laboratory ASHRAE.pdf

CHAPTER 14

LABORATORIES

Laboratory Types .................................................................... 14.1

Hazard Assessment ................................................................. 14.2

Design Parameters ................................................................. 14.2

LABORATORY EXHAUST AND CONTAINMENT

DEVICES ............................................................................. 14.3

Fume Hoods ............................................................................ 14.3

Biological Safety Cabinets ...................................................... 14.6

Miscellaneous Exhaust Devices .............................................. 14.8

Laminar Flow Clean Benches ................................................. 14.8

Compressed Gas Storage and Ventilation .............................. 14.8

LABORATORY VENTILATION .............................................. 14.8

Supply Air Systems .................................................................. 14.9

Exhaust Systems ...................................................................... 14.9

Fire Safety for Ventilation Systems ....................................... 14.11

Control .................................................................................. 14.12

Stack Heights and Air Intakes ............................................... 14.13

APPLICATIONS .................................................................... 14.14

Laboratory Animal Facilities ................................................ 14.14

Ancillary Spaces For Animal Laboratories .......................... 14.16

Containment Laboratories .................................................... 14.17

Scale-Up Laboratories .......................................................... 14.18

Teaching Laboratories .......................................................... 14.18

Clinical Laboratories ............................................................ 14.18

Radiochemistry Laboratories ................................................ 14.18

Operation and Maintenance ................................................. 14.18

Energy ................................................................................... 14.19

Commissioning ...................................................................... 14.19

Economics ............................................................................. 14.20

M ity, relative static pressure, air motion, air cleanliness, sound,

and exhaust. This chapter addresses biological, chemical, animal,

and physical laboratories. Within these generic categories, some

laboratories have unique requirements. This chapter provides an

overview of the HVAC characteristics and design criteria for labo-

ratories, including a brief overview of architectural and utility con-

cerns. This chapter does not cover pilot plants, which are essentially

small manufacturing units.

The function of a laboratory is important in determining the

appropriate HVAC system selection and design. Air-handling,

hydronic, control, life safety, and heating and cooling systems

must function as a unit and not as independent systems. HVAC

systems must conform to applicable safety and environmental reg-

ulations.

Providing a safe environment for all personnel is a primary

objective in the design of HVAC systems for laboratories. A vast

amount of information is available, and HVAC engineers must

study the subject thoroughly to understand all the factors that relate

to proper and optimum design. This chapter serves only as an

introduction to the topic of laboratory HVAC design. HVAC sys-

tems must integrate with architectural planning and design, electri-

cal systems, structural systems, other utility systems, and the

functional requirements of the laboratory. The HVAC engineer,

then, is a member of a team that includes other facility designers,

users, industrial hygienists, safety officers, operators, and mainte-

nance staff. Decisions or recommendations by the HVAC engineer

may significantly affect construction, operation, and maintenance

costs.

Laboratories frequently use 100% outside air, which broadens

the range of conditions to which the systems must respond. They

seldom operate at maximum design conditions, so the HVAC engi-

neer must pay particular attention to partial load operations that

are continually changing due to variations in internal space loads,

exhaust requirements, external conditions, and day-night vari-

ances. Most laboratories will be modified at some time. Conse-

quently, the HVAC engineer must consider to what extent

laboratory systems should be adaptable for other needs. Both eco-

nomics and integration of the systems with the rest of the facility

must be considered.

LABORATORY TYPES

Laboratories can be divided into the following general types:

• Biological laboratories are those that contain biologically active

materials or involve the chemical manipulation of these materials.

This includes laboratories that support such disciplines as bio-

chemistry, microbiology, cell biology, biotechnology, genomics,

immunology, botany, pharmacology, and toxicology. Both chem-

ical fume hoods and biological safety cabinets are commonly

installed in biological laboratories.

Chemical laboratories support both organic and inorganic syn-

thesis and analytical functions. They may also include laborato-

ries in the material and electronic sciences. Chemical laboratories

commonly contain a number of fume hoods.

