Basic Documentation

Laboratory Ventilation Codes and Standards
Siemens Industry, Inc. 44
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An Approach to Determining the Required Response Time for a VAV Fume Hood Control System, ASHRAE Transactions 1990 states “The result of this research indicates that
tracer gas did not escape the fume hood when control action was taken within 2 to 3 seconds…”
Topic Requirement(s) Commentary
Selection
Criteria and
Performance
Specifications
American National Standard for Laboratory Ventilation ANSI/AIHA Z9.5-2003
6.3 Specifications and procurement of laboratory chemical hoods shall be based on
tests conducted on the hood (or prototype hood) that demonstrate adequate hood
containment. The containment tests shall include:
• Exhaust Flow Measurements
• Hood Static Pressure Measurement
• Face Velocity Tests
• Auxiliary Air Velocity Tests (if applicable)
• Cross Draft Velocity Tests
• Airflow Visualization Tests
• Tracer gas Containment Tests
The tests shall be conducted under constant volume conditions where exhaust and
air supply flow are stable and exhibit no more than 5% variation from set-point.
American Conference of Governmental Industrial Hygienists (ACGIH)
INDUSTRIAL VENTILATION A Manual of Recommended Practice, 27th Edition
10.35.1 The ANSI/ASHRAE Hood Performance Test may be used as a specification.
The specified performance should be required of both the hood manufacturer and the
designer of the room air supply system.
The specification takes the form of: AUyyy, AIyyy, or AMyyy where:
AU identifies an “as used” test.
AI identifies an “as installed” test.
AM identifies an “as manufactured” test.
yyy = control level, ppm, at the breathing zone of the worker.
Any well-designed airfoil hood, properly balanced, can achieve <0.10 ppm control
level when the supply air distribution is good. Therefore it would seem appropriate
that the “AM” requirements would be <0.10 ppm. The “AU” requirement involves the
design of the room supply system and the toxicity of the materials handled in the
hood. The “AU” specification would be tailored to suit the needs of the laboratory
room location.
VS-35-02 (GENERAL USE LABORATORY HOODS)
C. Use corrosion resistant materials suitable for expected use.
E. Avoid sharp corners at jambs and sill. Tapered or round hood inlets are desirable;
an airfoil shroud at sill is important.
G. Bypass opening in hood is desirable to avoid excessive indraft under partially
closed sash condition. Opening to be baffled to prevent splash eruption in hood.
J. For air conservation, use horizontal sliding sash with airfoil sill.
K. All bench hoods should have a recessed work surface and airfoil sill.
The requirement of ANSI/AIHA Z9.5-2003 6.3
refers to “As Manufactured” tests which are
intended to be conducted by the various fume
hood manufacturers in accordance with the
procedure specified in ANSI/ASHRAE 110.
The test data is intended to provide potential
fume hood specifiers or buyers with a way to
compare the performance of various types,
models, sizes, etc. of fume hoods offered by
different manufacturers. In addition, the “As
Manufactured” tests are intended to provide
the necessary performance criteria so a
determination can be made as to whether a
given fume hood can meet the required level
of performance.
VAV Response Time: The major providers of
VAV fume hood control systems generally
attain a response time of approximately one
second, which is within the time necessary to
maintain safe containment as determined by
widely recognized research
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on this topic.
However, a specific fume hood’s response
time when installed (that is, 1.0 seconds, 1.1
seconds, 1.2 seconds, etc.) is dependent
upon the overall dynamics of the room
ventilation system. So it is advisable to
specify response time as a ‘not to exceed’
value such as 2 seconds.