(Cappe) Sefa-1 2002 PDF
(Cappe) Sefa-1 2002 PDF
(Cappe) Sefa-1 2002 PDF
Recommended Practices
SEFA 1 - 2002
This document was written with input from the following individuals.
SEFA 1 – 2002
Chairman: Ron Hill – HEMCO Corporation
Marv Garrett – Case Systems , Inc. Dana Dahlgren - Kewaunee Scientific Corp.
Steve Hackman – HERA, Inc. Ken Dixon - Air Control, Inc.
Chris Harp – Flad & Associates Dan Frasier - Phoenix Controls
Steve Hulsey – Campbell Rhea Jerry Koenigsberg – GPR Planners
Bill Stover – Mott Manufacturing Charles Kuhlman – Collegedale Casework L.L.C.
Gary Taylor – LSI
Foreword
Summary
1.0 Purpose
2.0 Scope
SEFA Profile
The Scientific Equipment and Furniture Association (SEFA) is a voluntary international trade
association representing members of the laboratory furniture, casework, fume hood and related
equipment industry. The Association was founded to promote the industry with improved
quality, safety and the timely completion of laboratory facilities in accordance with customer
requirements.
SEFA uses it best effort to promulgate recommended practices for the benefit of the public in
light of available information and accepted industry practices.
SEFA has developed a glossary of terms for the purpose of promoting a greater understanding
between designers, architects, manufacturers, purchasers and end users. The terms defined by
SEFA are frequently used in contracts and other documents, which attempt to define the products
to be furnished or the work involved. The Association has approved this glossary in an effort to
provide uniformity among those who use these terms.
SEFA encourages all interested parties to submit additional terms or to suggest any changes to
those terms already defined by the Association. The glossary should be used to help resolve any
disputes that may rise or to incorporate the applicable terms in any contract or related documents.
(Appendix – II)
SEFA, and its committees, is active in the development and promotion of recommended
practices having domestic and international applications. Recommended practices are developed
by the Association taking into account the work of other standard writing organizations. Liaison
is also maintained with other associations and governmental agencies in the development of their
specifications.
SEFA’s Recommended Practices are developed in and for the public interest. These practices
are designed to promote better understanding between manufacturers and purchasers and to assist
the purchaser in selecting and specifying the proper product to meet the user’s particular needs.
The existence of a SEFA Recommended Practice does not preclude any member or non-member
from providing products or services that do not conform to these recommended practices. SEFA
welcomes any proposed changes or additions to these recommended practices and encourages all
interested parties to participate in this important endeavor.
The Laboratory Fume Hood, as specified on the drawings, shall comply with SEFA 1-2002 in
material, components, type, and testing.
NOTE: Check your regional or local codes for jurisdiction and material allowance. There are
regional differences.
2
Uniform Building Code, 2000 Edition, International Association of Plumbing and Mechanical Officials,
20001 Walnut Drive, South Walnut, CA 91789 www.iapmo.org
3
NFPA 45 Standard on Fire Protection for Laboratories Using Chemicals, 2000 Edition, NFPA
Face velocity shall be adequate to provide containment. Face velocity is not a measure of
safety.
Refer to ASHRAE 110 - 1995 (or latest edition) for velocity measurement procedures.
Face Velocity Guide - Face velocities of laboratory fume hoods may be established on the basis
of the toxicity or hazard of the materials used or the operations conducted within the fume hood.
The most widely requested target average face velocity is 100 FPM. The measured deviation
across the face may vary + 20 FPM.
AM 0.05 can be achieved with a properly designed laboratory fume hood. It shall not be
implied that this exposure level is safe. Safe exposure levels are application specific and
should be evaluated by properly trained personnel.
Section 5.3 - Fume Hood Evaluation - As Installed - Precondition for Testing - Page 30
Precondition for As Installed (AI) Testing: The test of the fume hood should be performed
after the installation is complete, the building ventilation and control system has been balanced
and all connections made.
It is recommended that the user made provisions to have the following test performed on all
laboratory fume hoods. These tests should be performed by qualified personnel to verify proper
operation of the fume hoods before they are put to use. Testing should be repeated at least
annually, or whenever a significant change in the hood system occurs. Any unsafe conditions
disclosed by these tests should be corrected before using the hood. It is recommended that hoods
be tested in accordance with ASHRAE 110-1995 (or most current edition) before put into
service.
Although your organization's management is ultimately responsible for the health and safety of
laboratory personnel, a team approach is required to ensure proper performance of laboratory
fume hood systems.
2. SCOPE
Hazardous
Procedures
STORAGE
Laboratory
A Laboratory Ventilation System includes the Supply Air System; the Exhaust Air
System (which includes room air exhaust in addition to the laboratory fume hood
exhaust); the Laboratory; the Laboratory Fume Hood, and other ventilated enclosures.
A Laboratory Fume Hood is given here as the proper terminology. Other widely used
terms include --- Fume Hood, Chemical Hood, Chemical Fume Hood, Hood, and Fume
Cupboard.
Laboratory fume hoods are perhaps the most widely used and misused safety devices.
Fume hoods are available in many shapes, sizes, materials, and finishes. Their flexible
design enables them to be configured to accommodate a variety of chemical procedures.
However, the flexibility offered by different designs and operating configurations can
result in varying levels of performance and operator protection. Great care must be
employed by the user when using a laboratory fume hood. Consult the manufacturers
Recommended Practice for Specific Operation, Safety, and Maintenance Guidelines.
The laboratory fume hood is part of the ventilated laboratory safety device family and
can be sub categorized by type. Each type is connected to a laboratory ventilation
system. These “other” systems are described in Section 8.0.
This practice is organized to be consistent with the ASHRAE 110 protocol. "As
Manufactured" issues in this practice are directed to fume hood practices that are
pertinent to the hood manufacturers location. "As Installed" practice identify those
that occur in a newly constructed or renovated laboratory prior to the user occupying
the lab. The "As Used" section helps with issues after the installation is complete and
the hood is to be or is being used.
(4.2.2)
Radioisotope
Hood Hazard: Perchloric Acid Only
Volume Generation: Small to Moderate
(4.2.3) Perchloric Acid Effluent: Gases, Vapors, Mists
Hood
Other Ventilated
Laboratory Safety
Safety Devices
(8.1.1) Demonstration Oversized
Hood Table Top
Conventional
(8.1.2) California Balance
Hood Microscope
Robotic
(8.1.3) Ventilated Tissue Trimming
Enclosure
Local EXhaust
Ventilation
Hazard: Chemical
(8.2.1)
Canopy Hood Toxicity: None to Low
Volume Generation: Small
(8.2.2) Effluent: Gases, Vapors, Particulate, Powder
Slot Hood
Must be specifically designed.
