Filtration

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Filtration Duct Design

• October 25, 2009
• Mark Nunnelly

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FILTRATION
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Purposes of Filtration

• Keep AHUs system components clean (blower, motor
  and evaporator coils)
• Clean the air we breathe (dust, pollen, mold
  spores, etc.)
• Clean the air entering building environment for
  improved building maintenance and longevity

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Filtration Selection Considerations

• Efficiency requirements of project
• The types and dimensions of the particulates to
  be filtered
• Resistance to airflow
• Filter cost
• Physical size of the filter cabinet
• Location to permit service and replacement

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Filtration Rating Criteria(ASHRAE Standard
55.2-1999)

• Arrestance (weight fraction of dust removed)
• Dust spot efficiency (ability of filter to reduce
  soiling of fabrics and interior surfaces
• Fractional efficiency (percent of uniformly sized
  particles removed)
• Particle size efficiency (percent of dust removed
  with typical particle size distribution)
• Dust holding capacity

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Dirty Filters!
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Dirty Filters!
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Dirty filters increase friction loss and
therefore increase operating costs!
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Friction loss can be minimized by these design
practices

• Reduce face velocity below common equipment
  design velocity (i.e., 500 FPM) Downside
  Increases equipment width as coil module size
  increases (Formula 9.2, Page 129)

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Friction loss can be minimized by these design
practices

• Choose pleated or bag filters as in figure 9.5 of
  textbook Downside Higher cost filters,
  increase depth of filters
• Good practice is to include a differential
  pressure gauge across the filter and/or install
  an alarm of signal to alert the facility operator
  of a need for a filter change.

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Duct Design
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Challenging Activity of Designing Ducts

• Find a balance between the conflicting
  constraints of lowering the system installation
  cost by reducing the duct size (supply and
  return) with increasing air velocity, fan power
  requirements, and noise levels.
• The routing and sizing of supply and return
  ducting (i.e., the air distribution system) is
  one of the most challenging activities within the
  design of the HVAC system.

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Duct Design Methods(According to ASHRAE
Fundamentals Handbook)

• Equal Friction
• Static Regain
• T-Method Optimization

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Equal Friction Method

• Simplest
• Most commonly used
• Therefore, this is the method well use in this
  class

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Equal Friction Method

• A maximum target friction loss per unit length is
  selected and all ducts are sized to provide this
  loss (or slightly less)
• The conventional units are in inches of water
  per 100 of straight duct.
• Friction losses in fittings are expressed in
  equivalent length of straight duct.

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Duct Sizing example from Textbook

• Size a duct for a flow of 1000 cfm at a friction
  loss of 0.1w.c./100 . Determine the actual
  loss and velocity for the actual duct size.

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Duct Sizing example from Textbook

• Size a duct for a flow of 1000 cfm at a friction
  loss of 0.1w.c./100 . Determine the actual
  loss and velocity for the actual duct size.
• Solution The horizontal 0.1w.c./100 friction
  line intersects the vertical 1000 cfm line at a
  round duct diameter of 13.5. The next available
  round duct size is 14. When this duct is used,
  the loss will be 0.09w.c./100, and the velocity
  is 950 fpm at this intersection.
• Commonly used round duct sizes at
  5,6,8,10,12,14,16 and 20

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Fitting Losses K-factors

• More exact than using equivalent length method,
  but more cumbersome
• ?hK x (V2/2)
• K Geometry - and size-dependent loss
  coefficient
• V Average velocity, fps
• Text uses the simpler equivalent length method

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