Industrial Hygiene - Ventilation

Norhidayah binti AbdullOccupational Safety & Health ProgramFaculty of Manufacturing Engineering & Technology Management

Learning Outcome

At end of this topic, student will be able: Explain the principles of general ventilation

for controlling airborne contaminant Explain basic principles of local exhaust

ventilation for controlling airborne contaminant

Recognize whether general ventilation local exhaust ventilation is most appropriate for contaminant control for a given operation or process

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Purposes of Industrial Ventilation Control of toxic air contaminants

to acceptable levels ŠControl of noxious odors ŠControl of heat and humidity for

comfort and health ŠPrevention of fire and explosions

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Types of Industrial Ventilation ŠGeneral ventilation

Control of temperature, humidity and odors

ŠDilution ventilation Maintain control of low toxicity gases

and vapors below acceptable levels through dilution of concentration

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General Ventilation

Supply and exhaust of large volumes of air to dilute and displace contaminants and for ensuring thermal comfort Use for contaminants with low toxicity Passive systems that rely on natural

air movement distribute dilution air Active systems use fans and other

mechanical air-moving equipment

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General Ventilation

Examples of general ventilation Open windows, doors, etc. Build large partially open factory

buildings Heating ventilation and air

conditioning (HVAC) systems in buildings

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BEST Air Inlet BEST Exhaust

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BEST Air Inlet BEST Exhaust

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BETTER Air Inlet BEST Exhaust

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FAIR Air Inlet BEST Exhaust

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POOR Air Inlet POOR Exhaust

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Types of Industrial Ventilation �Local exhaust ventilation (LEV)

Capturing and removing contaminants at or near their sources of emission

Prevents the transmission of contaminant to worker

Given priority in “Hierarchy of Controls”

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What Makes Up An LEV System?

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Examples

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Examples

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Ventilation Terminology

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Capture velocity Air velocity at any point in front of the

hood necessary to overcome opposing air currents and to capture the contaminant at that point causing it to flow into the hood

Important hood/process design criteria

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Face velocity Air velocity at the hood or slot

opening An important design parameter Surrogate marker of performance

(i.e., can be tested)

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Duct velocity Air velocity through the cross section

of the duct Must be sufficient to prevent

gravitational settling of particulate contaminants

Important design parameter Can be measured

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Basic Ventilation Equation

Where: Q = air flow rate

(m3/min) A = cross-sectional

area of duct or opening (m2)

V = average air velocity (m/min)

Also referred to as continuity equation

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Basic Ventilation Equation Example:

If fan is unchanged and number of hoods is doubled, then the resulting hood face velocities will be ½ original velocity (possibly reducing air velocity to less-than-needed capture velocity)

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Hood Design

Purpose: keep contaminant out of breathing

zone Considerations:

minimize interference minimize pressure drop minimize exhaust volume

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Types of Local Exhaust Hoods

Enclosure - contains contaminants released inside the hood

Receiving hood - catches contaminants that rise or are thrown into it e.g. canopy type

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Types of Local Exhaust Hoods

Capturing hood - reaches out to draw in contaminants e.g. slot type

Downdraft hood

High velocity, low volume hood

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Canopy

Canopy designed as receiving hood will not control vapors from unheated tank

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Canopy

Airflow must be increased to use canopy as a capturing hood over unheated process

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Canopy

"Overflow" of contaminants from a receiving hood occurs if more contaminated air enters the hood than the fan exhausts

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Principles of Hood Design Enclose source as much as

practicable Capture/control contaminant with

adequate velocity Keep contaminant out of breathing

zone Discharge air away from fresh air

inlets Provide adequate make-up air

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Hood Proximityand Exhaust Volume To maintain desired capture

velocity, locate hood as close to source as possible

ŠAir volume requirement increases as square of the distance

ŠReduces required make-up air and associated costs

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Hood Proximityand Exhaust Volume

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Use of Enclosures

Using techniques such as enclosures, control capabilities are maximized

ŠAir volumes requirements are drastically minimized

ŠReduces required make-up air and associated costs

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Use of Enclosures

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Direction of Air Movement Direction of air movement should

carry air contaminants away from breathing zone

ŠResults in reduced worker exposure

ŠResults in better hood capture performance

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Direction of Air Movement

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Direction of Air Movement

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Design Velocities

All ventilation systems are designed to operate most effectively within a given air-flow range

