13.4.2 Steam-to-steam reheaters - part 5
Current designs of MSR have an outer shell welded together from cylindrical plate sections and dished ends, designed and constructed in accordance with pressure vessel codes. The vessels are generally mounted on two supports, one of which is anchored to the foundations. The other support is free to expand longitudinally but is restrained from moving in a transverse direction by means of a longitudinal key.
MSR vessels are always provided with manholes for access and inspection, and penetrations through the shell for connections to the tubenest headers are normally provided with devices to allow for relative thermal expansion. The number of reheated steam outlet connections from the vessel is governed by external layout considerations and the number of LP turbines.
With precautions taken to avoid high velocity wet steam, especially at the steam inlet, mild steel is a satisfactory material for the containing vessel and the internal support frames. It is obviously desirable to limit the need for erosion protection and therefore it is necessary to avoid high velocities within the shell. Any areas of the shell and main support frame which are exposed to wet steam erosion are always protected by stainless steel.
An internal support provides a framework for welding the separator sub-assemblies and a support for the tubenests and steam shroud. Longitudinal members which add stiffness to the frame often serve as runners for rollers which assist in the insertion of tubenests into the frame. These rollers also allow the tubenests to expand longitudinally during operation.
The costs of the separator and reheater are governed mainly by the reheater bundle, in other words by the size of the heating surface serving the heat exchange. The residual water after the separator must be evaporated before reheating. This requires additional heating surfaces and both live and bled-steam reheating.
To determine the type and the size of the reheater heat-transfer surface, it is necessary to establish the heat transfer and pressure drop correlations for both sides of the tubes. The relatively low rate of heat transfer between superheated steam and the tube generally dictates the use of externally-finned tubes.
Early designs of MSR were based on published predictions of the heat transfer and pressure drop for flow across tubes. However, the condensation process inside the tube produces a two-phase flow which at the time was not fully understood. The conditions were therefore simulated in tests which showed that the flow was initially stratified and progressed through wave-flow and slug-flow to plug-flow at the tube outlet.