Cooling towers are an integral component of many refrigeration systems, providing comfort or process cooling across a broad range of applications. They are the point in the system where heat is dissipated to the atmosphere through the evaporative process, and are common in industries such as oil refining, chemical processing, power plants, steel mills, and many different manufacturing processes where process cooling is required. They are also commonly used to provide comfort cooling for large commercial buildings including airports, office buildings, conference centers, hospitals, and hotels.
Cooling tower structures vary greatly in size and design, but they all function to provide the same thing: liberation of waste heat extracted from a process or building system through evaporation of water. In technical terms, cooling towers are engineered and designed based on a specified cooling load, expressed in refrigeration tons. The cooling load is determined by the amount of heat that needs to be extracted from a given process or peak comfort cooling demand. The cooling tower must be adequately sized to reject this same amount of heat to the atmosphere.
Cooling towers are used to reject heat through the natural process of evaporation. Warm recirculating water is sent to the cooling tower where a portion of the water is evaporated into the air passing through the tower. As the water evaporates, the air absorbs heat, which lowers the temperature of the remaining water. This process provides significant cooling to the remaining water stream that collects in the tower basin where it can be pumped back into the system to extract more process or building heat, thereby allowing much of the water to be used repeatedly to meet the cooling demand.
The amount of heat that can be rejected from the water to the air is directly tied to the relative humidity of the air. Air with a lower relative humidity has a greater ability to absorb water through evaporation than air with a higher relative humidity, simply because there is less water in the air. As an example, consider cooling towers in two different locations– one in Atlanta, Georgia, and another in Albuquerque, New Mexico. The ambient air temperature at these two locations may be similar, but the relative humidity in Albuquerque on average will be much lower than that of Atlanta’s. Therefore, the cooling tower in Albuquerque will be able to extract more process or building heat and will run at a cooler temperature because the dry desert air has a greater capacity to absorb the warm water.
Cooling towers can be split into two distinct categories: open circuit (direct contact) and closed circuit (indirect) systems. In open circuit systems the recirculating water returns to the tower after gathering heat and is distributed across the tower where the water is in direct contact with the atmosphere as it recirculates across the tower structure. Closed circuit systems differ in that the return fluid (often water, or sometimes water mixed with glycol) circulates through the tower structure in a coil, while cooling tower water recirculates only in the tower structure itself. In this case, the return fluid is not exposed directly to the air.