Fume cupboards play a pivotal role in the laboratory, protecting users from harmful substances. A typical laboratory uses an exhaust blower to pull air from inside the room through the open fume cupboard sash, taking hazardous and noxious gases with it. The contaminated air is then expelled or filtered for re-circulation.

The average chemical fume hood exhausts around 750 to 1,000 cubic feet of conditioned air per minute* placing a significant burden on the lab’s HVAC system and operational costs, but converting from a CAV to a VAV system can reduce this burden considerably.

VAV vs CAV

Constant air volume (CAV) ventilation provides a constant flow of air, regardless of whether or not the fume cupboard is in use and the sash is open or closed, wasting a significant amount of energy.

Closing the sash on a CAV hood will also increase the face velocity (or ‘pull’ at the front of the fume cupboard) with consequences including the disturbance of equipment, slowing of experiments and potentially even pushing contaminated air back in to the room.

In contrast, a variable air volume (VAV) system reduces the volume of air taken from the fume cupboard when it’s not being used and the sash is closed. This type of system, which can be fitted as part of a new installation or retrofitted to any existing laboratory, has several significant benefits.

6 Benefits of VAV fume cupboard ventilation

Energy efficiency
Using a VAV system, fume cupboards operate at minimum exhaust volumes when the sash is closed so energy usage is highly responsive to user needs. The exact saving depends on user behaviour but in general the less the hoods are open (in terms of height and time), the greater the energy savings.

Example:

A 1,500 mm wide fume cupboard with a maximum safe working sash height of 500 mm and face velocity of 0.50 m/s requires 300 l/s (litres per second) of conditioned air to be exhausted from the room.

Using a CAV system, the fume cupboard will exhaust 300 l/s regardless of sash position.

Using a VAV system: The fume cupboard will exhaust 300 l/s only when the sash is raised to its maximum sash working height but will reduce to around 60 l/s when the sash is down.

Assumption: in a typical working day – using an auto sash – the sash is only open 2 hours per day, 5 days per week.

Fume Cupboard Design Extract Volume (m3/week)

CAV 181,440

VAV 44,832

2. Carbon emissions reduced

With many educational and commercial laboratories under pressure to meet carbon reduction targets, VAV airflow controllers reduce energy wastage, ensuring reductions in carbon emissions.

3. Cost saving

VAV systems may incur higher initial costs than CAV systems but TEL’s own research demonstrates that customers fitting our VAV controllers can reduce energy costs by up to 85% which means they quickly pay for themselves.

A project at Reading University which saw the upgrade of 44 fume cupboards from CAV to VAV control has saved the university £223,958 per year in energy costs and means the new system will pay for itself in less than four years.

The higher the fume cupboard density, the greater the potential savings. This is because fume cupboards are significant contributors to the achievement of required overall air exchange rates.

4. Improved safety

VAV systems provide better protection for your staff from potentially hazardous fumes and substances, helping you stay compliant with relevant health and safety regulations.

5. Improved comfort

The on/off cycling of CAV systems causes laboratory temperature to fluctuate while in a VAV system the temperature remains constant, resulting in more precise space temperature control. In addition, because the air is exposed to cooling coils for a longer time in a VAV system, the air is passively dehumidified too, creating a more comfortable environment for laboratory users.

6. Reliability

TEL’s AFA4000 VAV Controller offers no inherent drift and incorporates a unique sensor able to provide stable readings over many years of operation without re-calibration.

This pioneering system, with integrated Auto Sash Controller, adjusts airflow according to need, decreasing the volume of air extracted when laboratory fume cupboards are not in use and minimising energy consumption by up to 85%. Featuring a full-colour, intuitive interface that’s easy to navigate, it facilitates simple two-step calibration and offers full alarm diagnostics and test functionality.

To find out more call TEL directly on 01457 865 635

* http://www.labmanager.com/lab-health-and-safety/2009/10/cav-rav-vav#.WgMQmVu0MdU