Abstract: A tubular heat exchanger preferably of a multi-pass design, has a cylindrical shell with open ends that are releasable covered and sealed by preferably two end caps. Located removably in the shell and axially between the end caps is a core having a plurality of outer tubes and preferably a plurality of inner tubes with each one of the inner tubes extending through a respective one of the outer tubes. A plurality of perforated plates disposed in the shell are sealed releasably to respective ends of the inner and outer tubes thereby forming a plurality of liquid tight chambers for the flow of a plurality of segregated mediums and the transfer of heat therebetween.