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Among the numerous potential dangers faced by workers in the laboratory, airborne hazards rank high for the possibility to cause severe injury or death. Industrial hygienist John Ostojic notes that airborne dangers in the laboratory include poisonous, reactive, flammable, and cryogenic gases; toxic fumes from pharmaceuticals; pathogens like viruses and bacteria; and compressed inert gases that can cause asphyxiation.1
Engineering controls
While lab safety programs often focus on personal protective equipment (PPE) such as goggles, respirators, and other gear, PPE is only one of four hierarchy of hazard controls OSHA identifies for protecting lab workers. The other items on the list are engineering controls, administrative controls, and work practices. Among this hierarchy of controls, OSHA explains that engineering controls “are preferred over all others because they make permanent changes that reduce exposure to hazards and do not rely on worker behavior.”2
A building’s engineering controls for ensuring safe air include chemical fume hoods and all aspects of the air supply and ventilation system. Together, these systems must:
- Exhaust airborne hazards from the lab
- Deliver sufficient fresh air
- Filter airborne contaminants
- Maintain pressure relationships between the lab and adjoining spaces.
A common HVAC component
In many laboratories, a common device installed in the HVAC system is a variable air volume (VAV) terminal unit/box. Dozens, or even hundreds, of these units are installed in a typical lab, depending on the facility size and configuration. VAV boxes regulate the amount of heated or cooled air entering a space in order to control temperature, humidity, and ventilation. A VAV box is essentially a rectangular sheet metal box with a butterfly damper/blade device inside that moves back and forth to regulate the amount of conditioned air flowing into a space.
Limitations of traditional airflow controls
VAV boxes have been used for decades in buildings worldwide, and perform well in many situations. However, in critical environments, such as laboratories, where precise airflow control is essential for safety, the units have several limitations. VAV boxes were designed for temperature control, and, while they do that well, the devices were never designed for speed of response and pressurization control.
One shortcoming of VAV boxes for laboratory airflow control is that the units can be slow to respond to changes in the indoor environment and within the HVAC system. Imagine a lab with fume hoods and an air supply system using VAV boxes that have the capability of supplying between 500 and 1500 cubic feet per minute (CFM) of air. Assume at the start of a work shift the valves are operating at 1000 CFM. A researcher planning to work with toxic chemicals opens a fume hood sash, which immediately requires a valve to supply 1500 CFM to ensure adequate fresh air supply and exhausting of the fumes. In many instances, VAV boxes cannot respond quickly enough, and the researcher could be exposed to hazardous levels of toxins before the system kicks in, especially if the HVAC system is operating in a low-pressure state, which facility managers sometimes do to reduce energy costs.
The reason VAV boxes can take seconds to respond to such changes is that the terminal units must receive signals to adjust their dampers from the building automation system (BAS). When the BAS tells the VAV box to adjust based on the fume hood opening, the box must move its damper, measure the new airflow rate, and then send a signal back to the BAS. As other VAV boxes in the HVAC system can also be adjusting to changes in the spaces they supply, overall system pressure changes, and the target VAV box must again measure and report airflow, taking critical time as signals go back and forth.
A second shortcoming is that, since VAV boxes must measure the airflow rate in order to adjust their damper positions correctly, it is crucial that the airflow sensors remain clean. As these sensors lie fully across the air stream passing through the terminal unit, they are susceptible to fouling with lint (Figure 1) and other items such as tissues when lab workers check to make sure the fume hood is working.
Figure 1 – Airflow sensor arms in a VAV box fouled by lint. (Credit: Phoenix Controls.)Despite the availability of fume hoods with sophisticated sensors and alarms, many researchers have been trained to double-check the hoods. The University of Texas at Austin explicitly instructs lab users to “Confirm that the hood is working before use by holding a [wipe], or other lightweight paper, up to the opening of the hood.”3 These items often get sucked into the HVAC system, where they can end up lodged on a VAV box’s airflow measurement arms. Keeping the sensors clean requires frequent and disruptive maintenance (Figure 2). In a facility with 500 VAV boxes, annual cleaning costs are on the order of $50,000–$100,000.
Figure 2 – Cleaning air supply valves in labs often entails costly and space-disruptive activities. (Credit: Abatement Technologies, Inc.)An alternative: venturi airflow valves
To ensure safe air in their facilities, more lab designers and owners are specifying venturi airflow valves as an alternative to traditional VAV boxes. Manufactured by several companies, venturi valves are tubes with a constricted neck, resulting in a shape like an hourglass (Figure 3). A cone on a spring assembly inside the valve is able to move back and forth parallel to the air stream passing through the valve.
As static pressure changes in the HVAC ducting, the spring/cone assembly instantly adjusts to regulate and maintain airflow within 5% flow accuracy. This mechanical pressure independence means the valve does not need to measure airflow, send a signal to the BAS, receive a return signal, and adjust the damper position via an actuator, as VAV boxes must do. Thus, the signal latency problem of VAV boxes is eliminated. The venturi valve’s fast response speed means it can instantly adjust to air supply needs to help ensure safe air for lab workers.
Figure 3 – Venturi airflow valves are easily recognized by their distinctive hourglass shape. (Credit: Phoenix Controls.)In addition to better airflow management, lab managers and designers also specify these valves to help reduce the HVAC system’s energy consumption. For example, to flow 1000 CFM of air into a lab space, the facility designers could specify a 10-inch VAV 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 of a VAV terminal box. Because a lab typically requires many air control devices, the energy cost difference between venturi valves and VAV boxes is substantial.
Further, venturi valves do not accumulate lint as happens with VAV boxes, so do not require frequent cleaning to operate properly, which can save tens of thousands of dollars per year.
Case study: Kansas University Integrated Science Building
To enable more researchers to pursue discoveries in life sciences and material sciences, Kansas University (KU) is building an Integrated Science Building (ISB). As with any laboratory design, sufficient airflow control is vital for protecting lab occupants from airborne hazards.
The airflow control system design needed to account for frequent high-demand loads to ensure safe air in a building with difficult-to-control spaces—numerous open labs and atriums—while also being energy efficient, noted Rex Mustain, president of airflow controls representative Associated Air Products (AAP). A key part of the HVAC system was more than 560 venturi valves (Figure 4), along with digital fume hood controls and electronic components linking the environmental control systems to a BAS.
Figure 4 – The HVAC system in Kansas University’s new Integrated Science Building relies on venturi valves (the red units in this image) for high-performance airflow control. (Credit: Associated Air Products.)“Safety is the highest priority in any lab,” said Mike Russell, director of KU Environmental Health and Safety. “The lab ventilation system, along with its various connected hoods and exhaust devices, must reliably capture, contain, and exhaust potential hazardous lab emissions. For more than 12 years [the brand of venturi valve-based systems we use] has performed exceptionally well across our campus, and provides an assurance of safety, as well as allowing us the ability secondarily to address energy savings.”
Conclusion
While thousands of laboratories around the world continue to use traditional airflow controls, for enhanced safety and energy savings and 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.
References
- https://www.ehstoday.com/industrial-hygiene/lab-safety-it-s-air
- https://www.osha.gov/Publications/laboratory/OSHA3404laboratory-safety-guidance.pdf
- https://ehs.utexas.edu/documents/Lab-Safety-Manual.pdf
Dave Rausch is market manager for Phoenix Controls, 75 Discovery Way, Acton, MA 01720, U.S.A.; tel.: 978-795-3400; e-mail: [email protected]; www.phoenixcontrols.com