Animal laboratories are areas for manipulation, surgical modi-

fication, and pharmacological observation of laboratory animals.

They also include animal holding rooms, which are similar to lab-

oratories in many of the performance requirements but have an

additional subset of requirements.

Physical laboratories are spaces associated with physics; they

commonly incorporate lasers, optics, nuclear material, high- and

low-temperature material, electronics, and analytical instruments.

Laboratory Resource Materials

The following are general or specific resource materials applica-

ble to various types of laboratories.

ACGIH. Industrial Ventilation: A Manual of Recommended Prac-

tice . American Conference of Governmental Industrial Hygien-

ists, Cincinnati, OH.

AIA. Guidelines for Design and Construction of Hospital and

Health Care Facilities. American Institute of Architects, Wash-

ington, D.C.

AIHA. Laboratory Ventilation. ANSI/AIHA Standard Z 9.5 .

American Industrial Hygiene Association, Fairfax, VA.

CAP. Medical Laboratory Planning and Design . College of

American Pathologists, Northfield, IL.

DHHS. Biosafety in Microbiological and Biomedical Laborato-

ries . U.S. Department of Health and Human Services Publication

(CDC).

EEOC. Americans with Disabilities Act Handbook . Equal

Employment Opportunity Commission.

NFPA. Hazardous Chemicals Data. ANSI/ NFPA Standard 49-94.

National Fire Protection Association, Quincy, MA.

The preparation of this chapter is assigned to TC 9.10, Laboratory Systems.

14.1

ODERN laboratories require regulated temperature, humid-

14.2

2007 ASHRAE Handbook—HVAC Applications (SI)

NFPA. Health Care Facilities. ANSI/NFPA Standard 99-99.

National Fire Protection Association, Quincy, MA.

NFPA. Fire Protection for Laboratories Using Chemicals. ANSI/

NFPA Standard 45. National Fire Protection Association,

Quincy, MA.

NRC. Biosafety in the Laboratory: Prudent Practices for Handling

and Disposal of Infectious Materials. National Research Council,

National Academy Press, Washington, D.C.

NRC. Prudent Practices in the Laboratory: Handling and Dis-

posal of Chemicals . National Research Council, National Acad-

emy Press, Washington, D.C.

NSF. Class II Biosafety Cabinetry. NSF/ANSI Standard 49.

OSHA. Occupational Exposure to Chemicals in Laboratories .

Appendix VII, 29 CFR 1910.1450. Available from U.S. Govern-

ment Printing Office, Washington, D.C.

SEFA. Laboratory Fume Hoods Recommended Practices . SEFA

1.2-1996. Scientific Equipment and Furniture Association, Hilton

Head, SC.

Other regulations and guidelines may apply to laboratory design.

All applicable institutional, local, state, and federal requirements

should be identified before design begins.

Potential changes in the size and number of fume hoods

Anticipated increases in internal loads

Room pressurization requirements

Biological containment provisions

Decontamination provisions

It is important to (1) review design parameters with the safety

officers and scientific staff, (2) determine limits that should not be

exceeded, and (3) establish the desirable operating conditions. For

areas requiring variable temperature or humidity, these parameters

must be carefully reviewed with the users to establish a clear under-

standing of expected operating conditions and system perfor-

mance.

Because laboratory HVAC systems often incorporate 100% out-

side air systems, the selection of design parameters has a substantial

effect on capacity, first cost, and operating costs. The selection of

proper and prudent design conditions is very important.

Internal Thermal Considerations

In addition to the heat gain from people and lighting, laboratories

frequently have significant sensible and latent loads from equip-

ment and processes. Often, data for equipment used in laboratories

are unavailable or the equipment has been custom built. Some com-

mon laboratory equipment information is listed in the appendix of

the ASHRAE Laboratory Design Guide (Dorgan et al. 2002). Data

on heat release from animals that may be housed in the space can be

found in Chapter 10 of the 2005 ASHRAE Handbook—Fundamen-

tals and in Alereza and Breen (1984).

