IH676 INDUSTRIAL VENTILATION

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IH676 INDUSTRIAL VENTILATION
jhpatel
IH676 INDUSTRIAL VENTILATION
CREDITS 5 (L = 3, P = 2)
SCHEME OF TEACHING
• SCHEME OF TEACHING
Topic
Name of Topics
Marks (Theory)
Lectures Hrs
(Theory)
No
1
Introduction of ventilation
5
2
2
General principles of ventilation
12
8
3
General Industrial Ventilation
8
5
4
Local exhaust ventilation systems
26
15
5
Non-Standard Condition
8
5
6
HVAC and makeup air systems
8
5
7
Testing of Ventilation Systems
8
5
75
45
Total
IH676 INDUSTRIAL VENTILATION
TOPICS AND SUBTOPICS
1.Introduction of ventilation
1.1 Purpose of ventilation
1.2Types of ventilation
2.General principles of ventilation
Introduction
Supply system, Exhaust systems, Basic Definitions such as static pressure, velocity pressure,
total pressure, etc.
Principles of Air flow
Acceleration of Air and Hood Entry losses
Duct Losses
Multiple-Hood Exhaust systems
Air Flow Characteristics of Blowing and Exhausting
3.General Industrial Ventilation
3.1 Introduction, Dilution Ventilation Principles, Dilution Ventilation for Health
3.2 Mixtures-Dilution Ventilation for Health
3.3 Dilution Ventilation for Fire and Explosion
3.4 Ventilation for Heat control, Heat Balance and Exchange
3.5 Ventilation system
IH676 INDUSTRIAL VENTILATION
4.0 Local exhaust ventilation systems
4.1 Applications, components of a local exhaust system, types of losses, losses and velocity
pressure, friction, elbow and branch entry losses
4.2 Hood design and selection, selecting and designing ductwork, fan selection
4.2.1 Hood Design
4.2.1.1 Contaminant Characteristics, Hood Types
4.2.1.2 Hood Design Factors, Hood Losses, Special hood Requirement
4.2.2 Duct
4.2.2.1 Types, Flow in Ducts
4.2.2.2 Losses, Correction in ductwork
4.2.3 Air Cleaning Devices
4.2.3.1 Selection of Dust Collection Equipment, Dust Collector Types
4.2.3.1 Control of Mist, Gas and Vapor Contaminants
4.2.3.2 Gaseous Contaminant Collectors
4.2.3.3 Selection of Air Filtration Equipment
4.2.4 Fans
4.2.4.1 Basic Definitions, Fan selection
5.0 Non-Standard Condition
5.1 Corrections for water vapor in air (Relative Humidity)
5.2 Density Correction factor
5.3 Air flow, Velocity Pressure, Vapor generation, System design
IH676 INDUSTRIAL VENTILATION
6.0 HVAC and makeup air systems
6.1 Introduction of HVAC
6.2 Makeup air systems
6.3 Placement of supply registers, Supplied-air islands
6.4 HVAC component and system types
7.0 Testing of Ventilation Systems
5
7.1 Introduction
7.2 Pressure Measurement, Volumetric Flow Measurement
7.3 Air Velocity Instruments, Calibration of Air/Measuring Instruments
7.4 Evaluating Exhaust Systems
5
IH676 INDUSTRIAL VENTILATION
Particles/Seminar/Work Practices
Hrs
Marks
Sr.
No.
Particles/Seminars/Work Exercise
1
2
Seminar on need of general ventilation in Factory
Seminar on need of local exhaust ventilation system in
factory
Exercise on emission source behaviour and problem
characterization of industrial ventilation
2
2
2
2
3
3
4
5
Seminar indoor air quality study
Exercise on air density, Velocity pressure, Duct diameter,
selection of fan and air horse power etc.
2
2
4
3
6
Exercise on Design of General and Local /Exhaust
Ventilation system
7
5
7
Introduction of Velometer and Anenometer
Practical on measurement of velocity and pressure from
LEV and General Ventilation installed for various
occupations
10
9
30
25
3
IH676 INDUSTRIAL VENTILATION
Topic No1 .Introduction of
ventilation
Purpose of ventilation
Definition
• Ventilation is defined as the process of supplying
air to, or removing air from, any space by natural
or mechanical means.
Defining
Ventilation is the mechanical system in a building that brings in "fresh"
outdoor air and removes the "contaminated" indoor air.
In a workplace, ventilation is used to control exposure to airborne
contaminants. It is commonly used to remove contaminants such as
fumes, dusts, and vapours, in order to provide a healthy and safe
working environment. Ventilation can be accomplished by natural
means (e.g., opening a window) or mechanical means (e.g., fans or
blowers).
Industrial systems are designed to move a specific amount of
air at a specific speed (velocity), which results in the removal
(or "exhaust") of undesirable contaminants. While all
ventilation systems follow the same basic principles, each
system is designed specifically to match to the type of work
and the rate of contaminant release at that workplace.
Industrial Ventilation at different angles!
 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
10
Purpose of 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.
General Principles
11
Purpose can also be referred as :
 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
General Principles
12
Why have an industrial ventilation system?
Ventilation is considered an "engineering control"
to remove or control contaminants released in
indoor work environments. It is one of the
preferred ways to control employee exposure to
air contaminants.
Other ways to control contaminants include:
eliminate the use of the hazardous chemical or
material,
substitute with less toxic chemicals,
process change, or
work practice change.




•
•
Ventilation
Industrial ventilation
–
Generally involves the use of supply and exhaust
ventilation to control emissions, exposures, and
chemical hazards in the workplace
Non-industrial ventilation systems commonly
known as heating, ventilating, and airconditioning
(HVAC) systems
–
Traditionally were built
to control temperature,
humidity, and odors
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
15
Pioneers in Industrial Ventilation
Year
Pioneer
1914
Dr. Willis Carrier
1925, 1933, 1938, 1945 Richard Madison
1932
1939
DallaValle and Hatch
John Alden
1951
1954
AGGIH
Wes Hemeon
1972
1985
Baturin
Goodfellow
2001
Goodfellow/Tahti
Area of Study
Fan Engineer
Fan Engineering
Airflow Equations
Design of Industrial Exhaust Systems
Industrial Ventilation Manual
Plant and Process Ventilation
Fundamentals of Industrial Ventilation
Advanced Design of Ventilation Systems
For Contaminant Control
Industrial Ventilation Design
Guidebook
1.2 Syllabus topic
• Types of ventilation
Types of Ventilation
Natural
 General
 Dilution
 LEV

Type of VS
– There are five basic types of ventilation systems:
1.
2.
3.
4.
5.
dilution and removal by general exhaust;
local exhaust (LEV)
makeup air (or replacement);
HVAC (primarily for comfort); and
recirculation systems.
Ventilation System Types
1. Dilution and removal by general exhaust
2. Local exhaust
3. Makeup air
– Replacement
4. HVAC
– Primarily for comfort
•
Ducts are square
5. Recirculation systems
TYPES OF VENTILATION
– Vertical
– Trench (strip)
– Basement
– Horizontal
– Natural
• Forced
• Mechanical positivepressure
• Mechanical negativepressure
• Hydraulic
What are the basic types of
Mechanical ventilation systems?
• There are two types of mechanical ventilation
systems used in industrial settings:
• Dilution (or general) ventilation reduces the
concentration of the contaminant by mixing
the contaminated air with clean,
uncontaminated air.
• Local exhaust ventilation captures
contaminates at or very near the source and
exhausts them outside.
Natural Ventilation
• Natural movement of air entering and leaving openings such as windows,
doors, roof ventilators as well as through cracks and crevices of a building
• Heated air rises, cool air below this creates flow of air in any systemNatural
ventilation
• Natural ventilation is the ventilation of a building with outside air without
the use of fans or other mechanical systems. It can be achieved with
openable windows or trickle vents when the spaces to ventilate are small
and the architecture permits. In more complex systems warm air in the
building can be allowed to rise and flow out upper openings to the outside
(stack effect) thus forcing cool outside air to be drawn into the building
naturally through openings in the lower areas. These systems use very little
energy but care must be taken to ensure the occupants' comfort. In warm
or humid months in many climates maintaining thermal comfort solely via
natural ventilation may not be possible so conventional air conditioning
systems are used as backups. Air-side economizers perform the same
function as natural ventilation, but use mechanical systems' fans, ducts,
dampers, and control systems to introduce and distribute cool outdoor air
when appropriate.
Topics N0.2.
General principles of ventilation
Introduction
Supply system, Exhaust systems, Basic Definitions
such as static pressure, velocity pressure, total
pressure, etc.
Principles of Air flow
Acceleration of Air and Hood Entry losses
Duct Losses
Multiple-Hood Exhaust systems
Air Flow Characteristics of Blowing and Exhausting
Topicno2 General principles of ventilation
•
– Introduction
– Supply system, Exhaust systems, Basic Definitions such
as static pressure, velocity pressure, total pressure,
etc.
