Heat exchanger and method of cooling Field of the invention

The invention relates to a heat exchanger for an electrical apparatus, which heat exchanger is arranged to transfer heat from one to the other of a first fluid flow and a second fluid flow. The invention also relates to an electrical transformer being provided with such a heat exchanger and with a method for cooling ah electrical transformer.

Background

Heat exchangers may be used for cooling different types of electrical apparatuses. Normally a cooling fluid such as oil is circulated to pass the heat generating components of the apparatus in order to absorb heat from these components . The so heated cooling fluid is thereafter circulated to pass a first side of heat-transferring elements in the heat exchanger. The heat is conducted through the heat-transferring elements and transmitted into the ambient air, which passes a second side of the heat-transferring element. By this means heat may be transferred from the cooling fluid to the air without any mixing of the two fluids.

Electrical transformers, such as power transformers is one example where such oil to air heat exchangers are used. The core and coil of the transformer are arranged in a tank, which also contains oil. One or several heat exchangers are arranged on the outside of the tank. Each heat exchanger comprises a number of planar heat transferring metal panels arranged side by side and in parallel with each other. The panels are sealingly arranged in pairs such that first inner side surfaces, mutually facing each other, of two adjacent panels define a channel for conducting the oil. Each channel is at its top and bottom connected to an upper and a lower common conduit, which conduits are connected to an upper and a lower portion respectively of the tank. The oil is circulated from

the tank, via the upper conduit down through the channels and back to the tank via the lower conduit. The circulation of oil may be effected by means of natural or free convection caused by density variations of the oil created by the heating and cooling of the oil. The circulation of oil may also be forced e.g. by means of an oil circulation pump. When the oil passes through the channels, heat is transferred from the oil into the panels via the first inner side surfaces. The heat is transferred through the panels and dissipated into the ambient air via second outer side surfaces of each panel. These outer side surfaces are exposed to the ambient air, which passes between adjacent sealingly arranged pairs of panels. The flow of air along the outer panel side surfaces may be effected by free convection or it may be forced, e.g. by means of a ventilator or fan.

It is essential to achieve sufficient cooling of the electrical components of such transformers. Enlarging the total area of the panels being in contact with the oil and air respectively may increase the cooling capacity of the heat exchangers. This may be done either by enlarging the area of the panels or by increasing the total number of panels . However, space limitations for the entire transformer installation may restrict the possibility to utilize heat exchangers with a large enough total area of the panels. The cooling capacity of the heat exchanger may also be increased by increasing the fluid flow of the oil and/or the air. The oil flow at free convection may be increased by arranging the heat exchanger at a level above the tank. Here again the space limitations for the total installation may restrict this possibility. The oil flow may also be increased by arranging forced circulation of the oil, e.g. by means of a circulating pump. Cooling capacity of the heat exchanger may also be

increased by arranging a fan or the like for creating a forced air flow to pass the outer side surfaces of the panels. However, sound requirements for the installation may limit the possibility to apply and use such fans.

EP 0 077 575 discloses an electrical transformer having an heat exchanger of the above-described type. According to this document, heat dissipation from the heat exchanger is said to be increased by the arrangement of circular nubs which are arranged inside the oil channels and extend from one inner side surface to the other inner side surface of the channel, in order to enhance the distribution of oil inside the channels .

US 4,019,572 discloses another transformer of the above described type. According to this document, elongate surface enlarging ridges and furrows are arranged on the panels. The ridges and furrows extend in parallel with the flow direction of the oil and in case the air is driven by natural convection, the general flow direction of the air will be parallel to the longitudinal direction of the ridges and furrows .

Summary of the invention

It is an object of the present invention to provide an improved heat exchanger for an electrical apparatus. It is a specific object to provide such a heat exchanger for transferring heat from one to the other of a first and a second fluid flow, wherein at least one fluid flow is a free convection flow, which heat exchanger provides a comparatively high heat transfer capacity.

These and other objects are achieved by a heat exchanger for an electrical apparatus, which heat exchanger is arranged to transfer heat from one to the other of a first fluid flow and

a second fluid flow, wherein at least the first fluid flow is a free convection fluid flow having a first general flow direction, which heat exchanger comprises a number of heat transferring panels arranged in parallel with each other to form mutually separated passages for the first and second fluid flow on respective sides of each panel, each panel having a first panel side surface exposed to the first fluid flow and a second panel side surface exposed to the second fluid flow. According to the invention at least one of said first panel side surfaces is provided with elongate surface protrusions which extend non-parallel to said first general flow direction.

