In power plant locations with adequate supply of cooling water the steam from the steam turbine is condensed in a water cooled condenser. In most instances circulating water from the cooling tower is used to condense the turbine exhaust steam. In other instances once through cooling is deployed wherein water from a lake, river or sea is used to condense the turbine exhaust steam. In water challenged locations or locations where wet cooling cannot be deployed due to permitting or regulatory issues, the steam from the steam turbine is condensed in an air cooled condenser (ACC) wherein ambient air is used to cool and condense the turbine exhaust steam. In a combined cycle plant, during normal operation, the water or air cooled condenser condenses the turbine exhaust steam. During bypass operation, when the steam turbine is out of service, the high-pressure steam from the HRSG is attemperated in a pressure reducing/desuperheating (PRD) valve and then admitted into the water cooled or air cooled condenser. The bypass steam flow is substantially higher than the design turbine exhaust steam flow and the duration of bypass operation can vary from a few hours to several weeks.
The requirements for admission of bypass steam into a water cooled condenser are substantially different from that for an air cooled condenser. In a water cooled condenser the bypass steam is admitted in the steam dome. The bypass steam as well as the turbine exhaust steam is condensed outside the tubes. In an air cooled condenser the bypass steam is admitted in the large diameter steam duct. The bypass, as well as the turbine exhaust steam (normal operation), is condensed inside the tubes. There are similarities and differences in the requirements for admission of bypass steam into a water cooled and air cooled condenser. The differences must be identified and addressed to ensure safe and reliable performance of the condenser.