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Abstract

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
What is industrial ventilation?
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.
What is the purpose of a ventilation system?
What is the purpose of a ventilation system?
There are four purposes of ventilation:
There are four purposes of ventilation:
Provide a continuous supply of fresh
Provide a continuous supply of fresh
outside air.
outside air.
Maintain temperature and humidity at
Maintain temperature and humidity at
comfortable levels.
comfortable levels.
Reduce potential fire or explosion
Reduce potential fire or explosion
hazards.
hazards.
Remove or dilute airborne contaminants.
Remove or dilute airborne contaminants.
Why have an industrial ventilation system?
Why have an industrial ventilation system?
Ventilation is considered an "engineering control"
Ventilation is considered an "engineering control"
to remove or control contaminants released in
to remove or control contaminants released in
indoor work environments.
indoor work environments.
It is one of the preferred ways to control employee
It is one of the preferred ways to control employee
exposure to air contaminants.
exposure to air contaminants.
Other ways to control contaminants include:
Other ways to control contaminants include:
eliminate the use of the hazardous chemical or
eliminate the use of the hazardous chemical or
material,
material,
substitute with less toxic chemicals,
substitute with less toxic chemicals,
process change, or
process change, or
work practice change.
work practice change.
What are the parts of an industrial
What are the parts of an industrial
ventilation system?
ventilation system?
Systems are composed of many parts including:
Systems are composed of many parts including:
an "air intake" area such as a hood or an
an "air intake" area such as a hood or an
enclosure,
enclosure,
ducts to move air from one area to another,
ducts to move air from one area to another,
air cleaning device(s), and
air cleaning device(s), and
fan(s) to bring in outside air and exhaust the
fan(s) to bring in outside air and exhaust the
indoor contaminated air.
indoor contaminated air.
What are the basic types of 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.
What are main features of dilution ventilation?
What are main features of dilution ventilation?
Dilution, or "general", ventilation supplies and
Dilution, or "general", ventilation supplies and
exhausts large amounts of air to and from an area or
exhausts large amounts of air to and from an area or
building. It usually involves large exhaust fans
building. It usually involves large exhaust fans
placed in the walls or roof of a room or building.
placed in the walls or roof of a room or building.
Dilution ventilation controls pollutants generated at
Dilution ventilation controls pollutants generated at
a worksite by ventilating the entire workplace. The
a worksite by ventilating the entire workplace. The
use of general ventilation distributes pollutants, to
use of general ventilation distributes pollutants, to
some degree, throughout the entire worksite and
some degree, throughout the entire worksite and
could therefore affect persons who are far from the
could therefore affect persons who are far from the
source of contamination.
source of contamination.
Dilution ventilation can be made more
Dilution ventilation can be made more
effective if the exhaust fan is located close to
effective if the exhaust fan is located close to
exposed workers and the makeup air is
exposed workers and the makeup air is
located behind the worker so that
located behind the worker so that
contaminated air is drawn away from the
contaminated air is drawn away from the
worker's breathing zone.
worker's breathing zone.
When used to control chemical pollutants, dilution
When used to control chemical pollutants, dilution
must be limited to only situations where:
must be limited to only situations where:
the amounts of pollutants generated are not very high,
the amounts of pollutants generated are not very high,
their toxicity is relatively moderate, and
their toxicity is relatively moderate, and
workers do not carry out their tasks in the immediate
workers do not carry out their tasks in the immediate
vicinity of the source of contamination.
vicinity of the source of contamination.
It is therefore unusual to recommend the use of
It is therefore unusual to recommend the use of
general ventilation for the control of chemical
general ventilation for the control of chemical
substances except in the case of solvents which have
substances except in the case of solvents which have
admissible concentrations of more than 100 parts per
admissible concentrations of more than 100 parts per
million.
million.
What are the limitations of dilution ventilation?
What are the limitations of dilution ventilation?
As a method for protecting workers, it is important
As a method for protecting workers, it is important
to know that dilution ventilation:
to know that dilution ventilation:
Does not completely remove contaminants.
Does not completely remove contaminants.
Cannot be used for highly toxic chemicals.
