LOCAL VENTILATION Chapter 10

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LOCAL VENTILATIONChapter 10

• LARS OLANDER
• LORRAINE CONROY
• ILPO KULMALA
• RICHARD P. GARRISON
• MICHAEL ELLENBECKER
• BERNHARD BIEGERT
• BERNARD FLETCHER
• HOWARD D. GOODFELLOW
• GUNNAR ROSÉN
• BENGT LJUNGQVIST
• BERIT REINMÜLLER
• ANTONIO DUMAS
• MICHAEL ROBINSON
• DEREK B. INGHAM
• ALBRECHT LOMMEL
• KATSUHIKO TSUJI
• IRMA WELLING
• XIANYUN WEN

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Contents of Chapter 10

• GENERAL
• Purpose and Function
• Modes
• Classification
• What Is Described
• EXHAUST HOODS
• General
• Exterior Hoods
• Enclosures
• References
• SUPPLY INLETS
• General
• Air Jets
• Low-Momentum Supply Systems
• Open Unidirectional Airflow Benches
• References

• COMBINED EXHAUST HOODS AND SUPPLY INLETS
• General
• Air Curtains
• Push-Pull Ventilation of Open Surface Tanks
• Aaberg Exhaust Hoods
• Low-Momentum Supply with Exterior Hoods
• Enclosures
• Enclosed Rooms
• Others
• References
• EVALUATION OF LOCAL VENTILATION SYSTEMS
• General
• Efficiency Measurements
• Other Methods
• References

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LOCAL VENTILATION
Short definition Local Ventilation systems are
used to transport contaminants or heat from the
occupancy zone. Long definition Local
Ventilation uses an air flow rate that is as low
as possible, but sufficient to minimize the
amount of airborne contaminants entering a
specified volume or passing specified point(s).
These are usually intended to be at the breathing
zone of occupants. This minimization of air flow
can be achieved either by capturing (or
containing) the airborne contaminant into an
exhaust hood before it enters the workspace, by
blowing non-contaminated air from a supply inlet
through the volume to prevent the contaminant
from entering the workspace, or a combination of
those.
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Local Ventilation Modes
All Local Ventilation systems can be manufactured
for use in one or more of three different
modes. Fixed systems are those where movement of
the hood or other changes to the system, except
perhaps opening and closing of lids and doors,
are not possible. One example is the laboratory
fume hood. Flexible systems are those where the
suction opening or supply device may be placed at
different locations inside a limited area or
volume. This is performed by connecting the
exhaust (or supply) opening and duct (or tube) to
the fan by flexible connections (ducts with
moveable elbows or flexible tube). One example is
a wall mounted hood for welding exhaust. Mobile
systems are those where the exhaust (supply)
opening may be placed almost anywhere inside a
workroom. This is performed by placing the whole
system (exhaust/supply opening, duct, fan) on
wheels or a portable frame. It can also be
accomplished by having a separate exhaust part
(opening and short tube) which could be connected
to a central duct system at many places (in
walls, on the floor or from the ceiling). One
example of the former is a welding exhaust (with
filter) on a small carriage and an example of the
latter is a centralized exhaust system for
connection to car exhaust pipes. Mobile exhaust
or supply systems also include portable systems.
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Velocity distributions outside round (top left),
round with flange (top right), and square
(bottom) exhaust opening.
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The air shower principle creates a zone of clean
air around the worker and pushes away the
contaminant plume from the breathing zone.
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Examples of principal arrangements of open
unidirectional airflow benches.
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A push-pull system is a combination of an exhaust
and an inlet system.
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A schematic representation of the Aaberg
principle in a combined exhaust-supply system.
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Principe for auxiliary air supply to a laboratory
fume hood.
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The Class III Biological Safety Cabinet is a
combined exhaust-supply system. A Glove ports
with O ring for attaching arm length gloves to
cabinet. B Sash. C Exhaust HEPA filter. D
Supply HEPA filter. E Double ended autoclave or
pass through box. Note Connection of the
exhaust to building exhaust system is required.
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Different configurations of supply air inlets and
exhaust air outlets in an abrasive blasting room.
A1 Supply in ceiling and exhaust through floor.
A2 Supply in ceiling and exhaust along floor
sides. A3 Supply in one end of ceiling and
exhaust through one opposite wall. A4 Supply in
one end of ceiling and exhaust in corners of
opposite wall. B1 Supply in one wall and
exhaust through opposite wall. B2 Supply in one
wall and exhaust in corners of opposite wall. B3
Supply in corners of one wall and exhaust
through opposite wall. B4 Supply in corners of
one wall and exhaust in corners of opposite
wall. C1 and C2 Not recommended (INRS, 1994).
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Characteristic geometrical parameters of air
curtains.
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EVALUATION OF LOCAL VENTILATION SYSTEMS
The evaluation methods could be direct, e.g.,
measuring a containment index, or indirect, e.g.,
measuring pressure loss or velocity distribution.
For some hood types, measurements usually seen
as indirect methods, are used to measure the
hoods performance to determine regulatory
compliance. For example, there are regulations
specifying minimum and maximum face velocities
for laboratory fume hoods and static pressure
(negative) inside enclosed hoods. Continuously
monitoring instruments can be connected to alarms
which sound when the measurement is outside the
specified limits.
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EVALUATION OF LOCAL VENTILATION SYSTEMS (contd)
Capture efficiency is the fraction of generated
contaminant that is directly captured by the
hood. Measurement of capture efficiency involves
measuring concentration of process-generated
contaminant or a tracer material. Using
process-generated contaminant requires use of
instruments suited to each specific contaminant
and its conditions (temperature, pressure,
concentration, form, etc). In order to facilitate
these measurements, a tracer is often substituted
for the process-generated contaminant.
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EVALUATION OF LOCAL VENTILATION SYSTEMS (contd)
A simple evaluation can be done by checking the
air flow rate into the opening, presuming the
source characteristics, the placement of the
exhaust, and other parameters (crossdraft, work
routines, supply air flow rate, etc) have not
changed since the detailed evaluation was done.
Since the static pressure loss for a hood is
dependent on form and flow rate it can be used
alone to monitor the flow rate into the hood. If
the flow rate and the pressure loss were measured
at the same time as the efficiency, the pressure
loss can be used for monitoring hood performance.
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EVALUATION OF LOCAL VENTILATION SYSTEMS (contd)
Another simple way to evaluate the exhaust is to
use smoke to visualize the air streamlines. It is
sometimes possible to see how far an exhaust
reaches by observing smoke movement when it is
generated at different distances from the hood
opening.
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