A plate fin heat exchanger comprises a circular section tubular shell. The shell comprises a plurality of first shell openings arranged along a length of the shell and a plurality of second shell openings arranged along a length of the shell. A first fluid plenum is provided on the shell in fluid communication with the first shell openings. A second fluid plenum is provided on the shell in fluid communication with the second shell openings. The heat exchanger further comprises a core extending axially within the tubular shell. The core comprises an axially extending first core passage and a second axially extending core passage isolated from the first core passage.