– Principles of Air flow
– Acceleration of Air and Hood Entry losses
– Duct Losses
– Multiple-Hood Exhaust systems
– Air Flow Characteristics of Blowing and Exhausting
Introduction
Purposes of ventilation
Maintaining human comfort and health are two key reasons for providing ventilation in work
environment\buildings. To achieve these purposes, a ventilation system should be able to meet
the following criteria:
1. provide sufficient supply of air/oxygen for the physiological needs of human beings (a
minimum of 0.2 l/s/person is required for breathing purpose) and/or livestock;
2. provide sufficient supply of air/oxygen for industrial, agricultural and other processes (for
example, provision of oxygen for burning and combustion processes);
3. remove the products of respiration and bodily odour (including those from smoking) of
human and/or animal occupants;
4. remove contaminants or harmful chemicals generated by processes or from building
materials; remove heat generated by people, lighting and equipment inside the occupied
space;
5. create some degree of air movement which is essential for feelings of freshness and comfort
(usually a velocity of 0.1 to 0.3 m/s is required).
Sub topics
• Supply system, Exhaust systems, Basic Definitions
such as static pressure, velocity pressure, total
pressure, etc.
Make up Air
• Fresh air supplied
into the breathing
zone of the
associate.
Types of Exhaust Systems
Basic Definitions
such as static pressure,
velocity pressure, total pressure, etc.
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
General Principles
32
Exhaust Systems
Types of exhaust systems:
 General exhaust system
 Local exhaust system
General Principles
33
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.
General Principles
34
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.
General Principles
35
Local Exhaust Systems(LES)
Components:
 Hood
 The duct system including the exhaust stack and/or recirculation duct
 Air cleaning device
 Fan, which serves as an air moving device
General Principles
36
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.
General Principles
37
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
38
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
General Principles
39
Basic Definitions
Air density
 It can be defined as the mass per unit volume of air, (lbm/ft3 or kg/cu.m. ). at
standard atmosphere (p=14.7 psfa or 760mm of Hg ), room temperature (70 F or
21 C) and zero water content. The value of ρ=0.075 lbm/ft3 or 1.20kg/cu.m.
 Example Calculate using chart
 IU example Calculate density of Air at T =150 o F and 5,000 feet altitude
 Solution from chart d=0.72 therefore density at STP*d 0.075lbs/cu.t * 0.72 =
0.054 lbs/cu.ftr
 SI unit example Calculate density of air at temp of 35 Celcius and 1.00km altitude.
 Solution STP Densityof air =1.20kg/cu.m *d solution is 1.20 *0.85 from table
=1.02 kg/cu.m.
General Principles
40
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.
General Principles
41
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.
General Principles
42
Pressure Relationships
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
General Principles
44
Static Pressure
Flow
Static pressure (SP)
is
exerted in all
directions.
SP
Static pressure can be positive or negative.
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.
General Principles
46
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.
General Principles
47
Velocity Pressure
Flow
Velocity Pressure
(VP) is
kinetic (moving
pressure) resulting
from air flow.
SP
VP
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
VP = (V/4.043)2 or V = 4.043√VP
Where
VP = velocity pressure, mm of water gauge (“wg)
V = flow velocity, mps
General Principles
49
Velocity Pressure
VELOCITY PRESSURE (VP) considering correction for density
VP = (V/4005)2 d or V = 4005√VP/d
Where
VP = velocity pressure, inches of water gauge (“wg)
V = flow velocity, fpm
530
BP
294
BP
d=------ * ---- (in IU) or
----- * ----- (SI Unit)
F + 460 29.92
C + 273 760
F and C temp inFer or Cel
density correction factor normally used 0.625at &km and temp 93o C or less
than depends on elevation and Temperature of gas (range 1.26 to 0.6250
VP = (V* /4.043)2 d or V = 4.043√VP/d
Where
VP = velocity pressure, mm of water gauge (“wg)
V = flow velocity, mps
General Principles
50
Exercises Estimate Density Correction factor of an air at temperature of100 o F and Elevation of
2000 ft above Mean sea Level (here barometric pressure bp is 27.8 inches of Hg)
530
BP
530
27.8
d=------ * ---- (in IU) or
----- *
----- (I Unit) =0.88
F + 460 29.92
100 + 460 29.92 F and r
Show the Use of chart char no.6
Estimate Density Correction factor of an air at temperature of 66 o C and Elevation of 1525m
above Mean sea Level (here barometric pressure bp is 722of Hg)
294
722
d=----- * ----- (SI Unit) = 0.72 Using Chart no. 6
66 + 273 760
Example on Velocity calculations
1. The average velocity pressure of an air stream in a duct is 1.00inch w.g.. What is is average
velocityu Assuming that Air is at STP and density correction is 1 : solution of the example
V = 4005√VP /d V= 4005 (1.0/1.0) 0.5 =4005fpm
2. The central velocity pressure of an air stream measured is 12 mm of mercury. What is its
central velocity ? (assuming d=1) Solution of problem
V = 4.043√VP/d 4.043(12/1)0.5 = 14 mps
Give Exercise here as home work and assignments
SAMPLE PROBLEM #1
Velocity Pressure
Given:
V = 4005 VP/df
V = 2200 fpm
df = 0.075 lbs./ cu ft = 1
Solve for VP in inches water gauge
2200 fpm = 4005 VP/df
• .55 = VP
• .302” w.g. = VP
Relationship between VP and ρ, density
V = 1096 VP/ ρ
What if…
• If VP = 1” wg
• If STP, ρ = 0.075 lb/ft3
V = 1096 1.00/0.075
= 4002 fpm
• V = 4005 fpm @ VP
= 1.00 inches of water gauge.
Solve for V at STP The average velocity pressure in a duct is
VP = 1.00” w.g.
What is the average velocity, V?
(Assume standard conditions,
ρ = 0.075 lbs./cu ft).
Given:
V = 1096 VP/ ρ
VP = 1.00” w.g.
ρ = 0.075 lbs./cu ft
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
General Principles
55
Total Pressure
Flow
SP
VP
TP
Total pressure (TP)
is the algebraic
sum of the VP and
SP.
Pressure Measurements in ducts
Relationships Explained
Calculation of Total pressures
Total pressure TP
-7.4
= Static Pressure SP + Velocity pressure VP
-8.1
+2.0
-9.4
+6.9
5
+2.00
7.2
-3.6
5,.2
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.
General Principles
60
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)
General Principles
61
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
General Principles
62
Volumetric Flow Rate
• The amount of air going through a system at a
certain point
–Q
– Given in Cubic Feet Per Minute (CFM)
• The amount of air flowing through any point has to be
the same
– Volume of air has to be the same, but the area and the velocity do no
remain the same
» If you increase the area you decrease the velocity
Q2
Q1
Q3
Basic Definitions
• Velocity
– Flow rate of air through duct
• V(fpm)
– Velocity = 4005 x Square Root of Velocity Pressure
» V = 4005 VP
• Area
– Area of duct
• A(ft²)
• Volumetric Flow Rate =
– Q(cfm or ft3/min)=VA
Velocity x Area
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
General Principles
65
Example
• The area of a of a round duct is 2.445sq.ft .
The average velocity of air flowing in duct is
V= 3500fpm at standard conditions . What is
Q?
SOLUTION
Q=V*A
3500fpm * 2.445sq.ft =8557.5 scfm say 8600
scfm for significant figure
Example in SI Units
• The diameter of a round duct is 25cm . The
average velocity of air flowing in duct is V=
21mps at standard conditions . What is Q?
SOLUTION
A= ∏D2/4 =3.14(0.25)2/4 = 0.0491sq.m.
Q=V*A
21mps * 0.0491 =1.04 scms
Example in SI System
The cross-sectional area of a duct is 27500 mm2.The
velocity of air flowing in the duct is 10.00 mps. What is
the volume?
From the given problem
A = 27500 sq. mm.
V = 10 mps
We know that
Q=V*A
Hence,
Q = 27500 * 10 /104 = 2.75 cumecs
Air Changes per minute (ACM) [or air changes per hour (ACH)] is generally used as a
way to measure the dilution ventilation rate. Air exchange rate means replacing the
entire volume of air in the workspace in one minute or one hour. The following
formula can be used to determine the air exchange rate:
Number of air
changes per hour =
Outside air intake rate in cubic feet per minute (cfm) x 60
Volume of the work space in cubic feet (ft3 )
In other words:
Outside air intake
rate in cubic feet per
minute (cfm) x 60 =
Volume of the work space in cubic feet (ft3 )
Required number of air changes per hour
For example if the air flow rate required in a workspace which
is 40 feet long, 40 feet wide and 12 feet high,
volume of the work space is 40 x 40 x 12 = 19,200 cubic feet.