Careful review of the equipment to be used, a detailed under-

standing of how the laboratory will be used, and prudent judgment

are required to obtain good estimates of the heat gains in a labora-

tory. The convective portion of heat released from equipment

located within exhaust devices can be discounted. Heat from equip-

ment that is directly vented or heat from water-cooled equipment

should not be considered part of the heat released to the room. Any

unconditioned makeup air that is not directly captured by an exhaust

device must be included in the load calculation for the room. In

many cases, additional equipment will be obtained by the time a lab-

oratory facility has been designed and constructed. The design

should allow for this additional equipment.

Internal load as measured in watts per square metre is the average

continuous internal thermal load discharged into the space. It is not

a tabulation of the connected electrical load because it is rare for all

equipment to operate simultaneously, and most devices operate with

a duty cycle that keeps the average electrical draw below the name-

plate information. When tabulating the internal sensible heat load in

a laboratory, the duty cycle of the equipment should be obtained

from the manufacturer. This information, combined with the name-

plate data for the item, may provide a more accurate assessment of

the average thermal load.

The HVAC system engineer should evaluate equipment name-

plate ratings, applicable use and usage factors, and overall diver-

sity. Much laboratory equipment includes computers, automation,

sample changing, or robotics; this can result in high levels of use

even during unoccupied periods. The HVAC engineer must evalu-

ate internal heat loads under all anticipated laboratory-operating

modes. Because of highly variable equipment heat gain, individual

laboratories should have dedicated temperature controls.

Two cases encountered frequently are (1) building programs

based on generic laboratory modules and (2) laboratory spaces that

are to be highly flexible and adaptive. Both situations require the

design team to establish heat gain on an area basis. The values for

area-based heat gain vary substantially for different types of labora-

tories. Heat gains of 50 to 270 W/m 2 or more are common for lab-

oratories with high concentrations of equipment.

HAZARD ASSESSMENT

Laboratory operations potentially involve some hazard; nearly

all laboratories contain some type of hazardous materials. Before

the laboratory is designed, the owner’s designated safety officers

should perform a comprehensive hazard assessment. These safety

officers include, but are not limited to, the chemical hygiene officer,

radiation safety officer, biological safety officer, and fire and loss

prevention official. The hazard assessment should be incorporated

into the chemical hygiene plan, radiation safety plan, and biological

safety protocols.

Hazard study methods such as hazard and operability analysis

(HAZOP) can be used to evaluate design concepts and certify that

the HVAC design conforms to the applicable safety plans. The

nature and quantity of the contaminant, types of operations, and

degree of hazard dictate the types of containment and local exhaust

devices. For functional convenience, operations posing less hazard

potential are conducted in devices that use directional airflow for

personnel protection (e.g., laboratory fume hoods and biological

safety cabinets). However, these devices do not provide absolute

containment. Operations having a significant hazard potential are

conducted in devices that provide greater protection but are more

restrictive (e.g., sealed glove boxes).

The design team should visit similar laboratories to assess suc-

cessful design approaches and safe operating practices. Each labo-

ratory is somewhat different. Its design must be evaluated using

appropriate, current standards and practices rather than duplicating

existing and possibly outmoded facilities.

DESIGN PARAMETERS

The following design parameters must be established for a labo-

ratory space:

Temperature and humidity, both indoor and outdoor

Air quality from both process and safety perspectives, including

the need for air filtration and special treatment (e.g., charcoal,

HEPA, or other filtration of supply or exhaust air)

Equipment and process heat gains, both sensible and latent

Minimum ventilation rates

Equipment and process exhaust quantities

Exhaust and air intake locations

Style of the exhaust device, capture velocities, and usage factors

Need for standby equipment and emergency power

Alarm requirements.

