CAV or VAV: Fume Hood Design Considerations

If you are part of the team on the procurement and designing (or retrofitting) of your laboratory, it is also your job to decide whether the fume hood you're planning to install should have a Constant Air Volume (CAV) or a Variable Air Volume (VAV). Why do we think it's important? An average fume hood exhausts around 750 to 1,000 cubic feet per minute of conditioned air, hence, imagine the amount of load it's putting on the lab's HVAC system. If not well planned, this will negatively impact the operating costs of the laboratory.

Don't be overwhelmed by these terminologies as these "jargons" will be thoroughly explained with photos I personally created.


Understanding Face Velocity
This topic will be talking a lot about the importance of face velocity inside the fume hood. Face velocity is simply defined as the pull of air that directs fumes from the hood through the ductwork.
One important consideration during the lab designing is on how to maintain a consistent, safe face velocity. Major European and American organizations recommend face velocity within 60 to 100 per minute ( 0.4 to 0.7 m/s), depending on the level of danger and toxicity of the particular contaminant.

Face velocity can be best described by the equation shown below:

Where: V = face velocity, m/s
             Q = volume flow rate, m3/s
             S = Area of the sash opening, m2


The efficiency of a fume hood to exhaust out fumes is directly related to the face velocity (m/s) of the air drawn towards the opening (sash) of the fume hood through the ductwork. Anything below or higher than the recommended face velocity is considered dangerous. 

Constant Air Volume (CAV) Fume Hood
A CAV fume hood exhausts the same amount of air regardless of the sash position. Going back to the equation above, Q is constant regardless of the sash position. Hence, closing the sash on a conventional CAV hood will increase the face velocity in order to draw out the same amount of air. Hence, we can deduced that the performance of the hood depends primarily on the sash position.

Conventional CAV Fume Hood

The photo shown above is a conventional CAV hood which is a basic enclosure with a movable sash (or window). If the sash is closed, face velocity can increase to a certain point where it can create turbulence and force contaminants out of the hood or disturb instruments and delicate apparatuses.

Bypass Hoods

To address the limitations of a conventional CAV hood, bypass openings are added above the sash in addition to an airfoil sill that will help redirect airflow as the sash is closed. These bypass openings help reduce the detrimental effect of higher face velocity by keeping the airflow equal as the sash is moved up or down.

Recently, there are new models of bypass hoods that can contain contaminants at a reduced face velocity of 0.3 m/s. These are called high performance, low-flow fume hoods, with safety features as well as an advantage for energy savings. This type of fume hood will be discussed separately.


 Variable Air Volume Fume Hood

VAV Fume Hood

A Variable Air Volume (VAV) fume hood, on the other hand, uses a VAV controller device that monitors the real-time airflow volume (Q). Once the desired face velocity (v) threshold has been set, this is kept constant regardless of the sash position.

VAV hoods change the exhaust volume by employing two common methods:

1.  By using a damper or a valve in the exhaust duct that opens and closes based on sash position. The damper, in other words, regulate the volume of air flowing inside the hood based on the position of the sash to maintain a constant face velocity. 
2. By using a blower (with an inverter) that changes speed to meet air-volume demands. The end goal is to regulate air volume to achieve the same face velocity. If the sash goes up, the blower rotates faster to increase airflow volume, otherwise.

Although VAV fume hoods are generally expensive to setup at the start and is more complicated than a traditional constant-volume hood, in the long run it can provide considerable energy savings by reducing the total volume of conditioned air exhausted from the laboratory.  VAV allows the blower to operate at a faster speed only when its necessary such as opened sash  but slows down when sash is in a closed position.
 
Summary
To summarize, again consideration must be given to CAV versus VAV when it comes to the designing or retrofitting of your laboratory. The advantages and disadvantages of each are outlined below:

Constant Air Volume (CAV) Fume Hood
    Advantages

1.  Straightforward design. Installation is not complicated.

   Disadvantages 

1.  In the long run, operating cost is higher than a VAV fume hood since it exhaust the same volume of air regardless of sash position. 

Variable Air Volume (VAV) Fume Hood
      Advantages

1. Increased safety due to having a constant face velocity (which can be set within a safe threshold).
2. Energy savings for the laboratory can be achieved due to reduced amounts of conditioned air being exhausted through the fume hoods.

    Disadvantages 

1.  Initial capital outlay is higher (but can be offset by the energy savings in the long run).