Mass and/or heat exchanger with thermal expansion relief

A mass and/or heat exchanger, comprising at least one bundle of capillaries having lumens, ends of said capillaries being embedded in a casting composition, and a housing in which the capillaries with the casting composition are arranged such that the casting composition forms a liquid-tight isolation between the lumens of the capillaries and space outside the capillaries within the housing, wherein an elastic ring flange which compensates for differences in changes of length of the bundle of capillaries and the housing, is readily removably provided between the casting composition and the housing, said elastic ring flange including an inner non-elastic ring, an outer non-elastic ring and an elastic ring located between and attached to said inner and outer non-elastic rings, said inner non-elastic ring, outer non-elastic ring and said elastic ring being firmly connected together to form one part, said elastic ring having no bores therethrough and said inner non-elastic ring being readily removably attached to said casting composition.