Abstract: In a power generation company, some researches and tests were carried out for changing the operation mode of the circulating water system of two 670MW units over from the unit system to the expanded unit system. Based on the thermodynamic test, the switching operating point which can make the condenser to operate in best back pressure was found. So the goal of economy and energy conservation was achieved. 

Key words: expanded unit system, best back pressure, switching operating point, optimal operation mode