5.3.1 Subjective design evaluation - part 1
The tubenest geometry and its disposition relative to the turbine exhaust have a considerable effect on the distribution of steam over individual groups of tubes which, in turn, influences local heat transfer rates, and tubenest ventilation. More perhaps than any other feature of large condensers, tubeplate and tubenest arrangements may differ considerably between one design and another.
This can be seen from the many arrangements employed on the 500 MW series, Fig 4.19. Each manufacturer of condensing plant arrives at his own solution independently. This highlights a point which emerges from experience, that tubenest arrangements which are apparently quite different can be of equally good performance, providing that proper attention is paid to a number of general principles.
In designing the tubenest arrangement, a number of external influences and constraints have to be taken into account. The main external constraint has already been illustrated in Fig 4.19, i.e., the choice between side-mounted and underslung condenser dispositions, which depends mainly on turbine design and turbine layout factors. Other factors which are also generally fixed at an early stage, without special reference to tubenest arrangement, include the condenser envelope interaction with the turbine support structure, operating floor height above basement, overall turbine hall dimensions, CW flow direction, number of passes, and the condenser alignment (i.e., axial or transverse). In addition, before the tubenest arrangement is developed in detail, the main performance characteristics of the condenser must be agreed, following optimisation studies as mentioned in Section 2 of this chapter. The main parameters settled at this stage are:
- Design heat load.
- Design CW inlet temperature.
- Design CW flow.
- Design condenser pressure.
- Total surface area.
- Tube length.
- Tube outside diameters, wall thickness, and material.
- Number of tubes.
- CW velocity in tubes.
Apart from the performance parameters, the type of tube/tubeplate fixing will have been decided, which in turn determines the minimum pitch of the tubes (Section 6 of this chapter), and also design values for the amount of air entering the condenser in the steam will have been established.