1. How Does a CAV Valve Maintain Stable Airflow?
A pressure-independent CAV valve usually uses a mechanical compensation mechanism or controller to automatically adjust the effective free area according to changes in duct pressure.
The basic control process is:
Pressure Change → Valve Adjustment → Stable Airflow
When the system pressure increases, the valve reduces its opening. When the pressure decreases, the valve increases its opening. However, constant airflow control is only effective within the operating differential pressure range specified by the manufacturer.
If the pressure differential is lower than the minimum requirement, the valve may not achieve the set airflow even when fully open. If the maximum allowable pressure differential is exceeded, noise, vibration, and increased component loads may occur.
2. How to Determine the Airflow Range?
The design airflow should be confirmed before selection rather than providing only the duct dimensions. It is recommended to provide at least:
Design Airflow: Design airflow;
Minimum Airflow: Minimum required airflow;
Maximum Airflow: Maximum required airflow;
Airflow Tolerance: Allowable airflow deviation.
The design airflow should remain within the valve’s effective control range. Long-term operation near the upper or lower limit should be avoided whenever possible.
An undersized valve will increase air velocity and pressure loss. An oversized valve may reduce control stability in the low-airflow range.
The airflow unit, such as m³/h, L/s, or CFM, should also be confirmed during selection to prevent conversion errors.
3. Why Are Static Pressure Parameters Important?
CAV valve selection should not be based only on rated airflow. The available differential pressure between the valve inlet and outlet must also be confirmed.
| Parameter | Function in Selection |
|---|---|
| Minimum Differential Pressure | Minimum differential pressure required to achieve the set airflow |
| Operating Pressure Range | Effective differential pressure range for constant airflow control |
| Maximum Allowable Pressure | Maximum pressure the valve body can withstand |
| Pressure Drop | Impact of the valve on total system resistance |
If terminal static pressure is insufficient, the fan’s available static pressure, filter resistance, duct resistance, and branch balancing should be checked instead of directly increasing the CAV airflow setting.
If the static pressure is too high, noise and energy consumption can be reduced by resetting the system static pressure, using variable-frequency fan control, or dividing the system into appropriate zones.
4. How to Match the Duct Size?
For circular CAV valves, the duct diameter should be confirmed. For rectangular CAV valves, the width, height, and flange type should be confirmed.
The relationship between airflow, effective area, and air velocity is:
Q = 3600 × A × V
Where Q is the airflow, A is the effective area, and V is the air velocity. Air velocity, noise, and pressure loss should all be evaluated during design.
Matching connection dimensions do not automatically mean the valve can meet the required airflow.
If a duct transition is required, a gradual transition should be used to reduce turbulence and local pressure loss.
5. How to Select CAV Valves for Different Projects?
General Air-Conditioning Supply Systems
Mechanical CAV valves can be selected for fixed-airflow zones. They have a simple structure and do not require an external control signal.
Hospitals and Cleanrooms
Attention should be paid to airflow accuracy, airtightness, material cleanliness, and maintenance convenience. When room pressure differential control is involved, the CAV valve must be coordinated with the supply-air, exhaust-air, and pressure-monitoring systems.
Laboratory Exhaust Systems
In addition to airflow and static pressure, the corrosiveness of the medium, fail-safe condition, and alarm requirements should be confirmed. Electronic CAV or Venturi Valve control solutions should be evaluated for high-risk laboratories.
Multi-Branch Systems
The design airflow and available static pressure of each branch should be calculated separately. The same valve model should not be used to cover operating conditions with excessively large differences.
6. What Should Be Considered During Installation?
Install the valve according to the arrow on the valve body to ensure the correct airflow direction;
Avoid installation immediately next to elbows, tees, fan outlets, or sudden duct transitions;
The required upstream and downstream straight-duct lengths should comply with the manufacturer’s technical documentation;
Ensure sufficient maintenance space for the valve shaft, adjustment mechanism, and actuator;
Duct connections should be concentric to prevent stress and deformation of the valve body;
Measure the actual airflow, pressure differential, and noise after installation;
Do not cover the scale, controller, or maintenance access points with insulation materials.
7. Common Procurement Questions
Does a CAV Valve Require an Actuator?
A mechanical CAV valve usually relies on its internal mechanism to maintain stable airflow and does not necessarily require an actuator. If remote switching, airflow resetting, or BMS control is required, a motorized model should be selected.
Can a CAV Valve Replace a VAV Valve?
Not completely. A CAV valve maintains a fixed airflow, while a VAV valve dynamically adjusts airflow according to the load. A VAV system is more suitable when energy-saving control or multiple operating conditions are required.
What Should Be Done If the Airflow Cannot Reach the Set Value?
First, measure the pressure differential across the valve and check the fan’s available static pressure, filter blockage, duct leakage, and installation direction. The valve should not be immediately assumed to be faulty.
8. Yueboyang CAV Solutions
Jiangsu Yueboyang Purification Equipment can provide circular or rectangular CAV valve selection according to the project airflow schedule, duct dimensions, static pressure conditions, and control requirements. Customization is available for materials, set airflow, and control methods.
When requesting a quotation, it is recommended to submit the Airflow Schedule, Duct Size, Available Pressure, and Installation Drawing. This allows the valve model to be accurately matched and reduces the risk of insufficient on-site airflow and repeated commissioning.
