Abstract: A heat exchanger for use in applications with high temperatures and rapidly changing thermal gradients. The heat exchanger has a shell, a head, a tubesheet, a tubesheet skirt, and a tube bundle. The tubesheet has a peripheral diameter smaller than the inner diameter of the interior of the head and is coupled to the head by the tubesheet skirt. The gap between the tubesheet and the head is sufficient to allow the removal of the tubesheet from the head after the tubesheet suffers typical thermal deformation. In a preferred embodiment, the head defines a chamber having a combined spherical-cylindrical geometry.