Abstract: A heat exchanger for transferring thermal energy between a first working fluid and a second working fluid. The heat exchanger has an outer shell that has a first port, a second port, a third port, and a fourth port. A set of tubes each extend within the outer shell and between the first and second ports, such that the first working fluid can flow in parallel through the tubes. A plenum space extends within the outer shell and between the third and fourth ports, and surrounding the tubes. The second working fluid is to flow through the plenum space. The heat exchanger has a central core region, a first transition region that extends between the first port and the central core region, and a second transition region that extends between the second port and the central core region.