A heat exchanger includes a casing having a first inlet, a first outlet, a second inlet, and a second outlet, and a plate assembly positioned between the first inlet and the first outlet and between the second inlet and the second outlet and at least partially in the casing, the plate assembly is being configured to transfer heat between a first fluid and a second fluid. The heat exchanger also includes a first plenum connecting a first side of the plate assembly and configured to direct the first fluid from first inlet to the plate assembly, and a second plenum connecting a second side of the plate assembly and configured to direct the first fluid from the plate assembly to the first outlet. An exterior of the second plenum is in contact with the second fluid, and the second plenum is configured to resiliently deflect in response to thermal expansion.