Air flow rate required per ACH = 19,200 / 60 = 320 cfm Or,
air flow rate required per ACM = 19,200 cfm Or,
if the ceiling height is 20 feet high
then the room volume is 40 feet X 40 feet X 20 feet high=
32,000 cubic feet and
the required air flow rate will be as follows:
Air flow rate required per ACH = 32,000 / 60 = 533 cfm Or,
air flow rate required per ACM = 32,000 cfm
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
General Principles
71
Considering Losses now
• DUCT LOSSES:
– Friction Losses – is due to little
complicated and is function of duct velo,
duct diam, air density air viscocity and
surface ruoghness of duct which combined
in to a dimensionless Number R or Rn
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
Moody has prepared a diagram combining
all together
This effect is given by a derived unit
Relative roughness = Absolute surface roughness (k) of a particular material of duct /
------------------------------------------------------------------------------------------( to) duct diameter(D)
Some standard value of absolute surface roughness used in ventilation system are given in the
Next slide which was combined by L.F. Moody in the form a single chart
Which give value of Reynold’s number and friction factor “f”
Darcy Weisbach Friction Coefficient
Equation
Hf or hL = f (L/d)VP
Where
hL Or hf = friction losses in a duct, “wg
f = friction coefficient (dimensionless)
L = duct length, ft
d = duct diameter, ft
VP = velocity pressure , ”wg
(Churchil has given formula for f
f = 8 [{8/Re} 12 + (A+ B) -3/2]1/12
A and B are also has its own relationship )
General Principles
75
Duct Losses
Typical equation of losses in ducts based on
Wright
(V/1000) 1.9
hL Or hf = 2.74----------------D1.22
General Principles
76
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
General Principles
77
Duct Losses
Types of losses in ducts
Friction losses
Dynamic or turbulence losses
General Principles
78
Duct Losses
Friction losses
Factors effecting friction losses:
Duct velocity
Duct diameter
Air density
Air viscosity
Duct surface roughness
General Principles
79
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
General Principles
80
Duct Losses
Turbulence losses are given by the following expression
Hl= FN*VP
Where
FN = decimal fraction
General Principles
81
The diagram in next slide is based on
standard air 0.075 lb/ft3 in clean round
galvanized metal ducts. With standard
conditions and assumptions of
1 inch water = 248.8 N/m2 (Pa)= 0.0361 lb/in2
(psi) = 25.4 kg/m2 = 0.0739 in mercury
1 ft3/min (cfm) = 1.7 m3/h = 0.47 l/s
1 ft/min = 5.08x10-3 m/s
1 inch = 25.4 mm = 2.54 cm = 0.0254 m =
0.08333 ft
The friction loss in a 20 inches duct with air flow 4000 cfm can be estimated to
approximately 0.23 inches water per 100 feet duct as shown in the diagram below.
The air velocity can be estimated to approximately 1850 feet per minute.
Using SI System The same diagram can be prepared as shown below
1 m/s = 196.85 ft/min
1 m3/s = 3600 m3/h =
1000 dm3(liter)/s = 35.32
ft3/s = 2118.9 ft3/min =
13200 Imp.gal (UK)/min =
15852 gal (US)/min
1 mm H2O = 9.81 Pa =
9.807x10-6 N/mm2 =
0.0987 10-3 bar = 1
kp/m2 = 0.09678 10-3 atm
= 1.422 10-3 psi (lbf/in2)
Example - Air Duct and
Friction Loss
The friction loss in a 500
mm main duct in comfort
system with air flow 1
m3/s can be estimated as
indicated below to 0.05
mm H2O/m (~ 0.5 Pa/m).
Example - Air Duct and Friction Loss
The friction loss in a 500 mm main duct in comfort system with air flow 1 m3/s can be
estimated as indicated below to 0.05 mm H2O/m (~ 0.5 Pa/m).
Fitting Losses
Entry ,elbow, exit and other fittings produce
loss in total pressure which is calculated by two
methods
(1) Velocity pressure method Hl =F VP ; F =coef
(2) The equivalent length method
Multiple Hood Exhaust System
It is observed that Four Basic Components of a Local Exhaust Ventilation System (LEV)
• Hood
• Duct System
• Air Cleaning Device
• Fan This is required as part of topic4 in detail
Effects of Airflow of
Blowing and Exhausting.
Air blown from a small opening retains its directional effect for a considerable distance beyond
the plane of the opening.
Process must be in proximity to the exhaust hood.
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
92
IH676 INDUSTRIAL VENTILATION
TOPIC NO3
3.General Industrial Ventilation
3.1 Introduction, Dilution
Ventilation Principles, Dilution
Ventilation for Health
3.2 Mixtures-Dilution Ventilation for
Health
3.3 Dilution Ventilation for Fire and
Explosion
3.4 Ventilation for Heat control,
Heat Balance and Exchange
3.5 Ventilation system
General Dilution Ventilation
The supply and exhaust of air in a building
Types of general dilution ventilation:
 Type1: dilution ventilation (D.V.)
 D.V. Is the dilution of contaminated air with uncontaminated air for
controlling potential airborne health hazards, fire and explosive
conditions, odors and nuisance type contaminants
 D.V. Also includes the control of airborne contaminants such as vapors,
gases and particulates generated within tight buildings
 D.V. Is not as satisfactory for health hazard control as is local exhaust
ventilation
 Type 2 : heat control ventilation
 It is the control of indoor atmospheric conditions found in hot industrial
environments. The purpose is to prevent discomfort or injury to workers
General Dilution Ventilation
94
Dilution Ventilation for Health




Dilution ventilation is generally used to control the vapors from
organic liquids with a TLV of 100 ppm or higher.
The limiting factors for D.V. For health are:
The quantity of contaminant generated must not be too great or the
air flow rate necessary for dilution will be impractical.
Workers must be at an appropriate distance from the contaminant
source or the exposed contaminant must be in sufficiently low
concentrations so that workers will not have an exposure in excess of
the established TLV.
The toxicity of the contaminant must be low.
The emission rate of contaminants must be reasonably uniform.
General Dilution Ventilation
95
Parameters Required for Determination of
Dilution Ventilation Rates
 Solvent vapor per minute (i.E. Evaporation rate )
 Specific gravity of liquid
 Molecular weight
 Acceptable health standard (threshold limit value i.E. TLV)
 K factor for incomplete mixing
General Dilution Ventilation
96
General Dilution Ventilation Equation
Rate of accumulation = Rate of generation – Rate of removal
Vdc = Gdt – Q’Cdt
Where
V = Volume of room
G = Rate of generation
Q’ = Effective volumetric flow rate
C = Concentration of gas or vapor in ppm
t = time
For steady state condition, change in concentration, dC = 0
Gdt = Q’Cdt
For constant concentration C and uniform generation rate G, the
above equation may be integrated as
Q’ = G/C
Now, G = (403 * SG * ER)/MW
General Dilution Ventilation
97
General Dilution Ventilation Equation
Q’ = (403 * 106 * SG * ER)/MW * C
Where
SG = Specific gravity
ER = Emission rate in pints/minute
MW = molecular weight
G = Rate of generation in cfm
C = Concentration of gas or vapor in ppm
Actual Ventilation Rate Q = Q’ * K
Where
K = factor for incomplete mixing and lies between 1 and 10 and depends on:
 Efficiency of mixing
 Toxicity of chemicals
 Duration of the process
General Dilution Ventilation
98
Contaminant Concentration Build Up
Vdc = Gdt - Q’Cdt
Rearranging the terms and integrating from time t1to t2 and
concentration C1to C2 , we get
ln[(G-Q’C2 )/ [(G-Q’C1 ) = -Q’/V(t2-t1)
Δt = t2 - t1
Δt = -V/Q’ * ln [(G-Q’C2 )/ [(G-Q’C1 )
If initial concentration C1 = 0 and Q’ = Q/K then
Δt = K(V/Q)ln [G )/ [G-((Q/K)C2 )]
Note: C is in parts /106
General Dilution Ventilation
99
Rate of Purging
For this case, rate of contaminant generation G = 0
VdC = -Q’Cdt
dC/ C = (-Q’/V)dt
Integrating from time t1to t2 and concentration C1to C2 , we get
ln(C2 / C1) = -Q’/V(t2-t1)
t2 - t1 = -(V/Q’) ln(C2 / C1)
If initial time t1=0 then
t2 = -(V/Q’) ln(C2 / C1) = -(V/Q’) ln(C1 / C2)
Q’= Q/K t2 = K(V/Q) ln(C1 / C2)
Where
t2=time, minutes
General Dilution Ventilation
100
Some practical example
Area of window opening = length x width
Area = 0.5 m x 0.5 m = 0.25 m2
Air velocity through window measured by
vaneometer = 1 m/s
Flow rate = Open window area x air velocity
= 0.25 m2 x 1 m/second
= 0.25 m3/s x 3,600 seconds per hour
= 900 m3 / hour
Room volume = width x depth x height
Volume of room =
3 m wide x 5 m deep x 3 m high = 45 m3
width x depth x height = 45 m3
ACH
= Air flow rate divided by room volume
= 900 m3/hour = 20 ACH
45 m3
Area of open windows == 2 m2 Window closed =1 m2
Average air velocity 0.15 m/sec
Average Flow Rate = Average air velocity 0.15 m/sec
X Area of windows 2 m2 X 3,600 sec/h = 1,080 m3 / h
Window area = length x width = 0.25 m2
Air velocity through window= 1 m/s
Air flow rate
= window area x air velocity = 900 m3/h
Door
0.20 m/s
0.10 m/s
Average air velocity =
0.20 +0.10 m/s
2
=0.15 m/sec
Example N02
Room volume 4m x 4m x 2.5 m =40 m3
ACH
= Air flow rate divided by room volume
= 300 m3/hour = ACH
40 m3
Flow/ volume
40 m3
40
1
40 m3
200
5
40 m3
300
7.5
40 m3
600
15
m3 x h
Flow/ volume
Mixtures-dilution Ventilation for Health
When two or more hazardous substances are present, then their