Cannot be used for highly toxic chemicals.
Is not effective for dusts or metal fumes or large
Is not effective for dusts or metal fumes or large
amounts of gases or vapours.
amounts of gases or vapours.
Requires large amounts of makeup air to be heated
Requires large amounts of makeup air to be heated
or cooled.
or cooled.
Is not effective for handling surges of gases or
Is not effective for handling surges of gases or
vapours or irregular emissions.
vapours or irregular emissions.
Regular "floor" or "desk" fans are also sometimes
Regular "floor" or "desk" fans are also sometimes
used as a method of ventilation, but these fans
used as a method of ventilation, but these fans
typically blow the contaminant around the work area
typically blow the contaminant around the work area
without effectively controlling it. Opening doors or
without effectively controlling it. Opening doors or
windows can be used as dilution ventilation, but
windows can be used as dilution ventilation, but
again, this method is not reliable since air movement
again, this method is not reliable since air movement
is not controlled.
is not controlled.
As a general note, the air or "volumetric" flow rate
As a general note, the air or "volumetric" flow rate
of dilution ventilation depends largely on the how
of dilution ventilation depends largely on the how
fast the contaminant enters the air as well as the
fast the contaminant enters the air as well as the
efficiency that fresh air mixes with workroom air.
efficiency that fresh air mixes with workroom air.
What is local exhaust ventilation?
What is local exhaust ventilation?
Local exhaust system is used to control air
Local exhaust system is used to control air
contaminants by trapping them at or near the
contaminants by trapping them at or near the
source, in contrast to dilution ventilation which lets
source, in contrast to dilution ventilation which lets
the contaminant spread throughout the workplace.
the contaminant spread throughout the workplace.
Local exhaust is generally a far more effective way of
Local exhaust is generally a far more effective way of
controlling highly toxic contaminants before they
controlling highly toxic contaminants before they
reach the workers' breathing zones.
reach the workers' breathing zones.
This type of system is usually the preferred
This type of system is usually the preferred
control method if:
control method if:
Air contaminants pose serious health risk.
Air contaminants pose serious health risk.
Large amounts of dusts or fumes are generated.
Large amounts of dusts or fumes are generated.
Increased heating costs from ventilation in cold weather
Increased heating costs from ventilation in cold weather
are a concern.
are a concern.
Emission sources are few in number.
Emission sources are few in number.
Emission sources are near the workers' breathing zones.
Emission sources are near the workers' breathing zones.
In a general way, a local exhaust system operates similar
In a general way, a local exhaust system operates similar
to a household vacuum cleaner with the hose as close as
to a household vacuum cleaner with the hose as close as
possible to the place where dirt would be created.
possible to the place where dirt would be created.
What are the components of local exhaust
What are the components of local exhaust
ventilation?
ventilation?
A local exhaust system has six basic elements:
A local exhaust system has six basic elements:
A "hood" or opening that captures the contaminant
A "hood" or opening that captures the contaminant
at the source.
at the source.
Ducts that transport the airborne chemicals through
Ducts that transport the airborne chemicals through
the system.
the system.
An air cleaning device that removes the contaminant
An air cleaning device that removes the contaminant
from the moving air in the system (not always
from the moving air in the system (not always
required).
required).
Fans that move the air through the system and
Fans that move the air through the system and
discharges the exhaust air outdoors.
discharges the exhaust air outdoors.
An exhaust stack through which the contaminated
An exhaust stack through which the contaminated
air is discharged.
air is discharged.
Make up air that replaces the exhausted air.
Make up air that replaces the exhausted air.
How do I know which type of ventilation
How do I know which type of ventilation
system is best for my workplace?
system is best for my workplace?
All industrial ventilation systems, when designed
All industrial ventilation systems, when designed
properly, should be able to provide long-term
properly, should be able to provide long-term
worker protection. The two types of ventilation,
worker protection. The two types of ventilation,
dilution and local exhaust, are compared in the
dilution and local exhaust, are compared in the
following table.
following table.
