their respective sides throughout the length of the exchanger which reduces the capital cost and the initial overall pressure drop of the exchanger it is most preferred to have an identical number and diameter of tubes in parallel flow section a and counter flow section b as the tubes run the entire length of the exchanger this limits the overall flexibility of the initial design of the exchanger in comparison to an arrangement as shown in fig1 the differential thermal growth between the shell and tubes of the exchanger in fig2 is on a similar scale to that of standard counter flow exchanger it is also not possible to replace only the parallel flow section of the exchanger arrangement shown in fig2 in contrast the arrangement shown in fig1 therefore an exchanger arrangement as shown in fig2 is better suited for steady operating conditions while an exchanger arrangement as shown in fig1 is better suited for use in unsteady operating conditions fig3 shows an alternate arrangement of a heat exchanger according to the invention wherein two hot gases are used in series to warm a single cold gas parallel flow section a comprises of parallel flow shell 12 contained within which is parallel flow shell side inlet vestibule 14 parallel flow shell side 16 and parallel flow shell side outlet vestibule 18 where through said parallel flow shell side 16 there passes parallel flow section tubes 20 connecting parallel flow tube side inlet vestibule 22 and parallel flow tube side outlet vestibule 24 counter flow section b is comprised of counter flow shell 26 contained within which is counter flow shell side inlet vestibule 28 counter flow shell side 30 and counter flow shell side outlet vestibule 32 where through said counter flow shell side.