A heat exchanger, comprising: a casing having a first inlet , a first outlet , a second inlet , and a second outlet ; 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, wherein the plate assembly is configured to transfer heat between a first fluid and a second fluid; a first plenum connecting a first side of the plate assembly and configured to direct the first fluid from the 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, wherein an exterior of the second plenum is in contact with the second fluid, and wherein the second plenum is configured to resiliently deflect in response to thermal expansion of the second plenum, the plate assembly, the casing, or a combination thereof; characterized in that : the plate assembly comprises a plurality of cells , each cell comprising: a first plate ; a second plate ; a header fin positioned between the first and second plates; a first baffle extending in a first direction and positioned between the first plate and the second plate; and a second baffle extending in a second direction, transverse to the first direction, and connected to the first plate, on an opposite face of the first plate from the first baffle. a casing having a first inlet , a first outlet , a second inlet , and a second outlet ; 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, wherein the plate assembly is configured to transfer heat between a first fluid and a second fluid; a first plenum connecting a first side of the plate assembly and configured to direct the first fluid from the 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, wherein an exterior of the second plenum is in contact with the second fluid, and wherein the second plenum is configured to resiliently deflect in response to thermal expansion of the second plenum, the plate assembly, the casing, or a combination thereof; characterized in that : the plate assembly comprises a plurality of cells , each cell comprising: a first plate ; a second plate ; a header fin positioned between the first and second plates; a first baffle extending in a first direction and positioned between the first plate and the second plate; and a second baffle extending in a second direction, transverse to the first direction, and connected to the first plate, on an opposite face of the first plate from the first baffle. a first plate ; a second plate ; a header fin positioned between the first and second plates; a first baffle extending in a first direction and positioned between the first plate and the second plate; and a second baffle extending in a second direction, transverse to the first direction, and connected to the first plate, on an opposite face of the first plate from the first baffle. <EOS>