Cooling tower

A cooling tower is a heat rejection device that rejects waste heat to the atmosphere through the cooling of a water stream to a lower temperature. Cooling towers may either use the evaporation of water to remove process heat and cool the working fluid to near the wet-bulb air temperature or, in the case of closed circuit dry cooling towers, rely solely on air to cool the working fluid to near the dry-bulb air temperature. Common applications include cooling the circulating water used in oil refineries, petrochemical and other chemical plants, thermal power stations, nuclear power stations and HVAC systems for cooling buildings. The classification is based on the type of air induction into the tower: the main types of cooling towers are natural draft and induced draft cooling towers. Cooling towers vary in size from small roof-top units to very large hyperboloid structures (as in the adjacent image) that can be up to 200 metres (660 ft) tall and 100 metres (330 ft) in diameter, or rectangular structures that can be over 40 metres (130 ft) tall and 80 metres (260 ft) long. The hyperboloid cooling towers are often associated with nuclear power plants, although they are also used in some coal-fired plants and to some extent in some large chemical and other industrial plants. Although these large towers are very prominent, the vast majority of cooling towers are much smaller, including many units installed on or near buildings to discharge heat from air conditioning. Cooling towers originated in the 19th century through the development of condensers for use with the steam engine. Condensers use relatively cool water, via various means, to condense the steam coming out of the cylinders or turbines. This reduces the back pressure, which in turn reduces the steam consumption, and thus the fuel consumption, while at the same time increasing power and recycling boiler-water. However the condensers require an ample supply of cooling water, without which they are impractical. The consumption of cooling water by inland processing and power plants is estimated to reduce power availability for the majority of thermal power plants by 2040–2069. While water usage is not an issue with marine engines, it forms a significant limitation for many land-based systems. By the turn of the 20th century, several evaporative methods of recycling cooling water were in use in areas lacking an established water supply, as well as in urban locations where municipal water mains may not be of sufficient supply; reliable in times of demand; or otherwise adequate to meet cooling needs. In areas with available land, the systems took the form of cooling ponds; in areas with limited land, such as in cities, they took the form of cooling towers. These early towers were positioned either on the rooftops of buildings or as free-standing structures, supplied with air by fans or relying on natural airflow. An American engineering textbook from 1911 described one design as 'a circular or rectangular shell of light plate—in effect, a chimney stack much shortened vertically (20 to 40 ft. high) and very much enlarged laterally. At the top is a set of distributing troughs, to which the water from the condenser must be pumped; from these it trickles down over 'mats' made of wooden slats or woven wire screens, which fill the space within the tower.' A hyperboloid cooling tower was patented by the Dutch engineers Frederik van Iterson and Gerard Kuypers in 1918. The first hyperboloid cooling towers were built in 1918 near Heerlen. The first ones in the United Kingdom were built in 1924 at Lister Drive power station in Liverpool, England, to cool water used at a coal-fired electrical power station. An HVAC (heating, ventilating, and air conditioning) cooling tower is used to dispose of ('reject') unwanted heat from a chiller. Water-cooled chillers are normally more energy efficient than air-cooled chillers due to heat rejection to tower water at or near wet-bulb temperatures. Air-cooled chillers must reject heat at the higher dry-bulb temperature, and thus have a lower average reverse-Carnot cycle effectiveness. In areas with a hot climate, large office buildings, hospitals, and schools typically use one or more cooling towers as part of their air conditioning systems. Generally, industrial cooling towers are much larger than HVAC towers.