Laboratories

14.3

Architectural Considerations

Integrating utility systems into the architectural planning,

design, and detailing is essential to providing successful research

facilities. The architect and the HVAC system engineer must seek

an early understanding of each other’s requirements and develop

integrated solutions. HVAC systems may fail to perform properly

if the architectural requirements are not addressed correctly. Qual-

ity assurance of the installation is just as important as proper spec-

ifications. The following play key roles in the design of research

facilities:

Modular Planning. Most laboratory programming and planning

is based on developing a module that becomes the base building

block for the floor plan. Laboratory planning modules are fre-

quently 3 to 3.5 m wide and 6 to 9 m deep. The laboratory modules

may be developed as single work areas or combined to form multi-

ple-station work areas. Utility systems should be arranged to reflect

the architectural planning module, with services provided for each

module or pair of modules, as appropriate.

Development of Laboratory Units or Control Areas. National

Fire Protection Association (NFPA) Standard 45 requires that lab-

oratory units be designated. Similarly, the International, Uniform,

and Building Officials and Code Administrators International

(BOCA) model codes require the development of control areas.

Laboratory units or control areas should be developed, and the

appropriate hazard levels should be determined early in the design

process. The HVAC designer should review the requirements for

maintaining separations between laboratories and note require-

ments for exhaust ductwork to serve only a single laboratory unit or

control area.

Additionally, NFPA Standard 45 requires that no fire dampers be

installed in laboratory exhaust ductwork. Building codes offer no

relief from maintaining required floor-to-floor fire separations.

These criteria and the proposed solutions should be reviewed early

in the design process with the appropriate building code officials.

The combination of the two requirements commonly necessitates

the construction of dedicated fire-rated shafts from each occupied

floor to the penthouse or building roof.

Provisions for Adaptability and Flexibility. Research objec-

tives frequently require changes in laboratory operations and pro-

grams. Thus, laboratories must be flexible and adaptable, able to

accommodate these changes without significant modifications to

the infrastructure. For example, the utility system design can be

flexible enough to supply ample cooling to support the addition of

heat-producing equipment without requiring modifications to the

HVAC system. Adaptable designs should allow programmatic

research changes that require modifications to the laboratory’s

infrastructure within the limits of the individual laboratory area

and/or interstitial and utility corridors. For example, an adaptable

design would allow the addition of a fume hood without requiring

work outside that laboratory space. The degree of flexibility and

adaptability for which the laboratory HVAC system is designed

should be determined from discussion with the researchers, labora-

tory programmer, and laboratory planner. The HVAC designer

should have a clear understanding of these requirements and their

financial impact.

Early Understanding of Utility Space Requirements. The

amount and location of utility space are significantly more impor-

tant in the design of research facilities than in that of most other

buildings. The available ceiling space and the frequency of vertical

distribution shafts are interdependent and can significantly affect

the architectural planning. The HVAC designer must establish these

parameters early, and the design must reflect these constraints. The

designer should review alternative utility distribution schemes,

weighing their advantages and disadvantages.

High-Quality Envelope Integrity. Laboratories that have strin-

gent requirements for the control of temperature, humidity, relative

static pressure, and background particle count generally require

architectural features to allow the HVAC systems to perform prop-

erly. The building envelope may need to be designed to handle rel-

atively high levels of humidification and slightly negative building

pressure without moisture condensation in the winter or excessive

infiltration. Some of the architectural features that the HVAC

designer should evaluate include

Vapor barriers—position, location, and kind

Insulation—location, thermal resistance, and kind

Window frames and glazing

Caulking

Internal partitions—their integrity in relation to air pressure,

vapor barriers, and insulation value

Finishes—vapor permeability and potential to release particles

into the space

Doors

Air locks

Air Intakes and Exhaust Locations. Mechanical equipment

rooms and their air intakes and exhaust stacks must be located to

avoid intake of fumes into the building. As with other buildings, air

intake locations must be chosen to minimize fumes from loading

docks, cooling tower discharge, vehicular traffic, etc.

LABORATORY EXHAUST AND

CONTAINMENT DEVICES

FUME HOODS

The Scientific Equipment and Furniture Association (SEFA

1996) defines a laboratory fume hood as a ventilated enclosed work

space intended to capture, contain, and exhaust fumes, vapors, and

particulate matter generated inside the enclosure. It consists basi-

cally of side, back and top enclosure panels, a floor or counter top,

an access opening called the face, a sash(es), and an exhaust ple-

num equipped with a baffle system for airflow distribution. Figure

1 shows the basic elements of a general-purpose benchtop fume

hood.