(8.2.3) for process.
Snorkel
Elephant Trunk
Laminar Flow
Hazard: Chemical
Hoods
(8.3) Toxicity: Low to High
Class I Weighing
Enclosure Volume Generation: Small to Large
Effluent: Particles
Biological
Safety Cabinets
(8.3.1)
Class I Cabinets
Hazard: Biological
(8.3.2) Class II Type A1 Toxicity: Low to High
Cabinets Volume Generation: Small
Effluent: Particulate, Powder
(8.3.2) Class II Type A2
Cabinets Class II Type B2
Limited Use with Gases, Vapors,
(8.3.2) Class II Type B1
and Radionuclides
Cabinets
Hazard: Chemical
Ductless (8.4) Toxicity: None to Low
Fume Hood
Volume Generation: Small
Effluent: Gases, Vapors, Particulate, Powder
There are a wide variety of fume hood designs. They generally share a number of similar
characteristics and components. The hood depicted in Figure #3 shows generalized
components of laboratory fume hoods.
Baffle
Liner
Aerodynamic Fascia Slots
Sash
Services
Hood Exterior
Work
Airfoil Sill Surface
Storage Cabinet
The common components of most laboratory fume hoods are described below.
The hood exterior is the external “skin” and is us ually made of painted
steel. Some hood exteriors are made of stainless steel, polypropylene,
wood, or phenolic. The exterior front of the hood is an important design
element for fume containment. Properly designed laboratory fume hoods
will have a contoured entry, which assists airflow into the hood and could
improve hood performance.
If the fume hood liner is not rated at 25 or less in accordance with ASTM-
E84 or there is a high risk potential of fire hazard in the fume chamber, for
safety reasons the fume hood should be equipped with automatic fire
suppression and alarm system or, in some cases, local jurisdiction may
require fire suppression system, wet or dry.
The baffle in the rear of the hood interior is designed to control airflow
distribution within the hood and through the face opening. The baffle
slots are sometimes adjustable.
The exhaust collar that connects the hood to the exhaust duct is located
behind the baffle at the top of the interior liner. The design of the exhaust
collar can affect the hood static pressure drop and noise level and shall be
made of a corrosion resistant material.
The number of exhaust collars varies depending on the length of the hood.
Typically hoods longer than six feet have more than one exhaust collar for
connection to the exhaust ducts.
Restricted Bypass: The restricted bypass serves the same function as the
open bypass, but the bypass is smaller. This is done to reduce the amount
of air required by the laboratory fume hood in the operating mode for
VAV systems, horizontal, and combination sashes. Eliminating the bypass
completely is not recommended due to the potential risk of contaminate
leakage. Minimum exhaust volume is recommended at 25cfm per square
foot of work surface. 4
Sashes are typically designed so that closing the sash does not restrict the
area beneath the airfoil sill. This leaves the area beneath the airfoil open
when the sash is fully closed.
Vertical Sash: A vertical sash has one or more panels that can slide up
and down to a height required by the operator. The sash controls the
opening area and it is generally advisable to lower the sash below the
breathing zone of the operator during generation of hazardous
contaminants. Hoods may be equipped with sash stops to restrict the
4
NFPA 45 Standard on Fire Protection for Laboratories Using Chemicals, 2000 Edition, NFPA
Horizontal Sash: A horizontal sash has typically two or more panels that
slide horizo ntally across the hood opening. The sash panels slide in tracks
located at the top and bottom of the face opening. Horizontal sashes are
used to restrict the maximum opening area of the face, but allow access to
the top interior of the hood enclosure.
Work surfaces are typically made of a material that provides good heat
and corrosion resistance and is easily cleaned and decontaminated. The
work surface should have a recessed area. The dished or recessed area is
designed to provide containment of small spills and provide demarcation
of the recommended work area inside the hood. Refer to SEFA 3 – Work
Surfaces.
Most fume hoods are equipped with some type of light. Lights come in a
variety of designs depending on the anticipated use of the hood. Most
lights are fluorescent tubes housed outside the hood chamber and
separated by a vapor resistant tempered glass panel in the top of the hood.
Access to re-lamping these types of lights should be from the hood
exterior. The light shall be designed to provide a minimum of 80
foot-candles on any part of the bench level (36” from the floor) work
surface. Incandescent vapor proof lights as well as incandescent and
fluorescent explosion proof lights are optional and available as specified.
Electrical outlets should not be located within the hood, especially when
flammable or corrosive materials may be present.
Connections for services will vary, depending on the point of origin and
number of fixtures. Service lines may be brought in from below, down
from the ceiling, or from the back wall.
NOTE: Check your regional or local codes for jurisdiction and material
allowance. There are regional differences.
• Any fire suppression system should be rated for all fire classes
(A, B, and C).
5
Uniform Building Code, 2000 Edition, International Association of Plumbing and Mechanical Officials,
2001 Walnut Drive, South Walnut, CA 91789 www.iapmo.org
6
NFPA 45 Standard on Fire Protection for Laboratories Using Chemicals, 2000 Edition, NFPA
7
NFPA 70 National Electrical Code, 2002 Edition, NFPA
All hoods shall have some type of monitor for indicating face velocity or
exhaust flow verification. The monitor can be a simple pressure gage
connected to a Pitot tube in the exhaust duct, one of many electronic
monitors, or a vaneometer.
STORAGE
A fume hood used for Beta and Gamma radiation shall be referred to as a
radioisotope hood. A radioisotope hood has the general characteristics of
a bench-top fume hood except the work surface and interior lining must be
type 304 stainless steel with coved seamless welded seams for easy
cleaning and decontamination. The hood design is identical to other hood
types in nearly all other respects.
The work surface shall be dished to contain spills and cleaning liquids and
shall be properly reinforced to support lead shielding and shielded
containers. The load-bearing capacity shall be 200 pounds per square foot
(976.5 Kg/m 2) minimum up to a total weight of 1,000 pounds (453.6 Kg)
per fume hood or base cabinet section.
In addition, the hood, duct, and fan must have a water wash down system
to remove perchlorates and prevent the build-up of potentially explosive
perchlorate salts. Drain outlet shall be designed to handle a minimum of
A distillation fume hood is designed for use with tall apparatus and
procedures that involve small to medium quantities of low to high toxicity
materials. A distillation hood has the same components as a bench-top
hood with the exception that the design provides a greater interior height.
