of the stresses set up by the cooling fluid on the internal walls of the u shaped duct and thus makes it possible to improve its solidity and as a result the longevity of the evaporator according to the invention it is clear that the invention is not restricted to the use of identical dimples 9 but extends equally to the use of dimples of varied shapes dimples of varied shapes will be used regularly distributed for example in a similar manner to the distribution of the dimples 9 in fig1 for example dimples with a circular base cylindrical dimples of various diameters can be used as well as dimples with elliptical oval polygonal regular or non regular polygons or other bases the use of dimples 9 of varied shapes also allows the acoustic energy to be distributed over a wide frequency band avoiding high amplitude acoustic peaks in fig3 however the preferred example of plates 13 according to the present invention can be seen similar to those of plate 13 in fig2 but including dimples 9 of varied shapes distributed in random manner in the u shaped concave region in the example shown cylindrical dimples 9 of various diameters are used dimples in the shape of a rectangular parallelepiped of various lengths and orientations and dimples with elliptical bases it is clear that other shapes of dimples for example dimples with a triangular or star shaped base of irregular shape or others do not depart from the.