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Mixed-Flow Fans Meet Biosafety Level Laboratory Requirements
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 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.
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 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.
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.
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.