A cooling tower is a heat rejection device used across various industries, including power generation, manufacturing, and large-scale commercial air conditioning systems. Its primary function is to remove waste heat by cooling a circulating water stream and releasing that heat into the atmosphere. The gas commonly released is often mistaken for smoke or other pollutants. The exhaust plume consists overwhelmingly of water vapor, the product of the tower’s evaporative cooling process.
The Primary Release: Water Vapor
The cooling tower operates on the principle of evaporative cooling, where a small fraction of the circulating water evaporates, carrying latent heat away from the remaining water. For every ten degrees Fahrenheit of cooling, approximately one percent of the water flow is lost to the atmosphere through evaporation. This evaporated water leaves the tower as invisible, gaseous water vapor, effectively acting as a highly efficient heat transfer medium.
The visible white cloud, often called a plume, is not true steam. Instead, the plume is created when the warm, saturated air released from the tower mixes rapidly with the cooler ambient air. This mixing causes the water vapor to condense into millions of microscopic liquid water droplets, which is essentially a man-made cloud or fog. Since this release is almost entirely composed of atmospheric moisture, it is considered benign in terms of air quality.
Stripping of Non-Condensable Gases
Beyond water vapor, the contact between water and air inside the cooling tower facilitates the release of gases dissolved in the water supply. This process, known as air stripping or degassing, naturally removes non-condensable gases from the circulating water. Gases commonly found dissolved in the makeup water, such as dissolved oxygen (\(\text{O}_2\)) and carbon dioxide (\(\text{CO}_2\)), are stripped out and vented along with the exhaust air.
The process drives these dissolved gases out of the water phase and into the air stream. The release of dissolved \(\text{CO}_2\) can affect the water’s pH balance, which influences the rate of corrosion and scale formation within the system. If the cooling water system has a leak from a process heat exchanger, volatile organic compounds (VOCs) like benzene or trichloroethylene can also be air-stripped and released into the atmosphere, requiring regulatory monitoring.
Volatile Chemicals from Water Treatment
The gaseous release also involves chemicals intentionally added to the water to maintain system integrity. Cooling tower water requires constant chemical treatment to prevent corrosion, scale formation, and microbial growth. Certain compounds used in this treatment are volatile, meaning they can transition from the liquid water phase to the gaseous air phase during the cooling process.
Biocides, such as chlorine or bromine compounds, are routinely added to control the growth of bacteria and algae, but a fraction of these chemicals can volatilize and be released as a gas. Similarly, some systems use neutralizing amines or ammonia for pH adjustment or as volatile corrosion inhibitors (VCIs). These compounds are designed to evaporate and protect metal surfaces in the air-vapor spaces of the system.
The volatility of these amines, particularly ammonia, is enhanced by the high air-to-water ratio within the tower, leading to release into the atmosphere. The amount of chemical release is highly dependent on the water’s pH and temperature, as well as the specific chemical agent used. Facilities must carefully manage these chemicals to ensure proper water quality maintenance without exceeding regulated air emissions limits.
Managing Released Materials and Health Risks
Controlling the release of material from cooling towers centers on evaporation and drift. While evaporation releases pure water vapor, drift consists of small, liquid water droplets carried out of the tower. Since these droplets have the same chemical composition as the circulating water, they contain dissolved solids, treatment chemicals, and biological contaminants.
To mitigate this release, cooling towers are equipped with drift eliminators, which are devices with a series of angled baffles that force the air stream to change direction multiple times. The inertia of the water droplets prevents them from following the sharp turns, causing them to collide with the baffle surfaces, coalesce, and drain back into the tower basin. Modern high-efficiency eliminators can reduce drift rates to as low as 0.001% of the total circulating water flow.
This control is paramount for public health, as drift droplets can carry the Legionella pneumophila bacteria, which causes Legionnaires’ disease. The bacteria thrive in the warm water environment of cooling towers, and if aerosolized in the drift mist, they can be inhaled by people nearby. The combination of chemical biocides and high-efficiency drift eliminators serves as the primary line of defense to prevent the dispersal of these potentially infectious aerosols over surrounding areas.