Patent Publication Number: US-9897383-B2

Title: Heat exchanger

Description:
RELATED APPLICATION 
     This application claims priority under 35 U.S.C. § 119 to European Patent Application No. 07115054.4 filed in Europe on Aug. 27, 2007, the entire content of which is hereby incorporated by reference in its entirety. 
     TECHNICAL FIELD 
     The present disclosure in general relates to a heat exchanger. For example, the present disclosure relates to a heat exchanger that can be used for power-electronics components. 
     BACKGROUND INFORMATION 
     Low voltage drive systems have a competitive market with many global players. This imposes a strict low cost condition to their design. In a typical system, power-electronics components such as discrete or integrated (i.e. module type) semiconductor devices, inductors, resistors, capacitors and copper bus-bars are assembled in close proximity. PCB panels and control electronics are also present in all designs. During operation, these components dissipate heat of varying quantities. In addition, these components are tolerant to temperatures of varying levels. The environmental conditions surrounding the drive system also varies in terms of air temperature, humidity, dust and chemical content. The thermal management and integration concept of a drive system has to consider all of these underlined factors in addition to the electrical performance of the system. 
     Semiconductor components and power resistors are commonly built with a plate-mount design to be bolted or pressed onto a flat surface that is kept at a suitably cold temperature. Fan-blown-air cooled aluminium heat sinks and pumped water cooled cold plates are typical examples of such heat exchange surfaces. Other components such as inductors, capacitors and PCB circuit elements are typically cooled by air-flow. 
     Typically, components such as the choke inductors, aluminium heat sinks and DC-link capacitors are allowed to protrude on one side of a drive system whereas the more delicate components are collected on the other side. The cooling air from the fan flows through the capacitors, heat sink and the choke which have temperature limitations in the reverse order (e.g. capacitors need to be kept colder than the choke). The delicate components can be further enclosed and cooled via an additional fan in the higher IP rated versions. 
     The degree of environmental protection that is offered by an electronic product is commonly expressed in terms of its “Ingress Protection (IP) Rating”. Many drive products are offered in IP20 or IP21 as standard with IP54 or higher protection ratings offered as optional. With lower IP ratings it is possible to design for through-flow of outside air within the drive enclosure while still providing adequate protection. Air filters may be employed to reduce the particles in the air. Down-facing air-vents on the enclosure walls prevent vertical water droplets from entering. With higher IP ratings, however, separation of outside air from the inside air of the drive enclosure becomes essential. For the highest protection levels, a water-tight enclosure is necessary. 
     An air-to-air heat-exchanger is commonly employed in high IP rated enclosures in order to dissipate heat to the ambient while completely separating the cabinet internal and outside air volumes. Heat-pipes and thermoelectric cooling elements are also used in such devices. 
     EP 0 409 179 A1 shows a heat pipe for computers with a conduit, which comprises an exterior and interior wall, which separates the evaporator and condenser tube. The device is only intended for a horizontal position of the evaporator section and the heat producing element. 
     In US 2007/0133175 a heat dissipation device with a heat transfer element is shown. The heat transfer element is made in form of a base plate, which is in contact to the heat producing element and a heat pipe. The base plate comprises grooves for better contact of the heat pipes and mounting holes for mounting the plate to a substrate, on which the electronic element is mounted. 
     SUMMARY 
     Exemplary embodiments disclosed herein are directed to a heat exchanger that allows an efficient heat removal. 
     A heat exchanger for removing heat energy from a heat generator is disclosed, comprising: at least one conduit for a working fluid, which is arranged in an upright position of at least 45°, each conduit having: an exterior wall and at least one interior wall for forming at least one evaporator channel and at least one condenser channel within the conduit; the heat exchanger further comprising a first heat transfer element for transferring heat into the evaporator channel; and a second heat transfer element for transferring heat out of the condenser channel. 
     A method of producing a heat exchanger is disclosed for removing heat energy from a heat generator, comprising: providing at least one conduit for a working fluid, each having an exterior wall and at least one interior wall for forming at least one evaporator channel and at least one condenser channel within the at least one conduit; and connecting to the at least one conduit a first heat transfer element for transferring heat into the evaporator channel and a second heat transfer element for transferring heat out of the condenser channel. 
