Abstract : This paper introduces the application of frequency converter in the renovation of central air conditioning refrigeration system, and designs PID closed-loop control for chilled water circulation, cooling water circulation, terminal air supply fan and return air fan. After use, the system has been proven to have significant energy-saving effect. Keywords : frequency converter, intelligent temperature controller, PID, central air conditioning 1 Introduction to the original system The original system uses Shanghai Carrier air conditioning units. The refrigeration station consists of 3 22kW compressors; the chilled water circulation system consists of 3 22kW motors, each with 1 water pump connected in parallel; the cooling water circulation system also consists of 3 22kW motors, each with 1 water pump connected in parallel; the air conditioning terminals consist of fan coil units. Since the working environment of the pharmaceutical plant is completely closed, each workplace has a return air fan, which works with the fan coil unit to form convection and keep the ambient air fresh. 2 Disadvantages of the original system (1) The temperature control of the original system determines the number of compressors to be put into operation based on the chilled water outlet temperature, while the control of the chilled water and cooling water circulation pumps is not designed with closed-loop automatic control. The number of pumps to be put into operation depends on the operator's experience. When the ambient temperature changes, the operator has to constantly start or stop the number of chilled and cooling water pumps, which greatly increases the operator's labor intensity, and the temperature is also constantly fluctuating. (2) The motors of the chilled water pump and cooling water pump in the system are all operating at rated full pressure, and the starting current is very large. When starting, it has a huge impact on the electrical and mechanical transmission equipment in the system, affecting the service life of the system, increasing the maintenance cost of the equipment, and the motor speed cannot be changed with the load. The flow rate of chilled water and cooling water can only be adjusted by changing the opening degree of the bypass valve. Therefore, the ambient temperature cannot be accurately controlled, and a lot of electrical energy is wasted. [b][align=center]For details, please click: Modification of Central Air Conditioning Refrigeration System[/align][/b]