5.3.1 Subjective design evaluation - part 5
'Three-dimensional' effects These effects have a significant detrimental effect on the performance of large condensers. The problem arises from the fact that, theoretically, the amount of steam which condenses at the cold end of the condenser is greater than the amount which condenses at the hot end. However, in practice this frequently cannot exist, because it requires that the pressure drop through the tubenest should be greater at the cold end than at the hot end, and this is not possible if the air extraction point pressures are the same at both the cold end and the hot end.
Although this phenomenon is sometimes accepted as a fundamental factor in condenser performance, various measures can be taken to minimise its effect:
- Use of a low CW temperature rise, and avoidanc of close approach between CW outlet temperature and vacuum temperature. This means the difference between the theoretical cold and hot end performances is not large, and the three-dimensional effect is therefore inherently small.
- Use of compartmented condensers As already briefly mentioned, this is a variation where the condenser is divided up into two or three compartments, one corresponding to each double-flow LP turbine, the total CW temperature rise is, therefore, divided into a number of separate steps. The various compartments operate at different vacua, and to achieve balance in the air extraction rates, it is usual to connect the air pumps directly to the cold end compartment, and then allow the other compartments to vent into the air suction line via restriction orifices.
- Use of three-dimensional design Figure 4.22 shows an example of the use of three-dimensional air cooling sections.
In addition, 'disc' baffles can be installed in the central axial passage. Steam which is flowing axial-ly from the hot end of the condenser is forced back into the tubebank, thereby increasing the resistance of the steam flow path from the hot end, and helping to achieve correct balance of pressure drops.
Section 10 of this chapter, looks at the position and functional requirements of the air cooling section in more detail, and describes its influence on the design of the air extraction equipment.