combined effect known as the additive effect should be given
primary consideration
If
(C1/TLV1) + (C2/TLV2) +……… (Cn/TLVn) > 1
then the threshold limit of the mixture is considered to be
exceeded
Where
C = observed atmospheric concentration
TLV = corresponding threshold limit
General Dilution Ventilation
104
Ventilation for Heat Control
History of heat stress
 Steel industry
 Glass industry
 Mining industry
 Paper industry
Heat load on a person
 Metabolism
 Conduction
 Convection
 Radiation
 Evaporation
General Dilution Ventilation
105
Heat Balance and Exchange
delta s = (M – W) + C + R - E
delta s = change in body heat content
(M-W) = total metabolism
C
= convection heat exchange
R
= radiative heat exchange
E
=evaporative heat loss
C and R are positive if delta s increases in heat
Data required:
Measurement of metabolic heat production
Air temperature
Air water vapor pressure
Wind velocity
Mean radiant temperature
General Dilution Ventilation
106
Methods of Heat Exchange
 Convection
 Radiation
 Evaporation
Convection
C = 0.65Va0.6 (ta-tsk)
Where
C = convective heat exchange, Btu/h
Va = air velocity, fpm
ta = air temperature, F
tsk = mean weighted skin temperature, usually assumed to be 95 F
General Dilution Ventilation
107
Methods of Heat Exchange
Radiation
R = 15.0 (tw - tsk)
Where:
R = radiant heat exchange , Btu/hr
tw = mean radiant temperature,F
tsk = mean weighted skin temperature(usually 95 F)
General Dilution Ventilation
108
Methods of Heat Exchange
Evaporation
E = 2.4Va0.6(ρsk - ρa)
Where:
E = evaporative heat loss, Btu/h
Va = air velocity, fpm
ρa = water vapor pressure of ambient air,mm Hg
ρsk= water vapor pressure on the skin, (assumed to be 42
mm Hg at a 95 F skin temperature)
General Dilution Ventilation
109
Acute Heat Disorders
1. Heat stroke
 A major disruption of central nervous function
 Lack of sweating
 Rectal temperature > 410C
Treatment
 Placing the patient in a shady area
 Removing the outer clothing
 Wetting the skin
 Increasing air movement
 Professional help
General Dilution Ventilation
110
Natural ventilation
Natural ventilation is based on the principle that the difference in pressure between the air inlet
and exhaust air vents in a building allows natural air exchange to take place. The pressure
differences arise due to:
the buoyancy forces in the building caused by differences in density between the air in the
building and the outside air, and the difference in height between the air inlet and exhaust air
vents,
the air currents in the building.
Accordingly, the volume of air flowing through the building depends on the surplus heat in the
building – caused by the convection heat that is emitted into the room air (internal cooling
load), the external cooling load (transmission through insolation), wind speed, wind direction
and building geometry.
The volume of air can be controlled and maintained within fixed parameters according to this
principle.
Fig. 1 and Fig. 2 in next slide show a simplified natural ventilation scheme. The neutral plane is
located where internal and external pressure is equal. Vents are not effective at this point.
Above the neutral plane, internal pressure is higher than external pressure, and this is where
the exhaust air vents are located. Underneath the neutral plane, external pressure is higher than
internal pressure, which is why the fresh air vents are located here.
When designing a natural ventilation system, it is necessary to predetermine the height of the
neutral plane and ascertain the proportion of pressure difference that is available to overcome
flow losses in the air inlet vents and the proportion that will be used by the exhaust air vents.
It is always necessary to calculate the position of the neutral plane.
A
B
His concentration has
lately turned to his
‘Takeaway’ designs,
which are factory
produced residential
houses custom designed
to the client’s
requirements. As stated
by his own website his
main objectives as an
architect are:
Air flow and ventilation
systems
Site aspect to suit the
climatic and
environmental
conditions
Provision for natural
light and shade. (Gabriel
Poole, 2009 )
Concept A
"Wind can blow air through openings in the wall on the windward side
of the building, and suck air out of openings on the leeward side and the
roof. Temperature differences between warm air inside and cool air
outside can cause the air in the room to rise and exit at the ceiling or
ridge, and enter via lower openings in the wall. Similarly, buoyancy
caused by differences in humidity can allow a pressurized column of
dense, evaporatively cooled air to supply a space, and lighter, warmer,
humid air to exhaust near the top."
Concept B
It is possible to design a building in such a way that the wind sucks the
stale air out and draws fresh air in, using the difference in air pressure at
different heights to create a flow of air. When this is done as part of an
overall energy strategy for a building, it can help cut the electricity
needed to keep the building running. Such ‘passive’ or ‘natural’
ventilation systems are typically associated with the design of new
buildings but there are also cases where existing buildings have been
adapted using the same principles.
Cross-section of a hypothetical educational facility with hybrid ventilation.
1Natural ventilation through open windows during mild weather.
2. South-oriented solar collectors or airflow windows preheat outdoor air during winter
months.
3. Warm air is delivered mechanically to north-oriented rooms.
4. Air is extracted by stack effect through a central atrium.
5. Exposed thermal mass.
6. Glazed buffer zone admits natural light and allows heat recovery of vitiated air before it is
extracted outside by stack and/or Venturi effects.
7 Standards allows unlimited operable window area between classrooms and
corridors if room/exit distance is calculated from any point in the building.
Originally ventilators were
part of the fabric of
an industrial building
This was followed by
separately manufactured
products such as ridge vents
Benefits of Natural Ventilation
• Virtually no maintenance
• Silent operation
• Low operational cost
• Long life span
• Not liable to breakdown
• Limited structural work necessary
• Can allow the entry of daylight
Natural ventilation is the process of supplying and removing air by means of purposeprovided aperture (such as openable windows, ventilators and shafts) and the natural
forces of wind and temperature-difference pressures.
Natural ventilation may be divided into two categories:
Controlled natural ventilation is intentional displacement of air through specified
openings such as windows, doors, and ventilations by using natural forces (usually by
pressures from wind and/or indoor-outdoor temperature differences). It is usually
controlled to some extent by the occupant.
Infiltration is the uncontrolled random flow of air through unintentional openings
driven by wind, temperature-difference pressures and/or appliance-induced pressures
across the building envelope. In contrast to controlled natural ventilation, infiltration
cannot be so controlled and is less desirable than other ventilation strategies, but it is
a main source of ventilation in envelope-dominated buildings.
Natural ventilation devices
1 High-efficiency ventilation: The science and design of curved turbine blades, as well as
excellent low resistance bearings, even in the breeze can continue to operate under, and
constantly indoor stale air exhaust
2 Reinforcement corrosion-resistant stainless steel body: high-quality 304 stainless steel body,
and can withstand a long period of wind and rain, can resist the erosion of corrosive gases
indoors, excellent low resistance bearings sealed, durable, eternal lubrication , without the need
for maintenance;
3 Simple and quick to install: Due to the unique angle-pipe neck design, the ventilation device
can be used in different tilt angles (0 ° ~ 22.5 °) roof, significantly reducing installation costs;
Principles of Natural Ventilation
For air to move into and out of a building, a pressure difference between the inside and outside
of the building is required. The resistance to flow of air through the building will affect the
actual air flow rate. In general, controlled natural ventilation and infiltration are driven by
pressure difference across the building envelope. The pressure difference is caused by:
1. wind (or wind effect);
2. difference in air density due to temperature difference between indoor and outdoor air
(stack or chimney effect); or
3. combination of both wind and stack effects.
Wind effect When air flow is due to wind, air enters through openings in the windward walls,
and leaves through openings in the leeward walls. The pressure distribution patterns due to
wind in a number of cases are illustrated in
Wind pressures are generally high/positive on the windward side of a building and low/negative
on the leeward side. The occurrence and change of wind pressures on building surfaces depend
on:
wind speed and wind direction relative to the building;
the location and surrounding environment of the building; and
shape of the building.
