Cleanroom HVAC Systems: Design and Classification

Fundamental Cleanroom Physics

Cleanrooms maintain controlled particulate contamination levels through engineered airflow patterns that continuously remove and dilute airborne particles. The fundamental principle relies on the particle mass balance equation:

$$\frac{dC}{dt} = G - Q \cdot C - \lambda \cdot C$$

where $C$ is particle concentration (particles/m³), $G$ is particle generation rate (particles/m³·s), $Q$ is air exchange rate (1/s), and $\lambda$ is particle deposition rate constant (1/s).

At steady state ($dC/dt = 0$), the equilibrium concentration becomes:

$$C_{eq} = \frac{G}{Q + \lambda}$$

This relationship reveals that particle concentration decreases with increased air exchange rate $Q$, forming the basis for cleanroom air change requirements.

ISO 14644 Cleanroom Classifications

ISO 14644-1 defines cleanroom classes based on maximum allowable particle concentrations at specified particle sizes. The classification number $N$ represents the maximum particle count (particles per cubic meter) at 0.1 μm diameter.

Classification Table

The particle count limit for any size follows the exponential relationship:

$$C_n = 10^N \times \left(\frac{0.1}{d_p}\right)^{2.08}$$

where $C_n$ is particle concentration limit (particles/m³), $N$ is ISO class number, and $d_p$ is particle diameter (μm). The exponent 2.08 derives from log-normal particle size distribution assumptions.

Airflow Patterns and Particle Removal

Cleanroom airflow patterns determine particle transport and removal efficiency. Two fundamental configurations exist: unidirectional (laminar) and non-unidirectional (turbulent mixing).

Unidirectional Flow (ISO 3-5)

graph TD
    A[HEPA Filter Ceiling] -->|Uniform Velocity| B[Work Zone]
    B -->|Vertical Flow 0.36-0.54 m/s| C[Perforated Floor]
    C --> D[Return Plenum]
    D --> E[AHU with HEPA]
    E --> A

    F[Particle Source] -->|Swept Away| C

    style A fill:#e1f5ff
    style B fill:#ffe1e1
    style C fill:#e1ffe1

Unidirectional flow maintains near-parallel streamlines with velocity $V_{flow} = 0.36-0.54$ m/s (70-106 fpm). This velocity exceeds particle settling velocities while avoiding turbulent vortices that trap particles.

The required face velocity to prevent particle recirculation follows from Reynolds number considerations:

$$V_{min} = \frac{\nu \cdot Re_{crit}}{L_c}$$

where $\nu$ is kinematic viscosity (1.5 × 10⁻⁵ m²/s for air), $Re_{crit} = 2,300$ for turbulent transition, and $L_c$ is characteristic length (typically 1-3 m). This yields $V_{min} \approx 0.01-0.03$ m/s, well below the standard 0.36 m/s specification, providing safety margin.

Non-Unidirectional Flow (ISO 6-8)

Non-unidirectional cleanrooms use turbulent mixing to dilute particle concentrations. Supply air from HEPA-filtered ceiling diffusers creates mixing patterns that reduce particle concentration through the exponential decay relationship:

$$C(t) = C_0 e^{-Qt/V}$$

where $C_0$ is initial concentration, $Q$ is supply airflow rate, $V$ is room volume, and $t$ is time. The time constant $\tau = V/Q$ represents the time to reduce concentration by 63.2% (1/e).

The required air changes per hour (ACH) to achieve classification follows from steady-state analysis:

$$ACH = \frac{G \cdot V}{C_{target} \cdot V} = \frac{G}{C_{target}}$$

Typical cleanroom ACH requirements:

HEPA and ULPA Filtration Requirements

High-Efficiency Particulate Air (HEPA) filters provide minimum 99.97% efficiency at 0.3 μm, corresponding to the most penetrating particle size (MPPS). Ultra-Low Penetration Air (ULPA) filters achieve 99.9995% efficiency at 0.12 μm.

Filtration Efficiency Physics

Filter collection efficiency $\eta$ depends on five mechanisms:

$$\eta_{total} = 1 - (1-\eta_{interception})(1-\eta_{impaction})(1-\eta_{diffusion})(1-\eta_{gravity})(1-\eta_{electrostatic})$$

At the MPPS (0.1-0.3 μm), inertial impaction and Brownian diffusion are weakest, creating the minimum efficiency point. The MPPS diameter $d_{MPPS}$ follows:

$$d_{MPPS} = \sqrt{\frac{18 \mu D}{\rho_p V_0 d_f}}$$

where $\mu$ is air viscosity, $D$ is particle diffusion coefficient, $\rho_p$ is particle density, $V_0$ is face velocity, and $d_f$ is fiber diameter.