Comparison of Ventilation Systems
Dilution Ventilation Local Exhaust Ventilation
Advantages Disadvantages Advantages Disadvantages
Usually lower
equipment and
installation costs.
Does not completely
remove contaminants.
Captures contaminant at
source and removes it from
the workplace.
Higher cost for
design, installation
and equipment.
Requires less
maintenance.
Cannot be used for highly
toxic chemicals.
Only choice for highly toxic
airborne chemicals.
Requires regular
cleaning, inspection
and maintenance.
Effective control for
small amounts of low
toxicity chemicals.
Ineffective for dusts or
metal fumes or large
amounts of gases or
vapours.
Can handle many types of
contaminants including dusts
and metal fumes.
Effective control for
flammable or
combustible gases or
vapours.
Requires large amounts
of heated or cooled
makeup air.
Requires smaller amount of
makeup air since smaller
amounts of air are being
exhausted.
Best ventilation for
mobile or dispersed
contaminant sources.
Ineffective for handling
surges of gases or
vapours or irregular
emissions.
Less energy costs since
there is less makeup air to
heat or cool.
In general, what are limitations of any
In general, what are limitations of any
ventilation system?
ventilation system?
Some limitations include:
Some limitations include:
The systems deteriorate over the years because of to
The systems deteriorate over the years because of to
contaminant build-up within the system, especially
contaminant build-up within the system, especially
filters.
filters.
Require ongoing maintenance.
Require ongoing maintenance.
Regular and routine testing is needed to identify
Regular and routine testing is needed to identify
problems early and implement corrective measures.
problems early and implement corrective measures.
Only qualified persons should make modifications
Only qualified persons should make modifications
to a ventilation system to make sure the system
to a ventilation system to make sure the system
continues to work effectively.
continues to work effectively.
The following is an example of changes that can
The following is an example of changes that can
affect how a system works.
affect how a system works.
What should I know about make-up air?
What should I know about make-up air?
An important and sometimes overlooked
An important and sometimes overlooked
aspect of local ventilation is the need to
aspect of local ventilation is the need to
provide enough air to replace the air that is
provide enough air to replace the air that is
exhausted from the workplace. If enough
exhausted from the workplace. If enough
make-up air is not provided when large
make-up air is not provided when large
volumes of air are exhausted, the workplace
volumes of air are exhausted, the workplace
becomes "starved" for air and negative
becomes "starved" for air and negative
pressure is created.
pressure is created.
Negative pressure in the workplace increases
Negative pressure in the workplace increases
resistance on the ventilation system causing it
resistance on the ventilation system causing it
to move less air.
to move less air.
Air will also enter a building through cracks
Air will also enter a building through cracks
around doors or windows or other small
around doors or windows or other small
openings to try to "equal" the rate of air
openings to try to "equal" the rate of air
being removed.
being removed.
The result is that workers may be exposed to
The result is that workers may be exposed to
cold air in the winter, and additional heating
cold air in the winter, and additional heating
costs may occur. One simple way to judge if
costs may occur. One simple way to judge if
a building is under an excessive negative
a building is under an excessive negative
pressure is if you have difficulty opening a
pressure is if you have difficulty opening a
door that pushed into the room or building
door that pushed into the room or building
(the air wants to force the door closed).
(the air wants to force the door closed).
A separate intake fan, located away from the exhaust
A separate intake fan, located away from the exhaust
fans, should be used to bring in fresh,
fans, should be used to bring in fresh,
uncontaminated air from outside. This air must be
uncontaminated air from outside. This air must be
clean and heated in winter or cooled in summer, as
clean and heated in winter or cooled in summer, as
needed.
needed.
For example if the air flow rate required in a
For example if the air flow rate required in a
workspace which is 40 feet long, 40 feet wide and 12
workspace which is 40 feet long, 40 feet wide and 12
feet high, volume of the work space is 40 x 40 x 12 =
feet high, volume of the work space is 40 x 40 x 12 =
19,200 cubic feet.
19,200 cubic feet.
