How should the expansion tank and pressurization unit of an air-conditioning water system be selected? How is the pressure at the pressurization point determined?
I am an HVAC expert. In HVAC system design, the selection of expansion tanks and pressurization devices, as well as the determination of the pressurization point pressure, are core elements for ensuring the safe and stable operation of water systems. If handled improperly, the consequences may range from difficult system makeup water and pipe vibration to pump cavitation, equipment damage, or even pipe burst accidents. The following is a detailed analysis based on codes and engineering experience.
First, according to Article 8.5.2 of the Code for Design of Heating Ventilation and Air Conditioning of Civil Buildings (GB 50736-2012), the choice of pressurization method shall be determined based on system scale, water temperature, building height, and operating requirements. Common schemes include high-level expansion tank pressurization (suitable for open systems or small closed systems) and pressure tank (or variable-frequency makeup water pump) pressurization (suitable for large closed systems). The pressurization point is usually set on the suction side of the circulating water pump (on the return pipe), in order to ensure that the pump inlet pressure is higher than the net positive suction head required, preventing cavitation during pump operation.
**I. Selection of Expansion Tanks and Pressurization Devices**
1. **High-level expansion tank**: Suitable for systems with a building height not exceeding 50 m. Its effective volume shall be calculated based on the system water capacity and water temperature variation. The formula refers to the second edition of the Practical Heating and Air Conditioning Design Manual: V = α·Δt·Vc / (1 - β), where α is the volumetric expansion coefficient of water (about 0.0006/℃), Δt is the system water temperature fluctuation range (usually 20-30℃), Vc is the total system water capacity, and β is the air expansion coefficient in the tank (about 0.2-0.3). For example, for a 1000 kW chilled water system with a water capacity of about 10 m³, if the water temperature rises from 5℃ to 35℃, the required minimum effective volume is about 0.28 m³. Actual selection requires a margin, and 0.5-1.0 m³ is often selected.
2. **Pressure tank (pneumatic tank)**: Suitable for high-rise buildings or systems where a high-level water tank cannot be installed. Its selection must consider both the regulating volume (makeup water volume) and the pressure rating. The regulating volume is calculated as 1.5-2 times the maximum hourly makeup water volume of the system, and the pressure rating must meet the pressure requirements of the highest point of the system. For example, for a hotel project with a building height of 80 m and a system static pressure of 0.8 MPa, a 1.0 MPa pressure tank is selected and equipped with variable-frequency makeup water pumps to ensure the pressure fluctuation range is within 0.05 MPa.
3. **Variable-frequency makeup water pressurization device**: Suitable for large systems, usually composed of makeup water pumps, pressure sensors, variable-frequency controllers, and a stabilization tank. The makeup water pump flow rate is selected as 1%-2% of the system circulating water volume, and the head must overcome the height of the highest point of the system plus a margin pressure of 0.05-0.1 MPa.
**II. Determination of Pressurization Point Pressure**
The pressurization point pressure P0 must satisfy three conditions:
- **Prevent the highest point of the system from running empty**: P0 ≥ static pressure at the highest point of the system + 0.01~0.02 MPa (GB 50736-2012 Article 8.5.4). For example, if the building height is 60 m, the pressurization point pressure shall be at least 0.6 MPa + 0.02 MPa = 0.62 MPa.
- **Prevent cavitation at the pump inlet**: P0 ≥ net positive suction head required (NPSHr) + 0.01~0.02 MPa. In an actual case, if the NPSHr of a chilled water pump is 4 m, the inlet pressure shall be ≥0.04 MPa + 0.02 MPa = 0.06 MPa.
- **Ensure the system operating pressure is within the pressure-bearing range of pipes and equipment**: P0 + pump head ≤ maximum working pressure of equipment (usually 1.0 MPa or 1.6 MPa). For example, if the pump head is 30 m, the maximum operating pressure is 0.62 MPa + 0.3 MPa = 0.92 MPa, and it is necessary to ensure that the pressure-bearing capacity of pipes and terminal equipment is not less than 1.0 MPa.
**Actual engineering case**: In a five-star hotel project with a building height of 45 m and a total system water capacity of 20 m³, high-level expansion tank pressurization was adopted. The effective volume of the water tank was calculated as 0.56 m³ according to the above formula, and 1.0 m³ was actually selected and placed on the roof. The pressurization point was set on the return pipe, with the pressure set at 0.47 MPa (i.e., 45 m static pressure + 0.02 MPa margin). During commissioning, it was found that because pipe water resistance was not considered, the pressurization point pressure needed to be fine-tuned to 0.49 MPa to ensure no negative pressure at the highest point. In another super high-rise project (120 m), a pressure tank + variable-frequency makeup water pump was adopted, the pressurization point pressure was set at 1.25 MPa, and a safety valve was configured to avoid frequent start-stop problems of the pressure tank.
Summary: The selection of expansion tanks or pressurization devices shall be based on system water capacity, building height, and operating parameters, and strictly follow the requirements of Section 8.5 of GB 50736-2012; the pressurization point pressure shall simultaneously satisfy anti-empty, anti-cavitation, and pressure-bearing limits. It is recommended to verify by hydraulic calculation software after preliminary calculation and reserve commissioning margins.
The above are reference opinions. The actual project conditions still need to be further comprehensively evaluated and implemented in combination with specific working conditions, site conditions, and other factors.