5.3.1  Subjective design evaluation - part 3

 

The disposition of the air cooling section In most condensers ¬ęthe air cooling section is a block of tubes located between the main condensing section and the air extraction point (or points), representing some 10% of the total number of tubes (Fig 4.19 (a)). In other condensers, however, the air cooling section is arranged adjacent to the CW inlet tubeplate, and consists not of separate tubes, but of the cold ends of all the tubes. The steam must travel axially along the condenser to reach the air cooling section, and this type of condenser requires design in three dimensions.

 

Causes of poor condenser performance

There are a number of possible reasons for poor condenser performance, the main parameters being:

  • High pressure drop in the tubenest.
  • High pressure drop before tubenest, and ineffective use of the incident velocity of the approaching steam.
  • Air blanketing and stagnation in the tubenest.
  • Inundation of tubes by condensate, and undercooling of condensate.
  • Three-dimensional effects.

The main options open to the designer will now be discussed briefly in relation to the various causes of poor performance listed above:

High pressure drop in the tubenest This is mainly concerned with the pressure drop in the main condensing section of the tubenest. Once the incondensable gases and residual steam have reached the air cooling section, it might be assumed that the volumetric flow is small, and that the pressure drop is therefore no longer a significant problem. This however can be a dangerous assumption (Fig 4.19 (b)).

High pressure drop before the tubenest This aspect of large condenser design originally tended to receive little attention, though its significance is now well recognised. If high velocity steam entering the condenser is not travelling naturally in the direction in which the designer would like it to go, the effective pressure drop caused by forcing it to turn in a different direction may be substantial. Figure 4.20 shows how one condenser designer has developed his tubenest layout to improve the use of the incident velocity in the steam. This problem is more apparent in underslung condensers than side-mounted or integral condensers. Conversely, due to irregularities in the velocity profile of the approaching steam, the pressure drop through the condensing section of the tubenest should not be too low. A moderate pressure drop helps to redistribute the steam flow beneficially and prevent high velocity jets of steam penetrating straight through into the air cooling section.

Comparison of tubenest designs to improve use of the incident velocity of the steam

 

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