Tunnel Ventilation

Tunnel ventilation systems maintain air quality during normal operation and provide critical life safety functions during fire emergencies. Design requirements vary significantly based on tunnel type, length, traffic characteristics, and fire safety objectives.

Ventilation Objectives

Normal Operation

During routine use, ventilation provides:

• Contaminant Dilution: Reduce vehicle emissions to acceptable levels
• Visibility: Maintain adequate sight distance
• Thermal Control: Remove heat from vehicles and systems
• Comfort: Acceptable conditions for occupants and workers

Emergency Operation

During fire or incident:

• Smoke Control: Clear evacuation paths
• Tenable Conditions: Support safe egress
• Firefighting Access: Enable emergency response
• Structural Protection: Limit fire-induced temperatures

Ventilation System Types

Longitudinal Ventilation

Air moves along tunnel length in one direction:

Components:

• Jet fans mounted in tunnel
• Portal fans (optional)
• Control systems

Advantages:

• Lower construction cost
• No transverse ducts
• Flexible operation
• Common for road tunnels <2 km

Limitations:

• Requires unidirectional traffic for fire mode
• Limited effectiveness for very long tunnels
• Back-layering concern during fires

Transverse Ventilation

Supply and exhaust through separate duct systems:

Full Transverse:

• Supply duct along tunnel
• Exhaust duct along tunnel
• Uniform distribution
• High construction cost

Semi-Transverse:

• Either supply or exhaust ducted
• Other through portals
• Reduced cost vs. full transverse

Advantages:

• Better contaminant control
• Bidirectional traffic compatible
• Enhanced fire control

Hybrid Systems

Combine longitudinal and transverse elements:

• Point extraction for fire emergencies
• Longitudinal normal operation
• Flexibility for varying conditions

Design Parameters

Air Quality Standards

Carbon Monoxide (CO):

Nitrogen Dioxide (NO₂):

• Short-term: 1.0 ppm
• Long-term: 0.25 ppm

Visibility:

• Extinction coefficient: K < 0.005-0.012 m⁻¹
• Sight distance: >100 m typical

Emission Factors

Vehicle emission rates depend on:

• Fleet composition (cars, trucks, buses)
• Traffic speed
• Grade (upgrade produces more emissions)
• Vehicle age and emission standards

Typical Values (modern fleet):

Airflow Requirements

Dilution Ventilation: $$Q = \frac{N \times E \times L}{C_{max} - C_{ambient}}$$

Where:

• N = number of vehicles
• E = emission rate
• L = tunnel length
• C = concentration limit

Typical Ranges:

• Road tunnels: 100-400 CFM per vehicle
• Transit tunnels: Based on train schedules

Jet Fan Systems

Operating Principle

Jet fans transfer momentum to tunnel air:

$$F_{thrust} = \dot{m} \times (V_{jet} - V_{tunnel})$$

Typical Specifications:

• Diameter: 800-1,400 mm
• Thrust: 500-2,000 N
• Jet velocity: 25-35 m/s
• Reversible operation

Sizing Considerations

Account for:

• Pressure losses (friction, obstacles)
• Traffic drag (piston effect)
• Meteorological effects (wind, buoyancy)
• Grade effects
• Fire scenario requirements

Installation

Mounting:

• Ceiling mounted in pairs
• Staggered arrangement
• Minimum spacing for thrust development
• Structural support for thrust loads

Fire Protection:

• High-temperature rated for fire zones
• Redundant units
• Rated for 250-400°C for 2 hours

Fire Emergency Ventilation

Design Fire Size

Smoke Control Strategies

Longitudinal Control:

• Push smoke downstream of fire
• Critical velocity: 2.5-3.5 m/s
• Requires one-way traffic

$$V_{critical} = K \times \left(\frac{g \times H \times Q}{c_p \times \rho \times T_{ambient} \times A}\right)^{1/3}$$

Point Extraction:

• Extract smoke at fire location
• Damper-controlled extraction points
• Maintains tenable conditions both directions

Emergency Operating Modes

Controls and Monitoring

Detection Systems

Air Quality Monitoring:

• CO sensors at regular intervals
• Visibility (opacity) meters
• NOx monitoring (some applications)

Fire Detection:

• Linear heat detection
• Video analytics
• Flame detectors
• Smoke detection (challenging)

SCADA Integration

Supervisory control includes:

• Automatic mode selection
• Manual override capability
• Sensor data trending
• Alarm management
• Emergency coordination

Operational Modes

• Normal: Maintains air quality
• Congested: Enhanced ventilation
• Emergency: Smoke control protocol
• Maintenance: Safe access conditions

Standards and Guidelines

Design Standards

• NFPA 502: Road Tunnels, Bridges, and Other Limited Access Highways
• ASHRAE Applications Handbook: Chapter on Enclosed Vehicular Facilities
• PIARC: World Road Association guidelines
• European Directive 2004/54/EC: Trans-European road tunnels

Testing Requirements

• Hot smoke tests
• Jet fan thrust verification
• Control system commissioning
• Emergency drill exercises

Tunnel ventilation design requires careful balance between normal operational requirements, emergency capabilities, and lifecycle cost considerations unique to these critical infrastructure systems.

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