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Abstract: A constant air volume valve is a mechanical self-acting device suitable for ventilation and air conditioning systems that require a fixed air volume. The air volume control of a constant air volume valve requires no external power; it relies on the airflow force within the duct to position the control valve, thereby maintaining the air flow at a preset rate across the entire pressure range. Keywords: constant air volume valve, control, flow. A constant air volume valve is a mechanical self-acting device suitable for ventilation and air conditioning systems that require a constant air volume. The air volume control of a constant air volume valve requires no external power; it relies on the airflow force within the duct to position the control valve, thereby maintaining the air flow at a preset rate across the entire pressure range. (1) Application in fresh air systems: At present, in China, the air conditioning system that combines fan-coil units with fresh air systems is still quite common, especially in hotel guest rooms; most offices and commercial buildings also use this approach. The common practice is to install fresh air handling units on each floor, lay fresh air main pipes in the corridors, and use dozens of branch pipes to connect to various rooms from the main pipes. Taking hotel rooms as an example, the fresh air volume per room is generally 100 m3/h. How can the air volume in each branch duct be made consistent? Generally, designers tend to install an air volume control valve on the fresh air branch pipe, hoping to achieve air volume distribution through subsequent tuning. Since fresh air systems generally have long main ducts and short branch ducts, the adjustment of air volume via control valves is neither intuitive nor precise enough. Moreover, the fresh air volume per guest room is only 100 m3/h, which is very low; as a result, such adjustments are practically impossible to carry out. The construction party can only measure the total air supply volume of the fresh air main ducts, to ensure that each air supply branch duct has a sense of airflow. To ensure that the amount of fresh air supplied to each room meets the design specifications, there is no need for the construction team to adjust the flow in each room individually; by installing a constant-volume valve on each fresh air branch pipe, these problems can be easily resolved. People who live or work in high-rise buildings are often concerned about insufficient fresh air volume, while designers often feel aggrieved. Because, as can be seen from the drawings, the standard value for fresh air volume is not low, but we often overlook one issue: how to ensure the actual desired results from a design perspective. The use of flow control valves in fresh air systems is an effective measure to address this. Since fixed-air-volume valves are currently mainly imported and thus relatively expensive, the author suggests that they are suitable for use in four- and five-star hotels as well as high-end office buildings. (II) Application in exhaust systems: A well-designed air conditioning system requires a rational exhaust system, yet this aspect is often overlooked. In civil buildings, especially high-rise buildings, the airtightness of the envelope is very good, so only a small amount of air flow is required to maintain a positive pressure in the rooms. Approximately 85–90% of the fresh air volume must be exhausted outdoors through organized exhaust systems, in order to maintain a balance between the volume of air supplied and that of air exhausted; otherwise, no amount of fresh air can be introduced into the room. In civil buildings, exhaust air is generally discharged outside through auxiliary rooms such as bathrooms and water heating rooms. In addition, sometimes an additional exhaust system should be installed to ensure a balance between air supply and exhaust. For bathroom ventilation, the common practice is to install one or two bathroom ventilators in each bathroom, which are interlocked with the exhaust fans located in the exhaust shafts. We know that the fresh air system in high-rise buildings should be in operation throughout the year, and similarly, the exhaust air system should also be in operation all year round. In other words, the person using the restroom cannot control the activation of the restroom ventilator, which eliminates the need for such a ventilator. Moreover, an exhaust system in which one large fan drives dozens of small fans is difficult to coordinate and not economical to operate. A bathroom ventilator with an air exchange rate of 400 m3/h generates a noise level of around 40 dB, which ruins the tranquility of the bathroom. The large number of bathroom ventilators also poses significant difficulties for maintenance. To solve this problem, we can remove the bathroom ventilator, install a constant-air-volume valve on each branch pipe at each floor of the exhaust shaft, and place an exhaust fan at the top of the shaft. Such an exhaust system ensures that the exhaust volume per floor is roughly equal, features simple system control and reliable operation, allowing the bathroom to remain very quiet. (III) Application in Variable Air Volume HVAC Systems: In variable air volume HVAC systems, variable air volume systems are generally used in the outer areas, while fixed air volume systems are employed in the inner areas. In a supply air system where air volume and air pressure are constantly changing, a fixed air volume design for the inner areas relies on the use of fixed air volume valves. (IV) Application in cleanroom air handling systems: It is crucial to maintain a positive pressure level in cleanrooms within air handling systems. By installing flow control valves in the exhaust or supply/exhaust systems, it is possible to ensure an appropriate positive pressure level in the cleanrooms. In a clean operating room, positive pressure must be maintained both when surgeries are being performed and when they are not. During surgeries, the amount of fresh air required is equal to the amount needed to maintain positive pressure plus the amount required by the people present; whereas when no surgeries are taking place, only the amount of fresh air necessary to maintain positive pressure is required. Therefore, a two-position constant-air-volume valve needs to be installed on the fresh air supply ducts. (5) Application in duct system balancing: In all-air systems, due to the influence of building conditions, the resistance between various branch ducts is inevitably uneven; three-way control valves or multi-leaf control valves are generally used to distribute the air volume. To ensure that the air volume in important rooms or main branches is neither too high nor too low, and to reduce the randomness in tuning, air volume valves can be appropriately installed as a substitute for three-way control valves or multi-leaf control valves. (VI) Characteristics of use with constant air volume: A constant air volume valve is an automatic mechanical mechanism that does not require external power; an electric actuator can be added to adjust the flow setting via remote control signals. Constant air volume valves can be used in both supply and exhaust air systems. The operating temperature is generally between 10 and 50°C, while the pressure difference range is 50 to 1000 Pa; in other words, there must be at least a 50 Pa pressure difference before and after the valve, otherwise the constant air volume valve cannot function