Colorado Public Works Journal, Winter 2021

RTD FASTRACKS Laurie Huff How is the air cleaned in Union Station’s bus concourse? You’ll be amazed In February 2016, it finally happened for diehard Denver Broncos fans: The team defeated the Carolina Panthers to win Super Bowl 50. Two days later, an estimated 1 million people converged in downtown Denver for a victory parade. Many fans took public transit to and from the celebration, which had them wading through the relatively new bus concourse at Union Station. Never before had so many people squeezed into the nearly 1,000-foot-long facility at once. That winter day, RTD facilities maintenance supervisor Clarence Pauls observed a line of about 200 people snaking through the concourse for a single bus, with another 1,000 people on the move. A concen- tration of people exhaling carbon dioxide (CO 2 ) is not a big issue for those occupying a space for a short time, but a high level of CO 2 – greater than 5,000 parts per million (ppm) – can lead to side effects such as a higher heart rate, headaches and drowsiness. The author Laurie Huff is the senior specialist of Public Relations at RTD and can be reached at It’s not clear what the CO 2 level was the day of the Super Bowl celebration, but no such symptoms were reported or noted during that time. A typical level of CO 2 in an occupied space with good air exchange ranges from 350 to 1,000ppm. Inside the Union Station bus concourse, it was about 300ppm during a pre-COVID-19 morning or afternoon rush. When Pauls checked the CO 2 level on a recent Friday with little foot traffic, it was 56ppm. During the pandemic, some of you have wondered about the quality of the air moving through the bus concourse because the facility is underground. Pauls notes that RTD has received no complaints about air quality in the space since it opened in 2014. A patron of this space might not give too much thought to how it functions, Pauls acknowledges. But if you could look behind the locked doors of this massive facility to glimpse it as he does, you would be astounded by the scale, the size, the sound. More than 80 vents pull air into a wind-tunnel-like plenum that runs the length of the facility, extending underground from the entrance to the Crawford Hotel to the light rail tracks. From there, three huge blue fans – each 200-horsepower, pushing 150,000 cubic feet per minute (cfm) of air – pull bus exhaust up and out of an air exhaust vent tube near Chestnut Pavilion, one of five entrances to the concourse. A typical bathroom exhaust fan, for comparison, runs at 75 cfm. We’ll bet you’ve noticed the trio of vent tubes near the light rail plat- form. The largest is for air exhaust from the fans, the smallest is for mechanical exhaust from plumbing and bathroom ventilation, and the third is an intake tube to bring fresh air inside. Several studies were conducted to determine the correct placement for these tubes, with safety, security and wind patterns factoring into the finished design. The air pumped into the passenger concourse is all fresh, brought in through the screened-in intake vent. The outside air is conditioned with three increasingly finer levels of filtration before RTD patrons breathe it. Air ultimately passes through pleated filters carrying a minimum efficiency reporting value (MERV) rating of 13, trapping par- ticles between 0.3 and 1 micron in size. Before the advent of COVID-19, a bus pulled away from the 22-bay facility every 48 seconds during morning and afternoon rush periods. Now, there’s significantly less vehicle and foot traffic as the pandemic continues. Regardless, RTD’s approach to airflow in this space r emains the same, with the only change made to infrastructure being the operating speed of the fans, to provide a higher exchange of the outside air coming in and increase the level of air moving out. Clarence Pauls, facilities maintenance supervisor Union Station’s bus concourse Winter 2021 | 55