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Mixed-flow impeller systems exhaust laboratory workstation fume hoods, prevent reentrainment into the facility and adjacent facilities, and help companies comply with appropriate pollution-control standards.
Concerns about emerging and reemerging infectious diseases, the national and international transfer of infectious microorganisms, and bioterrorism and biological warfare have led to an increase in the design and construction of biosafety level (BSL) laboratories at pharmaceutical and biotechnology organizations in recent years. These laboratories handle a wide range of biological agents, and their exhaust, especially at BSL-3 and BSL-4 facilities, may contain highly contagious disease-causing microorganisms (see sidebar, "Biosafety level basics"). To protect workers and the public, these biological contaminants must be handled according to strict procedures, and their escape into the workspace and the community must be prevented.
Biosafety level basics
Thus, a ventilation system for a new research facility (or a retrofit for an existing one) must be designed to effectively capture hazardous materials through the workstations' fume hoods, prevent exhaust reentrainment, and eliminate odors in the neighborhood. It is also important to keep maintenance and operation costs as low as practical. At some facilities, aesthetics such as the height of rooftop appurtenances and noise, are design considerations.
Mixed-flow impeller fan technology addresses these issues, and has been used successfully at several BSL facilities. This article explains what mixed-flow impeller technology is and how it works and discusses the benefits this type of ventilation system offers BSL laboratories.
Mixed-flow principle of operation
The sidebar "Characteristics of mixed-flow impeller technology systems," shows the operation of a mixed-flow impeller fan. 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. For example, a mixed-flow fan moving 80,000 ft3 /min of combined building and bypass air at an exit velocity of 6300 ft/min can send an exhaust air jet plume up to 120-ft high in a 10-mph crosswind. This extremely high velocity exceeds ANSI Z9.5 standards by more than twice the minimum recommendation of 3000 ft/min. Because as much as 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.
Characteristics of mixed-flow impeller technology systems (FIGURES ARE COURTESY OF THE AUTHOR)
A ventilation and exhaust system must remove contaminated air from the work area to ensure healthy indoor-air quality for workers (see Figure 1). This task must be done in a way that also prevents exhaust discharges from reentering the building through intake vents, windows, doors, and other openings. This is known as reentrainment, and it can be caused by inefficient roof exhaust fans, poor exhaust stack design and location, the position of building air intakes, weather and wind conditions, and various other factors.
Figure 1 (FIGURES ARE COURTESY OF THE AUTHOR)
The first line of defense against the release of microorganisms is the biological safety cabinets (BSC) in which research is conducted. These units are negative-pressure, ventilated cabinets operated with a minimum inward air velocity of 75 ft/min. Most are enclosed, although work with lower-risk (BSL-1 and BSL-2) agents may be conducted in an open-front BSC.
The air from within the BSC is exhausted through a high-efficiency particulate air (HEPA) filter, either into the laboratory or more commonly to the outside. Mixed-flow impeller fans can discharge the exhaust air well above the building rooflines to eliminate the possibility of reentrainment, regardless of wind and atmospheric conditions.
Furthermore, many BSL facilities use animals for research, which often creates odor problems. Although such odors are not a direct health hazard, they may be a significant nuisance. Dilution of the odiferous exhaust with ambient air and discharge of the resulting stream high into the atmosphere by a mixed-flow fan can be an efficient odor-management strategy. Iowa State University's College of Veterinary Medicine is very active with infectious disease research work. When it upgraded a BSL-2 laboratory to meet BSL-3 requirements, facility management was concerned that the existing centrifugal fans (one dedicated exhaust fan system and roof-mounted stack for each workstation) would not adequately eliminate the possibility of exhaust reentrainment and prevent toxic, noxious, or odiferous workstation exhaust from escaping into the neighborhood.
To address these concerns, Iowa State installed a mixed-flow impeller exhaust system (see Figure 2). George Mellen, the engineer in charge of mechanical and electrical design, cited two major factors in this decision: "First, we wanted to get the room and biological safety cabinet exhaust plumes up high enough so that there was no chance of it being ingested with other makeup air systems (reentrainment prevention), and, second, mixed-flow impeller technology allows us the opportunity in the future to install heat-recovery systems which should decrease heating costs."
Figure 2 (FIGURES ARE COURTESY OF THE AUTHOR)
Saving energy through heat recovery
Most BSL research laboratories require 100% conditioned makeup air because workstation exhaust cannot be reintroduced into the building. The makeup air must be filtered, heated or cooled, humidified or dehumidified, and so forth, depending on the circumstances. This process significantly increases energy costs—to as high as $6/ft3 solely for conditioning the makeup air.
Energy consumption in mixed-flow systems is about 25% lower than with conventional centrifugal fans. A typical energy reduction is $0.44/cfm at $0.10/kWh. This translates to a return on investment of approximately two years.
A mixed-flow impeller exhaust system with integral heat recovery can cut energy costs by reducing the consumption of natural gas, oil, or electricity. These devices, as shown in Figure 3, consist of a heat exchanger containing coils filled with a glycolsolution that extracts ambient energy from the workstation fume hood exhaust before it is discharged. The glycol–water solution is transferred to the supply-air handler to preheat the conditioned air entering the building during winter or pre-cool the air during summer.
