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

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Industrial VentilationObjectives

To introduce the basic terms To discuss heat control To design ventilation systems

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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.

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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.

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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

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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

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Terminal Or Settling Velocity

V = 0.0052(S.G)D2

WhereD = particle diameter in micronsS.G = specific gravityV = settling velocity in fpm