Abstract:
An expansion radiator for a hermetically closed electrical transformer or a throttle. A heat exchange fluid is delivered to the radiator via an inflow, passed through an expansion shaft cavity formed by an expansion shaft and an associated cover part, and then drained off via an outflow. A flow guiding part which steers a flow direction of the heat exchange fluid is arranged in a mouth region between the inflow and the expansion shaft cavity.

Description:
TECHNICAL FIELD 
     The invention relates to an expansion radiator for a hermetically sealed electrical transformer or a choke, to which expansion radiator a heat exchange fluid is supplied via an inflow means, passed through an expansion corrugation cavity formed by an expansion corrugation and an associated cover part, and then drained via outflow means. 
     PRIOR ART 
     Electrical transformers or chokes of the type used in power distribution systems are usually cooled using an insulating oil. In order to compensate the oil volume fluctuations occurring during operation, an equalizing vessel is provided in the top of the transformer tank. In this equalizing vessel, contact with the ambient air causes an increase in moisture and oxygen in the insulating oil in spite of dehumidifiers, thereby accelerating the aging process and reducing the service life of the insulating paper. In order to counteract this accelerated aging process of the insulating paper it is also known to seal off the transformer hermetically from the external environment. 
     To compensate volume and pressure fluctuations, cooling devices known as expansion radiators are used which are connected to the transformer tank by connecting pipes and can absorb a particular range of variation in the oil volume by bulging of their cooling elements. However, an expansion radiator cannot of course be constructed to compensate an expansion volume of any size, but only a maximum expansion volume associated with its structural shape. 
     A transformer cooling device in which individual radiator elements equalize fluctuating volumes by bulging is known from DE 100 10 737 C2, for example. It is comprised of two sheet metal sections provided with a plurality of grooves, wherein the two sheet metal sections are interconnected/welded at the edges and are additionally interconnected by welds in the grooves. In contrast to conventional radiators, the two metal sections are only interconnected at every second groove in order to obtain a greater expansion volume, and without loss of mechanical stability. In order to achieve the most efficient cooling possible, in DE 10 2005 002 005 B4 the bulges are limited using a spacer strip. In both embodiments the two sheet metal sections of the radiator elements are welded circumferentially to one another, making them costly to manufacture. 
     In comparison thereto, DE 10 2009 015 377 B4 discloses a more easily manufacturable cooling radiator consisting of a supporting sheet and a single corrugated sheet. The individual corrugations are supplied with heat exchange fluid (coolant) by collecting ducts, so-called collectors. However, said coolant does not flow immediately in the direction of the nose of the sheet metal sections, but obliquely downward into the cavity of an expansion corrugation. As a result, however, the upper part of the corrugated sheet only contributes to the cooling effect to a reduced extent. 
     SUMMARY OF THE INVENTION 
     The object of the present invention is to specify an expansion radiator for a hermetically sealed electrical transformer or a choke, said expansion producing a better cooling effect but at the same time being inexpensive to manufacture. 
     This object is achieved by an expansion radiator as claimed. Advantageous further developments of the invention are defined in the dependent claims. 
     According to the invention, the expansion radiator has expansion corrugations, each having a flow guiding part disposed in the orifice region. The flow guiding part directs the inflowing heat exchange fluid toward the outer edge as it enters the cavity of the expansion corrugation, i.e. toward the nose of the expansion corrugation. As a result, the cooling effect of the upper part of the radiator located close to the collector is comparatively greater. A better cooling effect in turn means overall that the operating temperature of the transformer oil is lower. Consequently, the range of variation for the oil volume in the transformer tank is reduced. However, a smaller volume fluctuation means that less bulging of the radiator is required. Material fatigue and stress of the joints of the individual parts of the expansion radiator are therefore lower, and/or a greater dissipation can be handled. This is advantageous in terms of service life. It is particularly advantageous that the flow guiding part can be easily incorporated into the manufacture of the collector, so that the total costs for the expansion radiator according to the invention can be kept comparatively low. 
     A preferred embodiment of the expansion radiator can be designed such that the flow guiding part is implemented such that the cross section of the flow guiding part projecting into the expansion corrugation decreases, viewed toward the side with respect to the out flowing collector. This causes the cooling medium to be efficiently directed in the direction of the nose, but with the further fluid flow being subject to a slight flow resistance. This effect of initial flow guidance in the direction of the nose and subsequent flow in the direction of a side of an expansion corrugation can be implemented by different shaping of the flow guiding part. 
     In a particularly preferred embodiment of the invention, the flow guiding part projecting into the cavity of the expansion corrugation can be a plate-like, e.g. trapezoidally shaped metal part. 
     In another preferred embodiment, the flow guiding part projecting into the cavity of the expansion corrugation can taper toward the nose, e.g. in the shape of a wedge. 
     In a most particularly preferred embodiment, the flow guiding part is planarly trapezoidal and its cross-section is made to taper toward the nose. 
     It is also conceivable for the flow guiding part to be implemented as a cone or similar. It is merely important that the guiding element projects into the expansion corrugation and directs the inflowing transformer oil toward the outer edge side where the heat dissipation is particularly good. 
     In terms of the manufacturing costs, it is particularly advantageous if the expansion radiator consists of two rows of expansion corrugations, wherein each expansion corrugation row is constituted by a single corrugated sheet welded to a cover sheet. 
     The expansion radiator according to the invention can be advantageously used for distribution transformers in power grids. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For further explanation of the invention, reference will be made in the following part of the description to drawings illustrating further advantages, embodiments, details and effects of the invention using non-limiting examples and in which: 
         FIG. 1  shows a perspective view of an expansion radiator according to the invention; 
         FIG. 2  shows the expansion radiator according to detail “M” in  FIG. 1 ; 
         FIG. 3  shows a side view of the expansion radiator according to  FIG. 1 ; 
         FIG. 4  shows a detail view according to section “P-P” in  FIG. 3 ; 
         FIG. 5  shows a cutaway view of an expansion corrugation wherein the flow guiding part is wedge-shaped according to a particular embodiment of the invention. 
     
