INDUSTRIAL VENTILATION Basic Principles of Airflow by: FRED PAUL DE

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INDUSTRIAL 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

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INDUSTRIAL 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

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INDUSTRIAL 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

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INDUSTRIAL 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.

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INDUSTRIAL 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

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INDUSTRIAL 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

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INDUSTRIAL 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.

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INDUSTRIAL 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.

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INDUSTRIAL 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

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Local exhaust consist of 5 parts
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LOCAL 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

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LOCAL EXHAUST SYSTEMS3 Basic Types of Hoods

1. Capture
2. Enclosing
3. Receiving (canopy)

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LOCAL EXHAUST SYSTEMS

• SEE PAGE 476
• FIGURE 21-2

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LOCAL 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)

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LOCAL 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

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LOCAL EXHAUST SYSTEMSPlain vs. Flanged

• SEE PAGE 447
• FIGURE 21-3

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LOCAL 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.

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LOCAL 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

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LOCAL 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.

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LOCAL 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.

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LOCAL 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.

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LOCAL EXHAUST SYSTEMS

• SEE PAGE 478
• FIGURE 21-6

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LOCAL EXHAUST SYSTEMS

• SEE PAGE 478
• FIGURE 27

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LOCAL EXHAUST SYSTEMS

• SEE PAGE 480
• FIGURE 21-8

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LOCAL EXHAUST SYSTEMS

• SEE PAGE 480
• FIGURE 21-9

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LOCAL EXHAUST SYSTEMS

• SEE PAGE 500
• FIGURE 21-26

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LOCAL 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

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LOCAL EXHAUST SYSTEMSDUCTS

• SEE PAGE 483
• FIGURE 21-12

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LOCAL EXHAUST SYSTEMSDUCTS

• 2 Solutions for balanced airflow
• Sizing the ductwork (preferred method)
• Use of blast gates

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LOCAL 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

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AIR 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!

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DUST 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.

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GASES VAPORS

• Gases and vapors are not particles
• Methods of collection
• Absorption liquid/chemical or solid particle
• Combustion
• Condensation

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FANS

• The 4th element in a local exhaust system
• Review of the others
• Hood
• Duct
• Air-cleaner
• Important considerations
• Noise and Fire

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FANS

• 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.

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FANS

• Centrifugal Fans
• Radial wheel (straight blades)
• Best for industrial ventilation
• Medium tip speed
• Medium noise
• Medium efficiency
• Backward-curved blades
• Forward blades

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FANS

• Axial-flow Fans
• Propeller fan
• Narrow-blade Propeller fan
• Tube-axial a modified propeller fan
• Vane-axial fan

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FANS

• SEE PAGE 490
• FIGURE 21-16

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EXHAUST SYSTEMPERFORMANCE

• No local exhaust system is foolproof
• Common faults
• Collectors not cleaned
• Ducts partially plugged
• Hoods battered or abraded
• Plant expansion or additions encroachment

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EXHAUST 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.

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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

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QUIZ

1. T F Local exhaust system are use to control an
   air contaminant near its source
2. T F No local exhaust can succeed unless the
   contaminant is drawn into the hood
3. T F There are 3 basic types of hoods
4. T F Ducts serve the purpose of carrying air
   captured by the hood to a cleaner or outside
5. T F There are 2 groups of fans, centrifugal and
   axial
6. Name one special operation requiring local
   exhaust ventilation. ____________________________
   ___