study guide

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 Engineering

 Elimination

 Substitution

 Process change

 Automation

 Isolation

 Ventilation

 Administrative

 Job rotation and job

enrichment

 Reduction of exposure time

 Good work practices

 Good housekeeping

 Regular preventive

maintenance

 Regular monitoring and

inspection

 Training and education of

employees

 Personal protective equipment

 Head

 Eyes

 Face

 Ears

 Body

 Hands and arms

 Feet and legs

 Respiratory system

EHST 3700/3701:

Industrial Hygiene

 To discuss the hierarchy of control strategies

 To explain the basic principles of dilution

ventilation

 To explain the basic principles of local exhaust

ventilation

 To discuss the evaluation of ventilation system

performance

 To discuss regulations and other standards

about ventilation system design and

performance

 To calculate the amount of dilution ventilation

required to reduce contaminants to acceptable

levels

 Hierarchy of control strategies

 Local exhaust ventilation systems

 Dilution ventilation for contaminant

control

Engineering Controls

Administrative

Controls

Personal Protective

Equipment

 Elimination

 Substitution

 Process change

 Automation

 Isolation and

enclosure

 Ventilation

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 Widely used and time-tested approach to

emission and exposure control

General exhaust ventilation (GEV)

Dilution ventilation

Local exhaust

ventilation (LEV)

Process Slot

 Local exhaust ventilation (LEV)

 To capture the contaminant

at the point of generation or

release

 Dilution ventilation

 Based on the use of an added

volume of air to dilute

contaminants without

removing them from the

work area atmosphere

Process Slot

 Designed to contain, control, or capture

emissions at or near their sources

 Attempts to eliminate emissions from the

workroom air

 Provides sufficiently rapid turnover of the

workplace air to achieve effective dilution of

airborne contaminants

 Allow emissions to occur and dilute

contaminants to some acceptable

concentration before the contaminated air

reaches the breathing zone

 Effectively removes contaminant from the

work area

 More efficient in terms of energy

consumption

 Less likely to impact the overall heating and

cooling requirements of a facility

Process Slot

Where airborne contaminants

 Pose a health, environmental, or

fire/explosion hazard

 Are irritating, or create an unacceptable

nuisance (i.e. impaired visibility)

 Create significant housekeeping problems

 Are released at irregular time and in

irregular volumes

 Occur in or near breathing zones of workers

 State, local or federal regulations require

that an LEV system be used

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 Hoods/Intake – captures and draws the contaminant into the system

 Ducts – carry the contaminants from the work area toward the

cleaner and exhaust

 Air cleaner – removes the contaminant from the air before it is

released to the environment

 Fan – provide the necessary movement of air through the system

 Exhaust/ Outlet – air is released from the system

Enclosing Hood Enclosing Hood

 Loss in kinetic energy of the air when it

enters the hood due to turbulence

 Vary with hood design

 Sharp-edged entry with highest losses

 Flange

 Reduces entry losses

 Causes more air mass to enter from the front

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 Velocity of air at the point of capture

 Related to the volumetric flow rate of air

that enters the hood according to the

equation: Q = VA

Higher capture

velocity,

Greater volume of

air into system

Source

Capture

Velocity

Range of Capture Velocities Condition of Dispersion

of Contaminant

Example Capture

Velocity, fpm

Released with

practically no velocity into quiet air

Evaporation from tanks;

degreasing, etc.

50 – 100

Released at low

velocity into moderately still air

Spray booths; intermittent

container filling; low speed conveyor transfers;

welding; plating; pickling

100 – 200

Active generation into

zone of rapid air motion

Spray painting in shallow

booths; barrel filling; conveyor loading; crushers

200 – 500

Released at high initial

velocity into zone at very rapid air motion

Grinding; abrasive blasting;

tumbling

500 – 2000

Duct Velocity

Source

Face Velocity

Capture Velocity

Source

Range of Minimum Duct Design Velocities Name of

Contaminant

Examples Design

Velocity, fpm

Vapors, gases,

smoke

All vapors, gases and smoke Any desired

velocity

(1000 – 2000)

Fumes Welding 2000 – 2500

Very fine light

dust

Cotton lint, wood flour, litho powder 2500 – 3000

Dry dusts and

powders

Fine rubber dust, Bakelite molding

powder dust, jute lint, cotton dust

3000 – 4000

Average

industrial dust

Grinding dust, coffee beans, shoe dust,

granite dust, silica flour, brick cutting

3500 – 4000

Heavy dusts Sawdust (heavy and wet), metal

turnings, sand blast dust, wood blocks

4000 – 4500

Heavy or

moist

Lead dusts with small chips, moist

cement dust, quick-lime dust

4500 and up

 Crossdrafts

 Operator movements/ position

 Thermal currents

 Machine motion

 Material motion

 Eddies at hood entrance

No Crossdraft

With Crossdraft

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Position # 2

Source Airflow

Position # 1

Source

Airflow

 Enclose the process as much as possible

 Place the hood so that contaminants are

drawn away from the breathing zone of the

worker

 Take advantage of process features which

can provide some initial movement of

contaminants toward the intake

 Locate the hood as close as possible to the

point of generation or emission

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 Friction loss

 Loss in some kinetic energy of the air in the

duct due to friction between the air and

side of the duct

 The longer the duct, the greater the total

friction loss

 Other duct features related to kinetic energy

loss

 Duct turns

 Duct tapers

 Connection to multiple ducts to the system

 Duct size

 Velocity pressure (VP)

 Pressure inside the LEV system created by

air movement

 Related to the velocity (V) of air

 V = 4005 x 𝑉𝑃

 V – velocity of air in the duct (fpm)

 VP – velocity pressure of air in the duct

(inches of water gage or inch w.g.)

 V = 4005 x 𝑉𝑃

 Problem 1:

 Velocity of air = 1500 fpm

 Velocity pressure = ?

 Problem 2:

 Velocity pressure = 0.2 inches w.g.

 Velocity of air = ?

 TP = SP + VP

 Velocity pressure (VP) – pressure inside the

LEV system created by air movement

 Static pressure (SP) – pressure exerted in all

directions by air moving inside the ducts on

an LEV system

 Total pressure (TP) – sum of VP and SP

1. Velocity-pressure method

 Sizing the ducts and

fittings to maintain the

desired velocity in the

system

2. Dampers or slide gates

 Blocking parts of the

system in order to

achieve the desired

airflow

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1. Velocity-pressure method

 Sizing the ducts and fittings to maintain the

desired velocity in the system

 Less susceptible to alteration or tampering by

workers

 Prevents clogging of the LEV system

 Required for LEV systems used to control

explosive or radioactive materials

2. Dampers or slide gates

 Blocking parts of the system in order to

achieve the desired airflow

 Drawbacks: erosion of slide gates; dead

spaces behind the gates throughout the

system (contaminant accumulation)

 Not recommended for contaminant control

 Ventilation is one of the most widely used

methods for controlling airborne hazards.

 2 approaches in ventilation: local exhaust

(LEV) and dilution ventilation

 Components of an LEV system: inlet/hood,

ducts, air cleaner, fan and exhaust.

 Velocity: V = 4005 x 𝑉𝑃

 Total pressure is the sum of the static

pressure and velocity pressure.