Title: INDUSTRIAL VENTILATION Basic Principles of Airflow by: FRED PAUL DE
1INDUSTRIAL VENTILATIONBasic Principles of
Airflowby FRED PAUL DEJEAN
- (Not necessarily HV / AC)
- Prime Purpose Protection of health
- Therefore a vital interest of every Safety
Health Professional
2INDUSTRIAL VENTILATIONBasic Principles of Airflow
- Note Before any exhaust system is installed,
every effort should be made to control the
contaminant by - isolation or
- by a change in the process,
- or by a substitution of less harmful materials.
- RECALL THE CLASS ON METHODS OF CONTROL
3INDUSTRIAL VENTILATIONBasic Principles of Airflow
- EXHAUST SYSTEMS
- Not part of the production equipment of the
operating unit - Regardless of building/plant size (large or
small) - a plant/project engineer will be the primary
designer the ventilation/exhaust system
4INDUSTRIAL VENTILATIONBasic Principles of Airflow
- The engineer may or may not take the time to
ensure the most efficient performance of the
exhaust system - In order to assist and advise the engineer, the
Safety Health Professional must be familiar
with the basics of industrial ventilation.
5INDUSTRIAL VENTILATIONLocal Exhaust Systems
- Purpose used to control an air contaminant by
trapping it near its source - As contrasted to dilution (general) ventilation
which lets the contaminant spread throughout the
workroom, later to be diluted by exhausting air
form the room
6INDUSTRIAL VENTILATIONLocal Exhaust Systems
- Local exhaust systems are preferred because
- Cleaner and healthier
- Smaller volume of air
- Smaller fan and dust arrester
- Less attendant heat loss
7INDUSTRIAL VENTILATIONLocal Exhaust Systems
- Use Local Exhaust if
- Air-samples show that the contaminant in the
atmosphere constitutes a health, fire, or
explosion hazard - State or city codes require local exhaust
ventilation at that particular process. - Maintenance of production machinery would
otherwise be difficult.
8INDUSTRIAL VENTILATIONLocal Exhaust Systems
- Use Local Exhaust if
- Marked improvement in housekeeping or worker
comfort will result. - Emission sources are large, few, fixed and/or
widely dispersed. - Emission sources are near the worker breathing
zone. - Emission rates vary widely by time.
9INDUSTRIAL VENTILATIONLocal Exhaust Systems
- Local exhaust consist of 5 parts
- HOODS - into which airborne contaminant is drawn
- DUCTS - for carrying contaminated air to a
central point - AIR-CLEANING DEVICE such as a dust arrester for
purifying the air before it is discharged - FAN to create the required air flow through the
system - STACK to disperse remaining air contaminants
10Local exhaust consist of 5 parts
11LOCAL EXHAUST SYSTEMSPrinciples of good design
- HOODS no local exhaust system can succeed
unless the contaminant is drawn into the hood. - Considered a critical component
- The local hood is the point of air entry into the
duct system. - Includes all suction openings
12LOCAL EXHAUST SYSTEMS3 Basic Types of Hoods
- Capture
- Enclosing
- Receiving (canopy)
13LOCAL EXHAUST SYSTEMS
14LOCAL EXHAUST SYSTEMS
- CAPTURE HOODS
- A snorkel like duct is connected to the inlet of
an exhaust fan (suction) at the opposite end. - Air form the work surface and the workroom moves
from all directions towards the duct orifice
(hood) - Air velocity is critical as it must carry the
contaminant the distance to the duct (hood)
15LOCAL EXHAUST SYSTEMS
- CAPTURE HOODS-Flanged
- A flange placed around the duct opening to
prevent the hood form drawing air from behind. - The same total amount of air is exhausted but the
greater amount comes from the front of the duct. - The rationale is that the contaminant problem is
in front, not behind the hood. - Efficiency can increase as much as 30-40
16LOCAL EXHAUST SYSTEMSPlain vs. Flanged
17LOCAL EXHAUST SYSTEMS
- ENCLOSING HOODS
- Enclosures Baffles help control cross drafts.
- The emission point (point of operation) is where
dust and fumes are controlled. - Beware of cross breezes.
- Unwanted cross drafts can easily blow the
contaminant away before it ever comes under the
influence of the hood.
18LOCAL EXHAUST SYSTEMS
- RECEIVING HOODS
- Best used where rising hot air carries the
contaminant into the hood. - Disadvantages
- Negative effect of cross drafts
- Greater volume of airflow required
- Air velocity drop-off
- Worker forced beneath the hood
19LOCAL EXHAUST SYSTEMS
- RECEIVING HOODS-Lateral Exhaust
- Best used with open surface tank operations
- Usually include a Push-Pull system
- Requires extreme caution as air jets in a
push-pull system can spread contaminant.
