Industrial VentilationGeneral Principles of Industrial Ventilation
General Principles 2
What Is Industrial Ventilation?Environmental engineer’s view: The design and application of equipment for
providing the necessary conditions for maintaining the efficiency, health and safety of the workers
Industrial hygienist’s view: The control of emissions and the control of
exposures
Mechanical engineer’s view:The control of the environment with air flow. This can be achieved by replacement of contaminated air with clean air
General Principles 3
Industrial VentilationObjectives
To introduce the basic terms To discuss heat control To design ventilation systems
General Principles 4
Why Industrial Ventilation?To maintain an adequate oxygen supply in the work area.To control hazardous concentrations of toxic materials in
the air.To remove any undesirable odors from a given area.To control temperature and humidity.To remove undesirable contaminants at their source
before they enter the work place air.
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Application Of Industrial Ventilation SystemsOptimization of energy costs.Reduction of occupational health disease claims.Control of contaminants to acceptable levels.Control of heat and humidity for comfort.Prevention of fires and explosions.
General Principles 6
Solutions To Industrial Ventilation ProblemsProcess modificationsLocal exhaust ventilationSubstitution IsolationAdministrative controlPersonal protection devicesNatural ventilation
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Ventilation Design ParametersManufacturing processExhaust air system & local extractionClimatic requirements in building design
(tightness, plant aerodynamics, etc)Cleanliness requirementsAmbient air conditionsHeat emissionsTerrain around the plantContaminant emissionsRegulations
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Source CharacterizationLocationRelative contribution of each source to the
exposureCharacterization of each contributorCharacterization of ambient airWorker interaction with emission sourceWork practices
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Types Of Industrial Ventilation Systems
Supply systems
Purpose:To create a comfortable environment in the
plant i.E. The HVAC systemTo replace air exhausted from the plant i.E.
The replacement system
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Supply SystemsComponentsAir inlet sectionFiltersHeating and/or cooling equipmentFanDuctsRegister/grills for distributing the air within the
work space
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Exhaust SystemsPurpose
An exhaust ventilation system removes the air and airborne contaminants from the work place air
The exhaust system may exhaust the entire work area, or it may be placed at the source to remove the contaminant at its source itself
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Exhaust SystemsTypes of exhaust systems:
General exhaust systemLocal exhaust system
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General Exhaust SystemsUsed for heat control in an area by introducing
large quantities of air in the area. The air may be tempered and recycled.
Used for removal of contaminants generated in an area by mixing enough outdoor air with the contaminant so that the average concentration is reduced to a safe level.
General Principles 14
Local Exhaust Systems(LES)The objective of a local exhaust system is to
remove the contaminant as it is generated at the source itself.
Advantages:More effective as compared to a general
exhaust system.The smaller exhaust flow rate results in low
heating costs compared to the high flow rate required for a general exhaust system.
The smaller flow rates lead to lower costs for air cleaning equipment.
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Local Exhaust Systems(LES) Components:HoodThe duct system including the exhaust stack
and/or re-circulation ductAir cleaning deviceFan, which serves as an air moving device
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What is the difference between Exhaust and Supply systems?
An Exhaust ventilation system removes the air and air borne contaminants from the work place, whereas, the Supply system adds air to work room to dilute contaminants in the work place so as to lower the contaminant concentrations.
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Pressure In A Ventilation SystemAir movement in the ventilation system is a
result of differences in pressure. In a supply system, the pressure created by
the system is in addition to the atmospheric pressure in the work place.
In an exhaust system, the objective is to lower the pressure in the system below the atmospheric pressure.
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Types Of Pressures In A Ventilation Systems
Three types of pressures are of importance in ventilation work. They are:
Static pressureVelocity pressureTotal pressure
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Basic Definitions Pressure
It is defined as the force per unit area.
Standard atmospheric pressure at sea level is 29.92 inches of mercury or 760 mm of mercury or 14.7 lb/sq.inch.
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Basic Definitions Reynolds numberR = ρDV/μWhereρ = density in lbm/ft3
D = diameter in ftV = velocity in fpmμ = air viscosity, lbm/s-ft
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Darcy Weisbach Friction Coefficient Equationhf = f (L/d)VPWherehf = friction losses in a duct, “wgf = friction coefficient (dimensionless)L = duct length, ftd = duct diameter, ftVP = velocity pressure,”wg
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Duct LossesTypes of losses in ducts
Friction losses Dynamic or turbulence losses
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Duct LossesFriction lossesFactors effecting friction losses:
Duct velocity Duct diameter Air density Air viscosity Duct surface roughness
General Principles 35
Duct LossesDynamic losses or turbulent losses
Caused by elbows, openings, bends etc. In the flow way. The turbulence losses at the entry depends on the shape of the openings
Coefficient of entry (Ce)
For a perfect hood with no turbulence losses Ce = 1.0I.EV = 4005ce√VP = 4005 √VP
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Duct LossesTurbulence losses are given by the
following expressionHl= FN*VP
WhereFN = decimal fraction
General Principles 37
Terminal Or Settling Velocity
V = 0.0052(S.G)D2
WhereD = particle diameter in micronsS.G = specific gravityV = settling velocity in fpm
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