ENCYCLOPEDIA OF ENVIRONMENTAL SCIENCE AND ENGINEERING - INDOOR AIR POLLUTION docx

In the design of a fume exhaust system utilizing hoods the following factors must be analyzed and evaluated: Capture velocities, Fume hood design, Seven basic hood designs, Makeup air so

I

INDOOR AIR POLLUTION

PART 1

Laboratory work and chemical testing involves procedures

that could contaminate the air inside occupied spaces The

nature of the contaminants varies widely: high humidity

from steam baths, odors from hydrogen sulfi de analyses,

corrosion capabilities of alkalies and acids, solubility of

acetone, explosive properties of perchloric acid, health

haz-ards of bacteriological aerosols, and poisonous properties of

nickel carbonyl Ideally, best procedure is not to emit; but

the next best is to remove or exhaust directly and as close

to the point of origin for safety of laboratory personnel and

protection of property

To achieve this end, the accepted methods used for

containment and removal of contaminants is by

restrict-ing the contaminant procedures to within an enclosure or

hood Simultaneously the air is drawn across the hood face

to capture and remove the contaminants before escaping

into the room

In the design of a fume exhaust system utilizing hoods the

following factors must be analyzed and evaluated: Capture

velocities, Fume hood design, Seven basic hood designs,

Makeup air source, Air distribution, Exhaust system, Exhaust

duct materials, Exhaust air treatment, Special systems

CAPTURE VELOCITIES

Air fl ow rates required for hood exhaust systems are based on

a number of factors, the most important of which is capture

velocity For most applications these will range from 50 to

200 fpm The lower fi gure is used to control contaminants

released at low speed into relatively quiet room air (15 to

25 fpm) The higher fi gure is used to control contaminants

released at high rates Under special conditions hood face

velocities as low as 25 fpm have been used with industrial

type hoods

Conclusions regarding optimum face velocity

selec-tion are rather mixed In conceptual design of a lab facility

this is given much thought and argument, especially when air conditioning is to be included For every 1000 cfm of air exhausted through hoods, 3 to 4 tons of refrigeration are required to be added to system capacity for makeup air

At $1,000 per ton of refrigeration the cost of exhausting

1000 cfm could range from $3,000 to $4,000 This cer-tainly adds to hood burden and capital outlay

Some design emphatically forbid hood face veloci-ties less than 100 fpm Attempts have been made to relate face velocity to hood service by compromising fume hood usage with the added responsibility of super-vision by laboratory personnel to insure that fume hood usage is restricted to the type contaminant for which face velocities were selected To this end, Brief (1963) offers

a method of hood classifi cation as a step toward economy

of design and operation He classifi ed type “S” hoods for highly toxic contaminants (threshold limit values less than 0.1 ppm) as having face velocities from 150 to 130 fpm Type “A” hoods for moderately toxic contaminants (TLV’s

of less than 100 ppm) can be sized for face velocities of

100 to 80 fpm Hoods for non-toxic contaminants, Type

“B” (TLV’s above 100 ppm), are sized for a face velocity from 60 to 50 fpm

It should be emphasized that TLV’s should be used with care and not as sole criteria since they represent airborne concentrations that most workers may be exposed to repeat-edly during a normal work day of 8 hours duration for a working lifetime

Fume hood effi ciency depends on the amount of air exhausted and hood design To assure fl exibility of operation and maximum safety to lab personnel, a fume hood should

be designed for exhaust air rates ample for complete removal

of all contaminants This may be a logical step when only one hoods or two are involved in a single facility However, with more than two, generous exhaust through all hoods can impose a heavy initial and operating cost penalty on the air conditioning system From actual experience with labora-tory design, it is diffi cult to select a one-hood design that will satisfy all situations

