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What Is VAV And Why Is It Important?

Fume cupboards are a crucial tool in laboratories, protecting users from harmful/toxic substances (fumes, dust etc). A typical laboratory fume cupboard pulls air from inside the room through the open sash, taking hazardous and noxious gases/particles with it. The contaminated air is then vented externally or passed through filters and recirculated back into the room.

Average chemical fume hoods exhaust approx. 20-30 m³/min of conditioned air* placing a significant load on the HVAC system and operational costs, but converting (or converting from) a CAV (Constant Air Volume) to a VAV (Variable Air Volume) system can reduce this load considerably.

VAV vs CAV vs RAV

CAV 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 huge 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.

RAV (Reduced Air Volume) is a limited and (in our opinion) a flawed half-hearted attempt to reduce energy usage and thereby costs. The system is calibrated to a lower face velocity (commonly 0.4 m/s instead of 0.5 or 0.6m/s) but this of course increases the risk of exposing users to fumes and only reduced exhausted air by 20%.

In contrast, a VAV system reduces the volume of air taken from the fume cupboard when not in use 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 as follows:

1. 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 1500mm wide fume cupboard with a maximum sash safe working height of 500 mm running at a face velocity of 0.50 m/s requires 0.3m³/s of air to be exhausted from the room.
  • Using a CAV system, the fume cupboard will exhaust 0.3m³/s regardless of sash position.
  • Using a VAV system: The fume cupboard will exhaust 0.3m³/s only when the sash is raised to its maximum sash working height but will reduce to around 0.03m³/s when the sash is closed.
  • 1500mm CAV cupboard will exhaust approx. 181,440 m³/week
  • With VAV and an auto-closing sash the volume should drop to around 18,144 m³/week – saving 163,296 m³ (90% reduction)

2. Carbon Emissions Reduced

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

3. Cost Saving

VAV systems may increase the initial costs but customers fitting VAV controllers can reduce energy costs by up to 90% which means a fast ROI.

Reading University upgraded 44 large fume cupboards from CAV to VAV and the annual energy saving was £223,958 which means the new system paid for itself in less than four years.

The higher the number of fume cupboards in a given room/area, the greater the potential savings because fume cupboards are significant contributors to the achievement of required overall air exchange rates.

The above example of a 1500mm CAV cabinet equates to the following saving:

  • The energy required to heat 1m³ through 1°C is approximately 0.00036KW
  • So energy loss due to air change is: 0.00036 x volume of air m³ x temperature difference
  • For temperature difference we will use an average of 12°C as a comfortable working temperature in a laboratory is around 22°C and the 2017 UK Average for outside temperature was around 10°C.
  • Therefore approximate heat requirement = 0.00036 x 181440 x 12 = 783.82KW
  • Gas heating costs say £0.03/KW = £23.52/week = £1222.76/year for just one cabinet!
  • With VAV that reduces to: 0.00036 x 18,144 x 12 = 78.38KW = £2.35/week = £122.20/year

Annual saving of £1,100.56 for one cupboard

On top of this, there is the real issue of sashes NOT being closed at night. We estimate the real-life cost saving of a 1500mm cupboard with an auto sash to be even higher than the above..

Universities often have much larger (2000mm+) units and the above real-life example saved £5090.00 per unit per year

5. Improved Safety

VAV systems ensure a constant face velocity regardless of sash position which safeguards the users from potentially hazardous fumes and substances. Not only does that offer peace of mind for all concerned but helps you stay compliant with relevant H & S regulations.

6. Improved Comfort

The on/off cycling of CAV or RAV systems causes laboratory temperatures to vary whilst a VAV system keeps it constant. This results 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 users.

7. Reliability

ISG’s VAV Controller with integrated Auto Sash Controller adjusts airflow according to need, decreasing the volume of extract when fume cupboards are not in use and reduces energy consumption by up to 85%. Featuring a full-colour, intuitive interface that’s easy to navigate, it facilitates simple calibration and offers full alarm diagnostics and test functionality.

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