The hood is suitable for work that can be conducted in a bench-top hood;
however, the greater interior height enables use of larger apparatus.
The name “walk- in hood” implies that the hood can be entered; however,
the name is a misnomer, as the same safety precautions should be applied
to this hood, as those required for a bench-top hood. The hood must never
be entered during generation of hazardous materials or while
concentrations exist within the enclosure. For this reason, we refer to
these structures as floor mounted fume hoods.
The auxiliary air system, when added to a standard laboratory fume hood,
shall function to reduce the consumption of conditioned room air. The
auxiliary air is typically introduced exterior to the fume hood face and
enters the fume hood through the face with the sash(es) open.
With the sash(es) closed, auxiliary air shall be drawn into the fume hood
interior in such a manner as to aid in the dilution of heat and fumes
generated in the work area.
The manufacturer shall include auxiliary air static pressure data for all
standard catalog models.
The ASHRAE 110 test is a method of testing the performance of laboratory fume
hoods. There are three test procedures incorporated into the 110 test; the first is
the face velocity grid test, the second is the flow visualization or smoke test and
the third is the tracer gas containment test. The ASHRAE 110 is the recognized
method for evaluating the performance of fume hoods; ASHRAE has defined
three modes, As Manufactured (AM), As Installed (AI), and As Used (AU). The
ASHRAE test should be conducted by an authorized person cognizant of each of
the three test procedures.
Refer to ASHRAE 110 – 1995 (or latest edition) for velocity measurement
procedures.
The manufacturer shall provide standard (AM) test data for all standard
hoods. This should be done in accordance with the most current
ASHRAE 110 Standard. The AM testing demonstrates what the hood is
capable of doing under controlled conditions. The report shall verify that
all laboratory fume hood types specified have been tested to ASHRAE
With sash at full-open position, static pressure loss through the fume hood
shall be no more than ¼ inch (6.35 mm) of water gauge when the fume
hood operates at face velocity of 75 feet per minute (.38 m/s), ½ inch
(12.70 mm) of water gauge at 100 feet per minute (.51 m/s), ½ inch
(12.70 mm) of water gauge at 120 feet per minute (.62 m/s). The
manufacturer shall state the design static pressure loss for all standard
catalog models. For all laboratory fume constant vo lume hoods equipped
with a bypass, static pressure loss and exhaust volume shall be relatively
constant regardless of sash position. The velocity when measured at the
sash opened six inches, shall be no more than three times the velocity at
the sash fully opened.
8 th
Industrial Ventilation: A Manual of recommended Practice, 24 Edition, American Conference of
Governmental Industrial Hygienists, 1330 Kemper Meadow Drive, Cincinnati, OH 45240 www.acgih.org
Laboratory fume hood exhaust systems should be balanced with room exhaust
systems and may be used in conjunction with room exhaust to provide the
necessary room ventilation. Constant operation of a fume hood will also provide
fume control during non-working hours.
Sufficient makeup air must be available within the laboratory to permit fume
hoods to operate at their specified face velocities.
Laboratory fume hoods are potential locations for fires and explosions due to the
types of experiments conducted in these units. As such, fume hoods should be
located within the laboratory so that in the event of a fire or explosion within the
fume hood, exit from the laboratory would not be impeded.
Laboratory fume hoods should be located away from high traffic lanes within the
laboratory because personnel walking past the sash opening may disrupt the flow
of air into the unit and cause turbulence, drawing hazardous fumes into the
laboratory.
Sufficient aisle space should be provided in front of the fume hood to avoid
disruption of the work or interference with the operating technician by passing
personnel.
Safety devices such as drench showers, eye wash stations, fire extinguishers, first
aid kits and fire blankets should be located convenient to the fume hood operating
personnel and plainly labeled as to their use and function.
Precondition for Testing: The test of the fume hood should be performed after
the installation is complete, the building ventilation and control system has been
balanced and all connections made.
It is recommended that the user make provisions to have the following test
performed on all laboratory fume hoods. These tests should be performed by
qualified personnel to verify proper operation of the fume hoods before they are
put to use. Testing should be repeated at least annually, or whenever a significant
change in the hood system occurs. Any unsafe conditions disclosed by these tests
should be corrected before using the hood. It is recommended that hoods be
tested in accordance with ASHRAE 110-1995 (or most current edition) before put
into service.
The ASHRAE 110 test is a method of testing the performance of laboratory fume
hoods. There are three test procedures incorporated into the 110 test; the first is
the face velocity grid test, the second is the flow visualization or smoke test and
the third is the tracer gas containment test. The ASHRAE 110 is the recognized
method for evaluating the performance of fume hoods; ASHRAE has defined
three modes, As Manufactured (AM), As Installed (AI), and As Used (AU). The
ASHRAE test should be conducted by an authorized person cognizant of each of
the three test procedures.
Check operation by moving sash(es) through its (their) full travel. Sash
operation shall be smooth and easy. Vertical rising sashes shall hold at
any height without creeping up or down, unless designed otherwise. Force
to open the sash shall be reasonable for the size and weight of the sash.
Typically a six foot hood with a vertical rising sash shall require no more
than five pounds of force to operate a sash.
On fume hoods with low flow warning devices, verify that monitor
functions properly and indicates unsafe conditions.
Determine specified average face velocity for fume hood being tested.
Perform the following test to determine if fume hood velocities conform to
specifications.
Refer to ASHRAE 110 – 1995 (or latest edition) for velocity measurement
procedures.
When fume hood test procedures detect improper function, the cause is frequently
due to insufficient quantity of air flowing through the hood, or due to room cross
drafts blowing into or across the face of the fume hood, or a combination of both.
The following suggestions are offered to help pinpoint and correct the problems.
Insufficient airflow through the fume hood can be caused by one or more
of the following conditions. Each condition should be checked, and
eliminated if possible; to determine which one or combination of
conditions may exists.
• Air moving through an open door located adjacent to the fume hood
can cause cross drafts.
• An open window or room air supply grill located to one side or across
from the fume hood can cause disturbing cross drafts.
• High velocity air from ceiling- mounted diffusers or room air supply
can cause cross drafts or downdrafts.
The exhaust unit should be sized to exhaust the volume of air necessary to
attain the selected fume hood face velocity at the total system static
pressure loss.
