General Principles of Industrial
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
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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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
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
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
Local exhaust ventilation
Personal protection devices
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Ventilation Design Parameters
Exhaust air system & local extraction
Climatic requirements in building design (tightness,
plant aerodynamics, etc)
Ambient air conditions
Terrain around the plant
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Relative contribution of each source to the exposure
Characterization of each contributor
Characterization of ambient air
Worker interaction with emission source
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Types Of Industrial Ventilation
To create a comfortable environment in the plant i.E.
The HVAC system
To replace air exhausted from the plant i.E. The
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Air inlet section
Heating and/or cooling equipment
Register/grills for distributing the air within the work
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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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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
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
More effective as compared to a general exhaust
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
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Local Exhaust Systems(LES)
The duct system including the exhaust stack and/or
Air cleaning device
Fan, which serves as an air moving device
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What is the difference between Exhaust and
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
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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
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Types Of Pressures In A
Three types of pressures are of importance in
ventilation work. They are:
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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
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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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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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 (VP)
VP = (V/4005)2 or V = 4005√VP
VP = velocity pressure, inches of water gauge (“wg)
V = flow velocity, fpm
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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
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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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
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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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Perfect Gas Equation:
P = ρRT
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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Volumetric Flow Rate
The volume or quantity of air that flows through a given location
per unit time
V = Q /A
A = Q/V
Q = volume of flow rate in cfm
V = average velocity in fpm
A = cross-sectional area in sq.ft
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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 = 3600 * 2.75 = 9900 cfm
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R = ρDV/μ
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
hf = f (L/d)VP
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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Types of losses in ducts
Dynamic or turbulence losses
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Factors effecting friction losses:
Duct surface roughness
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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
V = 4005ce√VP = 4005 √VP
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Turbulence losses are given by the following
FN = decimal fraction
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Terminal Or Settling Velocity
V = 0.0052(S.G)D2
D = particle diameter in microns
S.G = specific gravity
V = settling velocity in fpm
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