Laboratory airflow control systems

A laboratory's HVAC system has jobs that an office system never faces. It must capture hazardous fumes at fume hoods, keep the lab negative so contaminants do not spread into corridors, dilute any releases in the room, and do all of this while airflows change minute by minute as researchers open and close sashes. The laboratory airflow control system is the set of devices and logic that makes this happen.

Because labs often use several times the airflow per square metre of an office, and much of it is 100 percent outdoor air, airflow control is also the single biggest lever on a lab building's energy use. Good control is both a safety system and an energy system.

Components of a lab airflow control system

A typical lab room includes:

• Fume hood exhaust valves: one per hood, varying exhaust with sash position or face velocity.
• General exhaust valve: removes air from the room not exhausted by hoods, maintaining the minimum air change rate and room offset.
• Supply valve: delivers conditioned air to the room, often with a reheat coil downstream.
• Room controller: coordinates the valves, calculates totals and enforces offsets.
• Fume hood monitors: display face velocity or flow status at each hood and alarm locally.
• Sensors: sash position sensors, sidewall velocity sensors, room temperature, occupancy sensors, and sometimes a room pressure monitor or air quality sensors.

The airflow devices are commonly venturi air valves or VAV terminals with airflow stations. The comparison is covered in venturi valves vs VAV boxes with airflow stations.

What drives the room airflow

At any moment, the lab's supply airflow is set by the largest of several requirements. The room controller evaluates each and picks the governing one.

The controller then sets general exhaust so that total exhaust equals supply plus the required offset. If hood exhaust alone exceeds what is needed, general exhaust drops to its minimum and supply rises to track the hoods.

Hood-driven labs

In a lab with many hoods, hood exhaust governs most of the time. Reducing hood exhaust when sashes are closed, through VAV hoods and sash management, is the biggest saving. See fume hood face velocity control and sash sensing.

Load-driven labs

Labs packed with equipment such as freezers, incubators or analyzers may be governed by cooling load. In these rooms, decoupling cooling from ventilation, for example with fan coils, chilled beams or a dedicated recirculating cooling system, can allow air change rates to fall.

Ventilation-driven labs

Labs with few hoods and modest loads are often governed by the minimum air change rate. Here the choice of minimum ACH and whether it can be reduced at night has the largest effect.

Minimum air change rates

There is no single code value for lab air changes. Historically, designers often used 6 to 12 ACH occupied. Current practice, supported by ANSI/ASSP Z9.5 and ASHRAE laboratory design guidance, is that the owner sets minimum ventilation rates through a documented risk assessment within a laboratory ventilation management program. Many institutions use lower rates when the lab is unoccupied, and some use demand-based control with multipoint air quality sensing to raise airflow only when contaminants are detected.

Requirements can be affected by provincial occupational health and safety regulations, institutional policies and the authority having jurisdiction. Any reduction should be backed by the hazard assessment, not just energy goals.

Room pressurization

Labs are normally kept negative relative to adjacent corridors and offices so air flows into the lab, not out. The usual method is flow offset: exhaust is set higher than supply by a fixed volume. Some critical labs use direct pressure control or a cascade of the two. The trade-offs are explained in room pressurization control: offset vs direct pressure.

Corridors are often treated as part of the pressure scheme, supplied with transfer air that then flows into labs.

Control modes

Most lab airflow systems include several operating modes:

1. Occupied: normal minimum ACH, hoods at full face velocity when sashes open.
2. Unoccupied: reduced minimum ACH, triggered by schedule or occupancy sensors, with hoods still controlled to safe face velocity when opened.
3. Emergency or purge: maximum exhaust and supply, triggered manually after a spill.
4. Decontamination: shut-off valves isolate the room, used in biosafety and vivarium spaces.
5. Fire alarm response: defined by the fire safety plan; may continue exhaust, shut down supply, or maintain pressurization depending on the hazard.

Every mode change must be checked for transient pressure reversals. A mode change that drops supply faster than exhaust is generally safe; the reverse can briefly push lab air into corridors.

Integration with the building

Lab airflow control does not stand alone. It interacts with:

• Supply and exhaust fans: fan speed must follow total building demand and keep minimum pressure at the most demanding valve.
• Exhaust stacks: stack discharge velocity and height must avoid re-entrainment, often requiring bypass air at low flow. See manifolded lab exhaust systems.
• Heating and cooling: reheat coils at each lab, plus heat recovery on the exhaust where the hazard assessment allows.
• Building automation: room controllers usually expose points over BACnet for monitoring and trending.

Performance verification

A lab airflow system should be verified at commissioning and periodically after. Verification includes:

1. Fume hood face velocity and containment testing, often to ASHRAE 110 with CSA Z316.5 or ANSI/ASSP Z9.5 acceptance.
2. Measured supply and exhaust flows at each valve, compared with reported values.
3. Room offset and pressure direction under all modes.
4. Response time of hood exhaust to sash changes.
5. Alarm function at hood monitors and the BAS.

Annual fume hood certification is a good opportunity to check room-level performance at the same time.

Frequently asked questions

Why do labs use so much energy?

Labs typically use high airflow rates of mostly outdoor air, which must be heated, cooled and moved. In a Canadian winter, heating that air is a large load. Reducing airflow safely is the main opportunity.

Can lab exhaust air be recirculated?

Air from fume hoods is exhausted, not recirculated. Some general lab exhaust may be eligible for energy recovery through devices that limit cross-contamination, subject to the hazard assessment and code requirements.

What is a lab ventilation management program?

It is a documented program, described in ANSI/ASSP Z9.5, that assigns responsibility for lab ventilation, sets performance criteria, and requires routine testing and record keeping.

Key takeaways

• A lab airflow control system coordinates hood exhaust, general exhaust and supply to maintain safety, pressure and comfort.
• Room supply is set by the largest of hood demand, minimum ventilation, cooling load or other exhaust.
• Minimum air change rates should come from a documented risk assessment, not habit.
• Negative pressurization is normally achieved by flow offset, with direct pressure control used where needed.
• Reducing airflow safely through VAV hoods, occupancy setbacks and decoupled cooling is the main energy opportunity.