Mathematically, pressure on building surfaces may be expressed as:
where Pw = mean pressure on the building surface (N/m2 or Pa) Po = static pressure in
undistributed wind (N/m2 or Pa) vw = mean wind velocity (m/s) = density of air (kg/m3) Cp =
surface pressure coefficient
Stack effect
When air movement is due to temperature difference between the indoor and outdoor, the flow
of air is in the vertical direction and is along the path of least resistance. The temperature
difference causes density differentials, and therefore pressure differences, that drive the air to
move. During the winter season (see Figure 2a), the following stack effect occurs:
indoor temperature is higher than outdoor temperature;
the warmer air in building then rises up;
the upward air movement produces negative indoor pressure at the bottom;
positive indoor pressure is created on the top;
warmer air flows out of the building near the top; and
the air is replaces by colder outside air that enters the building near its base.
During the summer season
(see b), the reverse occurs
when indoor temperature
is lower than outdoor
temperature.
Figure below shows stack effect that may occur in different forms of buildings, including a
building with no internal partition, a building with airtight separation of each storey, and an
ideal building with vertical shafts and horizontal openings.
When thermal force is acting alone, a neutral pressure level (NPL) exists, where the interior and
exterior pressures are equal. At all other levels, the pressure difference between the interior
and exterior depends on the distance from the neutral pressure level and the difference
between the densities of inside and outside air.
where Ps = pressure difference due to stack effect (N/m2 or Pa) = density of air (kg/m3)
2
g = gravitational constant = 9.81 m/s
h = height of observation (m)
hneutral = height of neutral pressure level (m)
T = absolute temperature (K) (subscripts
i = inside and
o = outside)
Combined effect of wind and temperature difference
In most cases, natural ventilation depends on a combined force of wind and stack effects. The
pressure patterns for actual buildings continually change with the relative magnitude of thermal
and wind forces. Figure below shows the combined effect of wind and thermal forces. The
pressures due to each effect are added together to determine the total pressure difference
across the building envelope.
The relative importance of the wind and stack pressures in a building depends on building
height, internal resistance to vertical air flow, location and flow resistance characteristics of
envelope openings, local terrain, and the immediate shielding of the building structure.
Design for Natural Ventilation The design of controlled natural ventilation systems requires
identification of the prevailing wind direction, the strategic orientations and positions of openings
the building envelope. These openings include windows, doors, roof ventilators, skylights, vent
shafts, and so forth.
Ventilation rates
When designing a ventilation system, the ventilation rates are required to determine the sizes of
fans, openings, and air ducts. The methods that can be used to determine the ventilation rates
include:
(a) Maximum allowable concentration of contaminants
A decay equation can be used to describe the steady-state conditions of contaminant concentratio
and ventilation rate, like this
Ci = Co + F / Q
where Ci = maximum allowable concentration of contaminants
Co = concentration of contaminants in outdoor air
F = rate of generation of contaminants inside the occupied space (l/s)
Q = ventilation rate (l/s)
(b) Heat generation
The ventilation rate required to remove heat from an occupied space is given by:
where H = heat generation inside the space (W)
Q = ventilation rate (l/s)
cp = specific heat capacity of air (J/kg.K) = density of air (kg/m3)
Ti = indoor air temperature (K) To = outdoor air temperature (K)
(c)
Air change rates
Recommended
air change rates
Most related professional institutes and authorities have set up recommended ventilation
rates, expressed in air change per hour, for various situations. The ventilation rate is related to
the air change rate by the following equation:
where Q = ventilation rate (l/s)
V = concentration of contaminants in outdoor air
ACH = air change per hour
Table here gives some recommended air change rates for typical spaces.
Table in next slide provides some examples of outdoor air requirements for ventilation
Recommended air change
rates
Space
Carparks
Kitchen
Lavatory
Bathrooms
Boiler rooms
Air change rates per hour
6
20 - 60
15
6
15 - 30
Outdoor air requirements for ventilation Note: Data source: ASHRAE Standard 62-198
Outdoor air requirements
for ventilation
Application
Estimated maximum
Outdoor air requirements
occupancy (persons per 100 (l/s/person)
m2 floor area)
Offices
- office space
- conference room
Retail's Stores
- street level
- upper floors/arcades
7
50
10
10
30
20
5
5
50
150
20
8
8
8
10
20
13
15Note: Data source:
Education
- classroom
- auditorium
- library
Hospitals
- patient rooms
- operating rooms
ASHRAE Standard 62-1989,
Ventilation for Acceptable
Indoor Air Quality.
Flow caused by wind
Major factors affecting ventilation wind forces include:
average wind speed;
prevailing wind direction;
seasonal and daily variation in wind speed and direction;
local obstructing objects, such as nearby buildings and trees;
position and characteristics of openings through which air flows; and
distribution of surface pressure coefficients for the wind.
Natural ventilation systems are often designed for wind speeds of half the average seasonal
velocity because from climatic analysis there are very few places where wind speed falls below
half the average velocity for many hours in a year. The following equation shows the air flow
rate through ventilation inlet opening forced by wind:
where Q = air flow rate (m3/s)
A = free area of inlet openings (m2)
v = wind velocity (m/s)
Cv = effectiveness of the openings (assumed to be 0.5 to 0.6 for perpendicular winds and 0.25 to
0.36 for diagonal winds)
Flow caused by thermal forces
If the building's internal resistance is not significant, the flow caused by stack effect may be
estimated by:
where Q = air flow rate (m3/s) K = discharge coefficient for the opening (usually assumed to be
0.65) A = free area of inlet openings (m2) h = height from lower opening (mid-point) to neutral
pressure level (m) Ti = indoor air temperature (K) To = outdoor air temperature (K)
Guidelines for natural ventilation
a natural ventilation system should be effective regardless of wind direction and there must be adequate
ventilation even when the wind does not blow from the prevailing direction;
inlet and outlet openings should not be obstructed by nearby objects;
windows should be located in opposing pressure zones since this usually will increase ventilation rate;
a certain vertical distance should be kept between openings for temperature to produce stack effect;
openings at the same level and near the ceiling should be avoided since much of the air flow may bypass
the occupied zone;
architectural elements like wingwalls, parapets and overhangs may be used to promote air flow into the
building;
topography, landscaping, and surrounding buildings should be used to redirect airflow and give maximum
exposure to breezes;
in hot, humid climates, air velocities should be maximised in the occupied zones for bodily cooling;
to admit wind air flow, the long façade of the building and the door and window openings should be
oriented with respect to the prevailing wind direction;
if possible, window openings should be accessible to and operable by occupants;
vertical shafts and open staircases may be used to increase and generate stack effect;
openings in the vicinity of the neutral pressure level may be reduced since they are less effective for
thermally induced ventilation;
if inlet and outlet openings are of nearly equal areas, a balanced and greater ventilation can be obtained.
Hybrid Ventilation
In parallel with the hybrid heating research, NRC-IRC is using the upgraded test facility to study
combinations of natural and mechanical ventilation. This project provides the opportunity to
ensure that innovative hybrid ventilation strategies will be suitable for houses, with or without
good indoor air distribution provided by the (forced-air) heating system. This is an important
piece of work because ventilation and air-conditioning can account for up to 50% of residential
energy consumption and have a direct impact on occupants' health and comfort
Natural ventilation may result in too little or too much fresh air exchange, and may waste energy
in heating or cooling a space. Mechanical ventilation is easily controlled and enables heat
recovery and filtration but it consumes electrical energy and thereby promotes greenhouse gas
emissions. Hybrid ventilation, combining the advantages of both natural and mechanical
ventilation, may offer a way to reduce the energy used for building ventilation.
General Ventilation
• A method of improving or maintaining the
quality of air in the work environment with
airflow
• A room or an entire building is flushed by
supplying and exhausting large volumes of air
throughout the area
1. Supply or forced ventilation
2. Exhaust or induced ventilation
General or Dilution Ventilation
1. Natural
Ventilation
2. Mechanical
Ventilation
Preferred if significant
health hazards exist
Dilution Ventilation (DV)
• DV consists of general ventilation
• Uncontaminated outside air + inside air =
diluting and reducing the concentration of air
contaminants to acceptable levels to which a
worker can be safely exposed for eight hours a
day
Natural Ventilation
Calculation of rate of ventilation air flow
Q = H/(60 * CP * ρ * Δt) = H/1.08 * Δt
Where
H = Heat removed in Btu/hr
Δt = indoor outdoor temperature difference(oF)
CP = 0.245 Btu/lb/ oF
ρ = 0.075 lb/ft3
Natural Ventilation
149
Flow Due to Thermal Forces (Stack Effect)
Q = C * K* A * √ ( h * [ ( ti – to ) / ti ] )
Q = air flow in cfm
A = free area of inlets or outlets (assumed equal) in ft2
h = height from inlets to outlets, ft
ti = indoor air temperature, oF
to = outdoor air temperature, oF
C = Constant of proportionality =14.46
K = 65% or 0.65 for effective openings
= 50% or 0.50 for unfavorable conditions
Substituting the values for C and K the equation reduces to
Q = 9.4* A * √ ( h * [ ( ti – to ) / ti ] )
Q = 7.2 * A * √ ( h * [ ( ti – to ) / ti ] )
(for effective openings)
(for unfavorable conditions)
When to> ti replace denominator in equation with to.