     In another aspect, a heat-exchange arrangement is disclosed, comprising: at least one conduit for a working fluid, each having an exterior wall and at least one interior wall for forming at least one evaporator channel and at least one condenser channel within the at least one conduit; a first heat transfer element connected to the at least one conduit for transferring heat into the evaporator channel; and a second heat transfer element connected to the at least one conduit for transferring heat out of the condenser channel. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Exemplary embodiments of the present disclosure are depicted in the drawings and are detailed in the description which follows. 
       In the drawings: 
         FIG. 1  illustrates a first exemplary embodiment of the present disclosure; 
         FIG. 2  is a cross-sectional view of the exemplary embodiment shown in  FIG. 1 ; 
         FIG. 3  shows detailed view of a second exemplary embodiment of the present disclosure; 
         FIG. 4  shows further exemplary embodiment of the present disclosure; 
         FIG. 5  shows further exemplary embodiment of the present disclosure; 
         FIG. 6  shows further exemplary embodiment of the present disclosure; 
         FIG. 7  shows further exemplary embodiment of the present disclosure; and 
         FIG. 8  is a cross-sectional view of the exemplary embodiment shown in  FIG. 7 . 
     
    
    
     In the figures, same reference numerals denote the same or similar parts. 
     DETAILED DESCRIPTION 
     According to a first aspect the present disclosure provides a heat exchanger for removing heat energy from a heat generator, comprising at least one conduit for a working fluid, which is arranged in an upright position of at least 45°, each conduit having an exterior wall and at least one interior wall for forming at least one evaporator channel and at least one condenser channel within the conduit. Furthermore, the heat exchanger comprises a first heat transfer element for transferring heat into the evaporator channel and a second heat transfer element for transferring heat out of the condenser channel. 
     The present disclosure allows the use of a two-phase heat transfer principle in order to efficiently remove the input heat without the need for a pumping unit. This results in cost reduction and reliability improvement. The present disclosure provides a novel construction for a thermosyphon-type heat-exchanger that can be employed for cooling electric circuit components, e.g., for cooling low voltage AC drive systems. The present disclosure can be used as a loop-thermosyphon configuration by separating the upgoing and down-coming fluid streams in separate channels of multi-port conduit. Different numbers and sizes of channels can be used for the up-going and down-coming streams in order to optimize the boiling and condensation performance. 
     In an exemplary embodiment the first heat transfer element comprises a mounting element having a mounting surface for mounting the heat generator, and a contact surface for establishing a thermal contact to a portion of the exterior wall of the conduit associated with the evaporator channel. 
     In a further exemplary embodiment the at least one conduit is arranged in vertical position. The at least one evaporator channel and at least one condenser channel are aligned in parallel in the at least one conduit in another exemplary embodiment. 
     In a further exemplary embodiment the heat exchanger comprises a plurality of conduits. Furthermore, e.g., the second heat transfer element comprises cooling fins provided on a portion of the exterior wall of the conduit, e.g., only on a portion of the exterior wall of the conduit associated with the condenser channel. 
     In a further exemplary embodiment the heat exchanger comprises a distribution manifold, e.g., a header tube, which is connected to at least one end of at least one conduit. 
     Furthermore, e.g., the mounting element comprises a base plate having a planar mounting surface for mounting the heat generator and a contact surface opposite to the mounting surface comprising at least one groove conforming with a portion of the exterior wall of the conduit. Thus the heat exchanger is designed to efficiently discharge the heat generated by flat-plate mounted components for example to the ambient air while also allowing for the separation of the air volumes inside and outside the system enclosure. Thereby, e.g., the planar exterior sidewalls of the flat tube are oriented perpendicular to planar mounting surface of the base plate and that the mounting element comprises at least one mounting hole or at least one mounting slot on the mounting surface. Furthermore, e.g., the heat exchanger comprises two mounting elements, to allow for a compact design of the overall system. 
