Typical designs and liquid flow models in axisymmetrical DHSs of a cylindrical type with central supply of water to the header: a, b. Superconstricted DHSs without the central tube extension with and with no distributor respectively; c. constricted DHSs with the central tube extension and a header with a constricted inlet section; d. Constricted DHS with a distributor, the central tube extension and a header with a free inlet section; e, f. Constricted DHSs with a distributor, the central tube extension, and a header with constricted and free inlet sections; 1 – central tube; 2 – tube sheet; 3 – bundle tube; 4 – housing; 5 – stage; 6 – bottom; 7 – header; 8 – side annulus, 9 – distributor.