Fume Hood Safety and Containment

In a laboratory environment, a fume hood is the primary device used when researchers are working with hazardous materials. The main function of the fume hood is to protect the user from effects of toxic or flammable substances by exhausting the substances away from the researcher and offering a controlled space to prevent contamination to other areas within the laboratory.

In this article, we explore 3 factors to consider regarding fume hood safety and containment.

Face Velocity

Face velocity is a measure of how much air is pulled through the face of a fume hood. When face velocity is too low, contaminants can escape as the hood is not exhausting the air at a fast enough rate, which puts the fume hood user and other laboratory occupants at risk of exposure. If the face velocity is too high, the result is more turbulent airflow, which also allows contaminants to escape and uses more energy to maintain the high airflow rate.

Common industry guidelines for fume hood face velocity range from 60-120 feet per minute (fpm) with 100 fpm being accepted as the standard for safe operation. The ASHRAE Symposium CH-99-09 on Laboratory Verification and Testing offered the piece “Containment Testing for Occupied and Unoccupied Control of Fume Hoods” which showed significant findings that unoccupied fume hoods can contain fumes while operating at the reduced face velocity of 60 fpm. However, occupied fume hoods require higher velocities to achieve proper containment and protect researchers.

Operating the fume hood at high face velocities for extended periods of time can result in increased energy costs. In fact, fume hoods are incredibly energy intensive, using about 3.5x more energy than an average home (source). The annual cost for operating one fume hood is over $8,000 (source). This means that, while higher face velocities are required when users are present at the hood to maintain safe containment, constantly operating the hood at these levels can have a significant impact on a facility’s bottom line. However, employing Usage-Based Controls (UBC) allows you to better manage energy use by maintaining the airflow rate at minimum levels and increasing it only when needed by the presence of someone at the hood.

Speed of Response

The airflow control system must respond to changes in airflow command rapidly to ensure proper face velocity control. Opening and closing the sash, or the fume hood being occupied versus unoccupied, causes changes to the movement of the air within the hood. In order to compensate for these airflow changes, the face velocity must adjust quickly to ensure containment. A one second speed of response time is critical in these scenarios as there is almost no time for airflow to become turbulent, and no time for hazardous materials to escape and contaminate other laboratory spaces.

Speed of response also plays a role in room pressurization, which must be net-negative in laboratory facilities. If there is a discrepancy between the response time for the exhaust and supply valves, with the supply valve responding too slowly, the room can go positive, and contaminants can escape to adjacent spaces.

Accuracy & Stability

Precise control to maintain proper face velocity regardless of sash position is also required to protect researchers at the fume hood. The mechanically pressure independent venturi valve helps achieve this through a cone and spring assembly. When the pressure changes within the fume hood, the cone responds by adjusting automatically, either allowing more air through the valve (low pressure) or restricting airflow through the valve (high pressure) to maintain the desired flow range. The system must also maintain stability as it adjusts. If the system overshoots or undershoots the required flow, it causes oscillations, which are pulsations of face velocity that can adversely affect the hood's containment. The characterization process of the Phoenix Controls Venturi Valve allows for +/- 5% flow accuracy and less than 5% overshoot or undershoot over the entire flow range.

How Can Phoenix Controls Help?

Phoenix Controls has specialized in airflow control solutions for critical environments for nearly 40 years. Our mechanically pressure independent venturi valve provides a 1-second speed of response time, and stable, accurate airflow when there is no room for compromise. We also offer various fume hood control solutions including sash sensors and fume hood displays to improve visibility and control of your environment, along with usage-based controls to help improve your laboratory’s efficiency.

Contact us today to learn more about how we can optimize your research environment today.

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