Assumptions:-
1. No significant building internal resistance
2. Equation is valid for temperatures ti and to close to 80oF
Natural Ventilation
150
Factors affecting flow due to wind
 Average velocity
 Prevailing direction
 Seasonal and daily variation of wind speed & direction
 Terrain features (local)
Natural Ventilation
151
Calculation of Air Flow(due to Wind)
Q = EAV
Q = air flow in ft3/min
A = free area of inlet openings in ft2
V = wind velocity in ft/min
E = effectiveness of openings
= 0.5-0.6
perpendicular winds
= 0.25-0.35 diagonal winds
V for design practice = 1/2*seasonal average
Natural Ventilation
152
Flow Due to Combined Wind and Stack Effect
 When both forces are together, even without interference, resulting
air flow is not equal to the two flows estimated separately.
 Flow through any opening is proportional to the square root of the
sum of heads acting on that opening.
 Wind velocity and direction, outdoor temperature, and indoor
distribution cannot be predicted with certainty, and refinement
calculations is not justified.
 A simple method is to calculate the sum of the flows produced by
each force separately.
 Then using the ratio of flow produced by the thermal forces to the
aforementioned sum, the actual flow due to the combined forces can
be approximated.
 When the two flows are equal, actual flow is about 30% greater than
the flow caused by either force.
Natural Ventilation
153
Types of Natural Ventilation Openings
 Windows :
There are many types of windows.
Windows sliding vertically, sliding horizontally, tilting,
swinging.
 Doors, monitor openings and skylights.
 Roof Ventilators (weather proof air outlet).
 Stacks connecting to registers.
 Specially designed inlet or outlet openings.
Natural Ventilation
154
Natural Ventilation Rules
1.
Buildings and ventilating equipment should not usually be oriented
for a particular wind direction.
2.
Inlet openings should not be obstructed by buildings , trees,
signboards, or indoor partitions.
3.
Greatest flow per unit area of total opening is equal to inlet and
outlet openings of nearly equal areas.
4.
For temperature difference to produce a motive force, there must be
vertical distance between openings; vertical distance should be as
great as possible.
5.
Openings in the vicinity of the neutral pressure level are least
effective for ventilation.
6.
Openings with areas much larger than calculated are sometimes
desirable(e.g.hot weather,increased occupancy). The openings should
be accessible to and operable by occupants.
Natural Ventilation
155
Infiltration
Infiltration is air leakage through cracks and interstices, around windows
and doors, and through floors and walls into a building
Leakage rate (houses)0.2 to 1.5 air changes /hr in winter
Infiltration through a wall
Q = C*(ΔP)n
Q = Volume flow rate of air ft3/min
C = Flow coefficient(Volume flow rate per unit length of crack or unit area
at a unit pressure difference)
ΔP = Pressure difference
n = Flow exponent 0.5 –1.0 normally 0.65
Natural Ventilation
156
Pressure Difference Due to Thermal Forces
Pc = 0.52*P*(1/To-1/Ti).
Pc = theoretical PC = pressure difference across enclosure due to chimney
effect(inches of water).
P = atmospheric pressure lb/sq.inches.
h = distance from neutral pressure level or effective chimney height.
To = absolute temperature outside 0R.
Ti = absolute temperature outside 0R.
Apply for character of interior separations correction.
Natural Ventilation
157
Infiltration
 Air moves in and out of buildings at varying rates depending upon a
number of factors relating to both the structure and the local
meteorological conditions. Two terms are: infiltration and ventilation.
Both are measured as air exchange rate, or air changes per hour
(ACH).
 The ASHRAE defines infiltration as “uncontrolled airflow through
cracks and interstices, and other unintentional openings.”
 Infiltration occurs because no building is completely airtight; wind
pressures and temperature create driving forces which push or draw
the outdoor air through openings into the building.
 Infiltration is the rate of exchange of outdoor air with the entire
volume of indoor air, quantitated as ACH.
Natural Ventilation
158
Factors Affecting Air Infiltration
Type of structure and construction
 Meteorology
 Heating & cooling systems
 Occupant activity
Structural parameters
 Quality of construction
 Materials of construction
 Condition of the structure
Meteorological parameters
 The airflow rate due to infiltration depends upon
pressure
differences between the inside and outside of the structure and the
resistance to flow through building openings
Natural Ventilation
159
Wind Effects
 Shell and exterior air barriers.
 Interior barriers to flow that cause internal pressure buildup and thus
reduce infiltration.
 Lack of precise knowledge of the detailed wind pressure profiles on
building surfaces.
 Influence of complex terrain, presence of trees and other obstacles
that create channeling and may increase the magnitude of wind force
and alter its direction close to the structure.
 Sheltering, urban canyon and building wake phenomena due to
surrounding buildings and other neighborhood factors.
 Fluctuating winds, rather than linear wind forces, that may effect
infiltration rates through window cracks.
Natural Ventilation
160
Temperature Effects
 Temperature inside a structure is often different from the outside
ambient temperature.
 Maximum temperature differences occur when the indoor
environment is heated.
 Temperature differences cause differences in air density inside and
outside, which in turn produce pressure differences.
 In the winter when indoor air temperatures are high relative to those
outdoors, the warmer and less dense air inside rises and flows out of
the building at its top.
 This air is replaced by cold outdoor air that enters near the bottom of
the building or from the ground.This phenomenon is called the
building “Stack Effect”.
 During hot weather when air conditioning produces lower
temperatures inside than outside, the reverse process occurs.
Natural Ventilation
161
Humidity Effects
 Stricker in 1975 reported that homes with low infiltration
rates had high humidity.
 In a study by Yarmac et al. in 1987 in 25 houses in the
southern U.S., no apparent relationship was found between
relative humidity and air exchange rate. One explanation
for this lack of association is that absolute humidity, rather
than relative humidity, may be a better measure of any
effect the water content of the air has on infiltration.
Natural Ventilation
162
Pressure Difference Across the Building
Envelope
ΔP = Po+Pw-Pi
Where
ΔP = pressure difference between outdoors and indoors at the location
Po = static pressure at reference height in the undisturbed flow
Pw = wind pressure at the location
Pi = interior pressure at the height of the location
1. The more usual case is when both wind and indoor outdoor
temperature differences contribute to the ΔP across the building
envelope
Natural Ventilation
163
Pressure Difference Across the Building
Envelope
2.Temperature differences impose a gradient in the pressure differences
which is a function of height and the temperature difference
This effect is additive to the wind pressure expression and is expressed by
ASHRAE, 1989 as
ΔP = Po+Pw-Pi,r+ ΔPs
Where
ΔPs= the pressure caused by the indoor-outdoor temperature difference
(stack effect)
Pi,r = the interior static pressure at a reference height (it assumes a value
such that inflow equals outflow)
Natural Ventilation
164
Bernoulli’s Equation
PV = (Cp*ρ*V2)/2
Where
PV = surface pressure relative to static pressure in undisturbed flow,Pa
Cp = surface pressure coefficient
ρ = density of air,kg/m3
V = wind speed in m/s
Under standard conditions (100.3 Pa or 14.7 psi) and 200 C, this equation
reduces to:
PV = (Cp*0.601*V2)
Natural Ventilation
165
Bernoulli’s Equation
Cp varies with location around the building envelope and wind direction
The differences in air density due to temperature differences between
the interior and exterior of a building create the pressure difference
which drives infiltration
To estimate this pressure difference, ΔPs, it is necessary to know the NPL
This pressure difference can be expressed as:
ΔPs = ρi*g*h*(Ti-To)/ To
Where:
ΔPs = pressure difference, Pa
ρi = density of air, kg/m3
g = gravitational constant, 9.8m/sec2
Natural Ventilation
166
Bernoulli’s Equation
h=distance to NPL(+ve if above, -ve if below from the location of the
measurement
Subscripts:
i=inside
o=outside
It is difficult to know the location of the NPL at any one moment, but there
are some general guidelines
According to ASHRAE,1989, the NPL in tall buildings can vary from 0.3 to 0.7
of total building height
In houses with chimneys, it is usually above mid-height, and vented
combustion sources for space heating can move the NPL above the ceiling
Natural Ventilation
167
Measurement Techniques
 Tracer gas
 Fan pressurization
 Effective Leakage Area(ELA)
Natural Ventilation
168
Tracer Gas
 It is a different measure of air exchange rate.
