(d) Type B3: 70 percent recirculation within the cabinet, 30 percent exhaust
through a HEPA filter, common plenum configuration, must be exhausted to the
outside.
(e) Provide a dedicated exhaust system for each Type B1, B2 & B3 safety
cabinets.
(f) The exhaust air quantities and pressure drops for class II safety
cabinets Type A & Type B3 are listed below in Table 2-15, and Table 2-16.
Table 2-15 is based on direct connection to exhaust ductwork, and table 2-16
is based on indirect connection to exhaust ductwork. The exhaust air for Type
A and B3 Cabinets may be connected to the general exhaust system with
indirect connection or a direct connection. The indirect connection involves
an air gap between the safety cabinet and the exhaust duct. See Standard
Detail 15840-20. The exhaust system must pull more air than the exhaust air
through the cabinet to make-up airflow in through the gap. A hard connection
can also be used to minimize air exhaust from the laboratory, provided the
exhaust fan operates continuously. Type B1 and B2 Cabinets require direct
connections and should run continuously.
Table 2-15 Direct Connection (Type A & B3 Biological Safety Cabinets)-
Exhaust Air Requirements
Nominal Size
Exhaust Air
Pressure Drop Pa
mm (Ft)
Cu M/Min CFM)
(Inches of Water)
1200 (4)
10 (330)
35 (0.14)
1800 (6)
15 (510)
35 (0.14)
Table 2-16 Indirect Connection (Type A & B3 Biological Safety Cabinet)-
Exhaust Air Requirements
Exhaust Air
Pressure Drop Pa
Nominal Size
Cu M/Min CFM)
(Inches of water)
mm (FT)
1200 (4)
14 (490)
25 (0.10)
1800 (6)
20(700)
25 (0.10
(g) The exhaust air quantities and pressure drops for type B1 and B2 and
Class II cabinets are listed below:
Table 2-17 H12 Cabinet (Types B1 and B2) Exhaust Air Requirements
Type
Size
Exhaust Air
Pressure Drop Pa
mm (FT)
Cu M/Min (CFM)
(Inches of Water)
B1
1200 (4)
8 (270)
249 (1.0)
B1
1800 (6)
12 (410)
249 (1.0)
B2
1200 (4)
20 (730)
523 (2.0)
B2
1800 (6)
32 (1150)
523 (2.0)
(h) Notes:
(1) Class II, type B1 and B2 safety cabinets come with two sets of HEPA
filters, one for supply within the cabinet, and the other for exhaust from
the cabinet. The pressure drops include friction loss through clean exhaust
HEPA filters (not the supply HEPA filter within the cabinet as internal
blower takes care of this filter) and transition fitting on exhaust side.
HVAC DESIGN MANUAL
2-34
HVAC SYSTEMS AND EQUIPMENT
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