Exhaust units should be sized to achieve the lowest practical angular speed
of the impeller, thereby avoiding high impeller tip speed and minimizing
noise associated with this revolving member.
Fume hood and other exhaust devices shall not interconnect with
recirculating systems.
5.5 Maintenance
Lubrication of sash guides, cables, pulley wheels and other working parts should
be accomplished as required or in accordance with manufacturer’s
recommendations.
Flush all spills immediately using neutralizing compounds as required and clean
thoroughly.
The employer is responsible for ensuring that the hood meets satisfactory safety
standards. A hood operator is responsible for ensuring that the hood is used in a
safe manner and according to your organization’s safety guidelines. A hood
operator is also responsible for helping their organization maintain proper
operation of the hood systems.
The following guidelines are provided to help reduce your potential for exposure
when working with hazardous materials.
• What are the characteristics of the hazards associated with the procedure?
• Is this the right type of hood?
• Will the hood accommodate the equipment and experimental apparatus?
• Is the hood capable of capturing and exhausting the contaminants?
• What are the hood capabilities and limitations?
• What special precautions are required?
• Verify that the ventilation system is working properly.
For example, if you are going to conduct a procedure involving use of heated
perchloric acid, you must use a perchloric acid hood and the exhaust system must
be equipped with a water wash down system. Failure to use a perchloric acid
hood with a water wash down system could result in a future explosion or fire.
Another example is to be cautious with a heat generating processes. Generated
velocity due to the heat in a hood could result in counterproductive airflow. Is the
fume hood liner resistant to the heat loads?
Ensure that clothing and glove materials are appropriate for work with the
hazards. For example, vinyl gloves provide excellent resistance to formaldehyde,
but poor resistance to chloroform.
The ASHRAE 110 test is a method of testing the performance of laboratory fume
hoods. There are three test procedures incorporated into the 110 test; the first is
the face velocity grid test, the second is the flow visualization or smoke test and
the third is the tracer gas containment test. The ASHRAE 110 is the recognized
method for evaluating the performance of fume hoods; ASHRAE has defined
three modes, AS Manufactured (AM), As Installed (AI), and As Used (AU). The
ASHRAE test should be conducted by an authorized person cognizant of each of
the three test procedures.
Before generating hazardous materials within the hood, you should ensure that the
hood system is in good working order.
Check the hood integrity and verify adequate exhaust flow or face velocity. At a
minimum, check the hood inspection notice to ensure that the hood has been
recently tested and operation was satisfactory at the time of the tests.
Verifying proper system operation without a hood monitor is very difficult. All
hoods shall have some type of monitor to verify proper exhaust flow and/or
average face velocity. If yo ur hood does not have a monitor, request one.
Ultimately the ability of the hood to provide adequate protection depends on the
user. By utilizing proper work practices, the potential for exposure can be
reduced. Limitations inherent in many hoods and systems make proper work
practices required to optimize containment.
Baffle Baffle
ent
ent ipm
ipm Equ ent
Equ ipm
Equ
Plane of Sash
Researcher
HOOD
EXHAUST
Baffle
Generation Location
Procedure
Plane of Sash
Researcher Recessed
Reverse Flow Zone Work
Surface
• High heat loads create thermal drafts which increase face velocity
through the bottom of the fume hood opening and thus lower face
velocities at the top of the fume hood opening.
The hood user should always be aware of locations within the hood where
concentrations of contaminants can accumulate. The user should never
allow his head to break the plane of the sash because this will cause
contaminated air to pass through the breathing zone.
When materials are being generated in the hood, ensure that you slowly
approach and withdraw from the hood. The wake zone created by
movement near the hood opening can withdraw materials from within the
hood.
Rapid arm and body movements near the hood opening should be avoided.
Reducing the sash to below the user’s breathing zone provides a protective
barrier between the researcher and the experiment.
As air enters the opening of a hood with horizontal sash panels, turbulent
vortices develop along the vertical edges of the sash panels. The vortex,
readily visualized using smoke, can extend deep into the hood and draw
contaminants toward the edges of the sash panels.
High concentrations can develop near the edge of the sash panels
regardless of the generation location within the hood. Although escape is
not usually observed, rapid movements near the sash edge or turbulence
resulting from cross drafts could cause escape.
Line of Sight
Reverse Reverse
Flow Zone Flow Zone
Researcher
Baffle
Sash
Panel
A person walking past the hood can generate significant cross drafts.
When generating hazardous materials in the hoods, attempt to divert or
limit traffic past the hood. Inform other laboratory personnel about the
work being conducted in the hood.
• Always work at least six inches beyond the plane of the sash. The
farther into the hood the better.
• Ensure head and upper body remains outside the plane of the hood
opening at all times.
Group Responsibility
Lab Design Team and Engineering Identify needs and design/specify appropriate building
(6.6.4) systems, fume hoods, and laboratory components.
Laboratory Personnel and Laboratory Comply with Standard Operating Procedures (SOP).
Fume Hood Users (6.6.9)
The following list provides a summary of responsibilities for each group involved
with ensuring proper operation of laboratory fume hood systems.
6.6.1 Management
• Identify needs.
• Design appropriate building systems (architectural, mechanical,
electrical, plumbing, structural etc.).
• Design and specify appropriate fume hood system.
• Assist with pre-qualification of construction team.
• Review all proposed changes.
• Prepare “as built” documents.
• Ensure design intent is achieved and commissioned.
Laboratory ventilation systems include both exhaust and supply duct systems. The
purpose of a laboratory exhaust system is to exhaust a specific volume of air from
laboratory fume hoods or other exhaust devices and safely transport the contaminated air
from the building in a manner that reduces the potential for re-entrainment of exhaust
fumes into the fresh air intake in the building. According to a number of industry
standards, the supply air system must make up the air exhausted from the laboratory with
100% fresh outside air, conditioning it to provide a safe and comfortable work
environment for research. The amount of supply air delivered to a laboratory is
controlled to satisfy the demand for minimum ventilation (ACH) rate, hood flow demand
or cooling / heating load demand, whichever is greater. In order to maintain the negative
pressure requirement, the total exhaust volume for a lab must always exceed the supply
air volume by a specific volumetric offset or the flows must be controlled by a pressure
differential control system. The volumetric offset method is the most common. If the
total of all hood exhaust is less than the maximum possible supply flow, an additional
exhaust device, normally referred to as the general exhaust valve, is required.
