VENTILATION

DESCRIPTION

Ventilation refers to the movement of air within the tunnels and underground stations of the Metro system. It is sustained under two main conditions: normal and emergency.

  • Normal Ventilation: This occurs through the movement of trains in the tunnels, accompanied by mechanical fans to aid in the exchange of fresh air during operations. The train movement induces airflow, referred to as the piston effect, whereby air is forced ahead of the train and pulled in behind it. As a train nears a station, vent shafts enable airflow to dissipate upwards, preventing a rush of air from negatively affecting passengers awaiting the train.

  • Emergency Ventilation: This is primarily mechanically driven. Fans can function in either supply or exhaust modes as required. The main goal of the emergency ventilation system is to protect passengers and Fire/Rescue personnel from heat and smoke exposure during emergencies.

NORMAL VENTILATION IN THE UNDERGROUND SYSTEM

TUNNELS

The primary mechanism for air exchange in tunnels stems from the movement of trains. As a train proceeds forward:

  1. It forces air ahead (positive displacement).
  2. It draws air in behind it (negative displacement).
    This principle is known as the piston effect.

  • Vent Shafts: These shafts become crucial as a train approaches a station, enabling the air to exit the system. They also facilitate airflow between stations. Each time a train approaches a vent, it expels air upward through the shaft, subsequently drawing fresh air into the tunnel behind as it passes.

  • Louvers and Fans: The vent shafts are fitted with thermostatically controlled louvers and fans that regulate airflow. In normal mode, fans operate automatically based on thermostat settings, but during emergencies, the Operations Control Center (OCC) can manually activate them via computer controls.

MODES OF TUNNEL VENTILATION

Tunnel ventilation operates under four distinct modes based on temperature:

  1. Low Temperature Mode (below 50°F): No dampers or fans are operational; ventilation is solely due to train movement.
  2. Normal Temperature Mode (50°F to 95°F): Dampers remain open while fans do not operate; again, ventilation is solely due to train movement.
  3. High Temperature Mode (above 95°F): Dampers are open, and fans operate in exhaust mode to expel hot air.
  4. Emergency Mode: Dampers are open, and the fans function under OCC control, operating in the required direction. Once Fire/Rescue arrives, the Incident Commander assumes fan control.

STATIONS

In underground stations, the under-platform ventilation systems contribute significantly to air exchange (as referenced in Diagram 10-1). This system consists of fans designed to:

  • Exhaust and draw air from beneath trains and away from the platforms to keep hot air from affecting passengers.

  • Supply fresh air underneath trains or into the station when required.

  • Temperature Management: The station ventilation system operates by supplying cool air when temperatures exceed 70°F. The air is exchanged through the top vent domes equipped with fans and dampers. These dampers open automatically when the temperature rises, and close when temperatures drop below this threshold.

UNDER-PLATFORM VENTILATION SYSTEM DETAILS

There are specific stations where under-platform ventilation fans operate solely in the exhaust mode:

  • Dupont Circle
  • Farragut North
  • Metro Center
  • Judiciary Square
  • Gallery Place (upper level, Red Line, while the lower level, Green/Yellow Line, has reversible fans).

For other underground stations, the under-platform ventilation fans can function in either supply or exhaust modes as required. Each station has two control boxes managing half of the fans each, positioned at either end of the station. The control settings include:

  1. Emergency Exhaust: Manually operated to exhaust air.
  2. Emergency Supply: Manually operated to supply air.
  3. Automatic Operation: Controlled by thermostat or OCC; can operate in either mode as required.
  4. Off: Manually stops the fans (Note: Not all control boxes will have an "off" setting).

If the Fire/Rescue personnel need to operate the fans manually, they must communicate with OCC about the rationale for manual operation and revert to automatic mode post-operation.

EMERGENCY VENTILATION IN THE UNDERGROUND SYSTEM

TUNNELS

Emergency ventilatory measures are employed through fan shafts designated for smoke and air movement. These fans are:

  • Power-operated
  • Reversible in function
  • Equipped with between two to seven 60-inch fans capable of exerting up to 50,000 cfm (cubic feet per minute) for exhaust or 35,000 cfm for supply.

In case of emergency, the Fire/Rescue Incident Commander establishes control over these fans after contacting OCC. Initially, OCC will operate the fans based on reported conditions until Fire/Rescue units arrive and take over control.

FAN CONTROL BOX OPERATIONS

Each fan shaft has a control box that varies in location. Fire/Rescue personnel are encouraged to be acquainted with these controls in their assigned areas, with settings similar to those in stations:

  1. Emergency Exhaust: Manually exhausts air.
  2. Emergency Supply: Manually supplies air.
  3. Automatic Operation: Operates via thermostat or OCC control.
  4. Off: Stops the fans (some older models may lack an off setting).

In situations where fans must be activated manually, OCC should be notified. After emergency procedures are complete, devices should revert to automatic settings to ensure ongoing system regulation by OCC.

STATION VENTILATION DURING EMERGENCIES

During emergencies at stations, ventilation fans may be utilized to supply clean air or eliminate combustion products. Station fans can also exhaust smoke. When incidents occur on the platform level, OCC activates exhaust fans to clear evacuation paths by providing fresh air through the escalators, hence channeling smoke into tunnels. Conversely, if a fire occurs at the mezzanine level, OCC would switch the station fans to supply mode to expel smoke from the station and protect evacuation routes.

TACTICAL CONSIDERATIONS

When deciding which fans to utilize in emergencies, the following considerations should be factored in:

SUPPLY MODE

During air supply into the tunnel, one must always consider where the exhaust will lead, as it may obstruct passenger evacuation routes.

  • Fresh Air for Evacuations: It is advisable to channel fresh air into exit routes. This tactic ensures evacuees do not navigate through smoke-filled areas. If the supplying fan shaft serves as an evacuation route, an alternative shaft should be utilized for air supply to minimize smoke interference.
EXHAUST MODE

Exhaust fan shafts should be reserved for last-resort scenarios related to entrances or evacuations, particularly if a fire is present. Products of combustion will ascend via the shaft, creating risks to both firefighters and civilians. Evacuating through exhaust paths should not be customary.

OFF MODE

In the event of hazardous chemical or biological releases, it may be crucial to disable all ventilation systems. This action prevents contamination to unaffected areas and is imperative if exhaust operations might disperse contaminants into adjacent communities. Additionally, ceasing all train movement is critical to avoid air movement effects that could spread contamination.

  • Chimney Effect Awareness: With fans offline, shafts may engage in a chimney effect, drawing smoke toward exit points. Assessing air movement can dictate safer evacuation routes for passengers and containments for emergency personnel.
AUTOMATIC MODE

In this mode, fan operation relies on OCC supervision. Coordination between the Incident Commander and OCC is essential to determine the most effective operation mode, guiding both passenger evacuations and Fire/Rescue personnel entries.

THERMOSTAT OPERATIONS

On some occasions, fans may activate independently, even when instructed for a halt. During high temperatures (exceeding 95°F), thermostat settings may trigger fan operation. To override this, OCC may place fans in "Emergency Stop" mode remotely via their systems.