Fume hoods may be equipped with a variety of accessories,

including internal lights, service outlets, sinks, air bypass openings,

airfoil entry devices, flow alarms, special linings, ventilated base

storage units, and exhaust filters. Under counter cabinets for storage

of flammable materials require special attention to ensure safe

installation. NFPA Standard 30, Flammable and Combustible Liq-

uids Code, does not recommend venting these cabinets; however,

ventilation is often required to avoid accumulation of toxic or haz-

ardous vapors. Ventilation of these cabinets by a separately ducted

supply and exhaust that will maintain the temperature rise of the

cabinet interior within the limits defined by NFPA Standard 30

should be considered.

Types of Fume Hoods

The following are the primary types of fume hoods and their

applications:

Standard (approximately constant-volume airflow with variable

face velocity). Hood that meets basic SEFA definition. Sash may

be vertical, horizontal, or combination.

Application : Research laboratories—frequent or continuous use.

Moderate to highly hazardous processes; varying procedures.

Bypass (approximately constant-volume airflow). Standard vertical

sash hood modified with openings above and below the sash. The

openings are sized to minimize the change in the face velocity,

which is generally to 3 or 4 times the full-open velocity, as the

sash is lowered.

14.4

2007 ASHRAE Handbook—HVAC Applications (SI)

Fig. 1 Bypass Fume Hood with Vertical Sash and

Bypass Air Inlet

complete cleaning. The ductwork should have flanged neoprene

gasketed joints with quick disconnect fasteners that can be

readily dismantled for decontamination. High-efficiency partic-

ulate air (HEPA) and/or charcoal filters may be needed in the

exhaust duct.

Application : Process and research laboratories using radioactive

isotopes.

Perchloric Acid. Standard hood with special integral work sur-

faces, coved corners, and non-organic lining materials. Perchlo-

ric acid is an extremely active oxidizing agent. Its vapors can

form unstable deposits in the ductwork that present a potential

explosion hazard. To alleviate this hazard, the exhaust system

must be equipped with an internal water washdown and drainage

system, and the ductwork must be constructed of smooth, imper-

vious, cleanable materials that are resistant to acid attack. The

internal washdown system must completely flush the ductwork,

exhaust fan, discharge stack, and fume hood inner surfaces. The

ductwork should be kept as short as possible with minimum

elbows. Perchloric acid exhaust systems with longer duct runs

may need a zoned washdown system to avoid water flow rates in

excess of the capacity to drain the water from the hood. Because

perchloric acid is an extremely active oxidizing agent, organic

materials should not be used in the exhaust system in places such

as joints and gaskets. Ducts should be constructed of a stainless

steel material, with a chromium and nickel content not less than

that of 316 stainless steel, or of a suitable nonmetallic material.

Joints should be welded and ground smooth. A perchloric acid

exhaust system should only be used for work involving perchlo-

ric acid.

Application : Process and research laboratories using perchloric

acid. Mandatory use because of explosion hazard.

California. Special hood with sash openings on multiple sides (usu-

ally horizontal).

Application : For enclosing large and complex research apparatus

that require access from two or more sides.

Floor-Mounted Hood (Walk-In). Standard hood with sash open-

ings to the floor. Sash can be either horizontal or vertical.

Application : For enclosing large or complex research apparatus.

Not designed for personnel to enter while operations are in

progress.

Distillation. Standard fume hood with extra depth and 1/3- to 1/2-

height benches.

Application : Research laboratory. For enclosing tall distillation

apparatus.

Canopy. Open hood with an overhead capture structure.

Application : Not a true fume hood. Useful for heat or water vapor

removal from some work areas. Not to be substituted for a fume

hood. Not recommended when workers must bend over the

source of heat or water vapor.

Fig. 1 Bypass Fume Hood with Vertical Sash and

Bypass Air Inlet

Application : Research laboratories—frequent or continuous use.