The invention is based on the realization that heat transfer between a heat exchanger surface and a natural convection fluid flow may be enhanced by providing surface roughness to the heat exchanger surface. The thesis "Thermal and Hydraulic Performance of Enhanced Rectangular Tubes for Compact Heat exchangers", by the present inventor Carl-Olof Olsson and published 14 March 1997 (ISBN 91-7197-457-1) describes how heat transfer between a wall surface and a forced fluid flow may be enhanced by applying surface roughness in the form of elongate ribs to the wall surface. The improvement in heat transfer occurs when the forced fluid flow fulfils certain criteria in regard of flow characteristics, such as flow velocity, wall geometry and the Reynolds number for the forced flow.

US 6,179,276 discloses a heat transfer assembly for a rotary regenerative heat exchanger where such elongate ribs are applied to a heat exchanger surface which is exposed to a forced air flow.

Further, it is generally known that heat transfer between a surface and a fluid flow is increased when the flow rate is increased. At free convection flows, a major concern is and has therefore been to create a large enough flow rate. Any means, such as surface roughness, which may increase the pressure drop along the surface and thereby decrease the flow rate has therefore been avoided. In case ridges, ribs or the like have been applied to the surface, e.g. in order to increase the stability or the exposed area of the surface, such ridges or ribs have thus been arranged in parallel with the general flow direction of the free convection flow.

Contrary to such prior art teachings, the inventor of the present invention has now realized that surface roughness in the form of elongate protrusions that extend at an angle to the general flow direction may be applied also to heat exchanger surfaces being exposed to a natural or free convection fluid flow, to thereby enhance the total heat transfer between the surface and the free convection fluid flow.

The elongate surface protrusions according to the invention, which protrusions extend non-parallel to the first general flow direction, create vortices in the free convection fluid flow, which vortices increase the heat transfer between the first panel side surface and the free convection flow without increasing the pressure drop along the surface to any significant extent. The total heat transfer capacity of the heat exchanger is thereby increased, whereby satisfactory cooling of the electrical apparatus may be achieved with a comparatively small heat exchanger.

The protrusions may be formed as straight rib sections, which rib sections are arranged in pairs to form V-shaped ribs. The

V-shaped ribs may preferably be arranged side by side with respect to the first general flow direction to form rows of V- shaped ribs and a plurality of rows of V-shaped ribs may be arranged one after the other in the first general flow direction. By this means a particularly advantageous flow pattern is created, wherein a series of parallel longitudinal vortices are created. The longitudinal vortices have their axes of rotation aligned with the general flow direction of the free convection fluid flow between the first panel side surface of the first fluid flow passages. The fluid velocity at a point located off the axis of rotation has an angle to the general flow direction. Since the vortices would otherwise act against each other, adjacent vortices are counter rotating. Thereby an increased heat transfer is achieved while keeping the pressure drop low.

Gaps may be provided between adjacent V-shaped ribs in each row. Gaps may also or alternatively be provided between each two straight rib sections forming a V-shaped rib. Such gaps will cause less strain on the panel during manufacturing, especially if the V-shaped ribs are applied by deformation of panels having longitudinal ribs or corrugations for increasing the stability and strength of the panels.

The panels may be arranged such that adjacent first panel side surfaces facing each other are arranged at a mutual distance H, with the elongate surface protrusions protruding a distance h from each respective first panel side surface, and wherein the range of the ration h/H is 0,05 to 0,2. Herby a particularly advantageous relationship between the height of the protrusions and the distances between the side surfaces is achieved, which relationship enhances the formation of longitudinal vortices at the low flow velocities and flow rates that occur at free convection flows.

Pairs of the panels having adjacent second panel side surfaces facing each other may be sealed together to form sealed channels for said second fluid flow. The second fluid flow may then be formed of a liquid such as oil for efficient cooling of the electrical components of the electrical apparatus. The first fluid flow may advantageously be constituted by ambient air.

The heat exchanger may advantageously be arranged at an electrical transformer which transformer further comprises a tank enclosing a core and coil being cooled by a liquid such as oil, wherein the heat exchanger is arranged and connected to the tank such that the liquid forms the second fluid passing by the second panel side surfaces and ambient air forms the first fluid flow passing by the first panel side sections by free convection.

The invention also concerns a method of cooling an electrical transformer comprising a tank enclosing a core and coil and a heat exchanger. The method according to the invention is defined by the features specified in the appending independent claim 9.

Further objects and advantages of the invention appear from the following detailed description of embodiments and from the appended claims.

Brief description of the drawings

In the following a detailed description of exemplifying embodiments of the invention will be given with reference to the accompanying drawings, in which;

Fig. 1 is a schematic cross section of an electrical transformer provided with a heat exchanger according to one embodiment of the invention.