FUME HOOD DESIGN

The function of a hood exhaust system is to protect the lab

technician from exposure Thus, the heart of the system is

the hood and the design begins with the hood, which is,

at best, a compromise between the ideal and the practical

Basically, a hood is a simple box, Figure 1(a) Without

the necessary indraft shown for the basic ventilated hood,

Figure 1(b), the material inside the hood can become

airborne and be emitted into the room by one or a

combi-nation of the following normal laboratory operations:

ther-mal action and convection currents, mechanical agitation,

aspirating action by cross currents of the air outside the

box Material can escape from the basic hood only through

the door or opening in front However, in the simple

ven-tilated hood, contaminants are kept inside by the action of

the air fl owing into the opening To contain and keep the

material from escaping, suffi cient air must be exhausted

to create and maintain an indraft through the face of the

hood opening

Hoods should control contaminated air so that the

contam-inant does not reach the breathing zone of the lab technician in

signifi cant quantities

Nearly all hood designs presently in use attempt to

provide protection in three ways: a mechanical shield,

direc-tion of air movement, diludirec-tion of contaminant by mixing

with large volumes of air inside the hood The

mechani-cal shield comprises the hood sash When an experiment

is being set up the sash is in the raised position In many

experiments, the sash is lowered two-thirds the way down

or even closed off entirely while an unattended experiment

is being carried out Only the occasional visit by the

techni-cian is needed Care should be exercised not to lower the

sash to a level that can cause too high an indraft velocity

with attendant overcooling or snuffi ng out of a burner fl ame

Protection is provided by the direction of air fl ow across the

back of the worker and into the hood proper, past the

equip-ment within the hood and thence into the exhaust system,

Figure 1(c) Lastly, because large amounts of air are being

moved through the hood, dilution of the contaminated air

takes place readily and further reduces the hazard of

breath-ing hood air

SEVEN BASIC HOOD DESIGNS Seven basic hood designs are in use, all as shown in Figures 2a–2g

All exhausted air taken from the room This is the simplest, low in initial cost and effective However, high exhaust air rates place a heavy burden on air conditioning capital cost and operation

(Figure 2b)

An attempt to compromise hood effectiveness

to reduce air conditioning load chargeable to the hoods Although low in relative cost, it does reduce air conditioning load but its effectiveness in remov-ing fumes generated within the hood is weakened

Exhaust hoods may be needed at random inter-vals and it is not likely that they would be simul-taneously As with other types of air conditioning loads, there is a usage or diversity factor that is apparent, yet difficult to define precisely This factor depends upon judgment, experience, and logic For example, a large number of hoods in

a laboratory room does not necessarily mean all hoods will be operating at one time since the number of lab personnel will be limited and thus reflect on the number of hoods in operation On the other hand, it is the policy of some

laborato-ries to keep all hoods in operation 24 hours a day,

even though they are used intermittently So much depends on the management of the facility and it behooves the designer to explore the total opera-tion with the ultimate user

Required makeup air is fed directly inside the hood without affecting the overall room air con-ditioning This air need not be cooled in summer but merely tempered in winter Although an additional air handling system is required, the saving on the air conditioning load can offset

(a) Correct Distribution (b) Improper Distribution (c) Relevant to Worker

FIGURE 1 Flow directions through hoods.

this Cost of hood runs medium to high but unless

carefully designed and balanced fume removal

effectiveness can be poor

Because of the additional duct system required

such a system is relatively more expensive,

rela-tively low cost effect on air conditioning, and

because air is being exhausted across the hood

face, fume removal effectiveness is good

This allows ample opportunity for the conditioned

air to mix with room air and it becomes often

necessary to sensibly cool but not dehumidify this

auxiliary supply Because air is exhausted across

the hood face, fume removal effectiveness is good

Compared to the conventional hood with its

vertical sliding door, the horizontal sliding ash

unit presents much less area to be exhausted and

total exhaust is thereby reduced Relative cost of

the hood is low and since less air is exhausted, air

conditioning costs are low Air conditioning and

fume removal effectiveness are good

To keep hood face disturbances to a minimum high veloc-ity streams from the air conditioning system should not be permitted to disturb the even, smooth fl ow of air across the hood face

MAKEUP AIR SOURCE Makeup air to balance that exhausted is the most essential design feature of any hood exhaust system When a fume hood is operating poorly, closer analysis will most often show inadequate makeup air supply There is no air for the hood to

“breathe” and an improperly sized makeup system will starve the fume hood and restrict its intended operation

Some designs depend on air drawn from adjoining corridors and offi ce spaces Introduction of makeup air

by indirect means is an economical approach However, such a system can lead to balancing problems and cross-contamination between laboratory spaces Positive introduc-tion of air from corridors and offi ce spaces by use of trans-fer fans can improve this It has been found that the most reliable, fl exible, and easily maintained system arrangement

is that in which an adequate supply of outside conditioned

Conventional hood: All air taken from

the room.

