HVAC PRIME SOURCE, LLC
FILTRATION

Filter Efficiency -

Following are some of the common test methods for determining filtration efficiency –

  • Weight Arrestance %
    This method uses a specified test dust of a known quantity that is introduced at a
    controlled rate into the filter, and determines the dust weight that goes through the
    filter. This method is not used for high efficiency filters

  • Atmospheric Dust Spot %
    This method uses untreated atmospheric dust as the contamination that is introduced
    into the filter. The airflow upstream and downstream of the filter is directed separately
    towards identical targets of glass fiber filter paper, which results in a discoloration of
    the paper depending on the dust quantity in the air. The light transmission through the
    resulting dust spot on the two paper targets is used to determine the filter efficiency.
        
  • DOP Penetration
    This method is typically used for hi efficiency filters such as HEPA filters. A uniform
    smoke cloud of DOP (Di-Octyl Phthalate) with 0.3 micron particle diameter is introduced
    into the filter. The DOP concentration upstream and downstream of the filter is
    measured by light scattering photometer to determine the filter efficiency.

Filter Types
(Ref. Ashrae Equipment Handbook)

There are following three basic type of particulate filters –

  • Media
  • Centrifugal
  • Electrostatic

The most common filters used in the HVAC systems are the Media type filters. The Media
filters are divided into three basic categories by the level of filtration efficiency –

  • Group I, Low Efficiency filters – Viscous Impingement, panel type
    Dust spot efficiency = <20%, Weight Arrestance efficiency = 40 to 80%

  • Group II, Moderate Efficiency filters – Extended Medium, dry type
    Dust spot efficiency = 20 to 60%, Weight Arrestance efficiency = 80 to 96%

  • Group III, High Efficiency filters – Extended Medium, dry type
    Dust spot efficiency = 60 to 95%, Weight Arrestance efficiency = 96 to 99%

  • HEPA filters - Extended Medium, dry type
    Dust spot efficiency = 100%, Weight Arrestance efficiency = 100%
    DOP Penetration = 99.97%

There are also filters for special applications, such as charcoal filters for odor & fumes removal,
and chemical filters for toxic gas removal. These are not discussed in depth here since they are
not that commonly used in HVAC systems.

Typical Filtration Requirements -
(Ref. Nuclear Air Cleaning Handbook)

The most common applications of filtrations in HVAC systems are for airborne particulates. The
average airborne particulates composition by weight in the atmosphere is as follows –

  • Pollen – 10%
  • Dust – 40%
  • Skin flakes – 5%
  • Lint & Plant fibers – 10%
  • Smoke & Fumes – 15%

Typical airborne particulates can range in size from 0.001 microns to 100 microns. Following is
the range of size for the various airborne particulates –

  • Dust – 1 to 100 microns
  • Fumes – 0.001 to 1 micron
  • Smog – 0.01 to 2 microns
  • Smoke – 0.01 to 1 micron
  • Bacteria – 0.3 to 30 micron
  • Viruses – 0.003 to 0.05 micron

Typical range of application of most common filters –

  • Flat Panel Filters: 5 to 100 microns
  • Extended Media Filters: 1 to 100 microns
  • Bag Filters: 0.1 to 60 microns
  • HEPA Filters: 0.01 to 5 microns
  • Electrostatic Filters: 0.01 to 10 microns

Commonly Used Filters and Design Information -

Flat Panel or Viscous Impingement Type Filters –
    Efficiency: 15 – 20% Atmospheric dust spot
    Face Velocity: 500 FPM
    Initial Pressure Drop: 0.2” wg
    Final Pressure Drop: 0.5” wg
        
Extended Media Filters –
    Efficiency :
    20 – 35% Atmospheric dust spot
    60 – 65%
    80 – 85%
    90 – 95%
    Face Velocity: 500 FPM
    Initial Pressure Drop:
    20 – 35%         0.3” wg
    60 – 65%         0.5” wg     
    80 – 85%         0.6” wg
    90 – 95%         0.7” wg
    Final Pressure Drop:
    20 – 35%         1.0” wg
    60 – 65%         1.0” wg     
    80 – 85%         1.2” wg
    90 – 95%         1.2” wg
        
Bag Filters –
    Efficiency :
    40 – 45% Atmospheric dust spot
    50 – 55%
    60 – 65%
    80 – 85%
    90 – 95%
    Face Velocity: 500 FPM
    Initial Pressure Drop:
    20 – 35%         0.25” wg
    50 – 55%         0.35” wg
    60 – 65%         0.4” wg     
    80 – 85%         0.5” wg
    90 – 95%         0.6” wg
    Final Pressure Drop:
    20 – 35%         1.0” wg
    50 – 55%         1.0” wg
    60 – 65%         1.0” wg     
    80 – 85%         1.0” wg
    90 – 95%         1.0” wg
        
Roll-O-Matic Filters –
    Efficiency: 20 – 25% Atmospheric dust spot
    Face Velocity: 500 FPM
    Initial Pressure Drop: 0.3” wg
    Final Pressure Drop: 0.5” wg
        
HEPA Filters –
    Efficiency: 99.97% for > 0.3 micron particles, DOP Test
    Face Velocity: 250 FPM
    Initial Pressure Drop: 1.00” wg
    Final Pressure Drop: 3.00” wg
        
Electrostatic Filters –
    Efficiency:
    85% for 600 FPM (Atmospheric dust spot)
    95% for 400 FPM
    Face Velocity: 400 - 600 FPM
    Average Pressure Drop: 0.16 – 0.3” wg

Basic Classification of Clean Rooms -
(Ref. Federal Standard 209a)

There are three classifications for clean rooms. Classifications are based on particle count with
a maximum allowable number of particles per unit volume of size 0.5 micron and larger, or 5
microns and larger. Particle counts are to be taken during work activity period and at work
locations.

Class 100
Particle count not to exceed a total of 100 particles per cubic foot of a size 0.5 microns and
larger.

Class 10,000
Particle count not to exceed a total of 10,000 particles per cubic foot of a size 0.5 microns and
larger, or 65 particles per cubic foot of a size 5 microns and larger.

Class 100,000
Particle count not to exceed a total of 100,000 particles per cubic foot of a size 0.5 microns
and larger, or 100 particles per cubic foot of a size 5 microns and larger.