     In a further exemplary embodiment the conduit is flat tube having planar exterior sidewalls, e.g., a louvered fin-with-flat-tube design provides a high heat-transfer coefficient to air with small pressure drop in the air flow and in a compact size. 
     In a further exemplary embodiment the mounting element is made of aluminium or copper. Furthermore, the conduit can be made of aluminium. For example, brazed aluminium common in automotive industry can be used for reduced manufacturing cost, small size and good thermal-hydraulic performance. The present disclosure is suitable for automated manufacturing with heat-exchanger core assembly machines, commonly used in the automotive cooling industry. Such reuse of available series production equipment reduces the cost. 
     In a further exemplary embodiment the heat exchanger comprises a separation element for separating a first environment from a second environment, whereby the temperature of the first environment is higher than the temperature of the second environment. 
     According to a further aspect of the present disclosure a method of producing a heat exchanger is provided. Thereby, the method comprises the steps of providing at least one conduit for a working fluid, each having an exterior wall and at least one interior wall for forming at least one evaporator channel and at least one condenser channel within the at least one conduit, and connecting to the at least one conduit a mounting element, having a mounting surface for mounting the heat generator, and a contact surface for establishing a thermal contact to a portion of the exterior wall of the conduit associated with the evaporator channel. 
     In an exemplary embodiment of the inventive method components of the heat exchanger are joined together in a one-shot oven brazing process. Furthermore, the components of the heat exchanger can be covered with brazing alloy, e.g., an AlSi brazing alloy, before the brazing process. A flux material can be applied to the components of the heat exchanger before the brazing process, and that the brazing process is conducted in a non-oxidizing atmosphere. 
     In a further exemplary embodiment of the inventive method all components other than the mounting element are joined in a one-shot oven brazing process and the mounting element is pressed onto the exterior walls of the conduits with thermally conductive gap filling material in between. 
     A heat exchanger  100  according to a first exemplary embodiment of the present disclosure is described with reference to  FIG. 1 . 
     As shown in  FIG. 1  the heat exchanger  100  comprises a plurality of conduits  110  for a working fluid, each having an exterior wall  112  and each having interior walls  114  (see  FIG. 2 ) for forming at least one evaporator channel  120  and at least one condenser channel  130  within the conduit  110 . Furthermore, the heat exchanger comprises a first heat transfer element  150  for transferring heat into the evaporator channel and a second heat transfer element  180  for transferring heat out of the condenser channel. The conduits  110  are arranged in a vertical position, but other positions of at least 45° are also possible. The evaporator channels  120  and the condenser channels  130  are aligned in parallel in the conduits  110 . 
     In the exemplary embodiment shown in  FIG. 1  the first heat transfer element comprises a mounting element  150  having a mounting surface  160  for mounting a heat generator, and a contact surface  170  for establishing a thermal contact to a portion of the exterior wall  112  of the conduit associated with the evaporator channel  120 . 
     For example, in the exemplary embodiment shown in  FIG. 1  the mounting element  150  takes the form of a base plate having a planar mounting surface  160  for mounting the heat generator and a contact surface  170  opposite to the mounting surface comprising grooves  175  conforming with the exterior walls  112  of the conduits  110 . Furthermore, the second heat transfer element  180  comprises cooling fins provided on exterior walls  112  of the conduits  110  and two header tubes, used as distribution manifolds  190 , are connected to each end of the conduits  110 . In case of heat from the heat generator  200  the working fluid ascends within the evaporator channel to the upper distribution manifold  190  and from there to the condenser channels  130 , where the fluid condenses and drops to the lower distribution manifolds  190 . 
     In the exemplary embodiment shown in  FIG. 1  the conduits  110  take the form of flat multi-port extruded aluminium tubes. Thereby, the planar exterior sidewalls of the flat tube  110  are oriented perpendicular to planar mounting surface  160  of the base plate  150 . Two support bars  195  can also be attached at the side ends of the assembly. The side bars  195  add mechanical strength to the assembly and also enclose the side-most fins  180  in order to force the air-flow through them. 
