5.3.1 Subjective design evaluation - part 2
Preliminary considerations of tubenest arrangement
A number of decisions relevant to tubenest arrangement can be taken at this stage:
Subdivision of the total number of tubes This is the extent to which the total number of tubes may be subdivided into separate banks, each associated with its own tubeplates and waterboxes, air extraction points and cooling waterside isolation. It is common nowadays to restrict the number of tubes supplied from any waterbox to around 5000, which has been found through experience to be a number which does not inconvenience on-load leak searching or tube cleaning.
Subdivision of banks Consideration may also be given to subdividing the individual banks into two or more sub-banks, each with its own air extraction point (Fig 4.19 (b)).
Compartmentalisation of the condenser Steam from each LP turbine can be condensed in two or three separate compartments, which are baffled from each other on the steamside and operate at different back pressures, the lowest being at the end corresponding to the CW inlet. An alternative to this arrangement is the 'opposed flow' concept whereby the CW flow is in one direction in half the banks, and in the opposite direction in the remainder. This avoids the need for large-scale axial redistribution of steam within the condenser, towards the CW inlet end.
Detailed considerations of tubenest arrangement
As a first stage in tubenest design, it is necessary to choose a basic concept. The two main features by which the various possible concepts can be most easily characterised are the steam flow approach pattern to the tubenest and the disposition of the air cooling section:
The steam flow approach pattern to the tubenest The steam may enter the tubenest from just one direction, i.e., from the turbine exhaust, or it may be allowed to pass around the nest and enter it from all sides.
Figure 4.19 (c) clearly indicates that, for side-mounted condensers, a unidirectional steam approach has always been the natural choice, since the face area of the tubenest is fairly large. However, Fig 4.19 (b) indicates how underslung condensers have a small face area to a unidirectional steam approach, and a more common arrangement is for a substantial percentage of the steam to be allowed to pass round one or both sides of the nest, thereby increasing the face area over which the steam finally enters.