COOLING TOWER

A cooling tower is a heat rejection device which rejects waste heat of water to atmosphere through cooling of a water stream to a lower temperature. Cooling towers may either use 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 of cooling towers are used in power plants, oil refineries, petrochemical and other chemical plants and HVAC systems for cooling buildings. The classification is based on the type of air induction into the tower.

There are two types of cooling towers: 

a)   Natural draft cooling towers

b)  Induced draft cooling towers.

Cooling towers vary in size from small roof-top units to very large hyperboloid structures (stable under self weight and produce stack effect  ) that can be up to 200 metres tall and 100 metres in diameter, or rectangular structures that can be over 40 metres tall and 80 metres 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.

Industrial cooling towers can be used to remove heat from various sources such as machinery or heated process material. The primary use of large, industrial cooling towers is to remove the heat absorbed in the circulating cooling water systems used in power plants, petroleum refineries, petrochemical plants, natural gas processing plants, food processing plants, semi-conductor plants, and for other industrial facilities such as in condensers of distillation columns, for cooling liquid in crystallization, etc.

If that same plant had no cooling tower and used once-through cooling water, it would require about 100,000 cubic metres an hour and that amount of water would have to be continuously returned to the ocean, lake or river from which it was obtained and continuously re-supplied to the plant. Furthermore, discharging large amounts of hot water may raise the temperature of the receiving river or lake to an unacceptable level for the local ecosystem. Elevated water temperatures can kill fish and other aquatic organisms (see thermal pollution), or can also cause an increase in undesirable organisms such as invasive species of Zebra mussels or algae. A cooling tower serves to dissipate the heat into the atmosphere instead and wind and air diffusion spreads the heat over a much larger area than hot water can distribute heat in a body of water. Some coal-fired and nuclear power plants located in coastal areas do make use of once-through ocean water. But even there, the offshore discharge water outlet requires very careful design to avoid environmental problems.

TERMINOLOGY OF COOLING TOWERS:

DRY BULB TEMPERATURE: The temperature at which a thermometer reads freely in air but is shielded from radiation and moisture.

NATURAL DRAFT OR STACK EFFECT: When there is a large pressure differential between the outside air and air within a structure. Warm, moist air is less dense than dry air of the same pressure thus causing the worm, moist air to raise.

HOW NDCT WORKS

a)      As hot water enters the natural draft cooling towers, through a series of pumps, to the worm water inlet.

b)     The worm water is then distributed into through a packing or fill material inside the tower keeps the water evenly distributed. This plays a major role and contributes heavily to the efficiency of the tower i.e. better the distribution allows for more contact time, and greater surface area, which allows for more heat removal.

c)      As the differential is being produced by the shape of the tower itself, the hyperbolic shape, induces current upward.

d)     As the water is being distributed, the exposed lower portion of the tower allows for the ambient air over the cold water basin which actually creates the differential, thus cause the natural draft or stack effect.