A heat exchanger in one aspect includes a longitudinal shell and a transverse shell oriented transversely thereto. A J-shaped tube bundle carrying a tube-side fluid extends through the longitudinal and transverse shells from a first tubesheet in the longitudinal shell to a second tubesheet in the transverse shell. The first and second tubesheets are oriented perpendicular to each other. In a related aspect a dual heat exchanger unit includes a first longitudinal shell, a second longitudinal shell, and a common transverse shell extending transversely between and fluidly coupled to the longitudinal shells. The longitudinal shells may be parallel to each other. The shells are fluidly coupled directly together to form a common shell-side space between pairs of inlet and outlet tubesheets. A pair of J-shaped tube bundles is disposed in the dual heat exchanger unit for heating two tube-side fluids.