Atrium Smoke Control Systems

Atrium smoke control systems protect occupants in large-volume spaces by managing smoke movement and maintaining tenable conditions during fire events. These systems address unique challenges posed by multi-story open spaces where conventional compartmentation strategies cannot be applied.

Design Philosophy

Atrium smoke control relies on maintaining a smoke layer above the occupied zone while providing sufficient time for evacuation. The primary objective is to keep the smoke layer interface at least 6 feet above the highest walking surface or occupiable level within the atrium.

Design Approaches

Three fundamental strategies govern atrium smoke control design:

Fire Plume Characteristics

The smoke production rate from a fire determines exhaust requirements. The axisymmetric plume model provides the foundation for calculations.

Plume Mass Flow Rate

For fires not impinging on walls, the plume mass flow rate at height $z$ above the fire source:

$$\dot{m}_p = 0.071 Q_c^{1/3} (z - z_0)^{5/3}$$

Where:

• $\dot{m}_p$ = plume mass flow rate (kg/s)
• $Q_c$ = convective heat release rate (kW)
• $z$ = height above fire source (m)
• $z_0$ = virtual origin height (m)

Virtual Origin Correction

For fires with characteristic diameter $D$, the virtual origin:

$$z_0 = -1.02D + 0.083Q_c^{2/5}$$

This correction accounts for the physical size of the fuel source and prevents overestimation of plume entrainment near the fire base.

Exhaust Volumetric Flow Rate

The required volumetric exhaust rate depends on the plume mass flow and smoke layer temperature:

$$V = \frac{\dot{m}_p}{\rho_s}$$

Where $\rho_s$ is the smoke density at the layer temperature. For engineering calculations:

$$V = \dot{m}_p \left(\frac{T_s}{353}\right)$$

With $V$ in m³/s, $\dot{m}_p$ in kg/s, and $T_s$ in Kelvin.

System Configuration

graph TB
    subgraph "Atrium Space"
        A[Fire Source] -->|Heat & Smoke| B[Rising Plume]
        B --> C[Smoke Layer]
        C --> D[Smoke Layer Interface]
        D -.->|Maintained Above| E[Occupied Zone]
    end

    subgraph "Exhaust System"
        C --> F[Exhaust Inlets]
        F --> G[Smoke Exhaust Fans]
        G --> H[Discharge to Exterior]
    end

    subgraph "Makeup Air"
        I[Makeup Air Inlets] --> E
        I --> J[Low-Level Openings]
    end

    style C fill:#ff9999
    style E fill:#99ff99
    style G fill:#9999ff

Design Process Flow

flowchart TD
    A[Define Design Fire] --> B[Calculate Heat Release Rate]
    B --> C[Determine Clear Height]
    C --> D[Calculate Plume Mass Flow]
    D --> E{Natural Venting Feasible?}

    E -->|Yes| F[Size Natural Vents]
    E -->|No| G[Design Mechanical Exhaust]

    F --> H[Calculate Vent Area]
    H --> I[Verify Makeup Air Path]

    G --> J[Select Fan Capacity]
    J --> K[Design Ductwork]
    K --> I

    I --> L[Verify Layer Interface Height]
    L --> M{Acceptable?}

    M -->|Yes| N[Complete Design]
    M -->|No| O[Increase Exhaust Rate]
    O --> D

Code Requirements per NFPA 92

Design Fire Specifications

NFPA 92 provides guidance on design fire selection based on occupancy and fuel characteristics:

• Retail spaces: 5 MW steady-state fire
• Office atriums: 3 MW steady-state fire
• Assembly occupancies: Evaluate based on specific fuel loads

Design fires must consider both fire growth rate and steady-state heat release for time-dependent analysis.

Clear Height Requirements

Minimum clear height beneath the smoke layer:

• 6 feet above highest occupied level
• Additional height for thermal radiation consideration when layer temperature exceeds 200°F

Makeup Air Provisions

Makeup air must equal exhaust rate and enter below the smoke layer to prevent disruption of the stratified smoke layer. Maximum makeup air velocity: 200 fpm in occupied areas.

Mechanical vs. Natural Systems

Mechanical Exhaust Systems

Advantages:

• Precise control of exhaust rate
• Independent of environmental conditions
• Suitable for any atrium height
• Can integrate with HVAC systems

Design considerations:

• Fan temperature rating minimum 250°F
• Redundant fan capacity or backup power
• Inlet placement minimum 10 feet from plume centerline
• Ductwork slope to drain condensate

Natural Venting Systems

Required vent area for natural venting:

$$A_v = \frac{\dot{m}p}{\rho\infty C_v \sqrt{2g\Delta\rho h / \rho_\infty}}$$

Where:

• $A_v$ = vent area (m²)
• $C_v$ = vent coefficient (typically 0.6-0.7)
• $g$ = gravitational acceleration (9.81 m/s²)
• $\Delta\rho$ = density difference across vent
• $h$ = height of neutral plane above vent

Critical factors:

• Adequate atrium height (typically >60 feet)
• Reliable vent actuation mechanisms
• Wind effects on discharge
• Winter heating load impacts

System Integration

Atrium smoke control systems must coordinate with building HVAC:

1. Normal operation: Standard HVAC maintains comfort conditions
2. Smoke control mode: HVAC systems shut down or reconfigure to prevent smoke circulation
3. Pressurization: Adjacent spaces may require pressurization to prevent smoke migration

Control sequences must be programmed in the fire alarm system with hardwired interfaces to HVAC controls for reliability.

Performance Verification

Computational fluid dynamics (CFD) modeling validates design assumptions for complex geometries or unusual fire scenarios. Physical testing after installation confirms:

• Exhaust flow rates within 10% of design
• Smoke layer interface maintenance at design height
• Makeup air flow patterns do not disrupt stratification
• Control system activation and sequencing

Periodic testing per NFPA 92 requirements ensures ongoing system functionality and compliance with the original design intent.

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