Air flow rate required per ACH = 19,200 / 60 = 320
Air flow rate required per ACH = 19,200 / 60 = 320
cfm
cfm
Or, air flow rate required per ACM = 19,200 cfm
Or, air flow rate required per ACM = 19,200 cfm
Or, if the ceiling height is 20 feet high then the room volume
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
is 40 feet X 40 feet X 20 feet high= 32,000 cubic feet and the
required air flow rate will be as follows:
required air flow rate will be as follows:
Air flow rate required per ACH = 32,000 / 60 = 533 cfm
Air flow rate required per ACH = 32,000 / 60 = 533 cfm
Or, air flow rate required per ACM = 132,000 cfm
Or, air flow rate required per ACM = 132,000 cfm
The required air change rate is sometimes given in
The required air change rate is sometimes given in
ventilation regulations and ventilation design standards. For
ventilation regulations and ventilation design standards. For
example, a flammable storage room requires six air changes
example, a flammable storage room requires six air changes
per hour according to US OSHA requirements.
per hour according to US OSHA requirements.
What is a duct system?
What is a duct system?
The ventilation system in a building consists
The ventilation system in a building consists
of air moving devices such as fans and
of air moving devices such as fans and
blowers and a network of ducts to exhaust
blowers and a network of ducts to exhaust
the contaminated indoor air and to bring in
the contaminated indoor air and to bring in
air from the outside of the building.
air from the outside of the building.
What are some basic principles of duct design?
What are some basic principles of duct design?
Duct systems should be designed to have air flow
Duct systems should be designed to have air flow
through the ducts with as little friction or resistance
through the ducts with as little friction or resistance
as possible.
as possible.
The amount of air that flows through a duct depends
The amount of air that flows through a duct depends
on the cross section area (duct opening area) of the
on the cross section area (duct opening area) of the
duct and the air speed. Air moving too slowly will
duct and the air speed. Air moving too slowly will
allow contaminants such as dusts to settle and
allow contaminants such as dusts to settle and
accumulate and these particles will eventually clog
accumulate and these particles will eventually clog
the duct.
the duct.
Air moving too fast wastes power, can create noise
Air moving too fast wastes power, can create noise
problems, and may cause excessive abrasion by dust
problems, and may cause excessive abrasion by dust
particles hitting the ducts.
particles hitting the ducts.
Duct Design Principles
Principle Design for less resistance for
air flow Avoid design that causes more
resistance to air flow
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.
Duct Design Principles
Principle Design for less resistance
for air flow Avoid design that causes more
resistance to air flow
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.
Duct systems typically require large amounts of air
Duct systems typically require large amounts of air
to move relatively small amounts of contaminants.
to move relatively small amounts of contaminants.
The required volume of airflow depends of the
The required volume of airflow depends of the
acceptable concentration of air contaminants in
acceptable concentration of air contaminants in
the inside workspace.
the inside workspace.
A carefully designed system can achieve the
A carefully designed system can achieve the
required air concentration while using the least
required air concentration while using the least
amount of power.
amount of power.
Other design considerations include initial capital
Other design considerations include initial capital
costs, reliability, maintenance, and durability of air
costs, reliability, maintenance, and durability of air
handling equipment.
handling equipment.
Airflow changes direction abruptly:
Airflow changes direction abruptly:
Deposits are more common in short radius elbows
Deposits are more common in short radius elbows
and "T" type branch connections. The figures
and "T" type branch connections. The figures
below shows the examples of abrupt air direction
below shows the examples of abrupt air direction
changes.
changes.
However, below are some tips for conducting a
However, below are some tips for conducting a
simple inspection. Before you start, be sure you
simple inspection. Before you start, be sure you
have a drawing of the ventilation system (or make
have a drawing of the ventilation system (or make
one as you go). While you walk through the entire
one as you go). While you walk through the entire
system take note of the following:
system take note of the following:
Reduced ability to capture the contaminants
Reduced ability to capture the contaminants
("fugitive" contaminants can be measured or
("fugitive" contaminants can be measured or
sometimes been seen).
sometimes been seen).
Constant plugging of a duct. Tap the duct with a
Constant plugging of a duct. Tap the duct with a
stick to see if it has layers of build-up,
stick to see if it has layers of build-up,
Damaged ducts (dents, holes).
