fig1 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 fig5 ). 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 fig1 the upper collector 2 is cut away in the region of a detail “ m ”. fig2 shows an enlarged view of the detail “ m ” of fig1 . 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 ( fig4 ) toward the outer edge , i . e . the bend of the corrugated sheet , the nose 8 . the partially cut away representation in fig2 also shows that the collector 2 is constituted by two u - sections with their legs aligned to one another . fig3 shows a side view of the expansion radiator 1 according to the invention . fig4 shows a sectional view of a detail according to the line “ p - p ” of fig3 . 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 fig4 extracts more heat from the insulating coolant than was hitherto the case with cooling radiators . it can also be seen from fig4 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 . fig5 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 .