A heat exchanger comprising: a shell having a longitudinal axis and configured to receive a first fluid; a plurality of baffles each mounted in the shell at a helix angle H B to guide a first fluid flow into a helical pattern through the shell, wherein each of the plurality of baffles comprises: an outer circumferential edge longitudinally spaced apart from the outer circumferential edge positions of the rest of the plurality of baffles; a proximal radial edge spaced from a distal radial edge; a proximal side opposite from a distal side; and a plurality of spaced apart holes configured to be traversed by a plurality of axially extending tubes configured to carry a second fluid; and characterized by a first plurality of seal strips, each having a first end and a second end, radially disposed between the shell and the plurality of axially extending tubes and each respectively positioned between any two adjacent baffles, wherein each of the first plurality of seal strips is disposed to have the first end of each seal strip proximate to the distal side of a respective baffle and the second end of each seal strip proximate to the proximal side of a respective baffle at a helix angle H s that is greater than 5° and less than the baffle helix angle H B, where the helix angles H B and H s are defined as the angle of the respective baffle or seal strip relative to the longitudinal axis of the shell, and wherein each of the first plurality of seal strips have an angle greater than 0° up to 80° formed between each of the first plurality of seal strips and a line orthogonal to the proximal side of the respective baffle and the distal side of the respective baffle. a shell having a longitudinal axis and configured to receive a first fluid; a plurality of baffles each mounted in the shell at a helix angle H B to guide a first fluid flow into a helical pattern through the shell, wherein each of the plurality of baffles comprises: an outer circumferential edge longitudinally spaced apart from the outer circumferential edge positions of the rest of the plurality of baffles; a proximal radial edge spaced from a distal radial edge; a proximal side opposite from a distal side; and a plurality of spaced apart holes configured to be traversed by a plurality of axially extending tubes configured to carry a second fluid; and an outer circumferential edge longitudinally spaced apart from the outer circumferential edge positions of the rest of the plurality of baffles; a proximal radial edge spaced from a distal radial edge; a proximal side opposite from a distal side; and a plurality of spaced apart holes configured to be traversed by a plurality of axially extending tubes configured to carry a second fluid; and characterized by a first plurality of seal strips, each having a first end and a second end, radially disposed between the shell and the plurality of axially extending tubes and each respectively positioned between any two adjacent baffles, wherein each of the first plurality of seal strips is disposed to have the first end of each seal strip proximate to the distal side of a respective baffle and the second end of each seal strip proximate to the proximal side of a respective baffle at a helix angle H s that is greater than 5° and less than the baffle helix angle H B, where the helix angles H B and H s are defined as the angle of the respective baffle or seal strip relative to the longitudinal axis of the shell, and wherein each of the first plurality of seal strips have an angle greater than 0° up to 80° formed between each of the first plurality of seal strips and a line orthogonal to the proximal side of the respective baffle and the distal side of the respective baffle.