Analysis on Optimum Vacuum of Direct Air-cooled Unit Under Variable Working Conditions

To improve the thermal economy of direct air-cooled power unit, a determination method for optimum vacuum has been developed by calculating the increment of both the unit power generation and the auxiliary power consumption based on the variable condition model for cold-end system, at rising supply rate of air for condenser. According to the similarity law, the effect of face velocity on fan power consumption is determined firstly, then the influence of back pressure on unit power generation is obtained by analyzing and simplifying the mathematical model of cold end system, and finally the optimum vacuum corresponding to various face velocities is deduced at different ambient temperatures and different exhaust steam heat loads. Calculation results on a 330 MW unit show that both the optimum vacuum and corresponding air velocity increase with rising exhaust steam or ambient temperature; the optimum vacuum is greatly affected by the ambient temperature, when the temperature gets above 20 ℃.