Biosafety level (BSL) laboratories, particularly Levels 3 and 4, present special problems for managing workstation fumehood exhaust. Among them are exhaust reentrainment (into the facility or an adjacent building) and conformance to appropriate pollution abatement standards. All BSL laboratories are governed by rigid codes and standards formulated by organizations including the American National Standards Institute (ANSI); American Society of Heating, Refrigeration and Air Conditioning Engineers (ASHRAE); Centers for Disease Control (CDC), National Institutes of Health (NIH); and U.S. Department of Health and Human Services (DHHS).
Laboratory workstation fumehood exhaust must be managed safely, not only for workers’ (and neighbors’) health, but also from the perspective of possible litigation against virtually everyone associated with the facility. Fumehood exhaust reentrainment, for example, can be caused by factors such as inefficient roof fans, poor exhaust stack design and/or location, position of building air intakes, and weather/wind conditions.
BSL-3 and -4 laboratories require a dedicated air supply and exhaust system, which is critical to safety. The heating, ventilation, and air conditioning (HVAC) system would typically be independent of all other supply and exhaust systems within the building; the air inside of research areas must be fully conditioned and never reused. In addition to this conditioned “makeup” air, these laboratory facilities require safe and reliable methods of exhausting their workstations’ fumehoods to eliminate reentrainment and containment possibilities.
Hazards of BSL laboratory workstation exhaust
Roof exhaust reentrainment at BSL laboratories may be insidious, but it is often dangerous. This is especially true when highly contagious microorganisms may be present. The following discussion will focus on these facilities because of the more virulent substances typically associated with their functions.
Tighter standards and tolerances
Exhaust discharges from BSL laboratories may be highly toxic (or noxious) or both. Their danger to people covers a broad spectrum, which may be mildly annoying to seriously unhealthy. Also, government agencies are continually setting more stringent standards, with allowable exposure limits dropping lower and lower. Obviously there is no room for tolerance with regard to possible contamination from some agents that are exhausted at BSL Level 3 and 4 facilities. In many cases, even if the fumes are not toxic, public tolerance for odiferous discharges has decreased sharply in recent years.
Essentially, BSL-3 laboratory standards concern more serious (and potentially lethal) diseases that may be transmitted via inhalation. These laboratories would likely deal with bacterial agents, fungal agents, parasitic agents, and viral agents.
Agents used at BSL Level 4 laboratories are considered “dangerous and exotic . . . and pose a high individual risk to aerosol transmitted laboratory infections which result in a life threatening disease, or related agents with unknown methods of transmission.” This description is defined in the infectious agents list of the University of California, San Diego (UCSD) Biosafety Handbook.
While there are many other facilities and equipment standards for BSL-4 laboratories (including “staff with a level of confidence greater than one would expect in a college department of microbiology, and who have had specific and thorough training in handling dangerous pathogens . . .”), one key issue concerns specially designed and engineered features to prevent microorganisms from being discharged into the environment. These design features could include specially shielded isolation rooms under negative pressure with sophisticated airflow, temperature, pressure, and humidity control and monitoring systems; they would also require 100% conditioned makeup air to prevent reuse of the ambient air within an enclosed facility.
Achieving desired airflow and pressure differentials is essential when designing and building BSL laboratory containment facilities. Exhaust systems at Level 4 laboratories typically incorporate high-efficiency particulate air (HEPA) filters, usually mounted in series and placed as close as possible to the laboratory to minimize ductwork runs and the possibility of contamination reaching the roof-mounted exhaust fans. This is obviously a critical area when, for example, an animal colony might be infected with a virus because of its propensity to migrate and infect other animal colonies.
Figure 1 - Mixed-flow impeller system on the roof of Georgia State University Laboratories (Atlanta, GA).
Mixed-flow impeller technology at BSL laboratories
How can exhaust from BSL-3 or -4 laboratories best be managed with regard to worker and neighborhood safety, system performance, environmental compliance, and cost efficiencies? One approach that has become increasingly popular over the past few decades is known as mixed-flow impeller technology (in the form of low-profile roof exhaust systems). Mixed-flow impeller systems (see Figures 1–4) have made significant inroads at laboratories of all kinds throughout the world—for example, at colleges and universities; pharmaceutical research and manufacturing facilities; hospitals and vivariums; and many other nonmedical-related installations.
Figure 2 - Diagram of a typical mixed-flow impeller system (see sidebar).
Figure 3 - Three mixed-flow impeller systems with HEPA filters for BSL-3 and -4 laboratories.
Figure 4 - Mixed-flow impeller systems on the roof of the Toronto Medical Discovery Tower (Toronto, Ontario, Canada).
At a Level 3 or 4 biosafety laboratory—with possible serious implications associated with reentrainment—and the high costs of energy required to condition makeup air, the use of mixed-flow impeller technology for laboratory workstation fumehood exhaust has proved to be a practical solution. This technology offers substantial advantages over conventional centrifugal roof exhaust fans with dedicated, tall exhaust stacks on the roof.
Until a few decades ago, most laboratory fumehood exhaust applications were managed by these rooftop centrifugal fans with their tall stacks. While this approach was adequate (mainly because there were no practical alternatives), it also left much to be desired and thus presented a number of compromises. Among them are the need for expensive, complex mounting hardware (for the exhaust fans as well as the stacks), maintenance-intensive belt drives, inefficient performance (with regard to pollution abatement and reentrainment issues), high energy consumption, and undesirable noise levels at the property line. These shortcomings can be added to a relatively new concern in many locations, that is, the sight of tall exhaust stacks on a building’s roof, which usually imparts negative connotations in a community—in other words, another neighborhood polluter.
