principles of ventilation
TRANSCRIPT
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Principles of Ventilation
Patrick N. Breysse, PhD, CIHPeter S.J. Lees, PhD, CIH
Johns Hopkins University
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Section A
Introduction
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Opportunities for Control
source - transmission - receptor4Source: Peter Lees
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Purposes of Industrial Ventilation
Control of toxic air contaminants to acceptable levelsControl of noxious odorsControl of heat and humidity for comfort and healthPrevention of fire and explosions
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Types of Industrial Ventilation
General ventilation– Control of temperature, humidity,
and odorsDilution ventilation– Maintain control of low toxicity gases
and vapors below acceptable levels through dilution of concentration
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Types of Industrial Ventilation
Local exhaust ventilation– 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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Section B
Ventilation—Basic Principles
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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
Q = A*V
Where:– Q = air flow rate (ft3/min)– A = cross-sectional area of duct or
opening (ft2)– V = average air velocity (ft/min)
Also referred to as continuity equation
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Basic Ventilation Equation
Q = A*V
Example– If fan is unchanged and number of
hoods is doubled, then the resulting hood face velocities will be 1/2 original velocity (possibly reducing air velocity to less-than-needed capture velocity)
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Section C
Ventilation—Design and Testing
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Hood Proximityand Exhaust Volume
To maintain desired capture velocity, locate hood as close to source as possibleAir volume requirement increases as square of the distanceReduces 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 maximizedAir volumes requirements are drastically minimizedReduces 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 zoneResults in reduced worker exposureResults 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 rangeUsually 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)Avoid interference with exhaust hoods (currents and eddies may compromise exhaust systems)Air enter at living zone
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Principles of Supply Air Design
Supply air must be conditioned (temperature and humidity)Air entry points located away from source of contaminants to eliminate air currents which could interfere with exhaust
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Testing Ventilation Systems
Ensure it meets design criteriaComply with regulatory standardsDetermine system balanceDetermine if maintenance or repair requiredDetermine whether existing system is capable of handling additional hoods
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