Conventional hood with reduced face velocity.

Externally supplied hood.

Conventional hood with use

or diversity factor.

Perforated ceiling supply hood.

Horizontal sliding sash door hood.

(all room air make-up) Internally-supplied hood.

(f) (c)

(b)

(a)

(e)

FIGURE 2 Hood designs.

air as makeup is supplied to the laboratory space to balance

the air being exhausted It is good practice to supply a little

less makeup air this way than that being exhausted A slight

negative pressure will be maintained, drawing air through

door louvers from corridors or adjacent offi ces

Air exhausted from a hood is never recirculated so that

hood burden goes up Operating costs can be reduced by

supplying makeup air from an auxiliary source instead from

the cooling system The air handled is fi ltered and tempered

in winter only Of the seven basic hood designs, numbers 4d,

4e, and 4f make use of the auxiliary system An auxiliary

system can be either a central unit or unitary type with an

outside air inlet for each laboratory Correct selection of the

type of makeup air system can be made only by an

engineer-ing analysis and fl ow sheet of the hood exhaust system

One of the most important characteristics of an exhaust

system is that at some point the system must end and discharge

to atmosphere Unfortunately, while the exhaust system has

ended at this point, the problems associated with that exhaust

system may have just begun If too much air discharged from

an exhaust system is recirculated through the supply system

not much good has been accomplished If by poor design the

exhaust air is not properly located with respect to the intakes

of other supply systems, potentially disastrous results can

be attained Many poor designs are commonplace The real

cure for this type of problem is not higher exhaust velocities,

higher stacks, better weather caps, better separation of

dis-charge and intake openings, or other, although one or more of

these can contribute to the cure The real remedy must start

back at the source of contamination itself

Because the pattern of natural air fl ow around buildings

is not predictable, contamination by the location of vent effl

u-ents and air intakes is diffi cult to put to practice Halitsky 2

(1963) and Clarke 3 (1967) have advanced theoretical

knowledge and rule of thumb that aid greatly in the solution

of such problems

AIR DISTRIBUTION

In review, air movement within each room of a laboratory

complex must be such that a defi nite fl ow pattern will be

maintained throughout the building along with fl ow from

non-contaminated to potentially contaminated areas To

bring about this differential fl ow pattern, the nature barriers

between the various classes of rooms will assist The pattern

will also be assisted by supplying outside clean air to the

non-contaminated and semi-non-contaminated areas and by

exhaust-ing air only from the moderately and extremely contaminated

areas In general, supply fans should take suction from the

upper portions of the building Also, the exhaust fans should

discharge to the outdoors through stacks of varying heights

depending on adjacent structures To help, the building should

be maintained at a slight positive pressure with respect to the

outdoors Laboratory rooms should be maintained at a

nega-tive pressure with respect to the surrounding rooms

Only an adequate supply of makeup air to satisfy exhaust

needs will keep the building in balance This certainly implies

there must be an excess of supply over exhaust needs In actual installations, experience shows that when two fans are exhaust-ing from the same space with no provision for makeup air, the stronger fan will take command and outside air will enter the room through the weaker fan system When there are multiple exhaust hoods and no makeup air, with one hood off, outside air can downdraft through the idle fan When a fan must exhaust from a room without makeup, fan capacity will be reduced from design and will result in less control at the hood

EXHAUST SYSTEM The exhaust system being under negative pressure will cause leakage fl ow to be drawn into the system and contamination will be confi ned Best location for an exhaust fan serving hoods is on the roof Then all exhaust ductwork will be on the suction side of the fan and indoors But this is not always possible If the fan location must be indoors, say just above the hood, then careful attention must be paid to duct tight-ness on the discharge side When fl ammable material is han-dled, mounting fan on roof is a distinct advantage because explosion-proof construction may not be required of the fan motor However, fan wheel should be non-ferrous and inside casing should be epoxy coated for corrosion protection

