A heat exchanger comprising: a plurality of first fluid passages (45a-45n) configured to receive a first stream, and a plurality of second fluid passages each formed between two first fluid passages, wherein each second fluid passage is configured to receive a second stream, and wherein the second fluid passage is configured to allow heat to indirectly exchange between the first stream and the second stream, each first fluid passage comprising: a first plate; a second plate parallel to the first plate, wherein the first plate and second plate are connected by two closure bars; a fin pack having a plurality of fins connected to the first and second plate, and configured for the first stream to flow around the fins, wherein the fins comprise: a plurality of first fin base portions (66, 68) parallel and adjacent to the first plate; a plurality of second fin base portions parallel and adjacent to the second plate; characterised in that a plurality of first and second bowed fin portions (62, 64) connected between the first and second fin base portions; wherein the first and second bowed fin portions are oppositely bowed at non-operational temperatures and are configured to flex, forming a greater bow, under a thermal load without exceeding the tensile or fatigue strength of the fin. a plurality of first fluid passages (45a-45n) configured to receive a first stream, and a plurality of second fluid passages each formed between two first fluid passages, wherein each second fluid passage is configured to receive a second stream, and wherein the second fluid passage is configured to allow heat to indirectly exchange between the first stream and the second stream, each first fluid passage comprising: a first plate; a second plate parallel to the first plate, wherein the first plate and second plate are connected by two closure bars; a fin pack having a plurality of fins connected to the first and second plate, and configured for the first stream to flow around the fins, wherein the fins comprise: a plurality of first fin base portions (66, 68) parallel and adjacent to the first plate; a plurality of second fin base portions parallel and adjacent to the second plate; characterised in that a plurality of first and second bowed fin portions (62, 64) connected between the first and second fin base portions; wherein the first and second bowed fin portions are oppositely bowed at non-operational temperatures and are configured to flex, forming a greater bow, under a thermal load without exceeding the tensile or fatigue strength of the fin. a first plate; a second plate parallel to the first plate, wherein the first plate and second plate are connected by two closure bars; a fin pack having a plurality of fins connected to the first and second plate, and configured for the first stream to flow around the fins, wherein the fins comprise: a plurality of first fin base portions (66, 68) parallel and adjacent to the first plate; a plurality of second fin base portions parallel and adjacent to the second plate; characterised in that a plurality of first and second bowed fin portions (62, 64) connected between the first and second fin base portions; a plurality of first fin base portions (66, 68) parallel and adjacent to the first plate; a plurality of second fin base portions parallel and adjacent to the second plate; characterised in that a plurality of first and second bowed fin portions (62, 64) connected between the first and second fin base portions; wherein the first and second bowed fin portions are oppositely bowed at non-operational temperatures and are configured to flex, forming a greater bow, under a thermal load without exceeding the tensile or fatigue strength of the fin.