Transcript INDUSTRIAL VENTILATION

Industrial Ventilation
General Principles of Industrial
Ventilation
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
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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.
General Principles
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Solutions To Industrial
Ventilation Problems
 Process 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 plant
 Contaminant emissions
 Regulations
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Source Characterization
 Location
 Relative contribution of each source to the exposure
 Characterization of each contributor
 Characterization of ambient air
 Worker interaction with emission source
 Work 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 system
 To replace air exhausted from the plant i.E. The
replacement system
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Supply Systems
Components
 Air inlet section
 Filters
 Heating and/or cooling equipment
 Fan
 Ducts
 Register/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 system
 Local 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:
 Hood
 The duct system including the exhaust stack and/or
re-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 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.
General Principles
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Types Of Pressures In A
Ventilation Systems
Three types of pressures are of importance in
ventilation work. They are:
 Static pressure
 Velocity pressure
 Total 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 Pressure
VELOCITY 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 pressures
 SP 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*A
or
V = Q /A
or
A = Q/V
Where
Q = 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 air
flowing in the duct is 3600 fpm. What is the volume?
From the given problem
A = 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 = ρDV/μ
Where
ρ = density in lbm/ft3
D = diameter in ft
V = velocity in fpm
μ = air viscosity, lbm/s-ft
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Darcy Weisbach Friction
Coefficient Equation
hf = f (L/d)VP
Where
hf = friction losses in a duct, “wg
f = friction coefficient (dimensionless)
L = duct length, ft
d = duct diameter, ft
VP = 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 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 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 = 1.0
I.E
V = 4005ce√VP = 4005 √VP
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Duct Losses
Turbulence losses are given by the following
expression
Hl= FN*VP
Where
FN = decimal fraction
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Terminal Or Settling Velocity
V = 0.0052(S.G)D2
Where
D = particle diameter in microns
S.G = specific gravity
V = settling velocity in fpm
General Principles
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