EXHAUST DUCT MATERIALS

In many buildings ductwork is often concealed in ceilings or inside walls, making duct inspection and replacement a major problem Where this condition exists it is reasonable to use ductwork with long life expectancy For chemicals used in lab-oratories, galvanized iron and black iron ductwork are highly susceptible to corrosion Stainless steel, transite, polyvinyl chloride-coated steel or fi berglass- reinforced plastic (FRP) ductwork will not require early replacement for such corro-sive service but are costly Actually, selection of materials will depend on the nature and concentration of contaminants

or chemical reagents, space conditions, cost, accessibility Whatever materials are selected, duct joints must be leaktight and the ductwork should have ample supports For best ser-vice life all longitudinal duct seams should be run along the top panel An extensive duct system should have inspection and cleaning facilities Ducts that could develop condensa-tion loading should pitch toward a pocket in the bottom of the run and be provided with a trapped drain

Type 316 passive stainless steel may be used for bac-teriological, radiological, perchloric acid and other general chemical purposes 316 stainless steel is easy to work but

is not suitable for chemical hoods handling concentrated hydrochloric and sulfuric acids

EXHAUST AIR TREATMENT Gases that are bubbled through reaction mixtures and then discharge to the hood are generally, by their nature, reactive

enough to be completely eliminated by a scrubber of some

design For materials that are acidic, a simple caustic scrubber

is all that is necessary to assure essentially complete control

Similarly, for materials of a basic nature, an acid scrubber may

be used to advantage For those materials that do no react

rap-idly with either caustic or acidic solutions, a column fi lled with

activated charcoal will always provide the desired control

Perchloric acid is highly soluble in water and hoods

have been developed with packed sections built into the

hood superstructure and provided with water wash rings

in the ductwork downstream of the scrubber to prevent

buildup of perchlorates which are explosive on contact

Fume hoods handling highly radioactive materials should

have HEPA fi lters upstream and downstream of the hood

For highly hazardous bacteriological experiments safety can

be achieved only by incineration of the exhaust air stream,

which is heated to about 650°F to destroy the bacteria

SPECIAL SYSTEMS

Lowered Sash Operation

A hood exhaust fan maintains proper capture velocity when

the sash is wide open, but the exhaust’s hood’s vertically

slid-ing sashes are sometimes lowered to within a few inches of

the work surface when the hood is in operation A method in

use to reduce waste of conditioned air and also to achieve a

more constant face velocity over the range of sash positions

is the use of a 2-speed fan for each hood When the sash is

pushed up the fan runs at high speed A micro-switch mounted

in the hood is tripped by the sash when it is lowered below a

predetermined position The volume of air the fan will pull

on low speed is adequate to maintain desired face velocity for

the smaller cross-sectional area The proper placement of the

switch setting can be 50 to 60% of the vertical face opening,

i.e., the fan would go on low speed when the sash is lowered to

50 to 60% of opening This holds for all exhaust hoods despite

differences in hood dimensions and other variations in exhaust

systems It has been found to apply equally as well to hoods

with minimum face velocities of 80, 100, and 125 fpm The

volume of conditioned air that is normally lost is reduced by

about one-third when the sash is below the set point

In a conventional hood with a single speed fan, the

excessively high face velocities experienced at low sash

set-tings and the cooling effect on the backs of lab personnel

using the hood has an overall adverse effect Further, still,

when the laboratory technician stands in front of a hood in

operation his body presents an obstruction to the fl ow of

air into the hood Thus, a low pressure area develops in the

space between the man and the hood Under certain

condi-tions, the resulting low pressure area can cause fumes to be

aspirated from the hood and out into the room The

reduc-tion in face velocity using the 2-speed fan reduces the

prob-ability of such a hazardous condition developing

Type “S” hoods should be provided with fan speeds

so that at no point across the hood face should a velocity

greater than 250 fpm exist Another way to control this

velocity is to provide by-pass dampers in the exhaust duct just downstream of the hood itself By-pass hoods are made to accomplish this effect by providing this feature in the hood structure itself

