10.2 Determination of air extraction quantity
To ensure adequate removal capacity under operating conditions, the capacity of the air extraction plant must be greater than the maximum amount of air and non-condensable gases expected to be removed from the condensate.
The main parameters which determine the quantity of air to be extracted from the condenser and, hence, the design of the condenser air cooling system and air removal equipment are:
- The design air leakage into the condenser.
- The degree of sub-cooling of the air/steam mixture leaving the air cooling section.
- The limiting approach temperature difference between the saturation temperature θpc°C corresponding to the absolute pressure pc in the condenser, and the CW outlet temperature.
Table 4.6 indicates recommended values for typical fossil-fired power stations of varying size.
The design air leakage rates presented in Table 4.4 are known to be higher than those adopted in some other countries and, in many instances, are considerably in excess of air leakage rates realised on British plant in practice.
However, experience has shown that, on balance, the adoption of these ratings has been economically justified as a means of counteracting the following:
- High air leakage, due to valve packing failures, joint leaks, etc.
- Higher than design pressure loss through the condenser and air extraction pipework.
- Excessive steam penetration through the air cooling system.
- Adverse effects of part-load operation and low cooling water temperature conditions.
The considerable experience gained, particularly from 500 MW turbine condensers, has led to a much greater understanding of the factors influencing air inleakage and its effective removal from large condensers.
The limiting difference between the condensing temperature and the cooling water deserves brief mention. This limit is aimed at ensuring reasonable CW outlet-end condensing zone and air cooling section performance. Most large condensers in operation have temperature differences in excess of this figure; latest thinking suggests that a temperature difference closer to 5°C is desirable if pressure loss on the steamside and/or excessive steam carryover to the air extraction equipment is not to become a critical factor in determining condenser performance.