Abstract: Taking a 600-MW indirect air-cooling steam turbine unit for example, the calculation model of steam turbine, condenser and air-cooling tower is established on the basis of EBSILON software. Characteristics regarding to a variety of operation conditions are studied through the orthogonal test method, such that the variations of back pressure with respect to heat rate under the conditions of different ambient temperatures, circulating water flows and steam turbine loadings are obtained. The results show that, with constant circulating water flow and steam turbine load, as the ambient temperature goes up, the back pressure and the heat rate will keep rising. In particular the increasing rate in the high-temperature section is apparently higher than that in the low-temperature section. If the steam turbine load remains constant, the higher the ambient temperature, the higher the changing rate of the back pressure and the heat consumption rate under the same cyclic water flow. In the case of constant ambient temperature and the circulating water flow, with the raise of steam turbine load, the back pressure will increase while the heat rate will drop down. In the case of constant steam turbine load and circulating water flow, the rising rate of the outlet water temperature for the air-cooling tower is greater than that of the ambient temperature rise. Specifically, the former will be 1.11-1.15 times of the latter if the circulating water flow reaches full volume limit, which will lead to the rapid increase of the inlet water temperature at the condenser and the decline of the system economy performance.

Key words: indirect air-cooling system, steam turbine unit, off-design, back pressure, heat rate, EBSILON software