By-Pass Hoods

These provide for a constant rate of room exhaust and uni-form face velocities at any door position They stabilize the room exhaust and the room they supply The by-pass may be

an integral part of the hood itself As the hood door begins to close, the damper starts to open Another important aspect and advantage of the by-pass hood is that the hood interior

is continuously being purged of fumes even while the door

is closed tight For the by-pass hood see Figure 3

Supply Air Hoods

Two types are commercially available The fi rst has auxil-iary air introduced outside and in front of the sash, normally from the overhead position In this design the auxiliary air supply is drawn into the sash opening as a part of the room air Relative cost of this type compared to the conventional is high Relative cost of air conditioning is low because amount

of room air exhausted is reduced Air conditioning effective-ness, fume removal effectiveeffective-ness, and convenience to lab personnel are good However, acceptability to local authori-ties should be investigated See Figure 4(a)

In the second type, auxiliary air is fed directly into the hood

on the inside Relative cost is high, cost of air conditioning is low, air conditioning effectiveness is good, but fume removal effectiveness is poor Because effective face velocities can drop

TYPICAL BY-PASS HOOD

Safety Shelf

By-pass Damper

FIGURE 3

below the safe value needed to prevent leakage of fumes, its use

is discouraged by many health authorities See Figure 4(b)

Induction Venturi

For many fume exhaust applications such as those

involv-ing hazardous fumes or gases, the conventional exhaust

method of passing gases through the fan casing could be

potentially hazardous With exhaust from perchloric acid

fume hoods in particular, a build-up of crystals can occur

on duct walls and fan This crystalline growth is explosive

under normal conditions and special treatment of such a

system is mandatory

To overcome this, there are commercially available

induction venturi systems with water wash facilities Since

perchloric acid crystals are highly soluble this system is

provided with spray rings or nozzles and are washed down

internally at regular intervals Drainage is provided to a

trough attached to the back of the hood table (see Figure 5)

System operation is accomplished by introducing a high

velocity air stream jet inside a specially designed venturi

This in turn induces a fl ow of gas into the venturi inlet This

induced fl ow can then be used to exhaust the hood without

any of the gas having to pass through the fan Venturi is

usu-ally of stainless steel (316L) Blower is mild steel Such a

system used to exhaust 12000 cfm against ½ w.g required a

primary fl ow of clean air of 500 cfm and ¾ hp fan motor

Other perchloric acid fume exhaust systems use fans of

PVC construction but wash the gas stream upstream of the

fan Its construction is also PVC Each hood should be

pro-vided with its own exhaust system; no combinations should

be manifolded Organic compounds must be avoided in the

construction of the system as well as the chemical used in

testing inside the hood

Multihood Single Fan System

Should each fume hood be provided with its own exhaust

fan or should several hoods be serviced by one fan common

to all? A common exhaust duct and fan system may be

used if the facility handles similar and compatible chemi-cal reagents In the consideration of exhaust systems for

a chemical research facility, where the chemical nature of the reagents to be used cannot be predicted in advance, or cannot be controlled, safest procedure is to use separate and individual exhaust fans and ducts

DESIGN PROCEDURE For a most economical design and the use of the various cri-teria outlined herein the following procedure is suggested: 1) Set inside conditions of dry bulb temperature and relative humidity in the upper range of the comfort zone Since relative humidity is criti-cal to operating costs, place greater emphasis on this aspect

2) Select a hood face velocity sufficiently high to control the type hazard, using the recommen-dations outlined in reference 1 Review hood

FIGURE 4 Auxiliary air supply schematic.

Perchloric Acid Hood

Drain

Flushing Water

35 psi

Flushing rings every 10 to 12 ft in vertical as well as horizontal runs

of duct

Flushing Ring

Venturi

Roof

Eductor Nozzle

FIGURE 5 Induction venturi system.