Usually measured by hood face �velocity Laboratory hood = 75–100 ft/min

Operation at other than design �velocities can often have unintended (bad) consequences

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Principles of Supply Air

Design Supply air volume = exhaust air volume (balanced)

ŠAvoid interference with exhaust hoods (currents and eddies may compromise exhaust systems)

ŠAir enter at living zone

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Principles of Supply Air

Design Supply air must be conditioned (temperature and humidity)

ŠAir entry points located away from source of contaminants to eliminate air currents which could interfere with exhaust

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Ductwork

Carries contaminant from hood to discharge

Straight duct Elbows Entries Contraction/expansions

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Duct Considerations

Resultant air velocity in duct Maintain minimum transport velocity Minimize friction losses Shape is a factor (round is preferred) Diameter (determined by Q and

friction loss) Length (layout of process) Material of construction

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Duct Design Principles

Correct (less resistance) Incorrect (more resistance)

Streamline the system as much as possible to minimize turbulence and resistance

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Air Cleaning Equipment

Purposes: remove contaminant before discharge recover valuable materials

Selection depends on Material to be removed Degree of removal required Concentration of material Conditions of air stream Economics

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Types of Air Cleaners

Absorbers/adsorbers Filters Cyclones Electrostatic units Combustion units Wet scrubbers Combination units

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Absorbers

Contaminant-in-air contacts liquid Liquid dissolves or reacts with

contaminant and retains it Use packed towers/packed beds Typical uses: acid gases, chlorine,

etc.

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Adsorbers

Contaminant-in-air passes through bed of solid

Contaminant adheres to surface Examples: activated carbon; silica

gel Typical uses: organic vapors

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Filters

Contaminated air passes through fabric, which collects particles

Incorporated into “bag houses” Various materials used as filters Can be made very efficient Surface must be

replenished/replaced

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Cyclones

Centrifugal force used to separate particles

Good for large particles only

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Electrostatic Precipitators Voltage applied Charged particles are drawn to

plate Collector plates need to be

cleaned Good for very small particles

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Combustion Processes

For combustible contaminants Contaminant converted to harmless

form Thermal oxidation

Contaminant-in-air passes over flame Direct combustion

Contaminant-in-air used as fuel Catalytic oxidation

Contaminant-in-air passes over catalyst

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Wet Scrubbers

Particles contact water and are “washed” from the airstream

Minimizes secondary dust problem in disposal

Good for dusts

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Air Movers (Fans)

Fan is the “moving force” for the system

Many types available depending on the nature of contaminant, volume of air being moved and pressure drop through system

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Axial Flow Fans

Air enters and leaves fan moving in same direction

Types Propeller Tube-axial Vane-axial

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Centrifugal Flow Fans

Air exits 90 degrees to angle of entry; is “thrown” by force

Radial (paddle wheel)

Forward curved Backward curved

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Fan Selection Considerations Total quantity of air being moved Pressure requirements Presence of particulates? Explosive/flammable materials? Noise generated by air mover Others unique to the application

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Exhaust Stack

Vertical discharge cap When fan is OFF, little rain falls into stack

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Exhaust Stack

Deflector weathercap is no longer recommended since contaminants are not dispersed

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Testing Ventilation Systems Ensure it meets design criteria ŠComply with regulatory standards ŠDetermine system balance ŠDetermine if maintenance or

repair required ŠDetermine whether existing

system is capable of handling additional hoods

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Inspection

Is the fan belt broken or slipping? Is the fan wired backward (reversed

polarity)? Is ductwork clogged with dust? Is there holes, cracks or openings in

the ducting? Is the air cleaner clogged?

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Inspection

Are any dampers in the ductwork closed?

Is there insufficient makeup air? Has ductwork been changed to include

more length, more or sharper bends, or abrupt diameter changes?

Has the contaminant source been moved further away from the hood opening?

Are cooling fans causing cross drafts? 75

Inspection

Have additional hoods and ductwork been added? Without proper airflow balancing, some hoods in a multiple system may have inadequate flow. Or the fan may be too small to handle the additional resistance.

Is more contaminant being generated at the source?

Have employees modified the hood because it interferes with their job tasks?

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Testing

Airflow at the hood can be visually checked with inexpensive smoke generators (smoke tubes) or measured with air velometers

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Instrument

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Industrial Hygiene - Ventilation

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