Basic Documentation

Laboratory Ventilation Codes and Standards
Siemens Industry, Inc. 78
Topic Requirement(s) Commentary
Biological
Safety
Cabinets and
Classifications
(Continued)
Room air is drawn through the face opening of the cabinet at a minimum measured
inflow velocity of 100 fpm. As with the Type A1 and A2 cabinets, there is a split in
the down-flowing air stream just above the work surface. In the Type B1 cabinet,
approximately 70 percent of the down flow air exits through the rear grille, passes
through the exhaust HEPA filter, and is discharged from the building. The
remaining 30 percent of the down flow air is drawn through the front grille.
(Continued on Next Page.)
Since the air that flows to the rear grille is discharged into the exhaust system,
activities that may generate hazardous chemical vapors or particulates should be
conducted toward the rear of the cabinetwork area.
Type B1 cabinets must be hard-ducted, preferably to a dedicated, independent
exhaust system. As indicated earlier, fans for laboratory exhaust systems should
be located at the terminal end of the ductwork to avoid pressuring the exhaust
ducts. A failure in the building exhaust system may not be apparent to the user, as
the supply blowers in the cabinet will continue to operate. A pressure-independent
monitor and alarm should be installed to provide warning and shut off the BSC
supply fan, should failure in exhaust airflow occur. Since this feature is not supplied
by all cabinet manufacturers, it is prudent to install a sensor such as a flow monitor
and alarm in the exhaust system as necessary. To maintain critical operations,
laboratories using Type B1 BSCs should connect the exhaust blower to the
emergency power supply.
The Class II, Type B2 BSC: This BSC is a total-exhaust cabinet; no air is
recirculated within it. This cabinet provides simultaneous primary biological and
chemical (small quantity) containment. Consideration must be given to the
chemicals used in BSCs as some chemicals can destroy the filter medium,
housings and/or gaskets causing loss of containment. The supply blower draws
either room or outside air in at the top of the cabinet, passes it through a HEPA
filter and down into the work area of the cabinet. The building exhaust system
draws air through both the rear and front grills, capturing the supply air plus the
additional amount of room air needed to produce a minimum calculated or
measured inflow face velocity of 100 fpm. All air entering this cabinet is exhausted,
and passes through a HEPA filter prior to discharge to the outside. This cabinet
exhausts as much as 1200 cubic feet per minute of conditioned room air making
this cabinet expensive to operate. The higher static air pressure required to
operate this cabinet also results in additional costs associated with heavier gauge
ductwork and higher capacity exhaust fan. The need for the Class II, Type B2
should be justified by the research to be conducted.
Should the building exhaust system fail, the cabinet will be pressurized, resulting in
a flow of air from the work area back into the laboratory. Cabinets built since the
early 1980’s usually have an interlock system, installed by the manufacturer, to
prevent the supply blower from operating whenever the exhaust flow is insufficient;
systems can be retrofitted if necessary. Exhaust air movement should be
monitored by a pressure-independent device, such as a flow monitor.