operation carefully since not all hoods require

same face velocities

3) In cooperation with laboratory management

determine the minimum number of hoods

requir-ing continuous operation Determine if a hood or

hoods can operate intermittently or a minimum

and estimate if its exhaust flow can be eliminated

insofar as its effect on air conditioning load is

concerned

4) Avoid the use of hoods to store material and

merely provide local exhaust

5) Determine the acceptability of face screens or

shields or horizontal sliding panels

6) Locate hoods so that they are set clear of

door-ways and frequently traveled aisles

7) Determine if laboratory management is willing to

take a “slip” in room conditions when more air is

exhausted than is originally planned

8) Consider use of perforated ceiling supply hood

arrangement with conditioned air supply through

ceiling diffusers for spot cooling effect

General

Exhaust stack should be vertical and straight and discharge

up; no weather caps should be used Brief 1 suggests when

open-face velocities exceed 125 fpm, install an atmospheric

damper downstream of the hood just before the exhauster to

prevent excessive indraft velocities when conventional hoods

are used At high face velocities, laboratory equipment placed

within the hood should be set so that points of release of

con-taminant are at least 6 inches back of the hood face This can

be ensured by placing a ¼ inch thick edging 6 inches wide on

the bench top near the hood entrance face Brief 1 found that

concentrated head loads within the hood proper, exceeding

1000 watts per foot of hood width created thermal vectors

that require higher face velocities for control Obstruction of

hood face by large objects is discouraged; blockage causes

control problems

PART 2

Factors to Be Considered in Fume Hood Selection

In the selection of a fume hood the following factors should

always be considered:

Space

• What actual space requirements will be required?

• What are the future requirements?

• What physical space is available?

Function

• What chemicals and procedures will be involved

in this application? (Highly corrosive, TLV, etc.)

• High heat procedures?

• Extremely volatile?

Location

• Are your present ventilation capabilities ade-quate and will they be taxed by the new hood installation?

• Is the area where the hood will be installed adequately suited to the new installation? For instance, high traffic areas give rise to undesirable crosscurrents and cause materials to be drawn from hoods Hoods should not be installed next to doors but preferably in corners

• Is the operation such that the use of an auxiliary air system might compromise the safety of the oper-ator? Safety is paramount in any hood application

Hood Construction Materials

Although basic hood design has changed very little, many advances have been made in the materials from which hoods are constructed Here are some of the basic materials and their more distinctive features

Wood

• Generally poor chemical resistance

• Inexpensive to fabricate and modify in the field

• Can present a fire hazard in applications involving heat and flame

• Poor light reflectivity causes a dark hood interior

Sheet Metal (Cold rolled steel or aluminum)

• Requires secondary treatment for chemical resistance

• Demands extreme care to avoid damaging the coat-ing since corrosion can occur in damaged areas

• “Oil canning” due to thin-gauge metal causes noise in operation

• Relatively inexpensive

• Usually heavy and cumbersome to install

Fiberglass

• Excellent chemical resistance

• Lightweight for ease of installation or relocation

• Easily modified in field with readily available tools

• Sound-dampening because of physical construction

• Some inexpensive grades can cause fire hazards and are not chemically resistant

• Available with good light reflective properties for

a light and bright work space

• Shapes are limited to tooled mold configurations, and can be moulded with covered interiors

Cement/Asbestos (Transite)

• Excellent chemical resistance

• Has inherent sound dampening qualities

• Excellent fire resistance

• Heavy and difficult to install

• Extremely brittle, requiring care in handling to

avoid breakage

• Poor light reflectivity

• Stains badly when exposed to many acids, etc

• Easily modified in field with only minor tooling

difficulties

• Inexpensive

Stainless Steel

• Better general chemical resistance than cold rolled

steel

• Not well suited to many acid applications

• Generally provided in type 316 for specific

applications to which it is well suited such as

perchloric acid

• Heavy and expensive

• Difficult to modify in field

• Excellent fire resistance

Polyvinyl Chloride

• Excellent chemical resistance except for some

solvents

• Good fire-retardant properties

• Particularly well suited to acid digestion

applica-tions such as sulfuric and hydrofluoric

• Easily modified in field

• Generally not available in molded configurations

• Expensive

• Distorts when exposed to intense direct heat

Stone

• Excellent chemical resistance

• Excellent fire resistance

• Difficult and extremely heavy to install

• Extremely difficult to field modify

• Expensive

WALK-IN HOOD

This type of hood was not mentioned in Part I but will be

now included The walk-in hood is a standard hood whose

walls extend to the fl oor, thus providing suffi cient space to

accommodate a more elaborate experimental setup

requir-ing additional height Such hoods have double or triple hung

sashes, which may be raised and lowered to provide access

to any part of the setup while the remaining space is enclosed

to contain fumes The back baffl e of such a hood extends

over the full height of the hood and is equipped with at least

three adjustable slots to regulate the amount of air passing

over various parts of the setup

SPECIAL PURPOSE FUME HOODS

Perchloric Acid Fume Hood

Due to the potential explosion hazard of perchloric acid in

contact with organic materials, this type hood must be used

for perchloric digestion It must be constructed of relatively

inert materials such as type 316 stainless steel, Alberene stone,

or ceramic coated material Wash-down features are desirable since the hood and duct system must be thoroughly rinsed after each use to prevent the accumulation of explosive residue Air

fl ow monitoring systems are recommended to assure 150 fpm open face velocity operation An additional monitoring system for the wash-down facilities is also recommended

