Getting to know the SR 99 tunnel's biggest fans

Clean air is a critical element of a safe and well-functioning tunnel. In the new SR 99 tunnel, a complex system of fans will push and pull air into and out of the tunnel to ensure fresh air during routine operations and in emergencies.

The first element of the tunnel’s ventilation system is the very traffic that uses it. Vehicles traveling through the tunnel will act like pistons, pushing fresh air into the tunnel. But when that piston effect needs a boost, the tunnel will turn to its 17 jet fans.

Two of the tunnel's 17 jet fans

Ten environmental monitoring stations spread throughout the tunnel will measure air quality inside the tunnel. If traffic congestion begins to reduce tunnel air quality, these jet fans installed at the tunnel entrances will kick on (two fans are pictured above). Turning on two at a time, the jet fans will push additional air through the tunnel.

In addition to the jet fans, eight centrifugal fans, split between the tunnel’s two operations buildings, pull air out of the tunnel through dampers built into the tunnel wall. The photo below shows some of these dampers on the wall at right.

Dampers in the SR 99 tunnel wall

Each centrifugal fan pulls between 135,000 and 180,000 cubic feet of air per minute, venting it out the distinctive yellow stacks atop the operations building. During normal tunnel operations these fans will not be active. WSDOT estimates that during extreme traffic congestion (traffic speeds of 5 mph), these fans will turn on, two at a time, to clear car exhaust out of the tunnel. The centrifugal fans in the north portal operations building are shown below.

Centrifugal fans in the north portal operations building

Last but not least are the tunnel's four maintenance fans. These fans (shown in the picture below) have a different job: providing fresh air to the maintenance areas, emergency exit spaces and electrical rooms.

Maintenance fans in the north portal operations building

At least one maintenance fan will be on at all times, keeping the refuge and emergency exit corridor pressurized. This pressurization means that air flows from these areas into the tunnel, not the other way around, so exhaust and fumes don’t enter these spaces.

Construction of the ventilation and other tunnel systems is already underway. We’ll share more about these and other tunnel systems in future posts.