3.1 Phase 1
In this phase, which covers about half the total number of 500 MW turbines, the underslung condenser arrangement used on previous 120 MW and 200 MW units was retained. The reinforced concrete turbine generator foundation block was kept, but developments were made in several other aspects.
A general feature of these condensers was that they were all of single-shell design; otherwise they differed considerably. The most obvious differences relate to the basic tube arrangement. Figure 4.5 (a) illustrates a transverse arrangement, and Fig 4.5 (b) an axial arrangement. Each of these has advantages, as will be seen from later sections. When considering thermal performance the transverse design is simpler and easier to predict. A difficulty with the axial condenser is that the long tubes naturally result in a rather large difference in thermal performance between inlet and outlet ends, so making the effective extraction of non-condensables more difficult.
The axial arrangement, however, has a capital cоst advantage because of the smaller number of (longer) tubes involved. This results in a saving in the cost of manufacture of the expensive tube plates. It also allows a simple turbine-generator foundation arrangement in which the LP turbines are supported parallel foundation walls, one along each side. The process of tubing the condenser can use the space under the electrical generator, which is kept free of major plant items for this purpose.
Again, due to the length of tubing involved with an axial design, it becomes necessary to take measures to avoid large scale axial migration of steam towards the cold end, which may involve adopt opposed cooling water flow. This can be done having two separate water passes with the flows opposite direction, as illustrated in Fig 4.6. Such arrangement can complicate the culvert layout, cool water pipework and valve layout, and result in additional costs. Designing the condenser as a series of compartments to give a multi-vacuum design has generally been found to provide a more satisfactory solution and is now standard British practice for axial condensers.