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 (3, 4) 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 (3, 4) 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.