Damaged ducts (dents, holes).
Damaged or missing gaskets.
Damaged or missing gaskets.
Visible dust on equipment connected to
Visible dust on equipment connected to
ventilation system.
ventilation system.
Obvious add-ons to system (especially those that
Obvious add-ons to system (especially those that
were added on after the initial installation of the
were added on after the initial installation of the
system).
system).
Opened blast gates or other openings.
Opened blast gates or other openings.
Ducts cut off and covered with blank flanges.
Ducts cut off and covered with blank flanges.
Document any of the above problem(s) and
Document any of the above problem(s) and
possible causes from your walk around the
possible causes from your walk around the
system. Bring these problems to the attention of
system. Bring these problems to the attention of
the building maintenance staff, your supervisor,
the building maintenance staff, your supervisor,
or a ventilation expert if possible.
or a ventilation expert if possible.
What is a hood?
What is a hood?
A hood - correctly called a local exhaust hood -
A hood - correctly called a local exhaust hood -
is the point where contaminated air is drawn
is the point where contaminated air is drawn
into the ventilation system. The sizes and shapes
into the ventilation system. The sizes and shapes
of hoods are designed for specific tasks or
of hoods are designed for specific tasks or
situations. The air speed (velocity) at the hood
situations. The air speed (velocity) at the hood
opening and inside the hood must be enough to
opening and inside the hood must be enough to
catch or capture and carry the air contaminants.
catch or capture and carry the air contaminants.
To be most effective, the hood should surround
To be most effective, the hood should surround
or enclose the source of contaminant or be
or enclose the source of contaminant or be
placed as close to the source as possible.
placed as close to the source as possible.
What are the common types of hood?
What are the common types of hood?
The three common classes of hoods are:
The three common classes of hoods are:
enclosing,
enclosing,
receiving, and
receiving, and
capturing.
capturing.
Enclosing Hood
Enclosing Hood
Enclosing hoods, or "fume" hoods, are hoods
Enclosing hoods, or "fume" hoods, are hoods
surrounding the process or point where the
surrounding the process or point where the
contaminants are generated. Examples of
contaminants are generated. Examples of
completely enclosed hoods (all sides enclosed) are
completely enclosed hoods (all sides enclosed) are
glove boxes and grinder hoods. Examples of
glove boxes and grinder hoods. Examples of
partially enclosed (two or three sides enclosed)
partially enclosed (two or three sides enclosed)
hoods are laboratory hoods or paint spray booths.
hoods are laboratory hoods or paint spray booths.
The enclosing hood is preferred whenever possible.
The enclosing hood is preferred whenever possible.
Receiving Hood
Receiving Hood
These hoods are designed to "receive" or catch
These hoods are designed to "receive" or catch
the emissions from a source that has some initial
the emissions from a source that has some initial
velocity or movement. For example, a type of
velocity or movement. For example, a type of
receiving hood called a canopy hood receives
receiving hood called a canopy hood receives
hot rising air and gases as shown in Figure 2. An
hot rising air and gases as shown in Figure 2. An
example is a canopy hood located over a
example is a canopy hood located over a
melting furnace.
melting furnace.
Capturing Hood
Capturing Hood
These hoods are located next to an emission
These hoods are located next to an emission
source without surrounding (enclosing) it.
source without surrounding (enclosing) it.
Examples are a rectangular hood along the edge
Examples are a rectangular hood along the edge
of a tank or a hood on a welding or grinding
of a tank or a hood on a welding or grinding
bench table or a downdraft hood for hand
bench table or a downdraft hood for hand
grinding bench.
grinding bench.
What is meant by "capture velocity"?
What is meant by "capture velocity"?
The ventilation system removes
The ventilation system removes
contaminants by "pulling" the air (and the
contaminants by "pulling" the air (and the
contaminant) into the exhaust hood and
contaminant) into the exhaust hood and
away from the worker or the source.
away from the worker or the source.
