fumehood, biotransformation
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How To
Select The
Right
Laboratory
Hood System
An Industry Service Publication
Foreword
Table of Contents
Page Number
This booklet has been developed to serve as an aid in
selecting a laboratory fume hood ventilation system.
The information is intended to be unbiased and
generic in nature, compiled with help from experi-
enced architects, laboratory consultants, engineers
and laboratory hood users. The basic understanding
of hood systems you gain from reviewing this book-
let should prove valuable to you as you discuss your
needs with safety officers, engineers and hood
manufacturers.
Selecting The Proper Enclosure
What is a Laboratory Fume Hood
3
Laboratory Exhaust Systems and
Types of Laboratory Hoods
Constant Volume — Conventional, By-Pass,
Auxiliary-Air, Reduced Air Volume
3-4
Variable Air Volume
4
Special Application Laboratory Fume Hoods
Perchloric Acid Hoods
5
Radioisotope Hoods
5
Distillation and Walk-In Hoods
5-6
Not All Enclosures are Laboratory Hoods
Canopy Hoods
6
Our Method
Downdraft Hoods
6
Ductless Carbon-Filtered Enclosures
7
The table of contents outlines the various issues
which are addressed in this booklet. As you read
through the material, remember you are selecting a
laboratory hood system. A fume hood does not
function alone. A variety of factors external to a
hood influences its performance. Likewise, a hood
and the applications performed inside it can also
affect its surroundings. When selecting a fume hood,
you must consider the whole picture — the labora-
tory space, the building’s ventilation system, the
hood’s location in the room, to name a few.
While this booklet will raise the questions neces-
sary to identify your specific hood requirements, it
may not answer those questions. Only you, your
safety officer or industrial hygienist, and a qualified
design consultant can identify your laboratory’s
unique challenges.
We want this document to expand and improve
over time. If you have suggestions for additions or
improvements to this guide, please write Labconco
Corporation, 8811 Prospect Avenue, Kansas City,
MO 64132. Or call 800-821-5525 or 816-333-8811.
Biological Safety Cabinets and
Other HEPA-Filtered Enclosures
7
Clean Benches
7-8
Glove Boxes
8
Laboratory Hood Specifications
Hood Size
8
Liner Material
8-9
Sashes
8-9
Explosion-Proof vs.
Non Explosion-Proof Hoods
10
Service Fixtures
10
Electrical Receptacles
10
Lighting
10
Americans with Disabilities Act Requirements
10-11
Performance and Installation Considerations
Face Velocity and Containment Issues
11
Proper Techniques for Hood Use
12
Remote Blowers
12-13
Blower Sizing
13
Integral Motor/Blowers
13
Airflow Monitor
14
Exhaust Air Treatment
14
Ductwork
15
Base Cabinets
15
Work Surfaces
15
Sash Stops
16
By-Pass Blocks
16
Sash Position Alarms
16
Fire Extinguishers
16
Renovating Existing Laboratory
Fume Hoods and Ductwork
16
Planning Laboratory Space
Laboratory Layout
16
Sufficient Room Air
17
Energy Conservation
17
Noise Control
Conclusion
17
Laboratory Ventilation Standards
17-18
General References
18
Glossary
19
2
Selecting The Proper Enclosure
volume with the majority of exhaust air entering the
hood through the sash opening. Closing the sash
increases the speed of the air through the sash open-
ing so that high face velocities are to be expected
with the sash in the near closed position (Figure 1).
What is a Laboratory Fume Hood?
A laboratory fume hood is a ventilated enclosure
where harmful or toxic fumes or vapors can be han-
dled safely. The purpose of the hood is to capture,
contain and remove contaminants, preventing their
escape into the laboratory. This is accomplished by
drawing contaminants within the hood’s work area
away from the operator, so that inhalation and con-
tact are minimized.