Many factors affect the performance of hoods and laboratories, none of which receives
more discussion tha n the airflow control strategy. The flow control strategy significantly
impacts laboratory fume hood containment, room pressurization and energy usage.
There are three main airflow control strategies for laboratories with fume hoods.
The first and most widely used, Constant Volume (CV), has been in use since the
early 20th century. Second is Two-State Control (2SC), introduced in the 1960’s.
And finally, Variable Air Volume (VAV) has been gaining popularity and
effectiveness since the 1980’s. Specific applications are well suited to each.
A variable air volume fume hood control system is designed to vary the
hoods’ exhaust rate to maintain a constant average face velocity
throughout the sash travel. The complexity of this system requires fast,
stable control systems, which are more expensive, on an installed cost
basis, than constant volume control systems. Energy savings can be
further improved to potentially offset these higher costs.
If the minimum total hood flow for a laboratory is lower than the exhaust
flow required to maintain the negative pressure in the lab, a general
exhaust device may be required to provide minimum ventilation and
proper temperature control. In this case, the total exhaust (hoods plus
general exhaust) airflow rate is increased to overcome the added supply
requirements.
200 - 1000 C F M
Hood Exhaust Volume
Variable
CFM
Door
1000 Example:
5 to 1
900
Turndown
100 C F M
Offset Air
Example:
9 to 1
200
Turndown
Supply Air
100 Tracking
The cost of operating a laboratory fume hood is very significant and will
continue to be a major concern until alternative forms of renewable energy
are readily available. As of early 2002, the range of first pass estimates
range from $4 to $7 per CFM per year to operate the laboratory ventilation
systems. Reducing flows when appropriate, such as when the hood is not
in use, can result in significant cost savings.
The standards and guidelines stress the importance of room pressurization for
laboratory spaces. Laboratories that use laboratory fume hoods should be
maintained at a relative negative pressure to corridors and other adjacent spaces in
the building (such as the exception of clean room laboratories that may operate
under positive pressure).
7.3 Diversity
Both existing and new laboratories can benefit from applying diversity to the
HVAC design. Diversity allows existing facilities to add fume hood capacity
using the current HVAC systems. Diversity design in new construction allows
the facility to reduce capital equipment expenditures by downsizing the
mechanical systems during the design phase.
Diversity can be applied only after providing the required number of air changes
in the laboratory and the minimum flow to control room temperature. For these
reasons, some laboratories cannot reduce the total hood exhaust flow capacity.
For either type of facility, designers must develop a solution that best fits the
customers’ needs. However, some designers are hesitant to take diversity since
the savings are only realized when the sashes are lowered. Often, this has lead to
systems with methods of “forced” diversity that have proven problematic:
Sizing flow control system based on low face velocity settings. When
designing systems that provide fulltime low flows, sash control must be
managed.
A system that automatically switches between standard and setback flow based on
the presence or absence of a hood user can provide greater diversity than other
systems since some data indicates that researchers may be away from their hoods
more than 20 hours per 24- hour day. 9
• Control Method
§ Constant Volume CV
§ Varial Air Volume VAV
§ Two State Controls
§ Etc.
• Usage Pattern
§ Number of users per fume hood
§ Fume hood usage type
§ User compliance
• Sash
§ Sash type
§ Sash management
• Airflow Requirements
§ Face velocity
§ Cooling airflow rate
§ Minimum ventilation rate
9
Author: Varley, J.O. - ASHRAE Trans. 1993, Vol. 99, Part 2, Paper number DE -93-18-2, 1072-1080,
2 figs., 3 tabs., 6 refs. AND in Laboratory HVAC, 1995, 45-51, ISBN 1-883413-25-7. Author: Parker, J.A.,
Ahmed, O., and Barker, K. A. - Citation: ASHRAE Trans., 1993, Vol. 99, Part 2, Paper number DE -93-18-
3, 1081-1089, 11 figs, 2 tabs.
All ventilated devices used in a laboratory are safety devices and should be carefully
examined for application and safe working practice. Some experts believe that all
ventilated enclosures should be called a laboratory fume hood and tested to fume hood
standards. This is not possible because many enclosures are suitably made of flammable
materials, are sized for their application and operate safely for the intended purpose, but
not as a fume hood.
Products described in this section are not fume hoods by the definition in Section 3.
Testing of these products is not covered in the ASHRAE 110-1995 (or most current
edition) Standard. As such, great care must be taken to insure that the product being
evaluated is functioning safely for the intended purpose. It is not possible for SEFA to
presuppose all applications and as such this section is intended to be used as a
guideline only, not a definitive source. Contact your Chemical Hygiene Officer to
evaluate your specific application.
Special purpose hoods are hoods that are modifications of fume hoods. As such,
they fail to meet the exacting definition of a fume hood and shall be classified as a
special purpose hood. Common modifications to fume hoods include: baffle
designs, sash configurations and locations, size, and materials. Special purpose
hoods are designed specifically for that purpose, where a fume hood tends to
serve a more general application. Special purpose hoods shall be designed, tested,
and operated with their respective intended purpose in mind.
Examples – Multi Sided, Pass Through Hood, Dual Entry Hood, Trifacial
Hood
• Description
• Purpose or Application
• Reference Organization
None
• Testing Recommendations
Some hoods may be tested using the ASHRAE 110-1995 (or most
current edition) Standard. Others will require test modifications due to
size, sash location, and when to test for multiple sash positions.
Consideration must be made to the toxicity of the experiment and
acceptable exposure levels. The manufacturer should make
recommendations for the specific testing of this product including a
velocity profile, smoke visualization, and a filter integrity test if a filter
is part of the system.
• Additional Comments
Contact your Chemical Hygiene Officer for safe exposure levels and
for testing recommendations before working in a demonstration hood.
• Description
• Purpose or Application
• Reference Organization
None
• Additional Comments
• Description
• Purpose or Application
• Reference Organization
None
• Additional Comments
• Description
• Purpose or Application
10
A Down Draft Hood is a Table Top Hood that is vented down through the table top into an exhaust fan
system.
