Industrial Ventilation

General Principles of Industrial Ventilation

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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

exposuresMechanical 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 Ventilation

Objectives 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 Systems

Optimization 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 Problems

Process modificationsLocal exhaust ventilationSubstitution IsolationAdministrative controlPersonal protection devicesNatural ventilation

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Ventilation Design Parameters

Manufacturing 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 Characterization

LocationRelative 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 Systems

ComponentsAir inlet sectionFiltersHeating and/or cooling equipmentFanDuctsRegister/grills for distributing the air within the

work space

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Exhaust Systems

Purpose

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 Systems

Types of exhaust systems:

General exhaust systemLocal exhaust system

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General Exhaust Systems

Used 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 System

Air 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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Why is air considered incompressible in Industrial Ventilation design problems?

The differences in pressure that exist within the ventilation system itself are small when compared to the atmospheric pressure in the room. Because of the small differences in pressure, air can be assumed to be incompressible.

Since 1 lb/in2 = 27 inches of water, 1 inch = 0.036 lbs pressure or 0.24% of standard atmospheric pressure. Thus the potential error introduced due to this assumption is also negligible.

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Velocity Pressure

It is defined as that pressure required to accelerate air from rest to some velocity (V) and is proportional to the kinetic energy of the air stream.

VP acts in the direction of flow and is measured in the direction of flow.

VP represents kinetic energy within a system.VP is always positive.

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Static Pressure

It is defined as the pressure in the duct that tends to burst or collapse the duct and is expressed in inches of water gauge (“wg).

SP acts equally in all directions SP can be negative or positive

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Static pressure can be positive or negative.Explain.

Positive static pressure results in the tendency of the air to expand. Negative static pressure results in the tendency of the air to contract.

For example, take a common soda straw, and put it in your mouth. Close one end with your finger and blow very hard. You have created a positive static pressure. However, as soon as you remove your finger from the end of the straw, the air begins to move outward away from the straw. The static pressure has been transformed into velocity pressure, which is positive.

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Velocity PressureVELOCITY PRESSURE (VP)

VP = (V/4005)2 or V = 4005√VP

Where

VP = velocity pressure, inches of water gauge (“wg)

V = flow velocity, fpm

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Total Pressure

TP = SP + VP It can be defined as the algebraic sum of the

static as well as the velocity pressuresSP represents the potential energy of a system

and VP the kinetic energy of the system, the sum of which gives the total energy of the system

TP is measured in the direction of flow and can be positive or negative

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How do you measure the Pressures in a ventilation system?

The manometer, which is a simple graduated U-shaped tube open, at both ends, an inclined manometer or a Pitot tube can be used to measure Static pressure.

The impact tube can be used to measure Total pressure.

The measurement of Static and Total pressures using manometer and impact tube, will also indirectly result in measurement of the Velocity pressure of the system.

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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

Air density

It can be defined as the mass per unit volume of air, (lbm/ft3 ). at standard atmosphere (p=14.7 psfa), room temperature (70 F) and zero water content. The value of ρ=0.075 lbm/ft3

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Basic Definitions Perfect Gas Equation:

P = ρRT

Where

P = absolute pressure in pounds per square foot absolute (psfa).

ρ = gas density in lbm/ft3.

R = gas constant for air.

T = absolute temperature in degree Rankin.

For any dry air situation

ρT = (ρT)std

ρ = ρstd(Tstd/T) = 0.075 (460+70)/T = 0.075 (530/T)

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Basic Definitions

Volumetric Flow Rate The volume or quantity of air that flows through a given

location per unit time Q = V * AorV = Q /AorA = Q/V

WhereQ = volume of flow rate in cfmV = average velocity in fpmA = cross-sectional area in sq.ft

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Example

The cross-sectional area of a duct is 2.75 sq.ft.The velocity of air flowing in the duct is 3600 fpm. What is the volume?

From the given problemA = 2.75 sq. ft.V = 3600 fpm

We know thatQ = V * AHence,Q = 3600 * 2.75 = 9900 cfm

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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 Equation

hf = f (L/d)VP

Wherehf = friction losses in a duct, “wg

f = friction coefficient (dimensionless)L = duct length, ftd = duct diameter, ftVP = velocity pressure,”wg

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Duct Losses

Types of losses in ducts Friction losses Dynamic or turbulence losses

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Duct Losses

Friction lossesFactors effecting friction losses:

Duct velocity Duct diameter Air density Air viscosity Duct surface roughness

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Duct Losses

Dynamic losses or turbulent lossesCaused 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 Losses

Turbulence 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