EXHAUST
EXHAUST
ROOM
AIR
ROOM
AIR
Figure 1. Conventional hood with sash open and nearly closed
The conventional hood is generally the least expen-
sive, but its performance depends largely on sash
position. With the sash in the near closed position,
high velocity air passing through the sash opening
can damage fragile apparatus, disturb instrumenta-
tion, slow distillation rates, cool hot plates, disperse
valuable sample materials or result in turbulence
inside the hood.
Protector
®
60 Fiberglass Laboratory Fume Hood
Airflow into the hood is achieved by an exhaust
blower which “pulls” air from the laboratory room
into and through the hood and exhaust system. This
“pull” at the opening of the hood is measured as face
velocity. A baffle, air foil and other aerodynamically
designed components control the pattern of air
moving into and through the hood. Contaminated
air within the hood is then diluted with room air
and exhausted through the hood’s duct system to the
outside where it can be adequately dispersed at an
acceptably low concentration.
By-Pass
The by-pass hood generally operates at a constant
volume and is designed so that as the sash is closed,
the air entering the hood is redistributed, thereby
minimizing the high velocity air streams encoun-
tered in conventional hoods. The by-pass openings
above and below the sash area reduce fluctuations in
face velocity as the sash is raised or lowered (Figure
2). Therefore, the face velocity in by-pass hoods does
not generally reach levels which might be detrimen-
tal to lab fume hood procedures. By-pass type hoods
comprise the majority of hoods on the market.
Laboratory Exhaust Systems and
Types of Laboratory Hoods
EXHAUST
EXHAUST
All laboratory fume hoods’ operational airflow can
be described as one of two types: conventional and
by-pass. Auxiliary-air and reduced air volume hoods
are variations of the by-pass hood. Hoods use one of
two kinds of exhaust systems: constant volume or
variable air volume.
Constant Volume
ROOM
AIR
ROOM
AIR
Conventional
The conventional hood is a basic enclosure with an
interior baffle and movable front sash. The conven-
tional hood generally operates at a constant exhaust
Figure 2. By-pass hood with sash open and closed
3
Auxiliary-Air
A variation of the by-pass hood, the auxiliary-air
hood offers a means of providing up to 50% of the
air for the hood exhaust from outside the laboratory,
and limits the volume of tempered air removed from
the laboratory (Figure 3). This hood type has many
names including induced air, add-air, balanced air
and make-up air.
volume than by-pass hoods of the same size, they
require smaller blowers, which can be another cost
saving advantage.
RAV hoods should be used with caution. The
sash stop should be overridden only when loading or
cleaning the hood; never while in use. If the sash
stop is disengaged and the sash raised while the hood
is in use, the face velocity could drop to an unsafe
level (Figure 4). A sash position alarm is recommended.
AUXILIARY
OUTSIDE
AIR
AUXILIARY
OUTSIDE
AIR
EXHAUST
EXHAUST
EXHAUST
EXHAUST
BY-PASS
BLOCK
SASH
STOP
ROOM
AIR
ROOM
AIR
ROOM
AIR
ROOM
AIR
Figure 4. Reduced air volume hood with sash stop and by-pass
block shown with sash open to sash stop position and sash closed
Figure 3. Auxiliary-air hood with sash open and closed
One advantage to auxiliary air hoods is that they
feed air-starved laboratories, where room supply air
volume is not adequate to support a laboratory hood.
Another advantage to auxiliary-air is that, when prop-
erly applied, it can provide energy savings by limiting
the volume of heated or cooled room air exhausted by
the hood. The level of savings depends on the degree
to which the auxiliary air must be tempered.
Certain negative aspects of auxiliary-air hoods
should be considered. Because two blowers and two
duct runs are required, initial equipment and set-up
costs are higher than average. Since an oversized aux-
iliary-air system may overpower the exhaust system,
auxiliary-air systems require careful balancing to
prevent undesirable turbulence at the face of the
hood. In addition, temperature extremes, caused by
untempered auxiliary air, can adversely affect the
hood’s containment ability and cause user discom-
fort. Finally, the auxiliary air should be clean, dry
and tempered properly so it does not interfere with
analytical work being done in the hood.