Advantages of mixed-flow impeller technology
This has changed radically, and rapidly, with continuing advances in mixed-flow impeller system design. Laboratory workstation fumehood exhaust systems incorporating mixed-flow fans virtually eliminate all of these compromises. Their low-profile design (typically about 15 ft high vs 25+ ft for a conventional exhaust stack in many installations) eliminates the need for structural reinforcements on the roof. Because they are substantially shorter (and constructed modularly) than the tall, unsightly stacks they replace, their simplicity also helps reduce installation time and costs significantly. In fact, in many retrofit applications, there is virtually no downtime associated with their installation.
Mixed-flow impeller systems are also generally maintenance free (there are no belts, elbows, flex connectors, or spring vibration isolators to maintain), and expensive penthouses are not needed to accommodate maintenance personnel under adverse conditions. Modular construction reduces installation time substantially—in fact, in many retrofit applications, workflow at the laboratory workstation is not even interrupted—and the fans can be installed in as few as four hours, without cranes, helicopters, or other heavy construction equipment. As a result, additional savings of several hundreds of thousands of dollars over centrifugal-type fans may be achieved.
Mixed-flow impeller fans typically consume about 25% less energy than conventional centrifugal fans and offer faster payback periods as well. Typical energy reduction is $0.44 per cubic foot per minute (CFM) at $0.10/kilowatt-hour, providing an approximate two-year return-on-investment in many installations. These numbers do not include the substantial energy savings they can provide for conditioned makeup air facilities required at BSL-3 and -4 facilities.
Stack height aesthetics
Another consideration when retrofitting or designing new rooftop exhaust systems for laboratory workstation fumehoods includes stack height aesthetics. Obviously the lowest possible profile not only eliminates the smokestack look and negative perceptions by neighbors, but may actually be required (in some jurisdictions) to conform to applicable ordinances. Some communities restrict total building height and, by inference, the height of exposed stacks and other rooftop equipment. Elimination of tall, unsightly stacks (which are either prohibited by code or undesirable) is another worthwhile goal.
Mr. Tetley is Vice President/General Manager, Strobic Air Corp., A Subsidiary of Met-Pro Corp., 160 Cassell Rd., Harleysville, PA 19438, U.S.A.; tel.: 215-723-4700; fax: 215-723-7401; e-mail: email@example.com.
Characteristics of mixed-flow impeller technology systems
Mixed-flow impeller systems operate on the unique principle of diluting contaminated exhaust air with unconditioned, outside ambient air via a bypass mixing plenum. The resultant diluted process air is accelerated through an optimized discharge nozzle/windband where nearly twice as much additional fresh air is entrained into the exhaust plume before leaving the fan assembly. Additional fresh air is entrained into the exhaust plume after it leaves the fan assembly through a natural aspiration effect. The combination of added mass and high discharge velocity minimizes the risk of contaminated exhaust being reentrained into building fresh air intakes, doors, windows, or other openings.
As an example, a mixed-flow fan moving 80,000 CFM of combined building and bypass air at an exit velocity of 6300 ft/min can send an exhaust air jet plume up to 120 feet high in a 10-mph crosswind. This extremely high velocity exceeds ANSI Z9.5 Standards by more than twice the minimum recommendation of 3000 fpm. Because up to 170% of free outside air is induced into the exhaust airstream, a substantially greater airflow is possible for a given amount of exhaust, providing excellent dilution capabilities and greater effective stack heights over conventional centrifugal fans without additional horsepower.
Mixed-flow impeller systems also reduce noise, use less energy, and provide enhanced performance with faster payback over conventional laboratory fumehood exhaust systems. A typical reduction of $0.44 per CFM at $0.10/kW-hour provides an approximate two-year return on investment (ROI). Energy consumption for mixed-flow fans is about 25% lower than conventional centrifugal fans with substantially reduced noise levels, particularly in the lower-octave bands. They also conform to all applicable laboratory ventilation standards of ANSI/AIHA Z9.5 as well as ASHRAE 110 and NFPA 45, and are listed with Underwriters Laboratory under UL 705.
Mixed-flow systems are designed to operate continuously with a minimum amount of required maintenance, providing years of trouble-free performance under normal operating conditions. Direct-drive motor bearings have lifetimes of up to L10 400,000 hr. (This refers to a “sample” of 100 motors in which the bearings in ten motors [10%] would fail within a 400,000-hr time frame. It is a baseline for comparison of motor bearing lifetimes.) Nonstall characteristics of the system’s mixed-flow wheel make it ideally suited for constant volume or variable air volume (VAV) applications, along with built-in redundancy and design flexibility. VAV capabilities are achieved via the bypass mixing plenum or by using variable frequency drives to provide optimum energy savings.
Virtually maintenance-free operation eliminates the need for expensive penthouses to protect maintenance personnel under adverse conditions. Consequently, additional savings of several hundreds of thousands of dollars may be realized in a typical installation.
Mixed-flow impeller systems are available with a variety of accessories that add value, reduce noise, and/or lower energy costs substantially. For example, accessory heat exchanger glycol/water-filled coils for use in 100% conditioned makeup air (controlled environment) facilities add exhaust heat to intake ventilation air to save thousands (or hundreds of thousands) of dollars in annual energy costs.