Shell and tube heat exchanger apparatus comprising: an outer shell , a first tube bundle and a second tube bundle coaxial with each other, wherein: the tubes of said first tube bundle are straight; said second tube bundle is arranged externally around the first tube bundle , and the tubes of said second bundle are U tubes comprising straight outward sections and straight return sections which are parallel to the tubes of the first bundle ; the apparatus comprises a first inner shell and a second inner shell , said first inner shell surrounds said first tube bundle and is arranged between said first tube bundle and said second tube bundle ; said second inner shell surrounds said second tube bundle and is arranged between said second tube bundle and said outer shell ; said first and second tube bundles form a single tube side of the apparatus, which can be passed through by a fluid; the apparatus comprises first inlet and outlet interfaces for a first fluid circulating in the shell side on the outside of the tubes of said first and second tube bundles, and second inlet and outlet interfaces for a second fluid circulating in said tube side; the apparatus is configured such that: the first fluid flows along the shell side passing in sequence through a first space enclosed by the first inner shell and a second space defined between the first inner shell and the second inner shell ; the second fluid flows along the tube side passing, in sequence, first through the second tube bundle and then through the first tube bundle . the tubes of said first tube bundle are straight; said second tube bundle is arranged externally around the first tube bundle , and the tubes of said second bundle are U tubes comprising straight outward sections and straight return sections which are parallel to the tubes of the first bundle ; the apparatus comprises a first inner shell and a second inner shell , said first inner shell surrounds said first tube bundle and is arranged between said first tube bundle and said second tube bundle ; said second inner shell surrounds said second tube bundle and is arranged between said second tube bundle and said outer shell ; said first and second tube bundles form a single tube side of the apparatus, which can be passed through by a fluid; the apparatus comprises first inlet and outlet interfaces for a first fluid circulating in the shell side on the outside of the tubes of said first and second tube bundles, and second inlet and outlet interfaces for a second fluid circulating in said tube side; the apparatus is configured such that: the first fluid flows along the shell side passing in sequence through a first space enclosed by the first inner shell and a second space defined between the first inner shell and the second inner shell ; the second fluid flows along the tube side passing, in sequence, first through the second tube bundle and then through the first tube bundle . the first fluid flows along the shell side passing in sequence through a first space enclosed by the first inner shell and a second space defined between the first inner shell and the second inner shell ; the second fluid flows along the tube side passing, in sequence, first through the second tube bundle and then through the first tube bundle . <EOS>