 The gas concentration will decrease as dilution air flow
into the building.
 The rate of decrease is proportional to the infiltration
rate.
Natural Ventilation
169
Assumptions
 The tracer gas mixes perfectly and instantaneously
 The effective volume of the enclosure is known
 The factors that influence air infiltration remain
unchanged throughout the measurement period
 Imperfect mixing occurs when air movement is impeded
by flow resistances or when air is trapped by the effects of
stratification
 This causes spatial variation in the concentration of the
tracer gas within the structure, this may cause bias in
sampling locations
Natural Ventilation
170
Assumptions (contd…)
 Fans are often used to mix the tracer gas with the building
air.
 Effective volume is assumed to be the physical volume of
the occupied space.
 Areas which contain dead spaces that do not communicate
with the rest of the living space will reduce the effective
volume.
 Variations in conditions during the measurement
period,such as door openings or meteorological changes,
will cause a departure from the logarithmic decay curve
and the equation on which infiltration is calculated will no
longer hold.
Natural Ventilation
171
Types of Gases of Used As Tracers:
 Helium,Nitrous oxide, Carbon dioxide,Carbon monoxide,
Sulfur hexaflouride, and perfluorocarbons
 Non-toxic at concentrations normally used in such studies,
non-allergenic, inert, non-polar, and can be detected easily
and at low concentrations
 Most frequently used are SF6 and Perfluorocarbons
 Carbon dioxide or carbon monoxide can be used if initial
concentrations are substantially above background but well
below concentrations of health concern
Natural Ventilation
172
Tracer Gas Dilution: SF6
 Specific instructions for this method can be found in the
American Society of Testing Materials (ASTM)Standard
Method for Determining Air Leakage Rate by Tracer
Dilution (E741).
 The basic apparatus for this method includes: tracer gas
monitor, cylinder of tracer gas, sample collection
containers and pump, syringes, circulating fans, and a
stopwatch.
 Meterological parameters which are recorded include:
wind speed and direction, temperature (indoors and
outdoors), relative humidity barometric pressure.
Natural Ventilation
173
Tracer Gas Dilution: SF6
 For SF6 concentrations in the range of 1-500 ppm, a portable
infrared gas analyzer is used.
 For SF6 concentrations in the ppb range/a gas
chromatograph(GC)with an electron capture detector is used.
 A field GC is preferable so that the concentration of SF6 can be
immediately verified and optimum sample integrity maintained.
 If it is injected in undiluted form, SF6 may tend to sink and
accumulate in low areas.
 Documenting various structural parameters and occupant activities
which may be occurring during the sampling time as well as the
meterological parameters.
Natural Ventilation
174
Tracer Gas Dilution: SF6
 Structural parameters include: windows (number, location,
type), noticeable leakage paths, wall construction, location
of chimneys, vents and other direct indoor-outdoor
communication points, and type and capacity of the
heating and/or air conditioning systems.
 Occupant activity such as opening and closing of doors
(interior or exterior) or vents will affect the infiltration rate
as well as the distribution of the tracer gas within the
structure.
 Operational status of the heating or cooling system should
also be recorded.
Natural Ventilation
175
Calculation of Air Exchange Rate
 C=Co-It
Where:
 C = tracer gas concentration at time t
 Co= tracer gas concentration at time =0
 I = air exchange rate
 T = time
This relationship assumes that the loss rate of the initial concentration of
tracer gas is proportional to its concentration
If the ventilation system recirculates a fraction of the indoor air, then the
above assumption may not hold
Above equation then can be rearranged to yield the expression
 I = (1/t)*Ln(Co/C)
Natural Ventilation
176
Fan Pressurization
 It is sometimes also called depressurization.
 It is not a direct measure of infiltration.
 It characterizes the building leakage rate independent of
weather conditions.
 Measurements are made by using a large fan to create an
incremental static pressure difference between the
interior and the exterior of the building.
 The air leakage rate is determined by the relationship
between the airflow rates and pressure differences.
Natural Ventilation
177
Fan Pressurization (Contd…)
 The fan is usually placed in the door, and all direct openings in
the building envelope, e.g.,windows, doors, vents, and flues,
are sealed off.
 The airflow rate through the fan is determined by measuring
the pressure drop across a calibrated orifice plate.
 The resulting leakage occurs through the cracks in the building
envelope, and the effective leakage area can be calculated
from the flow profile.
Natural Ventilation
178
Advantages and Disadvantages of Fan
Pressurization
Advantages:
 It does not require sophisticated analytical equipment as
do the tracer techniques
 It allows for a comparison of homes based on their
relative leakiness irrespective of the prevailing weather
conditions at the time of measurement
 It can be used to measure the effectiveness of retrofit
measures
Disadvantages:
 This is an indirect measure of infiltration and hence
approximates the actual process through an inherently
artificial process, pressurization or depressurization
Natural Ventilation
179
General Steps
 Note the physical characteristics of the building.
 Close all normal openings (e.g.,windows, doors, vents, and
flues).
 Record meteorological conditions and indoor temperature
and relative humidity, and install the blower assembly.
 The blower should run at such speeds as to induce
pressure differences of 0.05 to 0.3 in. water (12.5 to 75
Pa).
Natural Ventilation
180
Effective Leakage Area(ELA)
 Another indirect method to estimate air infiltration.
 It can be interpreted physically as an approximation of the
total area of physical openings in the building envelope
through which infiltration occurs.
 The empirical model used to estimate air exchange is
based on pressure differences.
 The method involves measuring the dimensions of each
opening and converting this value to a leakage area
equivalent value.
Natural Ventilation
181
Calculation of ELA
ELA = Q4/(2*ΔP/ ρ)0.5
Where
ELA = effective leakage area,m2
Q4 = airflow at 4 Pa(m3/sec)
ΔP = the pressure drop causing this flow,I.e.,4 Pa
ρ
= density of air,1.2 kg/m3
Natural Ventilation
182
Pointers on the Use of Industrial
Ventilation
1. Air removed must be replaced by supply air
2. Short circuiting of air must be prevented
3. Lay-out of equipment and process should
be considered in relation to the direction of
air flow
Short Circuiting
Pointers on the Use of
Industrial Ventilation
4. Avoid cross drafts of air near exhaust
outlets
5. Contaminated air must be correctly
discharged outdoors such that its re-entry
inside the work environment is avoided
Local Exhaust Ventilation (LEV)
Removes airborne contaminants at the
point of dispersion or generation
before they become fugitive and
contaminate the work environment.
LOCAL EXHAUST
VENTILATION SYSTEM
Duct
Air Cleaning Device
Hoods
Fan
Pointers in
Local Exhaust Ventilation
1. Enclose the contaminant
2. Capture contaminant with
adequate air velocities
3. Keep contaminant out of the
worker’s breathing zone
4. Discharge contaminated air
outdoors
LEV
• Capture or contain contaminants at their
source before they escape into the work room
environment
• System consists of one or more hoods, ducts,
air cleaner and a fan
• LEVs remove contaminants rather than just
dilute them
Local Exhaust System
S
t
a
c
k
Fan
Duct
Hood
Barrel
Filling
Operation
Air
Cleaner
Local Exhaust System: Components
1.
2.
3.
4.
5.
–
–
–
–
–
–
–
Hood
Surrounds the contaminant
Capture the contaminant
Carries the contaminant to the duct
Duct
•
Transports the contaminants through the system
Most industrial ducts are circular
Fan
Moves the air
Cleaning device
Cyclones, Electrostatic precipitators
Stack
Puts the air outside of the factory
Use of Natural Ventilation
• Not suitable for processes which emit dust,
fumes, mists or gas
• Rooms for chemical storage +
• 25% of floor area
• Half the ventilating area should be between
floor level and a height of 2.25m from the
floor
Use of Dilution Ventilation
• DV is usually applied to the control of
contaminants in situation meeting these
criteria:
 To control vapours (+organic) from low toxicity
solvents
 To control contaminants released over such a large
area or in such a manner that LEV is impossible,
impractical or prohibitively expensive
Notes
 Small quantities of contaminants released into the
work room
 Rate of contaminant release should be reasonably
constant to avoid inadequate dilution during periods
of peak contaminant release
 No corrosion or other problems from the diluted
contaminants in the work room air
contd
 Sufficient distance from the worker to the
contaminants source to allow dilution to safe levels
Limitations
• DV is prohibited – control emission of very
toxic air contaminants e.g., formaldehyde or
other carcinogenic chemicals
• For effective DV, the exhaust outlet and air
supply must be so located that all the air
employed in the ventilation passes through
the zone of contamination
Contd.
• Avoid re-entrance of the exhausted air.