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
       FIG. 1  shows a perspective view of an expansion radiator  1  according to the invention. The expansion radiator  1  basically consists of two rows of expansion corrugations  9 , each disposed on either side of an upper collector  2  and a lower collector  12 . The backs of the expansion corrugations  9  are welded to a cover sheet  6  by means of a welded joint  11  in the inside area between the two collectors  2 ,  12 . The sides of the expansion corrugation are likewise welded top and bottom in a liquid-tight manner. The cross-section of the collector  2  is rectangular in the example shown and leads into a circular pipe having a flange for connection to a transformer or a choke. During operation, the heat exchange medium (insulating oil, e.g. transformer oil) flows into this flange in the direction of the arrow  10  and leaves the expansion radiator  1  at its lower collector  12 . The structural design is symmetrical with respect to an imaginary central plane running through the collectors  2 ,  12 . Both of the expansion corrugations  9  are made from a single corrugated sheet (folded sheet). In a practical example of a distribution transformer, an expansion corrugation (cooling fin) protrudes some 100 to 300 mm, has a thickness of approximately 10 mm (in the case of a sheet thickness of approx. 1 mm) and has a spacing of approximately 50 mm. Each expansion corrugation  9  encloses an elongated expansion corrugation cavity  7 , viewed in longitudinal direction of the expansion corrugation (see  FIG. 5 ). This cavity  7  is welded in a liquid-tight manner at the end and, as already stated, is connected to the upper and lower collector in a liquid-conveying manner  2 ,  12  (inlet and outlet). 
     In an expansion radiator, each expansion corrugation  9  has a dual function: on the one hand, the heat transported there by the cooling medium during operation of the transformer/choke shall be dissipated to the environment; on the other, any operationally related fluctuation of the pressure in the hermetically sealed transformer tank shall be compensated. This compensation of the oil volume or rather of the oil pressure is accomplished by a corresponding elastic dimensional change in the expansion corrugation  9 . Each expansion corrugation  9  is constructed like a kind of cushion. Overpressure in the transformer tank causes the cushion to bulge. In respect of volume compensation it is advantageous if the expansion corrugation  9  itself is made of an elastic, easily yielding material. In  FIG. 1  the upper collector  2  is cut away in the region of a detail “M”. 
       FIG. 2  shows an enlarged view of the detail “M” of  FIG. 1 . The sectional view reveals orifice regions  4  in the collector cavity  3 . The transformer oil flows into an expansion corrugation  9  through each of these orifice regions  4 , wherein according to the invention this flow is directed by a flow guiding part  5  ( FIG. 4 ) toward the outer edge, i.e. the bend of the corrugated sheet, the nose  8 . The partially cut away representation in  FIG. 2  also shows that the collector  2  is constituted by two U-sections with their legs aligned to one another. 
       FIG. 3  shows a side view of the expansion radiator  1  according to the invention. 
       FIG. 4  shows a sectional view of a detail according to the line “P-P” of  FIG. 3 . The flow guiding part  5  is clearly visible here. According to the embodiment of the invention shown here, it is a plate-shaped part in the form of a trapezium. It is welded to the lower U-section of the collector  2  and projects with progressively reducing height into the expansion corrugation cavity  7 . As a result, the cooling medium flowing into the orifice region  4  (arrow  14 ) is directed to the outer edge of the expansion corrugation  9 , i.e. the nose  8 . The flow direction then follows the arrow  15  toward the outlet. As regards the cooling effect, the upper region of an expansion corrugation  9  shown in cross-section in  FIG. 4  extracts more heat from the insulating coolant than was hitherto the case with cooling radiators. It can also be seen from  FIG. 4  that the upper collector  2  is constituted by two U-sections with their legs aligned to another. The collector cavity  3  is rectangular in cross-section. The flow part  5  is a metal piece welded to the lower of these U-sections. 
       FIG. 5  shows an enlarged cutaway spatial view of an expansion corrugation  9  and the associated cover sheet  6 . The expansion corrugation  9  consists of a metal sheet bent through 180° (corrugated sheet or folded sheet), the two legs running virtually parallel up to the nose  8 . Again visible in the expansion corrugation cavity  7  is the flow guiding part  5  which on the one hand tapers toward the nose  8 , but on the other is trapezoidally shaped. This provides a particularly efficient means of directing the heat exchange medium (insulating coolant, e.g. transformer oil) flowing in in the direction of the arrow  14  to the outer edge of the expansion corrugation  8  acting as cooling fins, thereby improving the cooling effect. 
     Although the invention has been described and illustrated in detail with reference to the preferred exemplary embodiment described above, the invention is not limited by the examples disclosed. In particular, other variations may be inferred therefrom by the average person skilled in the art without departing from the scope of protection sought for the invention. 
     LIST OF REFERENCE CHARACTERS USED 
     
         
           1  expansion radiator 
           2  collector 
           3  collector cavity 
           4  orifice region 
           5  flow guiding part 
           6  cover sheet 
           7  expansion corrugation cavity 
           8  nose 
           9  expansion corrugation 
           10  arrow 
           11  welded seam 
           12 ,  13 ,  14 ,  15  flow direction