A heat exchanger comprising: a longitudinally-extending first shell defining a first shell-side space and a first longitudinal axis; a longitudinally-extending second shell defining a second shell-side space and a second longitudinal axis, the second shell arranged parallel to the first shell; a transverse third shell fluidly coupling the first and second shells (101, 102) together, the third shell extending laterally between the first and second shells and defining a third shell-side space in fluid communication with the first and second shell-side spaces (108a, 108c); a tube bundle comprising a plurality of tubes each defining a tube-side space, the tube bundle extending through the first, second, and third shells (101, 102, 103); a shell-side inlet nozzle fluidly coupled to the first shell; a shell-side outlet nozzle fluidly coupled to the second shell; the third shell is orientated perpendicularly to the first and second shells (101, 102), and the third shell is fluidly coupled to a first terminal end of each of the first and second shells (101, 102); a first tubesheet coupled to a second terminal end of the first shell and a second tubesheet coupled to a second terminal end of the second shell; wherein a shell-side fluid flows in a path from the first shell-side space through the third shell-side space to the second shell-side space.; the heat exchanger being characterised in that it further comprises a first expansion joint coupled between the first tubesheet and the second terminal end of first shell; wherein the shell-side inlet nozzle is fluidly coupled to the first expansion joint, and wherein the shell-side fluid is introduced into the first shell through the first expansion joint in a radial direction. a longitudinally-extending first shell defining a first shell-side space and a first longitudinal axis; a longitudinally-extending second shell defining a second shell-side space and a second longitudinal axis, the second shell arranged parallel to the first shell; a transverse third shell fluidly coupling the first and second shells (101, 102) together, the third shell extending laterally between the first and second shells and defining a third shell-side space in fluid communication with the first and second shell-side spaces (108a, 108c); a tube bundle comprising a plurality of tubes each defining a tube-side space, the tube bundle extending through the first, second, and third shells (101, 102, 103); a shell-side inlet nozzle fluidly coupled to the first shell; a shell-side outlet nozzle fluidly coupled to the second shell; the third shell is orientated perpendicularly to the first and second shells (101, 102), and the third shell is fluidly coupled to a first terminal end of each of the first and second shells (101, 102); a first tubesheet coupled to a second terminal end of the first shell and a second tubesheet coupled to a second terminal end of the second shell; wherein a shell-side fluid flows in a path from the first shell-side space through the third shell-side space to the second shell-side space.; the heat exchanger being characterised in that it further comprises a first expansion joint coupled between the first tubesheet and the second terminal end of first shell; wherein the shell-side inlet nozzle is fluidly coupled to the first expansion joint, and wherein the shell-side fluid is introduced into the first shell through the first expansion joint in a radial direction.