A countercurrent flow single pass heat exchanger comprising: a bundle of channels comprising a first plurality of channels and a second plurality of channels, individual channels of the first plurality being arranged with respect to individual channels of the second plurality to form a checkerboard pattern substantially through a transversal cross-section of the bundle; the individual channels of the first plurality of channels being in fluid communication with first flowstream ports and individual channels of the second plurality of channels being in fluid communication with second flowstream ports, wherein the first flowstream ports are distinct from and not in fluid communication with the second flowstream ports; a first fluid receiving member disposed at each opposing end (10A, 10B) of the bundle of channels; and a second fluid receiving member disposed adjacent to each of the first fluid receiving members, the first fluid receiving members, the second fluid receiving members and the bundle of channels being arranged such that the first fluid receiving members are positioned between the second fluid receiving members and the ends of the bundle; and wherein a portion of the second plurality of channels extends past the end of the bundle and through the first fluid receiving members; the first plurality of channels extends through the bundle and is in fluid communication with a plenum area of the first fluid receiving members, the plenum area being fluidly connected to at least one of the first flowstream ports, and wherein at least a portion of the second plurality of channels has a different cross-sectional size or shape in a segment that runs through the plenum area compared to between the first fluid receiving members. a bundle of channels comprising a first plurality of channels and a second plurality of channels, individual channels of the first plurality being arranged with respect to individual channels of the second plurality to form a checkerboard pattern substantially through a transversal cross-section of the bundle; the individual channels of the first plurality of channels being in fluid communication with first flowstream ports and individual channels of the second plurality of channels being in fluid communication with second flowstream ports, wherein the first flowstream ports are distinct from and not in fluid communication with the second flowstream ports; a first fluid receiving member disposed at each opposing end (10A, 10B) of the bundle of channels; and a second fluid receiving member disposed adjacent to each of the first fluid receiving members, the first fluid receiving members, the second fluid receiving members and the bundle of channels being arranged such that the first fluid receiving members are positioned between the second fluid receiving members and the ends of the bundle; and wherein a portion of the second plurality of channels extends past the end of the bundle and through the first fluid receiving members; the first plurality of channels extends through the bundle and is in fluid communication with a plenum area of the first fluid receiving members, the plenum area being fluidly connected to at least one of the first flowstream ports, and wherein at least a portion of the second plurality of channels has a different cross-sectional size or shape in a segment that runs through the plenum area compared to between the first fluid receiving members.