Editor’s note: This three-part article is one of a series of Technical Bulletins and Best Practice Guides for laboratories, produced by Laboratories for the 21st Century (“Labs21”), a joint program of the U.S. Environmental Protection Agency and the U.S. Department of Energy. Geared toward architects, engineers and facility managers, these publications provide information about technologies and practices to use in designing, constructing, and operating safe, sustainable high-performance laboratories. For more information about these free resources, see: www.Labs21century.gov/toolkit/bp_guide.htm .
Part one, which was published in February, discussed standard practices in lab ventilation and an overview for preparing to optimize. Part two, below, covers implementation strategies for optimization. Part three, in the April edition, will provide some case studies and comments regarding commissioning.
The Labs21 website also provides full information about the agency’s 2010 annual conference, to be held in Albuquerque, Sept. 28-30. The co-meeting is sponsored by I2 SL, the International Institute for Sustainable Laboratories.
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Fig. 1. A laboratory’s airflow pattern shown by neutrally buoyant helium bubbles.
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There are many design factors to consider when optimizing lab ventilation. These include the lab’s layout (e.g., arrangement of equipment) and use (potential pollutants), control and removal of hazardous pollutants, and how to achieve adequate ventilation while attending to cooling load requirements.
The “good” and “better” practices outlined below begin with codes or standards as a starting point for designs, while facilitating the adoption of ventilation specifications that ensure safety and energy efficiency. Good practices emphasize lab-specific operations and control strategies, while better practices improve the ventilation design process with advanced computer or physical modeling techniques. These new techniques evaluate scenarios in which the system will need to respond to critical conditions (e.g., hazardous material spills, pollutant mixing factors), thereby reducing the guesswork involved in standard practice, and ensuring that the facility will perform well during emergencies.
Good practice strategies: control refinements
As with most pollution-abatement strategies, the most effective strategies begin with source control, containment and minimization. Ventilation is not a substitute for poor practices in handling hazardous materials, but the following control refinements can help maximize the effectiveness of ventilation while keeping energy costs in check:
- Occupancy control.
- Demand control.
- Hazardous banding control.
- Task ventilation control (localized exhaust ventilation, or LEV).
Occupancy control: occupied vs. unoccupied ventilation rates
The differences in ventilation requirements between occupied and unoccupied modes should be considered. The ASHRAE Laboratory Design Guide suggests that setback control strategies can be used in laboratories to reduce air changes hourly during unoccupied periods (e.g., at night and on weekends). The NFPA 45 Standard recommends a minimum ventilation rate of 4 ACH (air changes per hr) for unoccupied laboratories; some labs are designing for even lower rates.
Demand control: emergency override ventilation
Emergency override is a design refinement of the laboratory’s supply and exhaust system to provide increased airflow and negative pressurization in an emergency. Such a design can reduce both energy use and first cost, unlike designs for continuous operation under rare worst-case conditions. Emergency push-button overrides can be located near fume hoods and at the lab’s entrance(s), and should come equipped with indicator lights and audible alarms. The facility’s automated control system can:
- Increase airflow through the lab during an emergency.
- Notify the facility’s Environment, Healt and Safety (EH&S) staff.
- Discourage other workers from entering the laboratory.
Demand-controlled ventilation (DCV) is an emerging technology that utilizes pollutant sensors in order to provide real-time variableair-volume ventilation control. DCV differs fundamentally from typical practice, which “blindly” sets a fixed ventilation rate based on anticipated (but not verified) pollutant levels. Some key challenges in DCV design are correct sensor choice and placement.
A notable benefit of DCV—in addition to energy savings—is the introduction of monitoring equipment that can detect hazards and provide alarms and reporting. In addition to monitoring for spills and other accidents, DCV can also help identify malfunctioning fume hoods or poor lab practice (e.g., chemicals left out of fume hoods) that could otherwise go undetected.
Hazardous control banding: classifying chemicals and hazards
Control banding is a strategy for classifying and handling chemicals and hazards according to their associated health risks. A control band score is calculated by weighing a chemical’s level of toxicity, scale of use, and ability to become airborne under certain conditions. The control band score directs the user to appropriate control strategies.