Radiological Fume Hoods

Hoods used for radioactive applications should have integral bottoms and covered interiors to facilitate decontamination These units should also be strong enough to support lead shielding bricks in case they are required They should also

be constructed to facilitate the use of HEPA fi lters

Canopy Fume Hoods

Canopy fume hoods are a type of local exhauster which nor-mally has limited application in a laboratory Their main dis-advantage is the large amount of air required to provide an effective capture velocity Since the contaminant is drawn across the operator’s breathing zone, toxic materials can be quite dangerous A canopy hood can, however provide a local exhaust for heat or steam

INTEGRAL MOTOR-BLOWERS Many hoods are available with motors and blowers built directly into the hood superstructure From the standpoint

of convenience, the hood is relatively portable and can be installed easily A built-in motor-blower should not be used for highly toxic applications since it causes a positive pres-sure in the exhaust system ductwork and any leaks in the duct could spill the effl uent into the lab area There may be more noise associated with this type hood since the motor-blower is closer to the operator

Fume Discharge

Each individual exhaust fan on the roof should have its own discharge duct to convey the fumes vertically upward at a high velocity as far above the topmost adjacent roof as pos-sible Failure in this will result in potential recirculation

of fumes into building air intakes and will be particularly hazardous to personnel who use the roof for maintenance, research, or relaxation

As the wind blows over the leading edge of a roof para-pet, as shown in Figure 6, a disturbance is created that sweeps from the edge of the parapet up over the top of the building Above the boundary of this disturbance, wind fl ow is undis-turbed Below the boundary, the infl uence of the sharp edge

of the building creates eddy currents that can pocket fumes released at the roof This is known as the wake cavity Unless fumes are discharged into the undisturbed air stream above the boundary, where they can be carried away, they will remain relatively undisturbed and undiluted on the roof and in the lee

of the building, where they can enter the building air intakes either on the roof or at ground level When this happens, all the care taken in the design of a good fume exhaust system

may be nullifi ed And with the present concern over air

pollu-tion, failure to disperse the fumes may give rise to legal action

against the building owner

Fume absorbers such as charcoal have been proposed

to relieve the fume disposal problem, so have air washers

and catalysts These devices have not been used because

the kinds and amounts of fumes released are constantly

changing in research and are therefore unpredictable

Despite the number of warnings in the literature, rain

caps, cone shaped covers or hoods fastened to the tops of

ver-tical stacks—are still being used to prevent rain from

enter-ing exhaust stacks It is important that their use be avoided

completely There are several simple stack arrangements that

will prevent entry of rain into exhaust stacks when fans are

not operating One such arrangement is shown in Figure 7

BUILDING AIR INTAKES

In high-rise research buildings, mechanical equipment is

frequently installed in the penthouse and in the basement

Because of the possibility of recirculating fumes released from

or near the roof, outdoor air is often taken at the second fl oor

level on the prevailing wind side of the building, and away

from fume exhausts Assistance in determining the prevailing

wind direction at the building site may be obtained from the

local weather bureau

BASIC PERFORMANCE CRITERIA

The following may be used as a general guide for the

selec-tion of hood blower systems that will provide optimum

AIR FLOW PATTERNS AROUND A BUILDING

Wake cavity boundary

Wake boundary Free stream

Wind direction

Building

Peripheral flow

Cavity

Return flow

FIGURE 6

Very low toxicity level materials Noxious odours, nuisance dusts and fumes 80 fpm General lab use

Corrosive materials Moderate toxicity level materials (TLV of 10–1000 ppm)

100 fpm

Tracer quantities of radioisotopes Higher toxicity level materials (TLV less than 10 ppm)

