Advertisement

What is the most dangerous piece of laboratory equipment? In a provocative Popular Science article by that title1, reporter Joseph A. Bernstein recounts terrifying examples of serious injury and death caused by various common pieces of lab gear. Even so, he concludes the greatest danger in labs isn’t exploding glass containers or other equipment mishaps, but the people working there. “The most dangerous thing is human error,” then-University of Pittsburgh immunologist Gigi Gronvall told Bernstein.

Recognizing the role human error plays in compromising safety, the Occupational Safety and Health Administration (OSHA) emphasizes the crucial function of a lab’s engineering controls for ensuring safety: “These types of controls are preferred over all others because they make permanent changes that reduce exposure to hazards and do not rely on worker behavior.”

Given the plethora of potential airborne hazards in labs—from toxic gases to viruses—the engineering controls for maintaining safe air must accomplish multiple goals. Among these are ensuring appropriate ventilation and air exchange rates, filtration of airborne contaminants and maintenance of pressure relationships between the lab and adjoining spaces.

For decades, a key component of airflow control has been variable air volume (VAV) terminal units/boxes. Common in labs and other institutional/commercial buildings, VAV boxes are basic airflow devices using a blade damper that help control ventilation, temperature and humidity by measuring airflow pressure. In light of various technical limitations with these and similar devices that rely on blade dampers, more lab owners, managers and designers are opting for higher-performance airflow controls.
 

Venturi valves help ensure safe, energy-efficient airflow in laboratories and other critical environments. Image: Phoenix Controls

ADVANCED AIRFLOW CONTROL 

In 1797, Italian physicist Giovanni Battista Venturi published a scientific paper on fluid dynamics that included a discovery that now bears his name. The so-called “Venturi effect” is the reduction in pressure that results when a fluid’s velocity increases during flow through a constricted section of pipe. This effect has important ramifications for the design of high-performance airflow controls as well.

A “venturi valve” is a tube with a constricted neck, resulting in an hourglass-like shape. A spring/cone assembly inside the valve is able to move back and forth parallel to the valve’s body (and parallel to the air stream). The assembly responds immediately to changes in air pressure in the HVAC ducting, automatically adjusting to maintain airflow during pressure changes (within +/- 5 percent flow accuracy). 

Without going into an extensive technical discussion of fluid dynamics, venturi valves offer higher accuracy airflow control compared to traditional devices like VAV boxes, due to the venturi valve’s:

•    High speed of response, to both duct pressure and flow setpoint changes, which cannot be matched by valves requiring flow measurement (e.g., VAV boxes and other alternatives) due to inherent signal latency between the flow sensor, controller and actuator.
•    Mechanical pressure independence, which instantly maintains flow—even with constant changes in static pressure—so that a stable, reliable amount of directional airflow is not compromised. No movement of the actuator is needed, thereby extending the life of the entire assembly.
•    Factory characterized flow metering technology that provides higher turn-downs to achieve a number of stable, accurate room pressure states. The cone assembly quickly moves into position to achieve the flow set point vs. having to measure and find its position.
•    Volumetric offset guarantees directional airflow. Zone balance controls for some venturi valves track each other, maintaining a design offset between supply and total exhaust to ensure directional pressure in the space.

Higher turndown ratios mean the device has a wider range over which it can accurately provide the correct airflow. With this improved accuracy, labs can better manage ventilation and maintain air pressure relationships for worker safety and research/testing/production integrity, regardless of room state (occupied, unoccupied or purge condition).

VAV terminal box sensors can become fouled due to build-up of lint. Image: Phoenix Controls

WHAT ABOUT ENERGY COSTS?

While lab managers and designers usually prioritize safety as job one in their facilities, they also care about reducing energy consumption where possible.

To reduce energy consumption in the HVAC system, it is important to consider airflow. For example, to flow 1,000 cubic feet per minute (CFM) of air into a lab space, the facility designers could specify a 10-inch VAV terminal box or a 10-inch venturi valve. Based on the physics underlying the two valve types, a venturi valve can accurately exhaust as little as 50 CFM, compared to a minimum 250 CFM required from a VAV terminal box. Because a lab typically requires dozens of air control devices or more (depending on its size), and it costs a handful of dollars to vent each CFM, the energy cost difference between venturi valves and VAV boxes is substantial. Thus, to save energy while still also ensuring worker safety, venturi valves, are an excellent alternative to VAV boxes and other traditional airflow controls.

BE SURE TO WATCH OUT FOR THE “LOW PRESSURE MYTH”

In light of the above benefits of venturi valves, some manufacturers of traditional valves attempt to promote their products’ energy saving potential using a dubious premise. Such manufacturers tell customers their valves can control air at ultra-low pressure (0.1 inches of water column—wc). They contend this allows lab operators to turn down the building’s HVAC fans to save energy. But falling for this “low pressure myth” won’t get you any energy savings and can compromise safety in labs.

If you look closely at these manufacturer’s product data sheets, you’ll see the reported operating values are for situations virtually never seen in the real world. It appears they test their air terminal devices in the wide-open position at low pressure, and claim sufficient airflow control. Yet, by definition controlling airflow requires some level of impediment to the airstream, and typically differential pressure of 0.5 inches wc, not 0.1 inches wc. As a result, operating a lab’s ventilation system at ultra-low pressure could lead to inadequate ventilation/build-up of airborne hazards—and injury or death for lab workers.

MAINTENANCE ISSUES

In addition to higher-than-necessary energy consumption costs, a traditional airflow control system using VAV boxes incurs high maintenance costs. If not properly maintained, VAV boxes can accumulate lint on their sensors, which can result in poor airflow control that reduces lab worker safety. In a facility with 500 traditional VAV terminal boxes, annual cleaning costs are on the order of $50,000 to $100,000. The design of venturi valves eliminates this problem and the cost and hassle of regular cleaning.

CONCLUSION

While thousands of labs around the world continue to operate fine using traditional airflow controls, for enhanced safety and energy savings—as well as reduced valve maintenance—venturi valves offer a higher-performance alternative. Many HVAC system engineers throughout North America can discuss in detail ways to optimize airflow control using these valves and sophisticated building automation systems (BAS).

REFERENCES

1.    https://www.popsci.com/science/article/2011-07/fyi-what-most-dangerous-piece-laboratory-equipment 

Dave Rausch is the market manager for Phoenix Controls. He has more than 20 years of experience in the building industry, including engineering and product management roles in airflow controls and fire suppression systems. 
drausch@phoenixcontrols.com

Advertisement
Advertisement