Control banding can be applied to laboratory chemical operations. For a specific process and associated chemicals, the control band can specify which activities are permissible at a room air change rate, which activities require local ventilation, and which activities must be conducted in a fume hood at a particular flow rate. (Chemicals with the highest risk are handled at hood flows set for optimum containment, or performed in a glove box.) A laboratory might optimize airflows for work up to a prescribed control band, or designate specific hoods, based on airflow and contaminant containment, for work within a certain control band.
This new approach to classifying chemical hazards is being increasingly applied worldwide. For example, the United Kingdom has incorporated control banding into its recommended tools for compliance with regulations by the Control of Substances Hazardous to Health.
During the design of a new lab building and retrofit of an existing one, the Univ. of Rochester recently used control banding to identify a hazard level for each of its labs. After performing a detailed review and analysis of hazards being used in the university’s labs, the Health and Safety Officer used control banding to create a new air change rate standard. Based on this approach, an “A” lab has 8 ACH when it is occupied, and 6 ACH when unoccupied (8/6 ACH); a “B” lab has 6/4 ACH; and a “C” lab has 4/2 ACH. Control banding can also be done on a basis of CFM/ft2.
In the example of the Univ. of Rochester, the use of control banding is a step in the right direction, but it still reinforces the conventional wisdom that “more is better.” As described later, better ventilation design and evaluation strategies will provide greater protection from airborne chemical hazards than simply increasing air change.
Task ventilation control
Special-purpose laboratories provide an opportunity for designers to apply localized ventilation devices suited for a lab’s particular use. Examples include animal labs using cage ventilation as a task-specific ventilation or local exhaust ventilation (LEV) strategy, electronic clean rooms using mini-environments, or biomedical labs using biological safety cabinets (BSCs).
In the case of animal labs, studies such as those by Memarzadeh (1999) have shown that increasing a room’s ventilation rate does not have a significant effect on cage ventilation. In addition, Riskowski et al (1996) identified cage type as an important factor in determining the ventilation rate in an animal facility, and Zhang et al (1992) found that providing a quality environment for animal studies “was more dependent on cage design, room ventilation system design, and animal management practices than on room air exchanges.” (For a complete list of references, see the February 2010 digital edition at http://www.rdmag.com/General/Laboratory-Design-News-Archive/ )
Good practice therefore involves tailoring ventilation to a specific “task,” and to a location within a laboratory equipped with LEV. When this is done, general ventilation rates may be relaxed without compromising safety or comfort at the location of the task. Note that LEV systems can increase energy use if improperly designed, installed or operated due to high ventilation system pressure drop requirements, leaking devices and “open” unused LEV devices.
Better practice strategies: simulation methods
In an effort to optimize ventilation system layouts and laboratory designs, better-practice strategies apply real or virtual laboratory models that permit airflow pattern simulations. These performance-based approaches evaluate a simulated environment’s hazards, e.g., they determine a chemical’s clearing time by calculating the lab space’s “mixing factors” for a given spill scenario rather than simply applying a universal, prescriptive air change rate. This is an iterative process that accounts for facility design features that influence one another. The following simulation methods may be applicable:
- CFD simulations.
- Tracer gas simulations.
- Neutrally buoyant bubble simulations.
Computational fluid dynamics (CFD) simulations
For this better-practice approach, a geometric representation of the lab space is “built” within a computer. Then, a simulation of the airflow patterns inside the lab is modeled using a computational fluid dynamics (CFD) computer program. Results from the model help designers determine a lab’s airflow characteristics by:
- Developing “answers” to spill scenarios.
- Estimating residence time of a hazard.
- Evaluating the placement of major design elements, such as hoods, benches and registers.
- Eliminating stagnant dead zones in which air recirculates or in which exist “lazy” airflow patterns exist.
- Examining numerous “what if?” scenarios.
This virtual model can also be the basis of a full-scale construction of a laboratory space.
CFD simulation methods can help determine the lab’s airflow characteristics, spill clearing performance, and mixing factors including removal of fugitive emissions (e.g., small continual releases from an evaporating solvent in an uncovered beaker outside a fume hood). Importantly, CFD models can predict plume patterns of spill scenarios, and the required “clearing time” following a spill before it happens.
CFD modeling methods are useful for evaluating the dynamic effects of HVAC system features, layout and operation. Room geometry, HVAC system equipment, diffuser placement and laboratory equipment as well as operational procedures all influence air movement in the laboratory, particularly around the fume hood sash opening. A CFD model simulates the interaction of all of these variables—as well as the turbulence caused by a worker’s movements—to provide data that can be used to understand a laboratory’s temperature, air movement, relative pressure, regions of turbulence and contaminant concentrations.