Fig. 2 is a schematic isometric view illustrating some details of the heat exchanger shown in fig. 1.

Fig. 3 is a schematic side view in enlarged scale of the details shown in fig. 2.

Figs. 4a is a plan view illustrating the embodiment shown in figs 1-3.

Figs 4b-4g are plan views corresponding to fig.. 4a but showing different embodiments of the heat exchanger according to the invention.

Fig. 5 is a cross section along line 5-5 in fig 4d.

Fig. 6 is a cross section along line 6-6 in fig 4f.

Detailed description of embodiments

Fig. 1 illustrates schematically an electrical transformer comprising a tank 1. A transformer core 2 and a coil 3 are arranged in the tank and submerged in a liquid cooling media such as oil.

A heat exchanger 4 is arranged at an outside wall of the tank. The heat exchanger 4 comprises an upper header 5 connected to an upper portion of the tank and a lower header 6 connected to a lower portion of the tank 4. The heat exchanger further comprises a number of vertically arranged panels 7a, 7b, 8a, 8b, 9a, 9b, 10a, 1Ob ₇ 11a, lib. Each panel is formed of a sheet like material having high heat conductivity, such as steel or aluminium. The panels are arranged in pairs such that two adjacent panels are sealingly joined together to form vertical channels, which channels extend between and are connected to the upper 5 and lower 6 header. As seen in fig. 1, panels 7a and 7b form channel 12, panels 8a and 8b form

channel 13 and so on till channels 11a and lib which form channel 16.

By this means the cooling media is allowed to circulate from the tank 1, via the upper header 5, where it is distributed into the different channels 12, 13, 14, 15, 16. The cooling media flows downwards in the channels and is collected in the lower header 6, from which it is returned to the tank 1, at a lower portion thereof. This circulation of cooling media is indicated by dash-lined arrows A and a in fig. 1. In the shown example, the circulation of cooling media is driven by natural or free convection caused by the difference in temperatures between the comparatively hotter media in the tank and the comparatively cooler media in the channels. It is understood however that the circulation of cooling media may also be forced, e.g. by means of a cooling media circulation pump, if desired.

As also indicated in fig. 1, adjacent panels of neighbouring panel pairs together form vertical air flow passages 17, 18,

19, 20 them between. As seen in fig. 1 panels 7b and 8a form passage 17, panels 8b and 9a form passage 18, panels 9b and 10a form passage 19 and panels 10b and 11a form passage 20. The widths of the panels, in the direction perpendicular to the plane of the paper in fig. 1, is much greater than the corresponding width or diameter of the headers 5, 6. Thereby, ambient air may enter into the passages 17-20 from below and exit at the top of the heat exchanger as indicated by arrows B. Ambient air thus enters into passages 17-20, where it is heated by absorbing heat from the panels 7b-lla to thereby create a free convection air flow upwards in the passages 17-

20, as indicated by arrows b.

Figs. 2 and 3 illustrate a first example of how elongated surface protrusions 31 according to the invention are arranged on the heat exchanger panels 8a, 8b, 9a, 9b. In figs. 2 and 3 panels 8a, 8b, 9a, 9b of the heat exchanger shown in fig. 1 are illustrated. For reasons of clarity the means for forming sealed channels 13, 14 between panels 8a, 8b and 9a, 9b respectively are not shown in the figures. As illustrated in the figures 2 and 3, panel 8b has a first side panel surface 8b' which is exposed to the free convection air flow b in air passage 18 and a second panel side surface 8b' ' which is exposed to the oil flow in channel 13. Correspondingly, panel 9a has a first panel side surface 9a' exposed to the air flow b in passage 18 and a second panel side surface 9a' ' exposed to the oil flow a in channel 14. Also panels 7b, 8a, 9b, 10a, 10b and 11a have corresponding first and second panel side surfaces exposed to the air flow and oil flow respectively.

The elongated surface protrusions shown in figs . 2 and 3 are formed as transversal ribs 31 that extend essentially perpendicular to the general air flow direction b. The ribs 31 extend over essentially the entire width of the first panel side surfaces 8b', 9a'. The ribs 31 have a generally semi circular cross section and are formed of semi-circular bars joined to the first panel side surfaces 8b', 9a'. The ribs are arranged at a distance D from each other over essentially the entire height of the panels. The ribs 31 protrude a distance h from the first panel side surfaces 8b', 9a' into the passage 18. The passage 18 has a height H defined by the distance between the first panel side surfaces 8b' and 9a'. In the example shown in fig. 3 the ratio h/H is approximately 0,1.

