An assembly for mounting a plurality of modular heat exchangers between respective pairs of fluid openings in parallel opposed faces of an inlet header and an outlet header, including modules providing generally axial flow between the headers, each of which modules has an axially extending sleeve defining a fluid opening on one end and an end flange defining a fluid opening on the opposite end, and an end chamber having an axially flexible end wall and forming a connection between either or both of the end sleeve and the end flange and the heat exchanging portion of the module, the assembly including compressible sealing members attached to the end sleeve and the end flange and compression type mounting brackets by which the sealing members are compressed to seal the interface and provide a demountable connection between the module and the headers.