Moderate to highly hazardous processes; varying procedures.

Variable Volume (constant face velocity). Hood has an opening or

bypass designed to provide a prescribed minimum air intake

when the sash is closed and an exhaust system designed to vary

airflow in accordance with sash opening. Sash may be vertical,

horizontal, or a combination of both.

Application : Research laboratories—frequent or continuous use.

Moderate to highly hazardous processes; varying procedures.

Auxiliary Air (approximately constant-volume airflow). A plenum

above the face receives air from a secondary air supply that pro-

vides partially conditioned or unconditioned outside air.

Application : Research laboratories—frequent or continuous use.

Moderate to highly hazardous processes; varying procedures.

Note : Many organizations restrict the use of this type of hood.

Low or Reduced Flow (approximately constant-volume airflow

with variable face velocity). These hoods are designed to provide

containment at lower average face velocities.

Application : Research laboratories—frequent or continuous use.

Moderate to highly hazardous processes; varying procedures.

Process (approximately constant-volume airflow with approxi-

mately constant face velocity). Standard hood with a fixed open-

ing and without a sash. By some definitions, this is not a fume

hood. Considered a ventilated enclosure.

Application : Process laboratories—intermittent use. Low-hazard

processes; known procedures.

Radioisotope. Standard hood with special integral work surface,

linings impermeable to radioactive materials, and structure

strong enough to support lead shielding bricks. The interior must

be constructed to prevent radioactive material buildup and allow

Fume Hood Sash Configurations

The work opening has operable glass sash(es) for observation and

shielding. A sash may be vertically operable, horizontally operable,

or a combination of both. A vertically operable sash can incorporate

single or multiple vertical panels. A horizontally operable sash incor-

porates multiple panels that slide in multiple tracks, allowing the

open area to be positioned across the face of the hood. The combi-

nation of a horizontally operable sash mounted within a single verti-

cally operable sash section allows the entire hood face to be opened

for setup. Then the opening area can be limited by closing the verti-

cal panel, with only the horizontally sliding sash sections used dur-

ing experimentation. Either multiple vertical sash sections or the

combination sash arrangement allow the use of larger fume hoods

with limited opening areas, resulting in reduced exhaust airflow

Laboratories

14.5

requirements. Fume hoods with vertically rising sash sections should

include provisions around the sash to prevent the bypass of ceiling

plenum air into the fume hood.

The volume of auxiliary air must not be enough to degrade the

fume hood’s containment performance.

Auxiliary air must be conditioned to avoid blowing cold air on the

researcher; often the air must be cooled to maintain the required

temperature and humidity within the hood. Auxiliary air can

introduce additional heating and cooling loads in the laboratory.

Only vertical sash should be used in the hood.

Controls for the exhaust, auxiliary, and supply airstreams must be

coordinated.

Additional coordination of utilities during installation is required

to avoid spatial conflicts caused by the additional duct system.

Humidity control can be difficult: Unless auxiliary air is cooled to

the dew point of the specified internal conditions, there is some

degradation of humidity control; however, if such cooling is done,

the rationale for using auxiliary air has been eliminated.

Fume Hood Performance Criteria. ASHRAE Standard 110,

Method of Testing Performance of Laboratory Fume Hoods,

describes a quantitative method of determining the containment per-

formance of a fume hood. The method requires the use of a tracer

gas and instruments to measure the amount of tracer gas that enters

the breathing zone of a mannequin; this simulates the containment

capability of the fume hood as a researcher conducts operations in

the hood. The following tests are commonly used to judge the per-

formance of the fume hood: (1) face velocity test, (2) flow visual-

ization test, (3) large-volume flow visualization, (4) tracer gas test,

and (5) sash movement test. These tests should be performed under

the following conditions:

Usual amount of research equipment in the hood; the room air

balance set

Doors and windows in their normal positions

Fume hood sash set in varying positions to simulate both static

and dynamic performance

All fume hoods should be tested annually and their performance

certified. The following descriptions partially summarize the test

procedures. ASHRAE Standard 110 provides specific requirements

and procedures.