Figure 3 (FIGURES ARE COURTESY OF THE AUTHOR)
A typical system will reduce heating costs about 3% for each 1 °F of makeup-air preheat that is added. A 10 °F increase in intake-air temperature translates directly into a 30% energy saving, which could amount to considerable savings. Similar savings would be realized for cooling.
When the University of Richmond upgraded its biology, chemistry, and physics laboratories in its science center, it installed two separate and independent mixed-flow impeller systems. The renovation expanded the total laboratory space being ventilated by more than 20,000 ft2, yet the school did not need to increase its energy budget as a result of the added system.
Simpler, cheaper maintenance
Mixed-flow systems are designed to operate continuously with a minimum amount of maintenance under normal operating conditions. Direct-drive motor bearings have lifetimes of minimum L10 100,000 hours. (This refers to a sample of 100 motors in which the bearings in 10 motors [10%] would fail within a 100,000-hour timeframe. It is a baseline for comparison of motor bearing lifetimes.)
Because there are no belts, elbows, flex connectors, or spring-vibration isolators to maintain, there is no need for expensive rooftop penthouses to protect maintenance personnel working under adverse weather conditions. In addition, issues of worker safety problems are also eliminated—a key consideration. Typically, considerable savings may be realized in these kinds of applications.
Less-tangible aesthetic benefits
For efficient operation, centrifugal fans require tall exhaust stacks, which are generally expensive, complex, and heavy because of the associated mounting hardware, roof curbs, guy wires, and so forth. Their belt-driven motors tend to be maintenance-intensive, which is why they are often located in rooftop penthouses to protect maintenance personnel in inclement weather. Tall stacks on a building's roof are unsightly and are often perceived as "pollution generators."
Mixed-flow impeller systems, in contrast, have a low-profile design that is typically only about 15-ft high. This eliminates the smoke-stack look and the negative connotations associated with it. As a result, mixed-flow fans can be used where aesthetic considerations preclude the installation of tall stacks such as in jurisdictions that restrict building height.
A few years ago, Pfizer's La Jolla, California, facility eliminated rooftop stacks altogether. The use of conventional centrifugal fans for workstation exhaust was ruled out because of interior space limitations as well as a 30-ft total building-height limit in the community that prohibited tall rooftop stacks. To achieve the required ventilation, engineers recommended mixed-flow impeller systems, with the fans located inside the building, in specially designed fan rooms that require less than half the space of centrifugal fans. The exhaust stacks for the indoor fans extend only about 10 in. above the roofline, and a decorative parapet around the roof perimeter obscures them from view at the property line (Figure 4).
Figure 4 (FIGURES ARE COURTESY OF THE AUTHOR)
Moving the fans inside helped Pfizer solve not only the stack-height problem but also the noise problem. The facility was subject to a 50-dB sound limitation at the property line. This was addressed in two ways: each fan incorporates an acoustical nozzle silencer that substantially lowers attenuation (in the region of 15 net dBA), and the fan room sound was isolated with insulation and foam on the walls.
Most applications do not require such extensive noise-control measures. Because mixed-flow fans operate more efficiently than standard centrifugal fans, they are inherently quieter. Noise is also a function of blade-tip speeds, so mixed-flow fans, which rotate at significantly lower speeds for the same amount of work, generate less noise.
But for particularly sensitive areas, accessories that divert or absorb noise (e.g., chevron screen walls, acoustical screens and louvers, and in-line or nozzle silencers) can be integrated into the design of the mixed-flow system in a way that does not increase overall height. In addition, optimizing blade design and blade tip speed can also reduce noise levels.
Mixed-flow impeller exhaust systems remove contaminated air from the workplace and prevent reentrainment of the exhaust into the building or other structures, disperse odors so they will not be offensive to neighbors, are energy-efficient, require minimal maintenance, and operate quietly with acceptable aesthetics. When looking to upgrade, retrofit or construct new BSL laboratory facilities, they represent a practical and cost-effective way to protect employees and the public from exposure to dangerous biological agents.
Charlie Gans is assistant general manager at Strobic Air Corp., a subsidiary of Met-Pro Corp., PO Box 144, Harleysville, PA 19438, tel. 215.723.4700, firstname.lastname@example.org
Submitted: May 14, 2007. Accepted: Aug. 1, 2007.
1. "BMBL Section III - Laboratory Biosafety Level Criteria," in Biosafety in Microbiological and Biomedical Laboratories, 4th ed. (US Department of Health and Human Services, Centers for Disease Control and Prevention, and National Institutes of Health, US Government Printing Office, Washington, DC, April 1999), complete document available for download at www.cdc.gov/OD/ohs/biosfty/bmbl4/bmbl4toc.htm.
2. "Appendix A:Primary Containment: Biological Safety Cabinets," in Biosafety in Microbiological and Biomedical Laboratories, 4th ed. (US Department of Health and Human Services, Centers for Disease Control and Prevention, and National Institutes of Health, US Government Printing Office, Washington, DC, April 1999), complete document available for download at www.cdc.gov/OD/ohs/biosfty/bmbl4/bmbl4toc.htm.