Last edited 21 Oct 2020

Cooling tower

It can be necessary to provide cooling to buildings during warm weather, or where there are significant thermal gains (such as solar gain, people and equipment). This cooling is sometimes referred to as comfort cooling. Cooling may also be necessary for refrigeration or for some industrial processes.

Evaporative cooling is one of the methods that can be used to provide cooling. When water evaporates, it absorbs significant amounts of heat energy (latent heat, expressed in J/kg). The water itself does not change temperature, as the energy is consumed by the physical process of changing its state from liquid to gas. This produces a cooling effect in its surroundings. So when water evaporates from the surface of a building, or when sweat evaporates from the skin, this has a cooling effect. Conversely, when water condenses it releases heat.

Direct evaporative coolers (sometimes referred to as sump coolers, swamp coolers, or desert coolers), draw hot, dry air through a continually dampened pad and supply cool, humid air.

Indirect evaporative cooling can be achieved by using a heat exchanger to cool the supply air, by spraying water over the cooling coils of a conventional chiller or by cooling towers.

Cooling towers reject heat through the evaporation of water in a moving air stream within the cooling tower. The temperature and humidity of the air stream increases through contact with the warm water, and this air is then discharged. The cooled water is collected at the bottom of the tower. This process can achieve lower temperatures than air-cooled heat rejection systems.

Cooling towers, can be small-scale roof-top installations, medium-sized packaged units, or very large hyperboloid structures sometimes associated with industrial processes or power stations with their characteristic plume of water vapour in the exhaust air.

Cooling towers can be open or closed circuit. Closed circuit cooling towers (or fluid coolers) feed the ‘warm’ water over a 'fill' through which the air is flowing. The cool water is collected, and the humid air discharged remotely to prevent recirculation. This closed process relies entirely on the rejection of heat through the air. The flow of air through the fill can be horizontal (crossflow) or vertical (counterflow).

In open-circuit cooling towers (or wet cooling towers), the hot humid air is discharged direct to the atmosphere. This can be more straight forward, however, there is a risk of contamination as the cooling tower is open, and the proportion of water that evaporates must be replaced resulting in the progressive concentration of the water. The build up of minerals and other solids in the water must then be regulated.

Dry cooling towers use a heat exchanger to separate the water from the air.

The air flow in cooling towers can be mechanically driven (induced draft) or naturally driven (natural draft), relying on the buoyancy of the warm exhaust air to generate ‘draft’.

The water flow can be gravity driven or can be a pressurised spray.

In HVAC systems, the cooling tower is used to reject heat from chiller units which supply chilled water for comfort cooling systems. These will typically cool air in air handling units that is then ducted to the internal spaces of the building. Water-cooled chillers can be more efficient than air-cooled chillers, but more infrastructure (and so space) is required.

NB: Wet cooling towers have the potential for legionella growth. The Health and Safety Executive (HSE) require that appropriate measures are taken to prevent or control the risk of legionella. These must be at least as effective as the measures described in Legionnaires’ disease: The control of Legionella bacteria in water systems.

For more information, see Cooling tower design and construction.

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