industrial hvac

8/2/2019 Industrial HVAC

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Industrial Ventilation

General Principles of IndustrialVentilation


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General Principles 2

What Is Industrial Ventilation?

Environmental engineers view:

The design and application of equipment forproviding the necessary conditions for maintaining

the efficiency, health and safety of the workers Industrial hygienists view:

The control of emissions and the control ofexposures

Mechanical engineers view:

The control of the environment with air flow. Thiscan be achieved by replacement of contaminated airwith 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 workarea.

To control hazardous concentrations of toxicmaterials in the air.

To remove any undesirable odors from a given area.

To control temperature and humidity.

To remove undesirable contaminants at their sourcebefore 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 modifications

Local exhaust ventilation

Substitution Isolation

Administrative control

Personal protection devices

Natural ventilation


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Ventilation Design Parameters

Manufacturing process

Exhaust air system & local extraction

Climatic requirements in building design (tightness,plant aerodynamics, etc)

Cleanliness requirements

Ambient air conditions

Heat emissions

Terrain around the plantContaminant emissions

Regulations


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Source Characterization

Location

Relative contribution of each source to the exposure

Characterization of each contributorCharacterization of ambient air

Worker interaction with emission source

Work practices


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Types Of Industrial VentilationSystems

Supply systems

Purpose:To create a comfortable environment in the plant i.E.

The HVAC system

To replace air exhausted from the plant i.E. The

replacement system


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Supply Systems

Components

Air inlet section

FiltersHeating and/or cooling equipment

Fan

Ducts

Register/grills for distributing the air within the workspace


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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 largequantities of air in the area. The air may be temperedand recycled.

Used for removal of contaminants generated in anarea by mixing enough outdoor air with thecontaminant 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 removethe contaminant as it is generated at the sourceitself.

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 ageneral exhaust system.

The smaller flow rates lead to lower costs for aircleaning equipment.


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Local Exhaust Systems(LES)

Components:

Hood

The duct system including the exhaust stack and/orre-circulation duct

Air cleaning device

Fan, 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 airborne 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 ofdifferences in pressure.

In a supply system, the pressure created by thesystem is in addition to the atmospheric pressure inthe work place.

In an exhaust system, the objective is to lower thepressure in the system below the atmospheric

pressure.


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Types Of Pressures In AVentilation Systems

Three types of pressures are of importance inventilation work. They are:

Static pressure

Velocity pressure

Total pressure


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Velocity Pressure

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

VP acts in the direction of flow and is measured inthe 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 thattends 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 inyour 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 beentransformed into velocity pressure, which is positive.


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Velocity PressureVELOCITY PRESSURE (VP)

VP = (V/4005)2 orV = 4005VP

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 staticas well as the velocity pressures

SP represents the potential energy of a system andVP the kinetic energy of the system, the sum ofwhich 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 29.92inches of mercury or 760 mm of mercury or 14.7lb/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 =0.075 lbm/ft3


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Basic DefinitionsPerfect Gas Equation:P = RT

Where

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

= gas density in lbm/ft3.

R= gas constant for air.

T = absolute temperature in degree Rankin.

For any dry air situationT = (T)std

= std(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 locationper unit time

Q = V * Aor

V = Q /A

or

A = Q/V

WhereQ = volume of flow rate in cfm

V = average velocity in fpm

A = 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 airflowing in the duct is 3600 fpm. What is the volume?

From the given problemA = 2.75 sq. ft.

V = 3600 fpm

We know that

Q = V * A

Hence,

Q = 3600 * 2.75 = 9900 cfm


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Basic Definitions

Reynolds number

R = DV/

Where = density in lbm/ft3

D = diameter in ft

V = velocity in fpm = air viscosity, lbm/s-ft


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Darcy Weisbach Friction

Coefficient Equationhf= f (L/d)VP

Where

hf = friction losses in a duct, wgf = friction coefficient (dimensionless)

L = duct length, ft

d = duct diameter, ftVP = velocity pressure,wg


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Duct Losses

Types of lossesin ducts

Friction losses

Dynamic or turbulence losses


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Duct Losses

Friction losses

Factors effecting friction losses:

Duct velocity Duct diameter

Air density

Air viscosity

Duct surface roughness


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Duct Losses

Dynamic losses or turbulent losses

Caused by elbows, openings, bends etc. In the flowway. 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.E

V = 4005ceVP = 4005 VP


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Duct Losses

Turbulence losses are given by the followingexpression

Hl= FN*VPWhere

FN = decimal fraction


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Terminal Or Settling Velocity

V = 0.0052(S.G)D2

Where

D = particle diameter in micronsS.G = specific gravity

V = settling velocity in fpm

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