Mixed-Flow Fans Meet Biosafety Level Laboratory Requirements

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.
Nov 02, 2007

Biosafety level basics
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.

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

Characteristics of mixed-flow impeller technology systems (FIGURES ARE COURTESY OF THE AUTHOR)
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.

Preventing reentrainment

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.

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.