Filter Sizing and Pressure Drop

HEPA filter face velocity affects service life and pressure drop. The pressure drop across clean HEPA filters ranges from 0.5-1.0 in. w.g. at rated flow. Pressure drop follows the Darcy-Weisbach relationship:

$$\Delta P = K \cdot \rho \cdot V^2$$

where $K$ is filter resistance coefficient (0.5-1.5 for HEPA media) and $V$ is face velocity. As filters load with particles, pressure drop increases:

$$\Delta P(t) = \Delta P_0 \left(1 + \frac{m_p(t)}{A \cdot \alpha}\right)$$

where $m_p(t)$ is accumulated particle mass, $A$ is filter area, and $\alpha$ is dust holding capacity (100-300 g/m² for HEPA filters). Filters require replacement when pressure drop reaches 2.0-2.5 in. w.g. or efficiency degrades below specification.

Pressure Cascade Design

Cleanrooms maintain positive pressure relative to adjacent less-clean spaces, creating a pressure gradient that prevents contaminant infiltration. The pressure cascade follows the hierarchy:

graph LR
    A[Critical Zone<br/>ISO 5<br/>+0.08 in. w.g.] -->|ΔP = 0.03| B[Buffer Zone<br/>ISO 6<br/>+0.05 in. w.g.]
    B -->|ΔP = 0.03| C[Gowning<br/>ISO 7<br/>+0.02 in. w.g.]
    C -->|ΔP = 0.02| D[Corridor<br/>Unclassified<br/>0 in. w.g.]

    style A fill:#ccffcc
    style B fill:#e1ffe1
    style C fill:#ffffcc
    style D fill:#ffcccc

The minimum pressure differential $\Delta P_{min}$ must overcome door leakage and maintain directional airflow. ISO 14644-4 recommends:

$$\Delta P_{min} = 0.02-0.05 \text{ in. w.g. (5-12 Pa)}$$

The airflow required to maintain pressure differential against leakage follows the orifice equation:

$$Q = C_d A_{leak} \sqrt{\frac{2 \Delta P}{\rho}}$$

where $C_d = 0.6-0.65$ for door gaps and $A_{leak}$ is total leakage area. For a standard cleanroom with 200 ft² wall area and 0.1 cfm/ft² leakage rate, maintaining 0.05 in. w.g. requires approximately 20 cfm of makeup air.

Particle Generation and Removal Rates

Human personnel generate 100,000-10,000,000 particles/minute ≥0.5 μm depending on activity level and gowning protocol. Process equipment contributes additional particle loads.

Personnel Particle Generation

The particle removal rate in a cleanroom follows first-order kinetics:

$$\frac{dN}{dt} = G - \lambda N$$

where $N$ is total particle count, $G$ is generation rate, and $\lambda$ is removal rate constant. At equilibrium:

$$N_{eq} = \frac{G}{\lambda} = \frac{G \cdot V}{Q}$$

For a 1,000 ft³ ISO 5 cleanroom with one operator generating 500,000 particles/min at ≥0.5 μm, the required airflow to maintain classification (3,520 particles/m³ at 0.5 μm) is:

$$Q = \frac{G \cdot V}{C_{target}} = \frac{500,000 \text{ part/min} \times 1}{3,520 \text{ part/m}^3 \times 35.3 \text{ ft}^3/\text{m}^3} \approx 4,020 \text{ cfm}$$

This corresponds to approximately 240 ACH, consistent with ISO 5 requirements.

Temperature and Humidity Control

Cleanrooms maintain tight temperature and humidity tolerances for process control and personnel comfort. Typical specifications:

• Temperature: 20 ± 2°C (68 ± 3.6°F)
• Relative humidity: 45 ± 5% RH
• Temperature gradient: <0.5°C per meter height

Humidity control prevents electrostatic discharge (ESD) and controls hygroscopic particle behavior. The relationship between particle size and relative humidity follows the Kelvin equation:

$$\ln\left(\frac{RH}{100}\right) = \frac{4 \gamma M}{\rho RT d_p}$$

where $\gamma$ is surface tension, $M$ is molecular weight, $R$ is gas constant, $T$ is temperature, and $d_p$ is particle diameter. Hygroscopic particles grow significantly above 60% RH, complicating particle control.

