Fume hood face velocity control and sash sensing

A chemical fume hood protects its user by pulling air in through the open sash fast enough to capture vapours generated inside. That inward air speed, the face velocity, is the most familiar measure of hood performance. In a constant volume hood, face velocity rises sharply as the sash is lowered. In a variable air volume (VAV) hood, the exhaust changes with the sash opening so that face velocity stays near a target value.

VAV hoods save large amounts of conditioned air, because exhaust falls whenever sashes are lowered. They also add controls that must work every time. This guide explains how face velocity control works, the sensing methods available, and what to verify.

Face velocity targets

Face velocity is the average air speed through the open sash area. It is not a guarantee of containment, which also depends on hood design, room air currents and user behaviour, but it is a practical control variable.

Typical targets for conventional chemical fume hoods are in the range of 0.4 to 0.5 m/s (80 to 100 fpm), with many institutions using 0.5 m/s (100 fpm). ANSI/ASSP Z9.5 and CSA Z316.5 address face velocity and containment criteria. Some high-performance or low-flow hoods are designed and tested to operate at lower face velocities, but only where containment testing supports it. Very high face velocities are not better: above roughly 0.6 m/s (120 fpm), turbulence around the user's body can reduce containment.

The owner's chemical hygiene plan or lab ventilation management program normally sets the target, and provincial occupational health and safety rules may also apply.

How VAV face velocity control works

The required exhaust flow is simply the face velocity target multiplied by the open sash area. The hood controller needs to know one of two things: the open area, or the actual face velocity. That leads to two control approaches.

Sash position sensing

Sensors measure the sash opening, and the controller calculates open area and commands the exhaust valve to the matching flow.

• Vertical sashes: a cable-extension sensor or similar position transducer mounted above the sash measures height.
• Horizontal sliding sashes: sensors detect the position of each panel, often with proximity or strip sensors, so the controller can calculate the open gap.
• Combination sashes: both vertical and horizontal sensors are combined to calculate total open area.

Because exhaust is set directly from sash position, this approach is fast. When paired with a fast-acting air valve, the exhaust can reach its new setpoint within about one second of the sash moving. It relies on accurate characterization of the exhaust valve and correct entry of the hood's sash dimensions.

Sidewall velocity sensing

A sensor mounted in the side of the hood measures air velocity through a small bypass port, which correlates with face velocity. The controller adjusts the exhaust valve to hold the measured velocity at setpoint.

• Measures a value close to the actual performance variable.
• Does not need sash dimensions entered.
• Is slower, because it is a closed loop: the sash moves, velocity changes, the sensor detects it, and the controller responds.
• Can be affected by room drafts, sensor fouling and calibration drift.

Comparing the two

Many systems use sash sensing for control and a separate sidewall sensor or flow feedback for monitoring and alarms.

Minimum and maximum exhaust

A VAV hood cannot reduce exhaust to zero when the sash is closed. A minimum flow is maintained to dilute and remove vapours from the closed hood and to keep the hood interior negative. The minimum is set by the hood manufacturer's guidance and the owner's standards, informed by ANSI/ASSP Z9.5 and NFPA 45.

Maximum exhaust corresponds to the sash at its maximum design opening, often limited by a sash stop. Hoods may be designed for full opening during setup with an alarm if used above the stop during normal work.

Monitors and alarms

Every fume hood should have a monitor that tells the user whether it is working. ANSI/ASSP Z9.5 calls for a flow indicator on each hood. A typical VAV hood monitor:

1. Displays face velocity or a normal status indication.
2. Gives a local audible and visual alarm when face velocity or flow falls below a threshold for a set time.
3. Indicates when the sash is above the design opening.
4. Includes an emergency purge button that drives exhaust to maximum.
5. Reports status and alarms to the building automation system.

Alarm delays should be long enough to ignore brief transients but short enough to warn users of a real failure.

Occupancy and sash management

Some systems add a user presence sensor at the hood face. When no one is at the hood, the face velocity setpoint can drop to a lower value, saving energy while still maintaining containment. This must be supported by containment testing at the lower setpoint and the owner's hazard assessment.

Automatic sash closers lower the sash when no one is present for a set period. They require obstruction detection so they do not close onto equipment or users. Even without automation, sash management programs that encourage users to close sashes significantly reduce lab airflow.

Testing and certification

Face velocity control should be verified at commissioning and at each periodic certification. Common steps:

1. Measure face velocity on a grid across the sash opening at several sash heights, per ASHRAE 110 or the owner's protocol.
2. Confirm exhaust flow tracks sash position throughout the travel.
3. Measure response time from sash movement to stable face velocity.
4. Verify minimum flow with the sash closed.
5. Test monitor alarms by simulating low flow.
6. Perform smoke visualization and, where required, tracer gas containment testing.

ASHRAE 110 includes a VAV response test as well as face velocity and tracer gas tests. Annual fume hood certification is common practice in Canadian institutions. More on lab exhaust design is in laboratory HVAC and fume hood exhaust.

Frequently asked questions

Why does face velocity drop briefly when I raise the sash?

The exhaust needs time to respond. With sidewall sensing and a slow loop, the dip can last several seconds. With sash sensing and a fast air valve, it should be brief.

Is a lower face velocity unsafe?

Not necessarily. Containment depends on the whole system. Lower setpoints are acceptable when the specific hood passes containment testing at that setpoint in its installed condition.

Can constant volume hoods be converted to VAV?

Often yes, by adding sash sensors, a VAV exhaust valve, a hood monitor and matching changes to room supply. See retrofitting lab airflow controls.

Key takeaways

• VAV hoods vary exhaust with sash opening to hold a target face velocity, commonly about 0.4 to 0.5 m/s (80 to 100 fpm).
• Sash position sensing is fast and stable; sidewall velocity sensing measures performance directly but responds more slowly.
• A minimum exhaust is always maintained with the sash closed.
• Every hood needs a working flow monitor with local alarms.
• Verify response time, minimum flow, alarms and containment at commissioning and periodic certification.