None
• Testing Recommendations
• Additional Comments
• Description
• Purpose or Application
• Reference Organization
None
• Testing Recommendations
• Additional Comments
• Description
• Purpose or Application
• Reference Organization
None
• Testing Recommendations
• Description
• Purpose or Application
• Reference Organization
None
• Testing Recommendations
• Description
• Purpose or Application
• Reference Organization
None
• Testing Recommendations
• Additional Comments
• Description
• Purpose or Application
• Reference Organization
None
• Testing Recommendations
• Additional Comments
• Description
• Purpose of Application
• Reference Organization
• Testing Recommendations
• Additional Comments
• Description
11 th
Industrial Ventilation: A Manual of Recommended Practice, 24 Edition, American Conference of
Governmental Industrial Hygienists, 1330 Kemper Meadow Drive, Cincinnati, OH 45240 ww.acgih.org
• Reference Organization
None
• Testing Recommendations
Contact your Chemical Hygiene Officer for proper use of a slot hood.
The manufacturer should make recommendations for the specific
testing of this product including exhaust volume and smoke
visualization.
• Additional Comments
8.2.3 Snorkel
• Description
• Purpose or Application
Snorkel hoods are used for ventilating laboratory equipment and heat
or nuisance vapor exhaust only.
• Reference Organization
None
• Testing Recommendations
• Additional Comments
Examples: Clean Hoods, Class 10 Fume Hoods, Clean Air Chemical Hoods,
Trace Metals Analysis Hoods, Push/Pull Hoods.
• Description
An exhausted laminar flow (ELF) hood is one that is designed for critical
operations where both a clean air (class 10+) process environment is
necessary, along with adequate protection to the user, from fumes and
particles. ELF hoods are ventilated cabinets, which contain an integral
HEPA/ULPA filtered supply air source. ELF hoods are usually 100% outside
ducted, but may be recirculated in cases where particle entrapment is the
principle objective. ELF hoods contain vertically closing sashes, baffle
systems and often localized exhaust systems within the unit.
• Purpose or Application
ELF hoods are used to protect operators from potentially hazardous fumes,
typically associated with acid digestion or solvent parts cleaning, while
creating clean environmental conditions required for these types of critical
processes.
• Reference Organization
• Testing Recommendations
Because ELF hoods are hybrids between negative and positive pressure
environments, strict attention to balance testing is crucial. Testing to be done
against ASHRAE 110-1995 and Fed. Std. 209E (ISO 14644.1).
• Description
• Purpose or Application
• Reference Organization
• Testing Recommendations
• Additional Comments
• Description
12
NSF49-2002 Class II (Laminar Flow) Biohazard Cabinetry, NSF International
• Purpose or Application
Refer to the Center for Disease Control (CDC) and the National
Institute of Health (NIH) for application information. 13
13
Center for Disease Control and Prevention, 1600 Clifton Road, Atlanta, GA 30333 www.cdc.gov
National Institutes of Health, Bethesda, Maryland, 20892 www.nih.gov
• Testing Recommendations
• Additional Comments
• Description
• Purpose or Application
• Reference Organization
• Testing Recommendations
• Additional Comments
14
The American Glove Box society is a relevant organization and is listed in section 11.0 of this
document.
A ductless hood recirculates air back into the laboratory from the hood chamber.
• Description
A ductless hood is an open faced enclosure designed to protect the user from
laboratory and industrial airborne contaminates, similar to a laboratory fume
hood, but is not connected to a duct system (although options are available for
connecting to a duct system). Instead, the air is recirculated back to the room
atmosphere. The ductless hood's scope of use is limited to the capacity and
capability of the filtration system. The objective of the filtration system is to
reduce the levels of solids, gaseous or vapor constituent to that below the
acceptable TLV limit at the exhaust.
• Reference Organization
A&E – The “Architect and Engineer.” Generic term refers to designers of laboratory
building and ventilation systems.
Acceptable Indoor Air Quality – Air in which there are no known contaminants at
harmful levels as determined by appropriate authorities, and air with which 80% or more
of the people do not express dissatisfaction
Access Opening – That part of the fume hood through which work is performed; sash or
face opening.
ACFM – Actual cubic feet per minute of gas flowing at existing temperature and
pressure. (See also SCFM)
ACH, AC/H (air changes per hour), N – The number of times air is theoretically
replaced during an hour.
Airflow Monitor – Device installed in a fume hood to monitor the airflow through the
fume chamber of a fume hood.
Air Foil – A horizontal member across the lower part of the fume hood sash opening.
Shaped to provide a smooth airflow into the chamber across the worksurface.
Air Volume – Quantity of air expressed in cubic feet (ft3 ) or cubic meters (m3 ).
Baffle – Panel located across the rear wall of the fume hood chamber interior, and directs
the airflow through the fume chamber.
Bench Hood – A fume hood that is located on a work surface. (See superstructure)
Bypass – Compensating opening in a fume hood that functions to limit the maximum
face velocity as the sash is raised or lowered.
Combination Hood – A fume hood assembly containing a bench hood section and a
floor mounted section.
Combination Sash – A fume hood sash with a framed member that moves vertically
housing two or more horizontal sliding transparent viewing panels.
Cross Drafts – Air draft that flows parallel to or across the face opening of the fume
hood.
Diversity – Operating a system at less capacity than the sum of peak demand (ANSI
Z9.5)
Duct Velocity – Speed of air moving in a duct, usually expressed in feet per minute
(fpm) or meters per second (mps).
Exhaust Collar – Connection between duct and fume hood through which all exhaust air
passes.
Exhaust Unit – Air moving device, sometimes call a fan, consisting of a motor, impeller
and housing.
Face Velocity – Average speed of air flowing perpendicular to the face opening and into
the fume chamber of the fume hood and expressed in feet per minute (fpm), measured at
the plane of the face or sash opening.
Face – Front access or sash opening of laboratory fume hood. Face opening measured in
width and height. See sash or access opening.
Fan – Air moving device, usually called an exhaust unit, consisting of a motor, impeller
and housing.
Fan Curve – A curve relating pressure vs. volume flow rate of a given fan at a fixed fan
speed (rpm).
Friction Loss – The static pressure loss in a system due to friction between moving air
and the duct wall; expressed as inches w. g. 100 feet, or fractions of VP per 100 feet of
duct (mm w. g./meter; K/meter).
Fume Chamber – The interior of the fume hood measured width, depth and height
constructed of material suitable for intended use.
Gauge Pressure – The difference between two absolute pressures, one of which is
usually atmospheric pressure; viz, inches water gauge (in w. g.).
Glove Box – Total enclosure used to confine and contain hazardous materials with
operator access by means of gloved portals or other limited openings; this device is not a
laboratory fume hood.
Hood Entry Loss (He) – The static pressure loss, stated in inches w. g., when air enters a
duct through a hood. The majority of the loss is usually associated with a vena contracta
formed in the duct.
