Basic Documentation

Fume Hoods
Siemens Industry, Inc. 27
Topic Requirement(s) Commentary
VAV Diversity
American National Standard for Laboratory Ventilation ANSI/AIHA Z9.5-2003
5.1.2 The following issues shall be evaluated in order to design for diversity:
• Use patterns of hoods
• Type, size and operating times of the facility
• Quantity of hoods and researchers
• Sash management (sash habits of users)
• Requirements to maintain a minimum exhaust volume for each hood on the
system.
• Type of ventilation system
• Type of laboratory chemical hood controls
• Minimum and maximum ventilation rates for each laboratory
• Capacity of any existing equipment
• Expansion considerations
• Thermal loads
• Maintenance department’s ability to perform periodic maintenance
The following conditions shall be met in order to design a system diversity:
• Acceptance of hood-use restrictions by the user groups. Designers must take into
account the common work practices of the site users.
• A training plan must be in place for all laboratory users to make them aware of any
limitations imposed on their freedom to use the hoods at any time
• An airflow alarm system must be installed to warn users when the system is
operating beyond capabilities allowed by diversity.
• Restrictions on future expansions of flexibility must be identified.
6.5.3.4 System diversity shall be verified prior to use of laboratory chemical hoods.
The tests shall be designed to verify that users will be alerted when system capacity is
exceeded and unsafe conditions may exist.
If a VAV ventilation system is designed to
satisfy the likely maximum ventilation needs
rather than the theoretical absolute maximum,
the size of the HVAC system (that is, boilers,
chillers, fans, ducts, etc.) can be reduced to
yield a lower installed cost and less energy
consumption. This ‘downsizing’ of the
mechanical systems is based on the fact that
actual ventilation needs of the facility will be
less than the theoretical absolute maximum
(100%) during the vast majority of the time.
The ‘theoretical absolute maximum’ of 100%
would only occur only if every fume hood
sash was fully open on the hottest or coldest
day and the HVAC systems had to meet this
high demand condition. This might never
occur or only very rarely.
A laboratory VAV system could thus be fully
satisfactory even though its capacity is less
than the 100% value. Using an HVAC design
capacity less than the 100% but still likely to
meet the actual requirement is termed
applying a ‘diversity factor’. Experienced
design professionals generally feel that a
diversity factor of 60% to 70% is acceptable
for this purpose.
Note however that it cannot be guaranteed
that a VAV system design based on diversity
will not at some time be unable to meet actual
demand. Therefore an acknowledgement and
agreement of this possibility by the laboratory
staff is necessary before a designer
incorporates a diversity factor in the VAV
system design.