How Do Their Control Strategies Differ?
| Item | CAV — Constant Air Volume | VAV — Variable Air Volume | Venturi Valve |
|---|---|---|---|
| Core function | Maintains airflow at a setpoint | Adjusts airflow within a specified range according to demand | Acts as the valve that regulates airflow |
| Airflow variation | Remains essentially constant during normal operation | Changes with fume hood sash position, occupancy, and other conditions | Depends on the control configuration; can provide CAV or VAV control |
| Procurement considerations | Airflow setpoint and stability | Minimum and maximum airflow, and coordinated control | Operating differential pressure, airflow range, response, and control interfaces |
CAV is suitable for equipment or areas with relatively constant exhaust requirements. VAV adjusts supply and exhaust airflow according to actual demand, such as changing exhaust airflow when a fume hood sash moves. Venturi valves are commonly used for laboratory supply air, exhaust air, and room differential pressure control. Their specific performance must still be verified against the product model and system conditions.
Which Parameters Should Be Confirmed First?
First, determine the airflow range. For CAV, specify the design airflow. For VAV, provide minimum, maximum, and typical operating airflow separately. Do not select a valve based only on maximum airflow; also confirm that low-airflow conditions fall within its controllable range.
Second, verify the available differential pressure. Compare the valve’s required operating differential pressure range with duct system conditions at maximum and minimum loads. Insufficient differential pressure may prevent the valve from achieving the target airflow. Excessive differential pressure requires further assessment of noise and system energy consumption.
Third, define the control task. For fume hoods, consider exhaust response following sash movement. For rooms, calculate the airflow offset between supply and exhaust, and confirm control logic when doors open or equipment starts and stops. Simply stating “negative pressure required” is insufficient for accurate selection. ASHRAE notes that choosing CAV or VAV for a laboratory should consider ventilation requirements, the number of exhaust devices, and operational risks.
Fourth, confirm interfaces and materials. Specify control signals, building automation system communication requirements, feedback signals, exhaust media corrosivity, and cleanliness requirements. Interface incompatibility can make commissioning more difficult than replacing the valve itself.
How Should Different Engineering Applications Be Selected?
Fixed-flow exhaust equipment: If the process requires a constant airflow, evaluate a CAV solution first and verify airflow stability during system operation.
Fume hoods with changing operating conditions: If exhaust demand changes with sash position, evaluate a VAV solution. When using a Venturi valve, also confirm how the sensors, controller, and valve operate together.
Multiple fume hoods sharing an exhaust system: In addition to individual valve specifications, calculate total airflow during simultaneous operation, the fan’s modulation range, and room supply–exhaust balance. Even correctly selected local valves cannot compensate for insufficient total system airflow.
Frequently Asked Questions
Is a Venturi Valve Always a VAV Valve?
No. A Venturi valve can be configured for constant or variable airflow control. Specify the required control mode when requesting a quotation rather than relying on the name “Venturi Valve” alone.
Is VAV Always More Suitable Than CAV for Laboratories?
No. VAV is more suitable where loads vary significantly and demand-based adjustment is needed. CAV can be evaluated for fixed exhaust requirements. The final choice should consider safety requirements, minimum ventilation, control complexity, and operating conditions.
How Can Buyers Obtain Comparable Quotations?
Provide suppliers with a standardized parameter sheet covering: installation location, handled medium, minimum/design/maximum airflow, available differential pressure, duct dimensions, control mode, response requirements, communication interfaces, and material requirements. Request selection data and a control proposal for the specified conditions, and confirm whether the quotation includes actuators, controllers, and necessary feedback functions.
BoYang can help match CAV, VAV, and Venturi valve solutions to laboratory fume hoods, room supply and exhaust systems, and fixed-flow exhaust equipment. The operating parameters above are needed to determine valve sizes and the associated control configuration.
