Particulate Filtration

Particulate filtration removes airborne solid and liquid particles from HVAC airstreams, protecting both occupants and equipment. Understanding particle behavior and capture mechanisms enables effective filter selection for diverse applications.

Airborne Particle Characteristics

Particle Size Distribution

Indoor and outdoor air contains particles spanning several orders of magnitude in size:

Particle Sources

Outdoor Origin:

• Traffic emissions (ultrafine particles)
• Industrial processes
• Construction activities
• Natural sources (pollen, dust)

Indoor Generation:

• Human activity (skin cells, fibers)
• Cooking and combustion
• Printers and copiers
• HVAC system components

Health Impact by Size

Particle deposition in the respiratory system depends on size:

$$Deposition\ Fraction = f(d_p, breathing\ rate, physiology)$$

• PM10: Particles ≤10 μm penetrate lower respiratory tract
• PM2.5: Particles ≤2.5 μm reach alveoli
• Ultrafine (<0.1 μm): May enter bloodstream

Filtration Mechanisms

Interception

Particles following airflow streamlines contact filter fibers when passing within one particle radius:

$$\eta_R \propto \left(\frac{d_p}{d_f}\right)^2$$

Interception dominates for particles in the 0.1-1.0 μm range traveling at moderate velocities.

Inertial Impaction

Larger particles with sufficient inertia deviate from streamlines and impact fibers:

$$\eta_I \propto Stk^n$$ where $n \approx 2$

The Stokes number characterizes impaction behavior:

$$Stk = \frac{\rho_p d_p^2 U C_c}{18 \mu d_f}$$

Impaction is the primary mechanism for particles >1 μm.

Diffusion (Brownian Motion)

Submicron particles undergo random motion due to molecular collisions:

$$D = \frac{k_B T C_c}{3 \pi \mu d_p}$$

Diffusion collection efficiency increases with decreasing particle size:

$$\eta_D \propto \left(\frac{D}{U d_f}\right)^{2/3}$$

Electrostatic Effects

Charged particles experience attraction to filter fibers:

Coulombic Force (charged particle, charged fiber): $$F_c = \frac{q_p q_f}{4 \pi \epsilon_0 r^2}$$

Image Force (charged particle, neutral fiber): $$F_i = \frac{q_p^2}{16 \pi \epsilon_0 r^2} \cdot \frac{\epsilon_f - 1}{\epsilon_f + 1}$$

Electret filters exploit permanent electrostatic charges for enhanced efficiency at low pressure drop.

Gravitational Settling

Large particles may settle onto horizontal filter surfaces:

$$v_s = \frac{\rho_p d_p^2 g C_c}{18 \mu}$$

Significant only for particles >10 μm at low face velocities.

Most Penetrating Particle Size (MPPS)

The combination of mechanisms creates a minimum efficiency point:

graph TB
    A[Total Efficiency] --> B[Diffusion]
    A --> C[Interception]
    A --> D[Impaction]

    B -->|Increases with| E[Decreasing particle size]
    C -->|Increases with| F[Increasing particle size]
    D -->|Increases with| G[Increasing particle size]

The MPPS typically occurs between 0.1-0.3 μm, where neither diffusion nor impaction dominates. This worst-case size forms the basis for HEPA testing standards.

MPPS Shift with Velocity

Face velocity affects MPPS location:

• Higher velocity → MPPS shifts smaller (reduced diffusion time)
• Lower velocity → MPPS shifts larger (reduced impaction)

$$MPPS \propto \left(\frac{d_f \mu}{U \rho_p}\right)^{0.5}$$

Single Fiber Efficiency

Filter efficiency builds from individual fiber collection:

$$\eta_{fiber} = \eta_R + \eta_I + \eta_D - \eta_R \cdot \eta_I - \eta_R \cdot \eta_D - \eta_I \cdot \eta_D + \eta_R \cdot \eta_I \cdot \eta_D$$

For typical conditions, mechanisms act independently:

$$\eta_{fiber} \approx \eta_R + \eta_I + \eta_D$$

Filter Efficiency from Single Fiber

Overall filter efficiency relates to single fiber efficiency:

$$\eta_{filter} = 1 - exp\left(\frac{-4 \alpha \eta_{fiber} t}{\pi d_f (1-\alpha)}\right)$$

Where:

• $\alpha$ = filter solidity
• $t$ = filter thickness
• $d_f$ = fiber diameter

Particle Loading Effects

Filter Loading Stages

1. Clean Filter: Particles deposit on fiber surfaces
2. Transition: Dendrite structures form on fibers
3. Dust Cake: Surface accumulation dominates capture

Efficiency Changes with Loading

Most filters show improved efficiency as particles accumulate:

$$\eta(t) = 1 - (1-\eta_0) \cdot exp(-k \cdot m_d)$$

However, electret filters may lose efficiency as charge neutralization occurs.

Pressure Drop Progression

$$\Delta P(t) = \Delta P_0 + K_2 \cdot m_d$$ (surface loading)

$$\Delta P(t) = \Delta P_0 \cdot exp(\beta \cdot m_d)$$ (depth loading)

Application Guidelines

Residential and Light Commercial

Target Particles: Dust, pollen, mold spores Recommended Efficiency: MERV 8-11 Considerations: Balance IAQ improvement with system compatibility

Commercial Buildings

Target Particles: PM2.5, biological aerosols Recommended Efficiency: MERV 11-14 Considerations: Energy impact, maintenance access, code requirements

Healthcare Facilities

Target Particles: Bacteria, fungal spores, droplet nuclei Recommended Efficiency: MERV 14-16 (HEPA for critical areas) Considerations: ASHRAE 170 requirements, pressure relationships

Cleanrooms

Target Particles: Submicron particles affecting processes Recommended Efficiency: HEPA (H13-H14) or ULPA Considerations: ISO 14644 classification, leak testing requirements

Testing and Verification

Fractional Efficiency Testing

ASHRAE 52.2 measures efficiency across particle size ranges:

1. Generate test aerosol (KCl)
2. Size-segregate with optical counter
3. Measure upstream/downstream concentrations
4. Calculate efficiency for each size bin

Aerosol Photometry (HEPA)

DOP or PAO challenge verifies HEPA performance:

$$Penetration = \frac{C_{downstream}}{C_{upstream}} \times 100%$$

In-Situ Verification

Installed filter testing ensures system performance:

• Scan leak testing for HEPA installations
• Particle count verification downstream
• Pressure drop monitoring

Effective particulate filtration requires matching filter capabilities to specific particle removal requirements while considering system constraints and lifecycle economics.