Hood Static Pressure (sph) – The sum of the duct velocity pressure and the hood entry
loss; it is the static pressure required to accelerate air at rest outside the hood into the duct
at duct velocity.
Inches of Water (inch w.g.) – A pressure term, one inch of water is equal to 0.0735
inches of mercury, or 0.036 psi. Atmospheric pressure at standard conditions is 407
inches w. g. (water gauge).
Indoor Air Quality (IAQ) – The study, evaluation, and control of indoor air quality
related to temperature, humidity, and airborne contaminants.
Industrial Ventilation (IV) – The equipment or operation associated with the supply or
exhaust of air, by natural or mechanical means, to control occupational hazards in the
industrial setting.
Laboratory – The net assignable area in which diverse mechanical services and special
ventilation systems are available to control emissions and exposures from chemical
operations.
Laboratory Ventilation – Air moving systems and equipment which serve laboratories.
Laminar Flow (Also Streamline Flow) – Airflow in which air molecules travel parallel
to all other molecules; flow characterized by the absence of turbulence.
Liner – Interior lining used for side, back and top enclosure panels, exhaust plenum and
baffle system of a laboratory fume hood.
Local Exhaust Ventilation – An industrial ventilation system that captures and removes
emitted contaminants before dilution into the workplace ambient air can occur.
Loss - Usually refers to the conversion of static pressure to heat in components of the
ventilation system, viz., “the hood entry loss.”
Make -up Air – (See Replacement and Compensating Air). Air needed to replace the air
taken from the room by laboratory fume hood(s) and other air exhausting devices.
Minimum Transport Velocity (MTV) – The minimum velocity which will transport
particles in a duct with little settling; the MTV varies with air density, particulate loading,
and other factors.
Natural Ventilation – The movement of outdoor air into a space through intentionally
provided openings, such as windows, doors, or other non-powered ventilators, or by
infiltration.
Occupied Zone – The region within an occupied space between 3” and 72” above the
floor and more than two feet from the walls for fixed air conditioning equipment. (From
ASHRAE Standard 55-1981).
Odor – A quality of gases, vapors, or particles which stimulates the olfactory organs;
typically unpleasant or objectionable.
Outdoor Air (OA) – “Fresh” air mixed with return air (RA) to dilute contaminants in the
supply air (SA).
Particulate Matter – For this Recommended Practice, small lightweight particles that
will be airborne in low-velocity air [approximately 50 fpm (.25m/s)].
Pressure Drop – The loss of static pressure between two points; for example, “The
pressure drop across an orifice is 2.0 inches w.g.”
Relative Humidity (RH) – The ratio of water vapor in air to the amount of water vapor
air can hold at saturation. A “RH” of 100% is about 2.5% water vapor in air, by volume.
Replacement Air – (Also, compensating air, make-up air) Air supplied to a space to
replace exhausted air.
Respirable Particles – Those particles in air which penetrate into and are deposited in
the nonciliated portion of the lung.
Return Air – Air which is returned from the primary space to the fan for recirculation.
Room Air – That portion of the exhaust air taken from the room.
Sash – A moveable panel or door set in the access opening/hood entrance to form a
protective shield and to control the face velocity of air into the hood.
SCFM (Standard Cubic Feet Per Minute) – Airflow rate at standard conditions; dry air
at 29.92 inches Hg gauge, 70 degrees F.
Scrubber, Fume – A device used to remove contaminants from fume hood exhaust,
normally utilizing water.
Sulfur Hexafluoride (SF6 ). Tracer gas widely used for ASHRAE testing.
Slot Velocity – The average velocity of air through a slot. It is calculated by dividing the
total volume flow by the slot area; usually vs = 2,000 fpm.
Smoke Candle – Smoke producing device used to allow visual observation of airflow.
Spot Collector – A small, localized ventilation hood usually connected by a flexible duct
to an exhaust fan. This device is not a laboratory fume hood.
Standard Air, Standard Conditions STP Dry air at 70 degrees F, 29.92 in Hg.
Static Pressure Loss – Measurement of resistance created when air moves through a
duct or hood, usually expressed in inches of water.
Static Pressure (SP) – The pressure developed in a duct by a fan; SP exerts influence in
all directions; the force in inches of water measured perpendicular to flow at the wall of
the duct; the difference in pressure between atmospheric pressure and the absolute
pressure inside a duct, cleaner, or other equipment.
Suction Pressure – See Static Pressure (Archaic. Refers to static pressure on upstream
side of fan.)
Superstructure – That portion of a laboratory fume hood that is supported by the work
surface.
Thermal Anemometer – A device for measuring fume hood face velocity utilizing the
principle of thermal cooling of a heated element as the detection element.
Threshold Limit Value – Time Weighted Average (TLV-TWA) – The time weighted
average concentration for a normal 8-hour workday or 40-hour work week, to which
nearly all workers may be repeatedly exposed, day after day, without adverse effect.
Total Pressure (TP) - The pressure exerted in a duct as the sum of the static pressure
and the velocity pressure.
Transport Velocity – Minimum speed of air required to support and carry particles in an
air stream.
TWA (Time Weighted Average) – The average exposure at the breathing zone.
Velocity Pressure – Pressure caused by moving air in a laboratory fume hood or duct,
usually expressed in inches of water.
Velocity (V) – The time rate of movement of air; feet per minute.
Volume Flow Rate (Q) –The quantity of air flowing in cubic feet per minute, cfm, scfm,
acfm.
Work Space – The part of the fume hood interior where apparatus is set up and fumes
are generated. It is normally confined to a space extending from six inches (15.2 cm)
152 mm behind the plane of the sash(es) to the face of the baffle, and extending from the
work surface to a plane parallel with the top edge of the access opening.
Work Surface – The surface that a laboratory fume hood is located on and supported by
a base cabinet. In the fume chamber, the surface is recessed to contain spills.
This manual highlights the general principles of ventilation (including basic calculation),
supply systems, exhaust systems, principles of airflow, fans, construction guidelines, and
testing of ventilation systems.
This manual should be used in concert with the SEFA Recommended Practices.
SEFA recognizes and acknowledges the importance of government agencies that produce
documents concerning laboratory ventilation, laboratory fume hoods and laboratory
safety. These agencies include:
FS Federal Specifications
General Service Administration
Specifications and Consumer Information Distribution Center
(WFSIS)
Washington Navy Yard Building 197
Washington, DC 20407
The potential for chemical exposure of personnel in laboratories has promulgated a wide
variety of standards for ensuring proper operation of laboratory fume hood systems. The
requirements and value of the information contained in the different standards will vary
depending on your responsibilities.
