Subway ventilation system: the invisible guard protecting the air quality of underground space

In the urban transportation network, the subway is like an underground artery, carrying tens of millions of people traveling every day. However, in this relatively closed space, the quality of air is directly related to the health and comfort of passengers. As the "breathing system" of the underground space, the subway ventilation system is continuously purifying the flowing air through sophisticated equipment and technology.
I. Potential "air threats" in subway space
The particularity of the subway environment makes it a gathering place for a variety of harmful gases. First, the concentration of carbon dioxide will rise rapidly with the peak passenger flow. In a fully loaded subway car, hundreds of people continue to breathe, which will cause the concentration of carbon dioxide to exceed the national standard limit in a short period of time, causing dizziness, fatigue and other discomfort. Secondly, when the train is running, the high temperature generated by the friction of the braking system will cause the track dust to react with the air in a complex manner, releasing pollutants such as carbon monoxide and nitrogen oxides; and the volatile organic compounds (VOCs) slowly released by the interior and decoration materials of the car, such as formaldehyde and benzene, although the concentration is low, exist in the confined space for a long time.
It is worth noting that the problem of ozone pollution. When the fans and air-conditioning equipment of the subway ventilation system are discharged at high voltage or run at high speed, trace amounts of ozone may be generated due to the spark effect; ozone in the external atmosphere will also enter the underground space with fresh air. Ozone is highly oxidizing. At low concentrations, it can irritate the respiratory mucosa, causing coughing and chest tightness. Long-term exposure will cause damage to the lungs. These invisible gas threats require targeted treatment by the ventilation system.
II. Core equipment and purification principle of ventilation system
The subway ventilation system consists of a series of equipment with different functions. They are like precise "air filters" that purify the flowing air layer by layer.
(I) Fresh air and exhaust equipment: building an air circulation network
Axial flow fans and centrifugal fans are the "heart" of the ventilation system. Axial flow fans are installed at both ends of the tunnel and on the top of the station. They introduce fresh air from the outside through a strong air pressure difference and exhaust turbid air from the room, which can achieve several space changes per hour. Centrifugal fans are responsible for delivering fresh air to each carriage and platform. Their spiral impeller design can reduce noise while being highly efficient and energy-saving. In the interval tunnel, the piston air shaft uses the "piston effect" generated by the train running to drive the air flow without additional power, forming a natural ventilation auxiliary system.
(II) Air handling unit: a "purification center" with multiple filtration
The air handling units in stations and carriages are the core of refined purification. The primary filter first intercepts large particle pollutants such as dust and hair; the medium filter further captures micron-sized particles such as PM2.5 and bacteria; the activated carbon adsorption module targets VOCs and odor molecules, and reduces the concentration of harmful gases through the physical adsorption of the porous structure. For humidity control, the surface cooler and humidifier in the unit will adjust the air humidity to avoid the discomfort caused by dryness or humidity on the human body.
(III) Special pollutant treatment device: Accurate response to gas threats
For special gases such as carbon monoxide and ozone, the ventilation system is equipped with a catalytic purification device. Taking ozone treatment as an example, its core component uses a material with catalytic decomposition function. A large number of nano-level active sites are distributed on the surface of the material, which can adsorb ozone molecules and catalytically decompose them into harmless oxygen at room temperature. This process does not require additional energy consumption, and efficient purification can be achieved only through the catalytic reaction of the material, avoiding the energy waste of traditional treatment methods (such as high-temperature decomposition). Similar catalytic technology is also used in the treatment of carbon monoxide, which is converted into carbon dioxide through oxidation reaction to reduce toxic hazards.
III. Ozone treatment: the invisible guardian of catalytic technology
In the subway ventilation system, ozone treatment is particularly important. When ozone-containing air flows through the catalytic purification module, the active components on the surface of the material will undergo an electron transfer reaction with the ozone molecules. The oxygen-oxygen bonds in the ozone molecules are broken under the action of the catalyst and recombined into stable oxygen molecules. This catalytic reaction has extremely high efficiency and can continue to work even in a low-concentration ozone environment. The material itself is not consumed in the reaction and can operate stably for a long time. Data shows that the ozone concentration in the subway space after catalytic treatment can be controlled to less than one-third of the national standard limit (0.16mg/m³), fundamentally eliminating this potential hazard.
The application of this catalytic technology not only improves the targeted purification ability of the ventilation system, but also reflects the green and environmentally friendly design concept. It does not require the addition of chemical agents and does not produce secondary pollution. It only achieves purification through the physical and chemical properties of the material, which is highly consistent with the low-carbon operation goal advocated by the subway.
Fourth, breathing in underground space under the empowerment of science and technology
Today's subway ventilation system has long surpassed the simple "ventilation" function, but has integrated multidisciplinary technologies such as aerodynamics, materials science, and environmental engineering. From early natural ventilation to today's intelligent regulation, from single filtration to precise catalytic purification, every upgrade is creating a healthier travel environment for passengers. When we feel the fresh air on the platform, it is these invisible equipment and technologies that continue to filter every inch of flowing gas, dissolve harmful components such as carbon dioxide and ozone one by one, and keep the underground space at a suitable breathing rhythm.
The story of the subway ventilation system is a microcosm of the silent protection of people's livelihood by urban infrastructure. It uses the power of science and technology to build an invisible "air protection net" in the closed underground space, making every breath a safe journey. With the continuous advancement of environmental protection technology, this system will continue to evolve and build a more solid line of defense for the air quality of urban underground space.