125–150 fpm

Pathogenic microorganisms High alpha or beta emitters Very high toxicity level materials (TLV less than 0.01

ppm)

An enclosed glove box should be used

average face velocities for various exhaust materials Tables listing the TLV for various chemical compounds may be obtained from the American Conference of Governmental Industrial Hygienists

HOW TO CUT AIR CONDITIONING COSTS

As a rule of thumb, each 300 cfm of air exhausted through hoods requires one ton of refrigeration Current operating costs are about 50 to 60 dollars per ton of air conditioning for a four month period Installed equipment averages about

$1,000 per ton So, a hood exhausting at 900 cfm would require about three tons of air conditioning at a capital expense

of $150 to $180 per season However, if the same hood had

the Add-Air feature supplying 50% untempered air, $1,500

would be saved in capital equipment and $75 to $90 to annual

operating costs (Figures are in 1992 $.)

MAINTENANCE AND TESTING

Since the hood performance may be affected by the

cleanli-ness of the exhaust system and the direction of rotation of

the exhaust fan, it is important to provide a maintenance

schedule of inspections and performance testing throughout

the year to make certain that the fume hoods are operating

safely and effi ciently

If fi lters are used to remove dust and other particulates

from the exhaust air, they must be periodically inspected and

replaced if necessary Corrosion of ductwork and damper

mechanisms should be watched and debris should be removed

from inside the ducts, especially at startup time Excessive

cor-rosion of ducts may cause leakage of air into the system or the

failure of balancing dampers that will affect capture velocities

well below their design fi gures Remember to check fan

rota-tion since this most often causes poor exhaust performance

PERFORMANCE TESTING

Two performance tests should be conducted periodically

on all hoods One for fume leakage and the other for face

velocity The test for fume leakage consists of releasing

odorous fumes such as ammonia or hydrogen sulfi de within

the hood If fumes are detected outside the hood, especially

around the face opening, the capture velocity at the sash

opening may be inadequate, or there may be an interfering

air disturbance Cleaning the exhaust system, adjusting the

air fl ow damper, or increasing the fan speed may improve

the performance if low face velocity seems to be the prob-lem If, on the other hand, leakage seems to be caused by interference from an auxiliary air supply stream or other velocity near the sash, the nature of the interference may be investigated as follows: placing liquid titanium tetrachloride

on masking tape around the periphery of the sash opening Observations can then be made of the path of visible fumes

to determine where there is spillage into the room Smoke bombs have also been used to determine fl ow patterns at sash openings and to identify interference

A hot wire anemometer is usually used to measure actual face velocity This is done as a traverse over the entire sash opening, including especially all edges and corners The overall face velocity average is obtained by averaging the velocity readings at prescribed positions of the traverse

These testing procedures are diffi cult to standardize and are dependent on subjective observations Thus, they are

considered to be unadaptable and inadequate The American

Society of Heating, Refrigeration, and Air Conditioning Engineers (ASHRAE) has set up a research project for

developing fume hood performance criteria and new test procedures for such laboratory equipment

SAFETY FEATURES

Interconnection of Hoods

If two or more hoods independently serve a single room or

an interconnecting suite of rooms, all of the hoods in these rooms should be interconnected so that the operation of one will require the operation of all If this is not done, there is a strong possibility that fumes will be drawn from a hood that is not operating to makeup air demands of those in operation

Alarm for Hood Malfunction

All hoods should be equipped with safety devices such as sail switches to warn personnel that the air volume exhausted from the hood has dropped to a point where it will not pro-vide suffi cient capture velocity for safe operation

Fire Dampers

Most building codes require fi re dampers in all ducts that pass through fi re walls and fl oors However, it is important

not to install them in fume exhaust systems Should a fi re

occur in a hood, or if heat from a fi re nearby such a damper should cause the damper to close, the fume backup into the facility would prove disastrous

EXHAUST FROM LABORATORIES

A laboratory should exhaust 100% of the air fed to it If the materials that are being handled or tested in the laboratory are hazardous enough to need a hood, the presence of these materi-als in itself should dictate 100% exhaust An accidental spill or accidental release of materials at a bench or hood can result in

DRAIN TYPE STACK

Support

overlap

6 in min

1/2 in

drain

D + 1 in

D

FIGURE 7