Step 1: Identify the Fume Hood Exhaust Control Strategy
First, determine whether the fume hood uses constant air volume (CAV) or variable air volume (VAV) control. A CAV fume hood typically maintains a relatively constant exhaust airflow. A VAV fume hood adjusts exhaust airflow as the sash opening changes to meet the design face velocity and minimum exhaust requirements.
A Venturi valve is an airflow control device that can be used in different control strategies. Procurement documents should clearly specify the required strategy.
Converting an existing fume hood to VAV requires more than replacing the exhaust valve. Confirm that the sash position sensor, fume hood controller, room supply air system and exhaust fan can support coordinated modulation.
Step 2: Calculate Maximum and Minimum Exhaust Airflow
For preliminary sizing, VAV fume hood exhaust airflow can be estimated using:
Exhaust airflow Q (m³/h) = Effective sash opening area A (m²) × Design face velocity V (m/s) × 3600
For example, a fume hood has a design sash opening measuring 1.2 m wide × 0.5 m high. If the project specifies a design face velocity of 0.5 m/s, the preliminary exhaust airflow at this opening is:
1.2 × 0.5 × 0.5 × 3600 = 1080 m³/h
This calculation supports preliminary selection. Final setpoints should also consider the fume hood design, project safety requirements and on-site testing.
ASHRAE identifies sash opening area and target face velocity as one method for calculating exhaust demand in VAV fume hoods.
Provide at least three airflow values for procurement:
Minimum exhaust airflow with the sash closed
Design exhaust airflow at the normal operating sash opening
Maximum exhaust airflow at the maximum permitted sash opening
The valve’s controllable airflow range must cover these operating conditions. In particular, check whether the required minimum airflow falls below the model’s minimum stable control limit.
Minimum exhaust airflow should not simply be set to zero. It must be established through the safety design of the fume hood and laboratory.
Step 3: Verify Differential Pressure, Response and Control Interfaces
Valve Operating Differential Pressure
Ask the designer to provide the available differential pressure at the valve location under maximum and minimum total exhaust airflow conditions. Compare these values against the supplier’s specified operating differential pressure range.
Insufficient differential pressure at a remote duct branch may prevent the valve from delivering the design exhaust airflow, even when its rated maximum airflow is adequate. Excessive differential pressure requires an assessment of noise and fan energy consumption.
Response to Sash Movement
When an operator rapidly raises or lowers the sash, the controller must update the exhaust airflow setpoint, and the valve must respond accordingly.
Require the supplier to explain the test conditions for control response time, airflow stability limits, and behavior during power failure or communication loss. Avoid comparing products solely by a single advertised response-time figure.
System Integration Interfaces
Confirm how the valve connects to the sash position sensor, fume hood controller, room controller and building management system (BMS). List all required setpoint, feedback and alarm signals.
When fume hood exhaust airflow changes, laboratory supply airflow must adjust in coordination to maintain the intended airflow direction and room differential pressure relationships.
How Should Different Project Scenarios Be Configured?
New VAV laboratories: Calculate the minimum and maximum airflow for each fume hood. Then verify duct system differential pressure, exhaust fan capacity and room make-up air capacity under simultaneous sash-opening conditions.
Existing fume hood retrofits: Check the original fume hood type, sash configuration and exhaust duct diameter before assessing sensor and controller installation options. Commissioning after the retrofit should cover the closed sash position, normal operating opening and maximum opening.
Laboratories with corrosive exhaust: In addition to airflow requirements, provide the exhaust composition and cleaning requirements. Ask the supplier to confirm the suitability of materials, coatings and maintenance methods for all components exposed to the airstream.
Frequently Asked Questions
Can a Venturi Valve Alone Ensure Fume Hood Safety?
No. The valve controls exhaust airflow, but actual fume hood containment performance also depends on hood design, sash position, room supply air patterns and on-site commissioning.
After installation, verify airflow, face velocity and alarm functions at representative sash openings according to project requirements.
Is a Larger Valve Always a Safer Choice?
No. An oversized valve combined with a low minimum airflow requirement may place normal operating conditions outside its effective control range.
Select the valve according to the full minimum-to-maximum airflow range, rather than adding capacity based only on maximum airflow.
What Information Should Be Included in a Procurement Enquiry?
Provide the fume hood model and quantity, sash type and opening dimensions, design face velocity, exhaust airflow at the three operating conditions, exhaust duct diameter, available differential pressure at the valve location, exhaust composition, and control and communication requirements.
Require the quotation to identify the valve model, applicable airflow and differential pressure ranges, actuator and controller configuration, and scope of on-site commissioning.
BoYang can provide Venturi Valve selection and control integration recommendations based on fume hood exhaust requirements. Confirming valve specifications and the coordinated control strategy after receiving complete operating data helps reduce rework caused by incompatible airflow ranges or interfaces.