The Industrial Ventilation Manual provides one of the best sources of information
on hood and ve ntilation system design.
p. 10-40
“Selection of Hood Face Velocity – The interaction of supply air distribution and
hood face velocity makes any blanket specification of hood face velocity
inappropriate. Higher hood face velocities will be wasteful of energy and may
provide no better or even poorer worker protection.”
“For projected new building, it is frequently necessary to estimate the cost of air
conditioning early, before the detailed design and equipment specification are
available. For that early estimating, the following guidelines can be used.
* Hoods near doors are acceptable if 1) there is a second safe egress from the
room, 2) traffic past hood is low, and 3) door is normally open.
This standard is best suited for health and safety and engineering personnel
responsible for ensuring proper use and design laboratory fume hood systems.
• The ASHRAE 110 Test is the preferred test for initial evaluation of performance.
• New and renovated hoods must be equipped with flow measurement devices.
• Supply air velocities (cross drafts) should be limited to less then 50% of target
face velocity near hood openings.
• The sound pressure level of noise should be limited at worker locations to below
85 dBA. Room noise should be limited to below a noise criterial curve rating of
55 (NC 55).
• Further recommendations are provided for design and use of bypass fume hoods,
VAV hoods, auxiliary air hoods, floor mounted hoods, perchloric acid hoods, and
glove boxes.
ANSI /AIHA Z9.5 Committee issued a clarification letter to address this topic:
p. 1 – 3
Discourage the use of a numerical pressure differential between rooms as a basis
for design. Although it is true that the difference in pressure is the driving force
that causes airflow through any openings from one room to another, specifying
quantitative pressure differential is a poor basis for design. What is really desired
is an offset air volume. Attempts to design using direct pressure differential
measurement and control vs. controlling the offset volume results in either short
or extended periods of the loss of pressure when the doors are open or excessive
pressure differentials when doors are closed, sufficient to affect the performance
of low pressure fans. The direct pressure control systems are also hard to
stabilize, and can cause building pressure problems and result in excessively large
volume offsets in porous rooms. The need to maintain directional airflow at every
instant and the magnitude of airflow needed will depend on individual
circumstances. For example, “clean” rooms may have very strict requirements
while teaching laboratories may only need to maintain directional airflow during
certain activities or emergency conditions. In the later cases, one would simply
use the appropriate offset to maintain directional airflow as needed and
operational procedures during emergencies (i.e., close doors during a chemical
spill).
a) The airflow required to keep the room negative (or in some positive) with
regard to surrounding air spaces. The 10% offset suggested in the comments
may be appropriate in some cases, but has no general validity.
The standard is best suited for persons responsible for ensuring proper operation
of laboratory fume hoods, typically health and safety, engineering and
maintenance.
“As Installed” (AI) Tests – AI tests are conducted after experimental apparatus
have been placed in the hood. The tests are used to determine hood limitations
and the need for special work practices.
"As Used (AU) Tests - AU tests verify the function of the hood in the condition
that the user has established the hood.
p. 30.5
“All laboratory fume hoods and safety cabinets should be equipped with visual
and audible alarms to warn the laboratory workers of unsafe airflows.”
p. 13.11
“In order for the laboratory to act as a secondary confinement barrier …, it must
be maintained at a slightly negative pressure with respect to adjoining areas to
contain odors and fumes. Exceptions are sterile facilities of clean spaces that may
need to be maintained at a positive pressure with respect to adjoining spaces.”
P 45 – 28
Appendix “A-6.4.6. Laboratory fume hood containment can be evaluated using
the procedures contained in the ASHRAE 110, Method of Testing Performance of
Laboratory Fume Hoods. Face velocities of 0.4 m/sec to 0.6 m/sec (80 fpm to
120 fpm) generally provide containment if the hood location requirements and
laboratory ventilation criteria of this standard are met.”
p. 45-13
A measuring device for hood airflow shall be provided on each laboratory hood.
The measuring device for hood airflow shall be a permanently installed device
and shall provide constant indication to the hood user of adequate or inadequate
hood airflow.
The law requires that laboratory facilities have a written Chemical Hygiene Plan
that ensures protection for laboratory personnel, proper operation of laboratory
fume hood systems and training of all laboratory personnel in safe work practices.
p. 484
“4. Ventilation … direct air flow into the laboratory from non- laboratory areas
and out to the exterior of the building …”
p. 192
“In all cases, air should flow from the offices, corridors, and support spaces into
the laboratories. All air from chemical laboratories should be exhausted out-doors
and not recirculated. Thus, the air pressure in chemical laboratories should be
negative with respect to the rest of the building unless the laboratory is also a
clean room.”
15
Prudent Practices in the Laboratory: Handling and Disposal of Chemicals (1995), Committee on
Prudent Practices for Handling, Storage, and Disposal of Chemicals in Laboratories, National Research
Council.
p. 203
“If the hood and the general ventilating system are properly designed, face
velocities in the range of 60 –100 lfm will provide a laminar flow of air ove r the
floor and sides of the hood. Higher face velocities (125 lfm or more), which
exhaust the general laboratory air at a greater rate, are both wasteful of energy and
likely to degrade hood performance by creating air turbulence at the hood face
and within the hood. Such air turbulence can cause the vapors within the hood to
spill out into the general laboratory atmosphere.”
p. 204
“The optimum face velocity of a hood (also called the capture velocity) will vary
depending on its configuration. As noted above, too high a face velocity is likely
to increase the turbulence within the hood and cause gases or vapors to spill from
the hood into the room.”
p. 180
“Make sure that a continuous monitoring device for adequate hood performance is
present and check it every time the hood is used.”
p. 206
“After the face velocity of each hood has been measured (and the airflow
balanced if necessary), each hood should be fitted with an inexpensive manometer
or other pressure – measuring device (or a velocity- measuring device) to enable
the user to determine that the hood is operating as it was when evaluated. This
pressure measuring device should be capable of measuring pressure differences in
the range of 0.1-2.0 in. of H2 0 and should have the lower pressure side connected
to the duct above the hood and the higher pressure side open to the general
laboratory atmosphere.
…the design of the air exhaust system from a laboratory must be done carefully to
provide continuing replacement of fresh air in the room. The fume hood system
and the supplementary exhaust system should be interlocked to ensure a stable
room air balance at all times.”
forms/sefa-1-2001 frm