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 . <EOS>