Variable Air Volume
Variable air volume (VAV) hoods vary the amount
of room air exhausted while maintaining the face
velocity within a preset range. VAV hoods alter the
exhaust volume using various methods. One method
utilizes a damper that opens and closes based on
airflow and sash position. Another method involves
varying blower speed to meet air volume demands.
When multiple hoods share one common exhaust
blower, both methods may be utilized.
Fume hoods with VAV systems generally operate
as conventional hoods. Some VAV hoods include a
modified by-pass system which ensures that suffi-
cient airflow is maintained to adequately contain and
dilute fumes even at low sash positions (Figure 5).
EXHAUST
EXHAUST
DAMPER
Reduced Air Volume
A variation of the by-pass hood, the reduced air vol-
ume (RAV) hood uses a by-pass block to partially
obstruct the by-pass opening above the sash to
reduce the air volume exhausted thus conserving
energy. It is used in conjunction with a sash stop that
limits the height the sash may be opened during nor-
mal use so that the hood demands less air volume to
achieve safe velocity. Since these hoods use less air
ROOM
AIR
ROOM
AIR
Figure 5. Variable air volume hood with damper control and
modified by-pass
4
VAV systems are available built into the hood at the
factory or as an accessory added to the hood upon
installation. Some VAV hoods have electronics which
allow them to be connected to the laboratory build-
ing’s heating, ventilation and air conditioning (HVAC)
system for monitoring hood exhaust air and control-
ling laboratory air supply from a central location.
Although initial start up costs may be higher
due to building alterations, VAV hoods offer energy
savings over traditional by-pass and auxiliary-air
hoods. At the same time, they offer consistent airflow
regardless of sash position so they are a good choice
for complicated or lengthy experiments. In addition,
most VAV systems feature monitors/alarms which
alert the operator to unsafe airflow conditions.
Special Application Laboratory
Fume Hoods
Figure 6. Perchloric acid hood with washdown system
Unique features may be added to the hood and
exhaust system to accommodate special procedures
in the hood. Below are descriptions of a few of the
many special purpose hoods on the market.
Radioisotope Hoods
Hoods used for radioactive applications have integral
work surfaces and coved interiors to facilitate decon-
tamination. Liner materials, such as Type 304 stain-
less steel, should be impermeable to radioactive
materials. Cupsinks are sometimes provided in the
integral work surface, however local codes which
dictate the safe disposal of radioactive effluents
should be observed. These hoods should be sturdy
enough to support lead shielding bricks in instances
where they are required. They should also be
installed to facilitate the use of high efficiency partic-
ulate air (HEPA) or charcoal filters in the ductwork.
The laboratory’s safety officer should determine
which, if any, filters are required to trap the radioac-
tive materials emitted during a particular application.
Perchloric Acid Hoods
Perchloric acid hoods are dedicated for use with per-
chloric acid only. Organic materials should not be
used in a perchloric acid hood because an explosion
may occur when perchloric acid reacts with organic
materials. It must be constructed of relatively inert,
impervious materials such as Type 316 stainless steel,
Type 1 unplasticized polyvinyl chloride (PVC) or
ceramic-coated material. Hoods used for these appli-
cations have integral work surfaces, coved interiors,
and a drain for easy and thorough cleaning.
Washdown features are required since the hood and
duct system must be thoroughly rinsed after each use
to prevent the accumulation of potentially reactive
perchloric salts. (Figure 6). Horizontal duct runs and
sharp turns should be avoided so that washdown
residue drains thoroughly. Each perchloric acid hood
requires its own dedicated exhaust system with
washdown capability.
Distillation and Walk-In Hoods
Distillation and walk-in hoods are constructed with
additional interior height to accommodate large
apparatus. Distillation hoods typically mount on a
platform instead of a base cabinet or bench. A
California hood is a type of distillation hood with
sash entry on both sides (Figure 7). Walk-in hoods
mount on the floor, permitting roll-in loading of
heavy or bulk apparatus. Although called walk-in
hoods, the operator should never stand inside the
hood while fumes are being generated.
5
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