple manner adjacently of each other with constant spacing in housing  7 . The variation in the peripheral shape P and area A of tube  6  is found here within this constant envelope  17 . Wall  16  of tube  6  is deformed locally for this purpose. In the shown embodiment wall  16  is folded inward at three locations, whereby three recesses  18  are formed. These recesses  18  increase in depth and width as seen in the flow direction, whereby the sought-after reduction in the area A and the desired increase in periphery P is obtained. The cross-section of tube  6  in this way acquires the form of a three-pointed star with rounded tips ( FIG. 4 ). 
         [0037]    A circle has the smallest ratio of the periphery and the enclosed area. As can be seen from comparing  FIGS. 3 and 4 , the periphery P out  of the “star shape” is considerably longer than that of the circle P in . At the same time the area A out  enclosed by the star shape is considerably smaller than the area A in  enclosed by the circle—the difference being formed by the surface areas of recesses  18 . This is of course associated with the fact that the star shape falls within the same envelope  17  as the circle. The variation in the area A and the peripheral shape P of tube  6  is otherwise gradual so that there is no risk of flow release and turbulence in tube  6 . Wall  16  transposes gradually from a cylinder to a folded shape, after which the folds increase uniformly in size. 
         [0038]    In another embodiment of the invention tubes  6  are provided with four recesses  18  and end in a four-pointed star ( FIG. 8 ). The ratio of the periphery P and area A is hereby even larger because the wall  16  differs more from the circular shape. A greater number of recesses  18  results in a relatively longer periphery P, and so a larger heat-exchanging wall  16 . 
         [0039]    This embodiment of tube  6  is shown in combination with a heat exchanger  3  for a tap water system  20 . Inflow opening  10  of housing  7  is connected here to a conduit  21  which supplies cold water from a water source (not shown), for instance the water mains. This cold tap water is guided as heat-absorbing medium M 2  through heat exchanger  3  and brought therein to a desired temperature through contact with the medium M 1  (the flue gases) in tubes  6  (of which only some are shown). The heated tap water then leaves the heat exchanger through outflow opening  12  and flows through a conduit  22  to a draw-off point (not shown), for instance a drinking water tap. In this embodiment the tubes  6  once again also lie roughly transversely of the direction in which the medium M 2  flows through housing  7  from inflow opening  10  to outflow opening  12 . 
         [0040]    Further shown in this embodiment is a discharge opening  23  for condensation at the bottom of accumulation chamber  8  for the flue gases. When the flue gases relinquish their heat to the tap water and thereby cool, water vapour present in the flue gases will condense and the condensation will accumulate at the lowest point of heat exchanger  3 , so in the shown embodiment on the bottom of accumulation chamber  8 . Although not shown, such a condensation discharge can also be present in the first embodiment. 
         [0041]    Although the invention has been elucidated above on the basis of two embodiments, it will be apparent that it is not limited thereto but can be varied in many ways. The recesses thus run for instance in axial direction of the tube in the shown embodiments, although it is also possible to envisage them running at an angle to the axial direction, whereby the tube wall acquires something of a twisted appearance. In the shown embodiments the recesses are further distributed uniformly over the periphery of the tube, but this is not essential. Other distributions are also possible. It is also possible to opt for an initial shape of the tubes other than the shown circular shape. The inflow side of the tubes could thus take an elliptical form, optionally even with flattened sides. Non-curved peripheral shapes, such as optionally regular polygons, could also be envisaged. The tubes and heat exchangers equipped therewith can further also be used in applications other than CH installations and tap water systems. The variable cross-section of the tubes in longitudinal direction can also provide advantages in industrial process installations. 
         [0042]    The scope of the invention is therefore defined solely by the following claims.