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.
Figure 4 (FIGURES ARE COURTESY OF THE AUTHOR)
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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).
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.
Summary
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, cgans@strobicair.com
Submitted: May 14, 2007. Accepted: Aug. 1, 2007.
References
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
http://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
http://www.cdc.gov/OD/ohs/biosfty/bmbl4/bmbl4toc.htm.
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