• The advantages and disadvantages of DV are
as follows:
Advantage
• Simplicity
• Low original cost
Disadvantage
• Large volumes of
dilution air needed
• Employee exposures are
difficult to control near
source
4.0 Local exhaust ventilation systems
•
4.1 Applications, components of a local exhaust system, types of losses, losses and velocity pressure, friction, elbow
and branch entry losses
4.2 Hood design and selection, selecting and designing ductwork, fan selection
4.2.1 Hood Design
4.2.1.1 Contaminant Characteristics, Hood Types
4.2.1.2 Hood Design Factors, Hood Losses, Special hood Requirement
4.2.2 Duct
4.2.2.1 Types, Flow in Ducts
4.2.2.2 Losses, Correction in ductwork
4.2.3 Air Cleaning Devices
4.2.3.1 Selection of Dust Collection Equipment, Dust Collector Types
4.2.3.1 Control of Mist, Gas and Vapor Contaminants
4.2.3.2 Gaseous Contaminant Collectors
4.2.3.3 Selection of Air Filtration Equipment
4.2.4 Fans
4.2.4.1 Basic Definitions, Fan selection
5.0 Non-Standard Condition
5.1 Corrections for water vapor in air (Relative Humidity)
5.2 Density Correction factor
5.3 Air flow, Velocity Pressure, Vapor generation, System design
Local Exhaust System
• A local exhaust system is used to control an air
contamination by trapping it near its source,
as contrasted with dilution ventilation which
lets the contamination spread throughout the
work-room, later to be diluted by exhausting
quantities of air form the workroom.
Local Exhaust System
• A local exhaust system is often preferred to
ventilation-by-dilution because it provides a
cleaner and healthier work environment and
because it handles a relatively smaller volume
of air, with less attendant heat loss.
• A local exhaust system also uses a smaller fan
and dust arrester.
Local Exhaust System
• A LES is usually the proper method of
contaminant control if:
– Air-samples show that the contaminant in the
atmosphere constitutes a health, fire, or explosion
hazard.
– Maintenance of production machinery would
otherwise be difficult.
– Marked improvement in housekeeping or employee
comfort will result.
Local Exhaust System
• Emission sources are large, few, fixed and/or
widely spread.
• Emission sources are near the employee
breathing zone.
• Emission rates vary widely by time.
Comparison of Ventilation Systems
Dilution Ventilation
Local Exhaust Ventilation
Advantages
Usually lower
equipment and
installation costs.
Disadvantages
Advantages
Disadvantages
Does not completely Captures
Higher cost for
remove
contaminant at
design, installation
contaminants.
source and removes and equipment.
it from the
workplace.
Requires less
Cannot be used for Only choice for
Requires regular
maintenance.
highly toxic
highly toxic airborne cleaning, inspection
chemicals.
chemicals.
and maintenance.
Effective control for Ineffective for dusts Can handle many
small amounts of
or metal fumes or
types of
low toxicity
large amounts of
contaminants
chemicals.
gases or vapours.
including dusts and
metal fumes.
Effective control for Requires large
Requires smaller
flammable or
amounts of heated or amount of makeup
combustible gases or cooled makeup air. air since smaller
vapours.
amounts of air are
being exhausted.
Best ventilation for Ineffective for
Less energy costs
mobile or dispersed handling surges of since there is less
contaminant sources. gases or vapours or makeup air to heat or
irregular emissions. cool.
In general, what are limitations of any ventilation system?
•The systems deteriorate over the years because of to
contaminant build-up within the system, especially filters.
•Require ongoing maintenance.
•Regular and routine testing is needed to identify problems
early and implement corrective measures.
•Only qualified persons should make modifications to a
ventilation system to make sure the system continues to work
effectively.
Parts of Local Exhaust System
• Hoods, into which the airborne contaminants is
drawn
• Ducts, for carrying the contaminated air to a
central point.
• An air-cleaning device, such as a dust arrester for
purifying the air before it is discharged.
• A fan and motor to create the required airflow
through the system.
• A stack to disperse remaining air contaminants.
Hoods
• The local exhaust hood is the point of air entry
into the duct system.
• The term “hood” is used in a broad sense to
include all suction openings regardless of their
shape or mounting arrangement.
Types of hoods
• Capture type
• Enclosing type
• Receiving (canopy) type
Capture
Enclosing
Receiving
Plain opening
Flanged opening
Principles of hood design
• Enclose the operation as much as possible to
reduce the rate of air flow needed to control
the contaminant, and to prevent cross drafts
from blowing the contaminant away from the
field of influence of the hood.
• Locate the hood so the contaminant is moved
away form the breathing zone of the operator.
Principles of hood design
• Solvent vapors in health hazard concentration
are not appreciably heavier than air.
• Capture them at their source rather than
attempt to collect them at the floor level.
Ducts
• Duct serves the purpose of carrying the
contaminated air drawn by hood to an air
cleaner or to the outdoors.
• When air passes through any duct or pipe,
friction must be overcome (energy must be
expended).
Duct Design Principles
Principle
Streamline the system as much as possible
to minimize air turbulence and resistance.
Round ducts provide less resistance than
square ducts (less surface area).
Smooth, rigid ducts provide less resistance
than flexible, rough ducts.
Short runs of ducts provide less resistance
than long runs.
Straight runs offer less resistance than
runs with elbows and bends.
Duct branches should enter at gradual
angles rather than right angles. Duct
branches should not enter the main duct at
the same point.
Elbows with gradual bends provide less
resistance than sharp bends.
Large diameter ducts provide less
resistance than small diameter ducts.
Design for less resistance for air flow
Avoid design that causes more resistance to
air flow
Air cleaners
• Air cleaners or dust arrester fall into two broad
classes according to their use.
First type
• Industrial air cleaners are those whose
purpose is to remove airborne contaminants
(dust, fume, mist, vapor, gas or odor).
• These air cleaners can be cleaned of collected
contaminants.
• They operate at relatively moderate airflow
rate.
Second type
• Air cleaners that handle relatively high rates of
airflow.
• These are associated with heating, ventilating,
and air conditioning systems.
• This type of air cleaners removes particulates
form incoming outdoor air and re-circulated
air to provide “clean” air for the building.
• The filter medium cannot be cleaned and it is
disposable type.
Fans
• There are two groups of fans which are in use:
• Centrifugal flow fans = this fans can be used
where static pressure is medium to high. The air
leaves at right angle.
• Axial flow fans = this fans can be use where static
pressure is low. This are like the normal table fans
we use in which air leaves the straight way
opposite side.
Centrifugal fan
Axial fan
LES
Biological organisms
• When air is re-circulated for building ventilation,
the spread of communicable disease may be
accelerated by re-circulation of biological agents in
contaminated dust and droplets.
Biological organisms
• The bacterial content of the air in ventilating
ducts can be reduced by ultraviolet irradiation,
and perhaps by sprays of polyglycols, or by
efficient filters.
• Other biological agents, such as spores and
fungi, can be controlled by keeping air
distribution system dry, or chemically treated.
Heat
• With body temperature regulated at 37 C (98.6
F), and skin temperature at perhaps 33 C (92 F) in
winter and 35 C (95 F) in summer, the task of a
heating and ventilating system is not to heat the
body, but to permit the body heat to escape at a
controlled rate.
• This follows because air temperature in the mid20s C (mid –70s F) make up the comfortable
range for humans.
Poor fan location
Poor air inlet
Fair air inlet
Poor fan location
Good air inlet
Poor fan location
Poor air inlet
Fair air inlet
Poor fan location
Good air inlet
Good fan location
Poor air inlet
Fair air inlet
Good fan location
Good air inlet
Good fan location
pl
e
n
u
m
plenum
Best air inlet
Best air inlet
Good fan location
Plenum
Best air inlet
LOCAL EXHAUST
VENTILATION SYSTEM
Duct
Air Cleaning Device
Hoods
Fan
Use of LEV
• The most effective means of controlling air
contaminants is to capture and remove the air
contaminants at their source with LEV and to
prevent them from being carried away by air
currents into the breathing zones of the
worker
Some detail of LEV
• Major release (toxic) of localized sources of
contaminants.
• LEV consists of 4 parts:
1.
Hoods (most important)
2.
Ducts
3.
Air cleaner device
4.
Fan and motor
parts
• The hood consists of 3 main types to contain
and remove the air-borne contaminants.
1.
Enclosures
2.
Capturing hoods
3.
Receiving hoods
Conclusion
• Ventilation – an effective way to control toxic
air contaminants if substitution or enclosure
method of control is not possible
• DV reduces contaminant concentrations by
diluting them with fresh air. (not for toxic
emission)
Conclusion cotd
• LEV capture or contain contaminants at their
source before they are dispersed in the
workroom
• LEVs need to be maintained, inspected and
tested regularly to ensure that it is performing
adequately
Supply Systems
Components
 Air inlet section
 Filters
 Heating and/or cooling equipment
 Fan
 Ducts
 Register/grills for distributing the air within the work space
General Principles
252

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