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**Abstract:**
This paper investigates the failure cases of circulating pumps in central air-conditioning systems, highlighting the critical role of selecting an appropriate pump head. Through case studies, it emphasizes the importance of combining theoretical calculations with practical experience to avoid common mistakes in system design. The study provides practical guidance for engineers and designers to ensure efficient and reliable operation of air-conditioning systems. Keywords: Central air conditioning, Pump head, System resistance, Hydraulic calculation, Pump selection.
**Introduction:**
In central air-conditioning systems, the circulating water pump plays a vital role by delivering chilled or hot water to terminal units. Proper selection of the pump’s head is essential to ensure that the system operates efficiently and meets the required flow and pressure conditions. However, in many real-world projects, improper pump selection leads to inefficiencies, increased energy consumption, or even system failures. This article explores common issues related to pump head selection and offers practical solutions to improve system performance.
**Theoretical Background:**
The flow rate of the water in an air-conditioning system depends on the cooling load and the temperature difference between supply and return water. The system resistance is determined through hydraulic calculations, which help in selecting the appropriate pump. Ideally, the pump should operate within its high-efficiency zone, where the working point is the intersection of the pump curve and the system resistance curve. However, in practice, two common problems arise: overestimating the system resistance, leading to excessive pump head, or underestimating it, resulting in insufficient pressure and poor performance.
**Case Study 1:**
A high-rise building faced issues with its circulating pumps during commissioning. The motor current was excessively high, and the pump exhibited strong vibrations and abnormal noises. Upon investigation, it was found that the selected pump head was too large, causing the pump to work at a low head and high flow, which led to motor overload and cavitation. After adjusting the valve settings and replacing the pump with one of lower head, the system operated smoothly, and the motor current significantly decreased, demonstrating the importance of proper pump selection.
**Case Study 2:**
Another project involved a regional air-conditioning system with multiple buildings. During commissioning, the pump failed to circulate enough water, causing the chiller to shut down. Analysis revealed that the pump had an insufficient head due to excessive resistance caused by a small mesh filter installed at the inlet. After removing the filter and recalculating the system resistance, a new pump was installed, improving both flow and pressure levels. This case highlights the need for careful evaluation of system components and their impact on pump performance.
**Conclusion:**
Proper selection of the pump head in central air-conditioning systems is crucial for ensuring optimal performance and energy efficiency. Designers must combine theoretical calculations with practical experience rather than relying solely on assumptions or past practices. Additionally, as technology evolves rapidly, it is essential to stay updated with new products and their specifications to make informed decisions. By following these principles, engineers can avoid costly mistakes and enhance the reliability of air-conditioning systems.