Application and limits of a constant effectiveness model for predicting the pressure of steam condensers at off-design loads and cooling fluid temperatures

Abstract

The main steam condenser is an essential part of every steam power plant. It governs the exhaust pressure of the low pressure turbines and defines the minimum cycle temperature. The heat that is extracted from the cycle steam by the condenser is transferred to a cold stream such as air or water and discarded to atmosphere using a cooling device. The effectiveness of the condenser is key in determining steam temperature, and hence the turbine back pressure response at various operating conditions. Several methods have been proposed in literature to predict the change in backpressure due to load changes. This work examines the validity of a constant condenser effectiveness assumption method, for predicting the back pressure response at various loads, and/or for different cooling inlet temperature conditions. It also shows how the effectiveness of a condenser can be evaluated without detailed knowledge of the condenser internal design. The results of the model were validated against data from 2 Air Cooled Condensers and 1 Wet Cooled Condenser. The results indicate that the constant effectiveness assumption is valid for all heat load conditions, and for cooling inlet temperatures within a close range to the design cooling fluid inlet temperature. To further improve on the accuracy, a simple linear adjustment can be added if one extra low load condition is known, resulting in pressure prediction errors below 4% for the full operating range.