Abstract In air conditioning chilled water systems with variable flow rates and using a primary pump system, constant pressure control is not the most energy-efficient method when the primary pump adopts variable frequency speed control. To achieve maximum energy savings, variable pressure control should be used for the pump's variable frequency control, with the system pressure changing along the pipeline's operating characteristic curve. Keywords: Chiller system, variable flow rate, variable frequency pump, variable pressure control, energy saving 1 Chiller system, variable flow rate, primary pump, variable frequency speed control With the maturity of automatic control technology and products, variable flow water systems have become the most prevalent in air conditioning engineering, especially in large and medium-sized air conditioning systems, where they are almost 100% used. In a chiller unit with variable flow rate, the chilled water volume can vary within a certain range under partial load with almost no impact on the chiller unit's efficiency. This allows for design integration with the variable flow rate requirements of the chiller system's load side. This makes it possible to directly use variable frequency speed control for the primary pump in a chiller system with variable flow rate, fully utilizing the excellent energy-saving effect of the variable frequency pump. The overall energy-saving effect is significant, but the energy-saving effect varies depending on the control method used. The working principle of variable flow primary pump variable frequency speed control in a chilled water system is shown in Figure 1. When the chilled water system experiences variable flow, the terminal controls the opening and closing of its electric two-way valve based on the room temperature, thereby causing changes in the system flow rate and resulting in pressure changes in the chilled water system's supply and return main pipes. This allows for variable frequency speed control of the chilled water pump to adapt to the system's flow rate changes. A bypass electric two-way valve is still required on the supply and return main pipes as a backup. When the flow rate decreases to the chiller unit's minimum flow rate, the chilled water pump locks its frequency and stops variable frequency speed control to further reduce the flow rate. At this time, the auxiliary bypass electric two-way valve is activated to bypass a portion of the excess flow. Since there are usually one or more chiller units (e.g., two units), stopping one main unit already reduces the flow rate by 50%, and the chance of a single unit reducing it by 50% is unlikely. Therefore, the auxiliary bypass electric two-way valve is generally not activated and remains closed. 2. Energy-saving principle of variable frequency speed control pumps The working principles of constant speed pumps, variable frequency speed control pumps, and constant pressure control are shown in Figure 2. In a chilled water system with a variable flow rate primary pump, when the system flow rate changes from QA to QB, if a constant-speed pump is used, its operating point shifts from A to B′, increasing the system pressure. This results in the highest excess pressure head and the least energy-efficient operation. However, if the chilled water pump uses variable frequency speed control with constant pressure, when the set pressure is HA (HB), the pressure sensor detects the pressure difference (HB′ - HB) and issues a control command to reduce the inverter's output frequency, causing the pump's operating curve to change from n0 to n1, while maintaining the constant pressure value HA. This is significantly less energy-efficient than a constant-speed pump, reducing the pressure difference (HB′ - HB). The pressure loss in section B is significantly reduced compared to constant speed pumps, resulting in a noticeable energy saving effect. However, if the chilled water pump uses variable frequency speed regulation and pressure control, the pressure changes along the pipeline's operating characteristic curve. In this case, the control device issues a control command to change the pump's operating condition curve from n0 to n2, with the actual operating point being point B. This reduces the dynamic pressure loss (HB - HB") compared to constant pressure control, thus achieving the most significant energy saving effect. 3. Control and Energy Saving of Variable Frequency Speed ​​Control Pumps As seen in the above analysis, the constant pressure control method commonly used in variable frequency speed control pumps is relatively simple and easy to implement. It only requires setting a pressure value and a pressure feedback signal, and the system pressure can be maintained constant through variable frequency speed control. This method originates from the constant pressure water supply of variable frequency speed control pumps. Currently, there are quite mature standardized products and many manufacturers in China. However, directly applying this constant pressure control method to the control of air conditioning chilled water systems is not suitable, nor is it the most energy-efficient control method. The above analysis shows that the most energy-efficient control method is variable pressure control, where the system pressure changes along the pipeline's operating characteristic curve, resulting in the most significant energy-saving effect. Therefore, the variable frequency speed control device for chilled water pumps must use a programmable control chip to achieve this. After the chilled water system is installed, during commissioning, the system flow rate change, total system pressure difference change, and their relationship with the frequency of the chilled water pump are measured. The conditions of single-unit operation and parallel operation are measured. All the data measured during commissioning are summarized and simulated to form an approximate calculation formula for the pipeline working characteristic curve. This formula is then stored in the programmable control chip as the basis for frequency conversion speed regulation of the chilled water pump during operation. During operation, the principle of minimum pressure difference priority is adopted. When multiple units are connected in parallel, each chilled water pump always maintains the same frequency. When the frequency is converted to the minimum flow of the chiller unit, the frequency is locked. When the chilled water pump starts, it starts from a low frequency. In current engineering applications, if a building control system is used and the air conditioning system is integrated, achieving the aforementioned variable pressure control is not difficult. However, if the project does not use a building control system, it presents challenges because there are currently no standardized variable frequency speed control products with variable pressure capabilities. Due to the unavailability of suitable products, constant pressure control is often the only option. However, given current technological advancements, control devices with programmable logic controllers (PLCs) are readily available. Therefore, we urge control equipment manufacturers to develop standardized variable pressure control products for chilled water pumps, specifically tailored to the characteristics of air conditioning systems. This would facilitate the application of variable pressure control and achieve greater energy savings. 4. Conclusion For air conditioning chilled water systems with variable flow rates and chilled water pumps using primary pump variable frequency speed control, to achieve maximum energy savings, the variable frequency control of the chilled water pumps should employ a variable pressure control method. The system pressure should change along the pipeline's operating characteristic curve. We recommend that manufacturers develop corresponding standardized products.