6.8 Vertical high pressure heaters
A 500 MW unit vertical HP heater, as manufactured by NEIP for Ince В power station, is illustrated in Fig 3.54. Because of the economic factors prevailing when the overall turbine/feed system was evaluated, it was found that the provision of drain cooling sec¬tions was not cost effective.
The heater does, however, have a desuperheating section. The water header is of the all-forged 'bottle' construction, with the inlet and outlet branches and the support feet welded to the main forging. The desuperheating section is at the bottom of the heater surrounding the tubes on the outlet side of the U-tube bundle. As the desuperheat¬ing section wrapper plate and end section will be in contact with the drains water, a double-skin con¬struction is used to prevent direct contact between metal at steam superheat temperature on one side and condensate at saturation temperature on the other (see Fig 3.54 (a)).
A baffle plate is provided at the bled-steam inlet to prevent tube erosion/vibration and, as a further precaution, antivibration strips are placed between the tubesoppositethesteaminlet branch. Thesteaminlet is sealed to the shell by a bellows piece which is welded to the wrapper plate and the inlet branch. The steam flows through the desuperheater (as shown in Fig 3.54 (b)) and into a port ait in the back wall of the wrapper plate at the top. The steam and drains then follow the route indicated in Fig 3.54 (c) and (d). In both sections, the steam is constrained to flow across the tubes in such a manner as to maximise heat transfer. As the steam condenses, the drains flow down into the bottorn of the heater to accumulate in that half of the shell not occupied by the desuperheating section. A water level is maintained in the bottom of the heater, the purpose being to seal the drain outlet to prevent erosion due to two-phase flow adjacent to the drains outlet. The air vents are placed at the top of the shell as all non-condensable gases and air will rise to that point.
These two short descriptions of current HP heaters illustrate how the design principles, explained earlier in this section, are implemented in practice.