5.3.1 Subjective design evaluation - part 4
Air blanketing and stagnation in the tubenest Since air is a poor conductor of heat, it is essential to ventilate the tubenest adequately if a thermal performance, close to theoretical, is going to be achieved. Some air blanketing in the air cooling section is acceptable, but good ventilation will ensure that blanketing is minimised within the main condensing section.
Briefly, the main function of the air cooling section, apart from cooling the vapour/air mixture passing to the air extraction equipment, is to ensure that the condensing section is properly ventilated. Therefore, the total pressure drop between the turbine exhaust and the condenser air extraction point can be envisaged as being divided into two parts; one across the main condensing section, the other across the air cooling section. This concept is complicated by three-dimensional effects which are discussed later, but are the basis of two important general principles (see Fig 4.21):
- The resistance to steam flow along all actual flow paths between the turbine exhaust and the entry to the air cooling section should be equal. High resistance paths tend to cause stagnation, resulting in the poor thermal performance of associated tubes.
- There should be no major routes whereby steam can reach the air cooling section directly, without first passing through the main condensing section. If this occurs, it may become impossible to maintain a satisfactory pressure difference across the condensing section and it will tend to stagnate as a whole. Also the air cooling section will become overloaded with steam, and will itself manifest a high pressure drop.
Inundation of tubes This is a problem which is directly related to the number of tubes in the tube-nest. Reduce the number of tubes in each bank (particularly in the vertical plane) and inundation is reduced.
An alternative is to place collecting trays at key positions within the tubenest which will catch the condensate and divert it directly into the bottom of the condenser (condenser hotwell) through a system of gutters.
Condensate drainage can cause undercooling. Although this is not generally a serious problem, it can be avoided by allowing a small amount of steam to enter below the tubenest to reheat the condensate before it finally falls into the condenser hotwell.