In figs. 2 and 3 only panels 8a, 8b, 9a and 9b are shown. It is however understood that similar ribs are also formed on corresponding first panel side surfaces of panels 7b, 10a, 10b

and 11a which first panel side surfaces each form one side- wall of a passage for the free convection air flow. According to the invention it is sufficient that only one of the first panel side surfaces, which together with another first panel side surface form a passage for the free convection fluid, is provided with elongated surface protrusions. For optimal heat transfer efficiency it is however preferred that all first panel side surfaces delimiting an air passage for the free convection fluid flow is provided with elongated surface protrusions. Further more, "at the example shown in figs. 2 and 3 the ribs on opposing first panel side surfaces forming a passage are arranged in line such that opposing ribs are arranged at the same level. It is however also possible to arrange the ribs on opposing first panel side surfaces vertically staggered in relation to each other. Preferably, the ribs on one first panel side surface are then arranged at a level in-between the ribs on the opposing first panel side surface. Such a staggered arrangement of the protrusions on opposing first panel side surfaces may preferably be used at all the different protrusion configurations illustrated in figs. 4a-4g.

Figs 4a to 4g illustrates schematically different ways of arranging the elongated surface protrusions according to the invention. These figures are plan views of first panel side surfaces provided with surface protrusions and the arrows b indicate the general free convection flow direction in a corresponding passage for the free convection flow. Fig 4a illustrates transversal ribs 31 as shown in figs. 2 and 3. Fig. 4b illustrates elongated ribs 32 that extend diagonally over the first panel side surface. Figs 4c and 4d illustrate V-shaped ribs 33, 34, where each V-shaped rib extends over essentially the entire width of the first panel side surface.

The V-shaped ribs 33 in fig. 4c point in the flow direction, whereas the V-shaped ribs 34 in fig 4d point against the flow direction. In fig. 4e a plurality of V-shaped ribs 35 is arranged continuously in rows, which rows extend over essentially the entire width of the first panel side surface. Fig. 4f shows an embodiment which is similar to the one shown in fig. 4e, but where gaps 36a are arranged between each consecutive V-shaped rib 36 in the rows of V-shaped ribs. As indicated in fig. 4f the width of the gap 36a is G ₁ . In the embodiment shown in fig. 4g first gaps 37a are arranged between each consecutive rib 37, just as in fig. 4f and second gaps 37b are arranged at the point of each rib, between its two legs. The first 37a and second 37b gaps have the widths G ₁ and G ₂ respectively.

At the embodiments illustrated in figs 4a-4g the ribs 31-37 are arranged at a distance D, in the general flow direction b, from each other. The range of ratio h/H between the height h of the uribs and the distance H is preferably 0,05-0,2. The range of the ratio D/H between the distance D and the distance H (se fig. 3) is preferably 5-20. The ribs are arranged at an angle θ to the general flow direction b, which angle θ preferably is between 30-90 ° and more preferably between 30- 45 °. The pitch P between consecutive ribs in a row of ribs is preferably chosen such that the range of the ratio P/H is 0,75—3,0. The width G ₁ of the first gaps and the width G ₂ of the second gaps are preferably equal to or greater than h.

As described above the elongated surface protrusions may be formed by joining bars or the like to the first panel side surfaces. For ease of manufacturing the surface protrusions according to the invention may however also be formed by pressing or rolling of the panels comprising the first panel

side surfaces. Fig. 5 illustrates how the V-shaped ribs 34 shown in Fig. 4d are formed by pressing a panel 8a such that a rib 34 is formed by deformation of the panel 8a. The rib 34 has a V-shaped cross section and protrudes a distance h from the first panel side surface 8a' of the panel. In fig. 6 it is illustrated how each V-shaped rib 36 shown in fig. 4f are formed by rolling a panel 8a such that the rib 36 is formed by deformation of the panel 8a. The rib 36 has a rounded cross- section and protrudes a distance h from the first panel side surface 8a' . By forming the cross-section of the ribs V-shaped or rounded the risk of dust or other contaminations in the first fluid flow to collect on the ribs is reduced.

It is understood that all of the different elongated surface protrusions illustrated in figs. 4a-4g may be formed either by joining bars, threads, wires or the like to the respective first panel side surfaces or by deformation of the panels comprising the first panel side surfaces. Further, the cross- section of the elongated surface protrusions may be semicircular, V-shaped or rounded as described above. The cross- section of the protrusions may also exhibit other geometrical forms .

Above exemplifying embodiments of the invention have been described. It is however understood that the invention is not limited to these embodiments. Instead the invention may be varied in numerous ways within the scope of the appending claims .