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.