In addition, this modeled information can be further analyzed to study fume hood containment capabilities, challenges to the hood’s containment (e.g., supply temperature variations), residence time of air moving through the modeled lab, placement of ventilation inlets and outlets, and other factors.
Although costly, building a full-scale model of a laboratory module can be justified when the module will be replicated many times in one facility or in multiple facilities. Performance design methods including pre-construction CFD modeling and full-scale lab modeling, followed with in situ evaluations, can make laboratories safer and more energy efficient.
Tracer gas simulations
Once a scaled or full-size mockup is built, a lab’s ventilation system can be determined by using a tracer gas test, according to the ASHRAE Laboratory Design Guide. The tracer gas is evenly distributed throughout the laboratory, and the rate of decay in the tracer gas concentration is used to calculate air changes per hour (ACH). To implement this strategy, sensors are installed in the room, a tracer gas is introduced, and ventilation rates are increased until the desired rate of decay is obtained. (EH&S specialists typically determine the appropriate rate of decay.)
Neutrally buoyant helium bubble simulations
Using neutrally buoyant helium bubbles to study airflow patterns in a laboratory space (Fig. 1, above) is a relatively new method. Tiny helium-filled bubbles about 1/8-in. (2 mm) in diameter are generated at the rate of approximately 400 bubbles/min. These bubbles quickly reach room temperature and follow the slightest air current in the room. They persist for up to two minutes, providing designers an opportunity to study a lab’s ventilation system. Helium bubbles are also useful for evaluating the efficacy and placement of supply diffusers and return air grilles; their positions can be varied during the test in order to mitigate areas of stagnant air.
Control banding for optimizing laboratory ventilation rates
As discussed, control banding is a means of classifying and grouping substances used in a process or activity by health risk for the purpose of determining an appropriate control strategy. Risk is most often described as a function of the likelihood and consequences of an event.
For control banding, chemical classification has a similar risk basis. Toxicity (with consideration of the potential for skin absorption) is a measure of the consequence of exposure. The scale of use (quantity) and the ability to become airborne (volatility for liquids, or dispersibility for solids) are measures of the likelihood of exposure. Combinations of the different levels of toxicity, scale of use, and ability to become airborne under the conditions of use yield a score that equates to a control band. The control band, combined with the tasks involved, directs the user to the appropriate control strategies. Strategies are based on four key approaches:
- Employ good industrial hygiene practice.
- Use local exhaust ventilation.
- Enclose the process.
- Contact a professional industrial hygienist.
The control-banding concept can easily be applied to laboratory chemical operations, where the chemical use quantities tend to be small, and chemical toxicity and ability to become airborne vary widely with the chemicals of interest. For a specific process and associated chemicals, the control band might specify activities permitted with various room air change rates, activities that require local ventilation, and activities that must be conducted in a fume hood at various flow rates, with the highest risk at hood flows set for optimum containment of airborne contaminants.
A laboratory might have airflows optimized to do work only up to a certain control band, or specific hoods might be designated for work within a certain control band, based on airflow and contaminant containment.
Control banding for optimizing laboratory ventilation rates sectipn written by: John Piatt, PE, Pacific Northwest National Laboratory, Richland, Wash.
The upcoming April issue will discuss commissioning and provide some relevant case studies.
The author of this guide was Geoffrey C. Bell, PE, M. Arch., from the Lawrence Berkeley National Laboratory, Berkeley, Calif. (GCBell@lbl.gov). Contributors and reviewers were John Piatt, PE, Pacific Northwest Regional Laboratory, Richland, Wash.; John Peterson PE, Univ. of Texas System; Otto Van Geet, PE, National Renewable Energy Laboratory, Golden, Colo.; Paul Mathew, Lawrence Berkeley National Laboratory, Berkeley, Calif.; and Lou DiBerardinis, Massachusetts Institute of Technology, Cambridge.
For general information on the Labs21 program, contact Dan Amon (amon.dan@epa.gov) or William Lintner (william.lintner@ee.doe.gov).
For the full references list associated with this article, see the February 2010 digital edition at www.rdmag.com/General/Laboratory-Design-News-Archive/