     The mounting element comprises two mounting holes  165  for mounting a heat generating unit thereto. As an alternative to the mounting holes on the flat side of the base-plate  150 , T-shaped slots on the flat surface  160  can be used with to attach the components with bolts and nuts. The slots can be included as part of an extrusion to eliminate secondary machining steps needed to make mounting holes. The T-shaped slots can be designed to coincide with the areas over the fin columns such that their disturbance of the heat flow in the base-plate is reduced. 
     The heat exchanger  100  shown in  FIG. 1  works with the loop thermosyphon principle. The heat exchanger is charged with a working fluid. Any refrigerant fluid can be used; some examples are R134a, R245fa, R365mfc, R600a, carbon dioxide, methanol and ammonia. The device is mounted vertically or with a small angle from the vertical such that the fins  180  are situated higher than the base-plate  150 . The amount of fluid inside can be adjusted such that the level of liquid is not below the level of the base-plate  150 . 
     The grooves  175  of the base-plate  150  conduct the heat generated by the electrical components to the front side of the multi-port flat tubes  110 . As can be seen from  FIG. 2  only the sections of the flat tubes that are covered by the base-plate grooves  175 , which are the evaporator channels  120 , directly receive the heat. Some of the heat will may also be conducted through the walls of the flat tubes. The evaporator channels  120  are fully or partially filled with the working fluid, depending on the amount of initial charge. The fluid in the evaporator channels  120  evaporate due to the heat and the vapour rises up in the channel by buoyancy effect. Some amount of liquid is also entrained in the vapour stream and will be pushed up in the channels. 
     Above the level of the base-plate the flat tubes  110  have air-cooling fins  180  on both sides. These fins  180  are typically cooled by a convective air flow, commonly generated by a cooling fan or blower (not shown). It is also possible to use natural convection currents. In the case of natural convection, the system can be installed with an increased angle from the vertical. The mixture of vapour and liquid inside the evaporator channels  120  reaches the top side header tube  190  and the flows down the condenser channels  130 . While going through the condenser channels  130 , vapour condenses back into liquid since the channels  130  are cooled by the fins  180 . The liquid condensate flows down to the bottom header tube  190  and flows back into the evaporator channels  120 , closing the loop. 
     As with all thermosyphon-type devices, all air and other non-condensable gases inside can be evacuated (i.e. discharged) and the system is partially filled (i.e. charged) with a working fluid. For this reason discharging and charging valves (not shown) are included in the assembly. The free ends of the header-tubes are suitable locations for such valves. A single valve can also be utilized for both charging and discharging. Alternatively, the heat exchanger can be evacuated, charged and permanently sealed. In this case, a valve is not necessary. 
     In the exemplary embodiment shown in  FIG. 1 , the cooling fins  180  completely cover the sides of the flat tubes  110 . As a result, the up-going vapour in the evaporator channels  120  will start condensing as soon as it is above the level of the base-plate  150 . This may lead to a cross flow of up going vapour and down coming condensate liquid which may increase the pressure drop of the stream and hinder the operation of the heat exchanger. 
     To avoid this situation a further exemplary embodiment of the present disclosure is described with respect to  FIG. 3 . Thereby, the cooling fins  180  are provided only on a portion of the exterior wall  112  of the conduit  110  associated with the condenser channel  130 . For the same reason, the cooling air can flow in the direction shown in  FIG. 3  so that the coldest air stream hits the condenser channel side first. 
     The base-plate  150  can be made of a highly thermally conductive material such as aluminium or copper. It can be manufactured using extrusion, casting, machining or a combination of such common processes. The base-plate need not be made to the exact size of the flat-tube assembly. In fact, it may be preferred to make it larger in order to add thermal capacitance to the system. One side of the plate is contacting the flat tubes. The base-plate has grooves on this side that partially cover the multi-port flat tubes as shown in  FIG. 3 . The channels are shaped to conform to the flat-tubes. The other side of the plate is made flat to accept plate mounted heat-generating components  200  such as power electronics circuit elements (e.g. IGBT, IGCT, Diode, Power Resistors etc.). Mounting holes  165  with or without threads are placed on the flat surface to bolt down the components. 
