Basic Documentation

Siemens Industry, Inc. Page 5 of 8
Document No. 149-978
Laboratory Room Size
Table 1 covers three different sizes of laboratory
rooms separated by the double line divisions of the
table. Each group has eight identically sized
laboratory rooms. Rooms 1 through 8 represent
single module laboratory rooms that are 25 feet
long, 10 feet wide, and 9 feet high. Rooms 9 through
16 represent double module laboratory rooms with
twice the area of the single module rooms. Rooms
17 through 24 have three times the area of a single
module room. The increased room sizes are needed
to accommodate the increased number of fume
hoods in these rooms. The volume of each room is
given in order to determine the room's ventilation
rate in air changes per hour.
Excess Room Exhaust Airflow For
Negative Pressure
Each of the three laboratory room sizes has excess
exhaust air to maintain a negative static pressure
relationship of approximately 0.010 inches w.c. with
respect to the adjacent areas. Somewhat more
excess room exhaust airflow is required for larger
rooms since room leakage typically increases as the
size increases. For additional information on room
leakage areas and the exhaust offset required, see
Siemens Technology Report No. 3, Volumetric
Airflow Tracking Vs. Differential Pressure Sensing
(149-977).
VAV Fume Hoods in Room
The size and quantity of fume hoods assumed to be
in each room are indicated. Room 1 has one 4-foot
wide fume hood while Room 24 has six 8-foot wide
fume hoods. The varying quantities and sizes of
fume hoods result in considerable variation in the
room airflow requirements.
Total Fume Hood Exhaust
Each fume hood is assumed to be a VAV fume hood
with one vertical rising sash. This results in the
highest fume hood exhaust requirement and the
greatest turndown requirement for each fume hood.
Min. CFM is based upon a fully closed sash and the
minimum exhaust airflow as explained in footnote
[6]. Max. CFM is based upon a full open sash height
of 30 inches and an exhaust airflow rate that
provides a 100-foot per minute average face
velocity.
Total Miscellaneous Room Exhaust
Chemical laboratories typically have some
miscellaneous exhaust provisions in addition to the
fume hoods. This includes canopy exhausts and
other types of dedicated exhausts. Flexible bench
snorkel exhausts are perhaps the most common
having an exhaust airflow of approximately 100 cfm
each. The example in Table 1 assumes that each
room h
as at least two snorkels with the quantity
being proportionately increased in the larger size
rooms.
Total Room Exhaust
Total room exhaust is the sum of all room exhausts.
In the Table 1 examples, the Min. CFM is the su
m of
the total fume hood minimum exhaust and the
snorkel exhausts. The Max. CFM is the sum of the
total fume hood maximum exhaust and the snorkel
exhausts. In some instances, a laboratory room may
require a room general exhaust if additional exhaust
is needed to maintain the room's minimum
ventilation rate or negative room pressurization or to
enable sufficient supply airflow to maintain the
room's ambient conditions. (More information is
provided in the Room General Exhaust section.)
Room Minimum Air Changes Per Hour
The requirements most referenced for laboratory
room air changes per hour (ACH) are:
 National Institutes of Health:.....6 to 10 ACH
 NFPA 45....Unoccupied labs:....4 ACH
NFPA 45....Occupied labs: .......8 ACH
 U.S. OSHA:...............................4 to 12 ACH
 ANSI/AIHA Z9.5:..(No recommendation given)
Experienced ventilation designers generally prefer a
minimum of 10 to 12 ACH when a laboratory room is
occupied and a minimum of 4 ACH when the room is
unoccupied. Note that the combination of the
minimum airflow requirements for fume hoods
(Footnote 6) and the miscellaneous room exhaust
provisions very often prevent reducing the
unoccupied ventilation rate down to 4 ACH. In all of
the examples in Table 1 the minimum room
ventilation rat
e is considerably higher than 4 ACH.
Note that for Rooms 8, 16, and 24 the minimum
ventilation rate is 24 ACH.