| Air cooling unit has the advantages of saving a large number of cooling water etc.It can be instead of wet unit at dry and rich in coal region. It has a large developing prospects. In the air cooling unit, the direct air cooling unit has a wide range application. In order to improve the run economy of the direct air cooling unit. This paper introduce the structure of direct air cooling system. Establish the mathematical model at variable conditions of the cold side system of the direct air cooling unit, analyze the main factor of effecting the the vacuumof the cold end system. They are the tightness of the vacuum system, air-cooled condenser heat load, air-cooled condenser, fan system, the environment and exhaust systems. Obtain the tightness of the vacuum system and inside and outside fouling resistance's impact properties to the vacuum. Through analysis, exhaust heat load, face velocity and environmental factors are the main factors of affecting cooling system vacuum. Therefore, by simplifying, don't consider the impact factors such as fouling resistance. Study best vacuum mathematical model of the unit which run at variable conditions. Explore the calculation method of unit incremental power generation and wind power consumption. Take 330MW direct air cooling unit for example, for the two cases: exhaust capacity at 700t/h and ambient temperature change from -46℃to 36℃, calculate the unit's best vacuum.Obtain the corresponding results. The article also study and compare fan system's operation at the low—temperature. Compare running economy between the fan open and stopped some fans. The research results of the paper is a guide on economy operation of the unit.We can refer the results to regulate the system's operation mode in order to make it run in the best vacuum way. Achieve energy saving.
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