| Evaporators, boilers, and reboilers When the function of the heat exchanger is to cause one of the fluid streams to change from the liquid to the vapor phase, it is common for a modified shell-and-tube construction to be employed. Compared with the no-phase-change equipment discussed in Sec. B, vapor producing heat exchangers are usually supplied with enlarged spaces within which separation of the vapor from the liquid can occur. When the vapor is formed in the tube-side fluid, these phase-separation spaces are either enlarged upper-level headers, or else they are additional vessels connected with the headers. When it is the shell-side fluid that is evaporated, the shell may be provided with a Tube-free region, possibly of enlarged diameter; but there are many variants, with the shell vertical or horizontal, and with or without provision of external down comers, permitting liquid to recirculation to the bottom of the shell. Tube-side evaporators also often allow for such a recirculation, which may be either natural (i.e., driven by gravity) or assisted (e.g., by a pump). |
