A plate fin heat exchanger is disclosed. The heat exchanger includes a plurality of plates defining a set of hot fluid passages between adjacent plates of the plurality of plates and a set of cold fluid passages between adjacent plates of the plurality of plates. A hot fluid inlet and outlet are located at a first face of the heat exchanger. A barrier is located between adjacent plates defining the hot fluid passages. The barrier extends between the adjacent plates and extends from the first face of the heat exchanger at a location between the hot fluid inlet and the hot fluid outlet in a direction perpendicular to the first face, and defines a first pass of hot fluid passages on a first side of the barrier and a second pass of hot fluid passages on a second side of the barrier.