industrial hygiene - ventilation
DESCRIPTION
Occupational Safety & Health 1st Year, 2nd SemesterTRANSCRIPT
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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Ventilation Terminology
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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Ventilation Terminology
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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Ventilation Terminology
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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