6 High pressure feedwater heaters
6.1 Functional needs
The structural design of high pressure (HP) feedwater heaters is determined by two main needs:
- To contain the steam and HP feedwater at the appropriate cycle conditions.
- To provide the heat transfer surface to raise the feedwater temperature by the specified amount.
The temperature rise and the TTDs are determined by cycle economics, as shown in Chapter 1, and the means by which the consequent heat transfer surface areas are found is shown in Section 6 of this chapter.
Figure 3.43 illustrates the way in which the feed temperature increases as it passes through a typical two-pass horizontal heater of the U-tube type. The heater has both integral drain cooling and desuper-heating sections as shown (Fig 3.44).
The desuperheating section is placed on the outlet end of the U-tubes in order that the incoming superheater steam can raise the feedwater near to or above the saturation temperature of the body pressure before it leaves the heater. The drain cooling section is placed at the inlet end of the tubes to allow the outgoing drains to be cooled to as near to the incoming feedwater temperature as needed. Steam enters the desuperheating section and is reduced in temperature by transferring its heat to the feedwater to within 27°C of the temperature of saturation of the condensing section pressure. The steam then flows to the condensing section, where it leaves as water at saturation temperature to enter the drain cooling section. A water seal is maintained at the inlet to the drain cooling section by a level control system to prevent loss of prime in the section.
In the drain cooling section, the condensate is cooled to the drain outlet temperature and then discharged to the next lowest pressure heater.
Each section within the heater is provided with baffles to ensure flow across the outside of the tubes by the heating medium.
As the heating steam is condensed in the heater, non-condensable gases are released. Unless correctly vented these would rapidly blanket the heat transfer surface and would impair the heater thermal performance. To remove these gases, vents connected to the condenser are provided at strategic points throughout the heater tubenest.
Special care is needed in the venting of horizontal heaters, as air pockets can accumulate under baffles, etc. If lower initial heater cost outweighs the higher running cost due to the loss of cycle efficiency, heaters can be constructed without drain cooling sections or separate desuperheating sections.
Vertical HP heaters employ the same basic layout with regard to disposition of the desuperheating and drain cooling sections. However, there are some points of difference in construction which are highlighted in the following section on construction of specific heater designs.
Figure 3.45 shows a typical arrangement of a vertical HP heater. The desuperheating section is below the working water level in the heater. To prevent water inleakage, seals are provided between the tubes and the end plates of the desuperheating section, trunking is also needed to carry the steam from the desuper-heater to the condensing section if needed. These provisions can be avoided by placing the desuperheating section in the dotted position shown in Fig 3.45 but, as it is not now adjacent to the tubeplate, the feedwater must travel through tubes immersed in water at the saturation temperature equivalent to the body pressure. This limits the steam TTD as, if a negative TTD were employed, the feed would tend to be cooled back down to saturation temperature. The horizontal design of heater does not suffer from this problem as the desuperheating section is above the water level by virtue of the heater attitude.
The reduction in the complication of heater internal construction is another point in the favour of horizontal attitude HP heaters.
As the high pressure feedwater heaters are on the discharge side of the feed pumps, the feedwater within the water headers and the tubes is at boiler pressure plus the pressure rise between the heater and the boiler. To contain this high pressure, various designs of water header have been used in the past, but virtually all current 660 MW units employ hemispherical-headed heaters with a flat tubeplate. The tubes are welded onto the back of the tubeplate by the 'Foster Wheeler fusion welding process' illustrated in Fig 3.46. This method of tube attachment has been used for many years and, once initial difficulties with the quality of welds were overcome, it has proved a cost effective method of tube attachment.
Current HP heaters and associated systems use welded joints wherever practicable, as bolted joints have proved difficult to maintain leak-free when subjected to thermal cycling. It is now accepted that to maintain heater internals, the heater shell has to be cut off and likewise any defective valves, etc., have to be cut from the pipework. The subsequent re-welding of the heater shell has proved to be less difficult than the reassembly of complicated bolted joints, which require special techniques to ensure precise bolt ten-sioning and thereafter periodic checking and possible retensioning to allow for gasket relaxation.
In the following section, examples of current HP heater construction are given for the two main contractors who have supplied heaters for 500 and 660 MW units, which shows how their designs meet the feed system needs.