       FIG. 3  shows a further exemplary embodiment of the present disclosure. In this variation of the basic design, two base-plates are assembled facing opposite directions. Each base-plate has grooves  165  that overlap evaporator channels  120  on both sides of the flat tubes. This configuration brings major benefits in the electric circuit layout as it minimized the inter-component distances. Similar to the configuration in  FIG. 3 , the cooling fins  180  are aligned to cover only the condenser sections. 
     It is noted that not both of the base-plates need to be designed to accept plate-mounted heat generating components as illustrated above. It is also possible that one of the plates is used only to as a block of mass, in order to increase the thermal capacitance of the system. 
     The multi-port flat tubes shown in  FIGS. 1 to 4  have a symmetric layout of the internal channels, whereby the up-going and down-coming streams in the loop thermosyphon configuration share the same multi-port tube. For this reason the channels can be configured for these two streams independently. For example, the largest pressure drop in the flow of the refrigerant vapour-liquid mixture is created inside the evaporator channels  120 . For this reason larger channel cross-sectional area can be allocated to these channels as can be seen in  FIG. 5 . 
     For the condenser channels  130 , smaller channels with dividing walls or additional fin-like features on the inner-wall surfaces can increase the inner channel surface thus increasing the heat-transfer surface, as can be seen in  FIG. 6 . 
     When using different size channels inside the multi-port tube it may be necessary also to have different wall thickness around the periphery of the tube so that all sections are equally strong against internal pressure. For example, the wall thickness around a larger sized evaporator channel can be increased while using a thinner wall thickness around the small condenser channels. In comparison to using a uniform and thick evaporator thickness, this approach can save on material costs. Typical wall thicknesses used in aluminium multi-port extruded flat tubes are in the order of 0.2 to 0.75 mm. 
     According to a further aspect of the present disclosure a method of producing a heat exchanger  100  is provided. Thereby, the method comprises the steps of providing at least one conduit  110  for a working fluid, each having an exterior wall  112  and at least one interior wall  114  for forming at least one evaporator channel  120  and at least one condenser channel  130  within the conduit  110 , and connecting to the conduit  110  a mounting element  150 ,  183 , having a mounting surface for mounting the heat generator, and a contact surface for establishing a thermal contact to a portion of the exterior wall of the conduit associated with the evaporator channel. 
     After the assembly, the heat-exchanger components can be joined together in a one-shot oven brazing process. Soldering and brazing of aluminium on to aluminium is particularly challenging because of the oxide layer on aluminium that prevents wetting with solder alloy. There are various methods employed to accomplish this task. The base aluminium material can be covered with an AlSi brazing alloy (also called the cladding) that melts at a lower temperature (around 590° C.) than the base aluminium alloy. The aluminium tubes are extruded with the cladding already attached as a thin layer. A flux material is also applied on the tubes, either by dipping the tubes into a bath or by spraying. When the parts are heated in the oven, the flux works to chemically remove the oxide layer of the aluminium. The controlled atmosphere contains negligible oxygen (nitrogen environment is commonly used) so that a new oxide layer is not formed during the process. Without the oxide layer, the melting brazing alloy is able to wet the adjacent parts and close the gaps between the assembled components. When the parts are cooled down, a reliable and gas-tight connection is established. Furthermore, the cooling fins and the tubes are also bonded to ensure a good thermal interface between them. 
     It is highly desirable that there is good thermal contact interface between the base-plate and the flat tubes. It would be ideal if the base-plate channels are also brazed onto the flat tubes during the oven brazing process. In fact, it is possible to use the base-plate as the holding fixture for the flat tube assembly while the assembly goes through the brazing oven. Assembling the whole device and brazing it at one shot would ensure that the channels on the base-plate are exactly matching the location of the flat tubes. Alternatively, a second, lower temperature soldering process can be employed to join the base-plate with the flat tubes after the heat-exchanger core is brazed. The lower temperature soldering is needed to make sure that the brazed joints do not come off during re-heating for soldering. 
