Air Conditioning Cold Water System Frequency Control Pump Control

**Abstract:** In an air conditioning cold water system that operates with variable flow, a primary pump system is typically used, and the pump speed is controlled via frequency control. However, using constant pressure control is not the most energy-efficient method. To achieve maximum energy savings, variable pressure control should be implemented alongside variable frequency control. This approach allows the system pressure to adjust according to the pipeline characteristics curve, resulting in improved efficiency. **Keywords:** air conditioning, cold water system, pump, variable frequency control, variable flow, primary pump, pressure control, energy saving With the advancement of self-control technology and the availability of mature automatic control products, variable flow water systems are now widely used in air conditioning projects, especially in large and medium-sized systems, where they are almost universally applied. In variable flow chilled water unit operations, the units can adjust their flow within certain limits during partial load conditions, with minimal impact on efficiency. Combining variable flow on both the chiller side and the load side can significantly enhance energy savings. Using a single pump controlled by frequency conversion can fully utilize the energy-saving benefits of variable speed pumps, leading to substantial overall energy reduction. However, the control method employed directly affects the level of energy savings. In a variable flow cold water system, the electric two-way valve at the terminal adjusts based on room temperature, causing fluctuations in system flow and resulting in changes in the pressure between the supply and return manifolds. To adapt the chilled water pump’s frequency control to these flow changes, a bypass valve is usually installed as a backup. When the flow drops below the minimum required for chillers, the pump frequency stops decreasing further, and the bypass valve opens to redirect excess flow. Since chillers are often multiple, shutting down one unit can reduce flow by 50%, and further reducing it by another 50% has limited effect, so the bypass valve remains closed most of the time. **Principle of Energy Saving with Variable Frequency Pump Control** When the system flow changes from QA to QB, the operating point shifts from A to B', causing the system pressure to increase. At this point, there is the most surplus pressure head, which leads to the highest energy-saving potential. If the chilled water pump uses constant pressure control, and the set pressure is HA (HB), the pressure sensor detects the difference (HB’ - HB) and sends a command to lower the inverter output frequency. The pump curve then shifts from n0 to n1, maintaining HA as a constant pressure. Compared to a constant-speed pump, this reduces dynamic pressure loss during the (HB’ - HB) period, resulting in noticeable energy savings. However, if the variable frequency pump uses variable pressure control, the pressure follows the pipeline characteristic curve. The control device then adjusts the pump curve from n0 to n2, reaching the actual operating point at B''. This results in even greater energy savings, as it reduces the dynamic pressure loss during the (HB - HB'') period. **Energy Saving through Frequency Control** From the above analysis, it's clear that while constant pressure control is simple to implement and commonly used in variable frequency pump systems, it is not the most efficient method. It only requires setting a pressure value and receiving a feedback signal to maintain constant pressure. This approach was originally developed for water supply systems and has become quite mature in the market. However, applying this method directly to air conditioning and cold water systems is not optimal. Based on the analysis, the most efficient control method involves adjusting the system pressure along the pipeline characteristic curve, which maximizes energy savings. For this, the variable frequency speed control device must include a programmable control chip. During the installation of a cold water system, the first step is to measure the relationship between system flow, total pressure difference, and pump frequency during commissioning. Data from single and parallel operations are collected and simulated to derive an approximate formula for the pipeline operating characteristic curve. This formula is stored in the programmable control chip, serving as the basis for variable frequency speed regulation of the chilled water pump during operation. The principle of minimum pressure difference is preferred, and the average frequency is adjusted to lock the chiller at its minimum frequency. The chilled water pump starts at a low frequency and gradually increases as needed. In current engineering applications, if the project includes a building control system with integrated air conditioning control, implementing the described variable pressure control is relatively straightforward. However, if no building control system is used, the lack of standardized variable speed control products makes implementation more challenging. This often leads to the use of constant pressure control, despite its inefficiency. With today’s technological advancements, programmable control devices are now easily available, making it feasible to achieve variable pressure control. Manufacturers are encouraged to develop specialized products tailored for variable pressure control of chilled water pumps, enabling more efficient and widespread application of such systems. **Conclusion** In an air conditioning cold water system with variable flow, using a variable frequency-controlled chilled water pump offers the greatest energy-saving potential. Implementing variable pressure control, where the system pressure adjusts according to the pipeline characteristics curve, is the most efficient method. It is recommended that manufacturers develop appropriate standardized products to facilitate the adoption of this control strategy, enhancing both performance and energy efficiency.

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