Airflow toward the hood opening must be
Airflow toward the hood opening must be
fast or high enough to "catch and transport"
fast or high enough to "catch and transport"
the contaminant until it reaches the hood
the contaminant until it reaches the hood
and ducts. The required air speed is called
and ducts. The required air speed is called
the "capture velocity".
the "capture velocity".
Any air motion outside of the hood and
Any air motion outside of the hood and
surrounding area may affect how the air
surrounding area may affect how the air
flows into the hood.
flows into the hood.
The ventilation system will require a higher
The ventilation system will require a higher
airflow speed to overcome air disturbances.
airflow speed to overcome air disturbances.
As much as possible, the other sources of
As much as possible, the other sources of
air motion should be minimized or
air motion should be minimized or
eliminated effectively
eliminated effectively
Common sources of external air movement
Common sources of external air movement
include:
include:
thermal air currents, especially from hot
thermal air currents, especially from hot
processes or heat-generating operations,
processes or heat-generating operations,
motion of machinery such as by a grinding
motion of machinery such as by a grinding
wheel, belt conveyor, etc,
wheel, belt conveyor, etc,
material motion such as dumping or filling,
material motion such as dumping or filling,
movements of the operator,
movements of the operator,
room air currents (which are usually taken at 50
room air currents (which are usually taken at 50
fpm (feet per minute) minimum and may be
fpm (feet per minute) minimum and may be
much higher), and
much higher), and
rapid air movement caused by spot cooling and
rapid air movement caused by spot cooling and
heating equipment.
heating equipment.
Most of the capture velocities are around 100 feet
Most of the capture velocities are around 100 feet
per minute (fpm). How fast is 100 fpm? Blowing
per minute (fpm). How fast is 100 fpm? Blowing
lightly on your hand so that you can just barely
lightly on your hand so that you can just barely
feel air movement is about 100 fpm. It is easy to
feel air movement is about 100 fpm. It is easy to
see how it will take very little air movement from
see how it will take very little air movement from
other sources to affect how well a hood can
other sources to affect how well a hood can
capture contaminants.
capture contaminants.
What are general rules for hood design?
What are general rules for hood design?
The shape of the hood and its size, location, and rate
The shape of the hood and its size, location, and rate
of airflow each play an important role in design
of airflow each play an important role in design
considerations. Each type of hood also has specific
considerations. Each type of hood also has specific
design requirements, but several general principles
design requirements, but several general principles
apply to all hoods:
apply to all hoods:
The hood should be placed as close as possible to the
The hood should be placed as close as possible to the
source of contamination, preferably enclosing it. The
source of contamination, preferably enclosing it. The
more completely enclosed the source is, the less air
more completely enclosed the source is, the less air
will be required for control. The required volume
will be required for control. The required volume
varies with the square of the distance from the source.
varies with the square of the distance from the source.
The air should travel from source of the contaminant
The air should travel from source of the contaminant
and into the hood with enough velocity (speed) to
and into the hood with enough velocity (speed) to
adequately capture the contaminant.
adequately capture the contaminant.
The hood should be located in a way that the operator
The hood should be located in a way that the operator
is never between the contaminant source and the
is never between the contaminant source and the
hood.
hood.
The natural movement of contaminants should be
The natural movement of contaminants should be
taken into consideration. For example, a hood should
taken into consideration. For example, a hood should
be placed above hot processes to trap rising gases and
be placed above hot processes to trap rising gases and
heat. A grinding wheel or woodworking machine
heat. A grinding wheel or woodworking machine
should be equipped with a partial enclosure to trap
should be equipped with a partial enclosure to trap
the flying particles where they spin off.
the flying particles where they spin off.
The flanges or baffles should be used around the hood
The flanges or baffles should be used around the hood
opening to increase the capture effectiveness and
opening to increase the capture effectiveness and
reduce ventilation air requirements.
reduce ventilation air requirements.
Air-cleaning
Air-cleaning
Devices
Devices
What are air-cleaning devices?
What are air-cleaning devices?