20LOCAL EXHAUST SYSTEMSPrinciples of good design
- DESIGN CRITERIA
- Enclose the operation as mush as possible.
- Locate the hood so the contaminant is moved away
from the breathing zone. - Locate and shape the hood so the initial velocity
of the contaminant will throw it into the hood
opening.
21LOCAL EXHAUST SYSTEMSPrinciples of good design
- DESIGN CRITERIA
- Do not attempt to collect solvent vapors at the
floor level. - Locate the hood as close as possible to the
source of contaminant. - The hood should not interfere with the worker.
22LOCAL EXHAUST SYSTEMS
23LOCAL EXHAUST SYSTEMS
24LOCAL EXHAUST SYSTEMS
25LOCAL EXHAUST SYSTEMS
26LOCAL EXHAUST SYSTEMS
- SEE PAGE 500
- FIGURE 21-26
27LOCAL EXHAUST SYSTEMSDUCTS
- Purpose is to carry contaminated air to a cleaner
or outdoors. - The problem with FRICTION
- The problem with Multiple Hoods.
- The design objective for ducts
- Hold power consumption to minimum
- Maintain proper velocity
- Keep the system balanced
28LOCAL EXHAUST SYSTEMSDUCTS
- SEE PAGE 483
- FIGURE 21-12
29LOCAL EXHAUST SYSTEMSDUCTS
- 2 Solutions for balanced airflow
- Sizing the ductwork (preferred method)
- Use of blast gates
30LOCAL EXHAUST SYSTEMSDUCTS
- 3 Terms relative to duct velocity
- Static pressure
- created by the duct fan
- Is the energy source of the system
- Converts to velocity pressure
- Velocity pressure
- created by moving air
- By converting static pressure to air movement
- Duct friction loss
- creates extremely high power demands
- Smaller the diameter higher the friction loss
31AIR CLEANERS
- 2 Broad Classes
- Industrial air cleaners
- used to remove airborne contaminants (dust,
fume,mist, vapor, gas, or odor) - May require a contaminant permit
- Heating, Ventilating, Air Conditioning
- used domestically (not a concern of this course)
- AVOID POLLUTION, DISPOSAL MAY BE REGUALTED!
32DUST ARRESTERS - COLLECTORS
- Centrifugal Collectors
- Not good for collecting fine particles
- Wet Collectors
- Water may be expensive
- Freezing
- Corrosion
- Settling Chambers
- Good for pre-cleaners for HEPA filters
- Filters
- 1. Disposable but not suitable for industrial
exhaust - 2. Cloth allows a dust buildup that acts as a
filter.
33GASES VAPORS
- Gases and vapors are not particles
- Methods of collection
- Absorption liquid/chemical or solid particle
- Combustion
- Condensation
34FANS
- The 4th element in a local exhaust system
- Review of the others
- Hood
- Duct
- Air-cleaner
- Important considerations
- Noise and Fire
35FANS
- Two groups of fans are in use
- Centrifugal
- Air leaves at right angles to the direction in
which it enters the blades - Axial
- Air leaves in the same straight line in which it
enters.
36FANS
- Centrifugal Fans
- Radial wheel (straight blades)
- Best for industrial ventilation
- Medium tip speed
- Medium noise
- Medium efficiency
- Backward-curved blades
- Forward blades
37FANS
- Axial-flow Fans
- Propeller fan
- Narrow-blade Propeller fan
- Tube-axial a modified propeller fan
- Vane-axial fan
38FANS
- SEE PAGE 490
- FIGURE 21-16
39EXHAUST SYSTEMPERFORMANCE
- No local exhaust system is foolproof
- Common faults
- Collectors not cleaned
- Ducts partially plugged
- Hoods battered or abraded
- Plant expansion or additions encroachment
40EXHAUST SYSTEMPERFORMANCE
- Baseline test are important for comparisons
- Test static pressure or suction each time the
system is cleaned - Compare readings to determine if anything has
gone wrong. - Perform smoke test to trace the flow of air into
hoods - Refer to NSC Data Sheet 428 for more info.
41 INDUSTRIAL VENTILATION SPECIAL APPLICATIONS
- Open-Surface Tanks Spray Booths
- Welding Fume Control Foundry Dust Control
- Grinding Operations Woodworking
- Oil Mist Melting Furnaces
- Material Conveying
- Fog Removal Kitchen Range Hoods
- Internal Combustion Engine Ventilation
42QUIZ
- T F Local exhaust system are use to control an
air contaminant near its source - T F No local exhaust can succeed unless the
contaminant is drawn into the hood - T F There are 3 basic types of hoods
- T F Ducts serve the purpose of carrying air
captured by the hood to a cleaner or outside - T F There are 2 groups of fans, centrifugal and
axial - Name one special operation requiring local
exhaust ventilation. ____________________________
___