Fume Hood Performance

Containment of hazards in a fume hood is based on the principle

that a flow of air entering at the face of the fume hood, passing

through the enclosure, and exiting at the exhaust port prevents the

escape of airborne contaminants from the hood into the room.

The following variables affect the performance of the fume hood:

Face velocity

Size of face opening

Sash position

Shape and configuration of entrance

Shape of any intermediate posts

Inside dimensions and location of work area relative to face area

Location of service fittings inside the fume hood

Size and number of exhaust ports

Back baffle and exhaust plenum arrangement

Bypass arrangement, if applicable.

Auxiliary air supply, if applicable

Arrangement and type of replacement supply air outlets

Air velocities near the hood

Distance from openings to spaces outside the laboratory

Movements of the researcher within the hood opening

Location, size, and type of research apparatus placed in the hood

Distance from the apparatus to the researcher’s breathing zone

Air Currents. Air currents external to the fume hood can jeop-

ardize the hood’s effectiveness and expose the researcher to materi-

als used in the hood. Detrimental air currents can be produced by

Air supply distribution patterns in the laboratory

Movements of the researcher

People walking past the fume hood

Thermal convection

Opening of doors and windows

Caplan and Knutson (1977, 1978) conducted tests to determine

the interactions between room air motion and fume hood capture

velocities with respect to the spillage of contaminants into the room.

Their tests indicated that the effect of room air currents is significant

and of the same order of magnitude as the effect of the hood face

velocity. Consequently, improper design and/or installation of the

replacement air supply can lower the performance of the fume hood.

Disturbance velocities at the face of the hood should be no more

than one-half and preferably one-third the face velocity of the hood.

This is an especially critical factor in designs that use low face

velocities. For example, a fume hood with a face velocity of 0.5 m/s

could tolerate a maximum disturbance velocity of 0.25 m/s. If the

design face velocity were 0.3 m/s, the maximum disturbance veloc-

ity would be 0.15 m/s.

To the extent possible, the fume hood should be located so that

traffic flow past the hood is minimal. Also, the fume hood should be

placed to avoid any air currents generated from the opening of win-

dows and doors. To ensure the optimum placement of the fume

hoods, the HVAC system designer must take an active role early in

the design process.

Use of Auxiliary Air Fume Hoods. AIHA Standard Z9.5 dis-

courages the use of auxiliary air fume hoods. These hoods incorpo-

rate an air supply at the fume hood to reduce the amount of room air

exhausted. The following difficulties and installation criteria are

associated with auxiliary air fume hoods:

The auxiliary air supply must be introduced outside the fume

hood to maintain appropriate velocities past the researcher.

The flow pattern of the auxiliary air must not degrade the contain-

ment performance of the fume hood.

Face Velocity Test

The safety officer, engineer, and the researcher should determine

the desired face velocity. The velocity is a balance between safe

operation of the fume hood, airflow needed for the hood operation,

and energy cost. Face velocity measurements are taken on a verti-

cal/horizontal grid, with each measurement point representing not

more than 0.1 m 2 . The measurements should be taken with a device

that is accurate in the intended operating range, and an instrument

holder should be used to improve accuracy. Computerized multi-

point grid measurement devices provide the greatest accuracy.

Flow Visualization

1. Swab a strip of titanium tetrachloride along both walls and the

hood deck in a line parallel to the hood face and 150 mm back

into the hood. Caution : Titanium tetrachloride forms smoke and

is corrosive to the skin and extremely irritating to the eyes and

respiratory system.

2. Swab an 200 mm circle on the back of the hood. Define air move-

ment toward the face of the hood as reverse airflow and lack of

movement as dead airspace.

3. Swab the work surface of the hood, being sure to swab lines

around all equipment in the hood. All smoke should be carried to

the back of the hood and out.

4. Test the operation of the deck airfoil bypass by running the cot-

ton swab under the airfoil.

5. Before going to the next test, move the cotton swab around the

face of the hood; if there is any outfall, the exhaust capacity test

(large capacity flow visualization) should not be made.

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