In
In a ventilation system, an air-cleaning device
a ventilation system, an air-cleaning device
removes or captures the contaminants that are
removes or captures the contaminants that are
present in the air. The type of air cleaner used
present in the air. The type of air cleaner used
will depend on:
will depend on:
type of air contaminant to be removed,
type of air contaminant to be removed,
concentration of the contaminant in the air,
concentration of the contaminant in the air,
how much contaminant must be removed to meet
how much contaminant must be removed to meet
any regulations or standards,
any regulations or standards,
type and concentration of toxic chemical
type and concentration of toxic chemical
contaminants,
contaminants,
type and size of dust particles,
type and size of dust particles,
temperature, humidity, etc.,
temperature, humidity, etc.,
fire safety and explosion control, and air
fire safety and explosion control, and air
pollution control regulations.
pollution control regulations.
What are air-cleaning devices for gases and
What are air-cleaning devices for gases and
vapours?
vapours?
Gases and vapours can be removed by using the
Gases and vapours can be removed by using the
following processes:
following processes:
Adsorption
Adsorption
The removal of a contaminant by contact with other
The removal of a contaminant by contact with other
materials such as activated alumina, activated
materials such as activated alumina, activated
charcoal and silica gel (referred to as adsorbers).
charcoal and silica gel (referred to as adsorbers).
Absorption
Absorption
Absorbers remove soluble or chemically reactive
Absorbers remove soluble or chemically reactive
gases from the gas stream by close contact with an
gases from the gas stream by close contact with an
appropriate liquid so that one or more of the air
appropriate liquid so that one or more of the air
contaminants will dissolve in the liquid.
contaminants will dissolve in the liquid.
Catalytic conversion
Catalytic conversion
In this process, a catalyst converts a contaminant to a
In this process, a catalyst converts a contaminant to a
chemical form not considered to be hazardous. Catalysts
chemical form not considered to be hazardous. Catalysts
are substances that alter the rate of a chemical reaction
are substances that alter the rate of a chemical reaction
without being affected by the chemical reaction.
without being affected by the chemical reaction.
Thermal Oxidation (Combustion)
Thermal Oxidation (Combustion)
The combustion process (also called incineration)
The combustion process (also called incineration)
converts volatile organic compounds (VOCs) to carbon
converts volatile organic compounds (VOCs) to carbon
dioxide and water vapour by burning them. It is a very
dioxide and water vapour by burning them. It is a very
effective means of eliminating VOCs. Typical
effective means of eliminating VOCs. Typical
applications for incineration devices include odour
applications for incineration devices include odour
control, reduction in reactive hydrocarbon emissions,
control, reduction in reactive hydrocarbon emissions,
and reduction of explosion hazards.
and reduction of explosion hazards.
What should be considered when selecting an air-
What should be considered when selecting an air-
cleaning device?
cleaning device?
Following are some tips for selecting an air-cleaning
Following are some tips for selecting an air-cleaning
device in your workplace. Remember that a qualified
device in your workplace. Remember that a qualified
professional should make final decisions regarding
professional should make final decisions regarding
the suitability of an air-cleaning device.
the suitability of an air-cleaning device.
Before the air cleaning device is selected, it is very
Before the air cleaning device is selected, it is very
important to know maintenance and access
important to know maintenance and access
requirements, the physical size of the equipment and
requirements, the physical size of the equipment and
how it will be installed in the plant as well as the
how it will be installed in the plant as well as the
methods of removing the collected contaminants.
methods of removing the collected contaminants.
The air cleaner must be reliable. Many installations
The air cleaner must be reliable. Many installations
require monitoring or proof of continual operations
require monitoring or proof of continual operations
by measuring conditions in the system.
by measuring conditions in the system.
Maintenance and operating costs must be considered.
Maintenance and operating costs must be considered.
The air cleaner must operate in stable conditions as
The air cleaner must operate in stable conditions as
well as variations such as plant start-up and shut
well as variations such as plant start-up and shut
down. Considerations also include if it must be
down. Considerations also include if it must be
accessible for maintenance or if the air cleaner must
accessible for maintenance or if the air cleaner must
continue to operate while maintenance or repairs are
continue to operate while maintenance or repairs are
being done.
being done.
The device must meet local and national regulations
The device must meet local and national regulations
(at start-up and over time)
(at start-up and over time)
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