Sensible cooling load in cleanrooms includes:

$$Q_{sensible} = Q_{lights} + Q_{equipment} + Q_{people} + Q_{infiltration} + Q_{supply\ fan}$$

Latent load from personnel and infiltration requires dehumidification capacity:

$$Q_{latent} = m_{moisture} \times h_{fg}$$

where $h_{fg} = 1,060$ BTU/lb is water vaporization enthalpy. A typical person generates 200 BTU/hr latent heat at light activity.

Supply Air Distribution

Supply air distribution affects both particle removal efficiency and thermal uniformity. Ceiling-mounted HEPA filter modules (2×4 ft or 2×2 ft) provide 100% coverage for ISO 4-5 unidirectional flow cleanrooms.

Filter Coverage Ratio

The filter coverage ratio determines airflow uniformity:

$$Coverage = \frac{A_{filter}}{A_{ceiling}} \times 100%$$

Non-unidirectional cleanrooms use perforated diffusers or low-velocity displacement diffusers to minimize turbulent jet mixing that resuspends settled particles.

Energy Consumption and Optimization

Cleanrooms consume 10-100 times more energy per square foot than conventional buildings due to high airflow rates and 100% outside air requirements (in some applications). Annual energy consumption ranges from 50-200 kWh/ft²·year.

Energy Recovery Opportunities

Heat recovery from exhaust air reduces makeup air conditioning loads. The heat recovery effectiveness $\varepsilon$ follows:

$$\varepsilon = \frac{T_{supply} - T_{OA}}{T_{exhaust} - T_{OA}}$$

Rotary heat exchangers achieve $\varepsilon = 0.75-0.85$ but risk cross-contamination. Plate heat exchangers provide $\varepsilon = 0.50-0.70$ with complete separation of airstreams.

Fan power consumption scales with airflow and pressure drop:

$$P_{fan} = \frac{Q \times \Delta P_{total}}{6,356 \times \eta_{fan}}$$

where $P_{fan}$ is fan power (hp), $Q$ is airflow (cfm), $\Delta P_{total}$ is total system pressure (in. w.g.), and $\eta_{fan}$ is fan efficiency (0.60-0.75). High-efficiency ECM motors and variable frequency drives reduce energy consumption by 30-50% compared to constant-volume systems.

Contamination Control Strategy

Effective cleanroom operation requires integrated contamination control addressing sources, pathways, and receptors:

Source Control

• Personnel gowning protocols minimize particle shedding
• Equipment selection emphasizes low-particle-generating designs
• Material transfer procedures prevent contaminant introduction

Pathway Control

• Pressure cascades prevent airborne migration
• Airlocks isolate transition zones
• Sticky mats capture floor-level particles

Receptor Protection

• Unidirectional flow shields critical work zones
• Localized HEPA-filtered workstations create microenvironments
• Process isolation in barrier systems or isolators

The defense-in-depth approach combines multiple control layers, ensuring single-point failures do not compromise cleanliness.

Validation and Monitoring

ISO 14644-3 specifies cleanroom performance tests conducted during installation qualification (IQ), operational qualification (OQ), and performance qualification (PQ) phases.

Critical Performance Parameters

1. Airflow velocity and uniformity: Measure at 80% of filter face locations
2. HEPA filter leak testing: DOP or PAO aerosol challenge at 99.97% efficiency
3. Particle count testing: Sample at representative locations, multiple size channels
4. Pressure differential verification: Record cascade across all barriers
5. Recovery time: Measure time to return to classification after particle challenge
6. Air change rate: Calculate from volume and measured airflow

The recovery time $t_{recovery}$ to reduce concentration from ISO 8 to ISO 5 levels follows:

$$t_{recovery} = \frac{V}{Q} \ln\left(\frac{C_{initial}}{C_{target}}\right) = \frac{1}{ACH/60} \times \ln\left(\frac{C_8}{C_5}\right)$$

For ISO 8 to ISO 5 recovery (concentration reduction factor ≈1,000) at 360 ACH:

$$t_{recovery} = \frac{60}{360} \times \ln(1,000) \approx 1.15 \text{ minutes}$$

Continuous monitoring of differential pressure, particle counts, temperature, and humidity ensures sustained compliance between periodic recertification cycles (typically 6-12 months).

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