Abstract: A heat exchanger with a housing defining a main fluid flow passage therethrough and a tube bundle access opening to the fluid flow passage, a tube bundle unit removably positioned in the housing in the fluid flow passage through the access opening, and a cover for removably closing the access opening so that heat exchange fluid flowing through the tube bundle unit is in a heat exchange relation with the fluid flowing through the housing main fluid flow passage around the tube bundle unit. Also shown is an economizer for a stream boiler incorporating a heat exchanger.