13.4.2 Steam-to-steam reheaters - part 2
The heat transfer coefficients are very different on the inside and the outside of the reheater tubes. On the inside, where the heating steam is condensing, the heat transfer coefficient is much higher than on the outside which is in contact with the nominally dry steam at low pressure.
Externally-finned tubing is therefore used to enhance the heat transfer capability on the shell-side and hence reduce the total number of tubes, reheater size and shell-side pressure loss, compared with an unfinned tube arrangement. The length and number of tubes and the fin profile are chosen on the basis of a cost optimisation. The critical variable in this cost optimisation procedure has been found to be the pressure drop across the reheater, rather than the heat transfer coefficient of the extended tube surface.
On recent wet steam machines, economic factors dictate two stages of reheat. The exhaust steam from the external separators is directed across the outside of the tubes in a single pass in a manner which is very close to pure crossflow. The reheating steam is supplied to the upper header of each tube element and condenses on the inside of the tube. The condensate is piped from the bottom header and passed to the feed train. The heating steam supply to the elements through which the separator exhaust steam first passes, is obtained from a tapping on the HP cylinder. The shell-side steam, then passes through further elements, which derive their heating steam from the live steam pipework upstream of the HP stop valves. The heating elements impart a small degree of superheat to the shell-side steam, which then enters the LP cylinders through the LP interceptor valves in a condition almost identical to LP inlet conditions in a fossil-fired plant.
Occasionally a small percentage of inlet steam from the separator exhaust is allowed to pass on either side, and also around the top and bottom, of the tube bank and finally exits with the superheated outlet steam. This by-pass steam cools the reheater outer shell and minimises the vessel deformation which can be caused by the temperature rise in the shell-side steam as it passes from one side of the vessel to the other.
On NEI-Parsons reheaters in Canadian machines, all components, with the exception of the tube element header-gaskets, are made from carbon steel. In order to protect the heat transfer surfaces on the inside and outside of the tubes from rusting during prolonged shutdown periods, heated air is blown through the reheater elements on the shell side to keep the ambient internal temperature above the dew point. This prevents rust forming on the outside of the tubes and the shell internals. The tube bores are protected during shutdown periods by nitrogen blanketing. External erosion-corrosion of the tubes, tubeplates and vessel at full and part-load is minimised by limiting the venting and flow velocities. Wet steam erosion of the tube bores is minimised by using low reheating steam velocities of the order of 1 m/s and a downward flow direction which results in a steady annular flow pattern.