     A potential disadvantage of a soldered or brazed connection can be the deformation (i.e. warping) of the flat surface of the base-plate. Refinement of the surface may require a post-brazing surface machining operation. Alternatively, the base-plate channels can be press-fit onto the flat tubes or a glue material with gap filling ability and high thermal conductivity can be used. 
     Furthermore, flat, multi-port tubes with louvered fins can be used. The flat tubes introduce less pressure drop to the air flow compared to round tubes. In addition, the multi-port design increases the internal heat-transfer surface. Louvered fins increase the heat-transfer coefficient without significant increase in pressure drop (louvers are twisted slits on the fin&#39;s surface). The fins are cut from a strip of sheet aluminium and bent into an accordion-like shape as shown. The pitch between the fins can be easily adjusted during assembly by “pulling on the accordion”. Two round header tubes at the ends of the flat tubes constitute the distribution manifolds. Most importantly, the stacking and assembly of all these elements of the heat-exchanger core can be done in a fully automated way. 
     A heat exchanger  100  according to a further exemplary embodiment of the present disclosure is described with reference to  FIG. 7 . 
     As shown in  FIG. 7  the heat exchanger  100  comprises a plurality of conduits  110  for a working fluid, each having an exterior wall  112  and each having interior walls  114  for forming at least one evaporator channel  120  and at least one condenser channel  130  within the conduit  110 . Furthermore, the heat exchanger comprises a separation element  250  for separating a first environment  270  from a second environment  260 , whereby the temperature of the first environment  270  is higher than the temperature of the second environment  260 . 
     As can be seen from  FIG. 8  cooling fins  180  are provided on a portion of the exterior wall  112  of the conduit  110  associated with the condenser channel  130  and heating fins  183  are provided on a portion of the exterior wall  112  of the conduit  110  associated with the evaporator channel  120 . The heating fins  183  and the cooling fins  180  work as first and second heat transfer elements, respectively. 
     The heat exchanger  100  shown in  FIGS. 7 and 8  again works with the loop thermosyphon principle. The heat exchanger is charged with a working fluid. Any refrigerant fluid can be used; some examples are R134a, R245fa, R365mfc, R600a, carbon dioxide, methanol and ammonia. 
     The heating fins  183  conduct the heat from first environment  270  to the evaporator channels  120  of the heat exchanger  100 . Some of the heat may also be conducted through the walls of the flat tubes. Then evaporator channels  120  are fully or partially filled with the working fluid, depending on the amount of initial charge. The fluid in the evaporator channels  120  evaporate due to the heat and the vapour rises up in the channel by buoyancy effect. Some amount of liquid is also entrained in the vapour stream and will be pushed up in the channels. 
     The mixture of vapour and liquid inside the evaporator channels  120  reaches the top side header tube  190  and flows down the condenser channels  130 . While going through the condenser channels  130 , vapour condenses back into liquid since the channels  130  are cooled by the fins  180  situated in second, cooler environment. The liquid condensate flows down to the bottom header tube  190  and flows back into the evaporator channels  120 , closing the loop. 
     It will be appreciated by those skilled in the art that the present invention can be embodied in other specific forms without departing from the spirit or essential characteristics thereof. The presently disclosed embodiments are therefore considered in all respects to be illustrative and not restricted. The scope of the invention is indicated by the appended claims rather than the foregoing description all changes that come within the meaning and range and equivalence thereof are intended to be embraced therein. 
     LIST OF REFERENCE NUMERALS 
     
         
           100  Heat exchanger 
           110  conduit 
           112  Exterior wall of conduit 
           114  Interior wall of conduit 
           120  Evaporation channel 
           130  Condenser channel 
           150  First heat transfer element 
           160  Mounting surface 
           165  Mounting hole 
           170  Contact surface 
           175  Groove 
           180  Second heat transfer element 
           183  Heating fin 
           190  Distribution manifold 
           195  Support bar 
           200  Heat generator 
           250  Separation element 
           260  Second environment 
           270  First environment