Basic Documentation

Page 2 of 8 Siemens Industry, Inc.
Document No. 149-978
Thus, it is important not to equate higher turndown
ratios
with superior control performance. Rather,
control devices should be compared on the basis of
how closely their range or turndown ratio matches the
application. In fact, the range or turndown ratio of a
controller or control system should ideally be only
slightly more than what the control application calls
for. Turndown ratios greater than the actual need are
generally undesirable.
VAV Fume Hood Airflow
Conventional fume hoods (in difference to low flow
fume hoods) typically need a face velocity of
approximately 100 feet per minute
1
to provide fume
containment.
2
The maximum airflow through a fume
hood will be the exhaust airflow rate necessary to
provide the required face velocity at the maximum
sash opening.
VAV Fume Hood Maximum Exhaust
Airflow
In contrast to horizontal sliding sash fume hoods,
fume hoods with vertical rising sashes provide a
totally unobstructed face opening. As a result, vertical
sash fume hoods require the highest exhaust airflow
rate for a given fume hood width. Vertical rising
sashes of bench type fume hoods generally have a
maximum full open height of about 30 inches or 2.5
feet. When the sash is fully open, this results in 2.5
square feet of open sash area for each foot of sash
width. The exhaust airflow needed to maintain an
average face velocity of 100 fpm for each foot of sash
width is:
2.5 ft
(Height)
× 1 ft
(Width)
× 100 ft/min = 250 cfm
Thus, a fume hood with a fully open vertical sash will
have a maximum exhaust airflow of approximately
250 cfm for every foot of sash width.
1. American National Standard for Laboratory Ventilation
ANSI/AIHA Z9.5, 3.3.1: "Design face velocities for laboratory
chemical hoods in the range of 80
 100 FPM (0.41  0.51m/s)
will provide adequate face velocity for a majority of chemical
hoods." "This is the range recommended for a majority of
laboratory chemical hoods."
2. Safety professionals also point out that the face velocity
required is also dependent upon other factors that affect fume
hood containment such as room air currents, the fume hood's
airflow characteristics, the chemicals used and the type of
experiments being conducted.
VAV Fume Hood Minimum Exhaust
Airflow
Even when the sash is fully closed, minimum fume
hood exhaust airflow is needed to prevent a buildup
of flammable or explosive fumes in the hood. The
current versions of Standards NFPA 45 and
ANSI/AIHA Z9.5 recommend that a minimum fume
hood airflow of at least 25 cfm per square foot of
work surface always be maintained whenever
chemicals are present in the hood.
The typical bench type fume hood has a working
depth of approximately 2 feet. Thus, the airflow
necessary to maintain 25 cfm per square foot of
internal work surface for each foot of internal width
becomes:
2 ft
(Depth)
× 1 ft
(Width)
× 25 cfm/ft
2
= 50 cfm
Thus, a fume hood with a fully closed sash requires a
minimum exhaust airflow of approximately 50 cfm for
every foot of internal width.
3
VAV Fume Hood Exhaust Airflow
Turndown
As indicated, the airflow through a vertical sash type
fume hood will range from a maximum of about 250
cfm to a minimum of 50 cfm for each foot of sash
width. The turndown ratio for maximum fume hood
exhaust to minimum exhaust is about 5-to-1.
Therefore, a VAV fume hood's exhaust air terminal
and associated controls must provide a maximum
turndown ratio of about 5-to-1. If the fume hood
sash's full open height is less than 30 inches, the
maximum exhaust required will be less and so will
the required fume hood exhaust turndown ratio. If the
internal work surface area is more than 2 square feet
per ft. of sash width, the minimum exhaust will
increase and thus decrease the required fume hood
exhaust turndown ratio.
3. The internal work surface is a few inches wider than the sash
opening width. This extra area (although small) should also
be considered when determining a fume hood's minimum
airflow. In addition, some fume hoods may have a working
area deeper than 24 inches and if so, this should be
considered when determining the minimum airflow.