This section deals with miscellaneous problems in the design of evaporators and their ancillary plant.

A. Hot fluid space

When the source of heat is a condensing vapour on the shell side of a shell-and-tube heat exchanger, an impingement plate should be installed opposite the inlet nozzle to protect the tubes in that region from erosion; see Figure 270.8. Following the recommendations of TEMA, an impingement plate is also required for single phase fluids on the shell side if the nozzle momentum flux exceeds 2,250 kg/m²s. See also Section 251, Section 252, Section 253, Section 254, Section 255 and Section 256. Alternatively, the fluid may be introduced through a vapour belt; See Figure 270.5; with this arrangement the tubes completely fill the shell, whereas without a vapour belt, some tubes must be omitted in the region of the inlet nozzle to avoid excessive pressure drop there.

Segmental baffles are fitted on the shell side to maintain an adequate velocity and to support the tubes. In the condensation of pure vapours, a high velocity gives little enhancement to heat transfer, but it helps with the purging of inert gases from the shell during start-up. When significant quantities of non-condensable gas are present in the vapour, the baffles should preferably be progressively closer together in successive passes, as shown in Figure 270.4. This is to counter the fall-off in film coefficient due to mass transfer effects in condensation in the presence of inerts; see Section 141B.

The main function of the baffles is to support the tubes and to prevent damage due to flow-induced vibrations; see #%SECTION_4.2.7_%#. This is especially important in the region of the steam inlet.