A cooling tower is a heat rejection device that cools warm water by bringing it into contact with air and evaporating a small share of it. As the water trickles down over surfaces called fill, air flows through it; the water that evaporates draws the heat it needs from the water that remains. This lets a tower cool water below the temperature of the outdoor air, down to within a few degrees of the air's wet-bulb temperature. Cooling towers are used in factories, power plants and the central cooling systems of large buildings to release the heat coming from machines and condensers into the atmosphere.
Evaporative cooling: why a tower is so effective
Evaporating water takes a lot of energy. At ambient temperature, 1 kg of water absorbs about 2,400 kJ of heat as it evaporates, while cooling the same 1 kg of water by 1 °C only requires removing 4.2 kJ. Roughly speaking, when one percent of the water circulating in a tower evaporates, the rest cools by 5–6 °C. That is why a tower can reject a large amount of heat for a small loss of water.
The limit of evaporation is set by how much more moisture the air can take up. This limit is shown by the wet-bulb temperature: the reading of a thermometer whose bulb is wrapped in a wet wick and exposed to moving air. In dry air the wet-bulb reading is clearly below the ordinary (dry-bulb) temperature; in humid air the two come close. For example, air at 35 °C and 25% relative humidity has a wet-bulb temperature of about 21 °C, while at the same temperature and 55% humidity it rises to around 27.5 °C.
Tower water never gets down to the wet-bulb temperature. The difference between the water leaving the tower and the wet-bulb is called the approach; designs usually work with 3–5 °C, and the smaller the approach you ask for, the larger the tower becomes.
The parts of a cooling tower
- Hot water distribution: Warm water returning from the plant is sprayed through nozzles at the top of the tower or released by gravity onto the fill from a perforated distribution basin.
- Fill: Plastic packs that spread the water into a thin film or break it into small droplets. They increase the contact area and contact time between water and air; most of the heat exchange in the tower happens here.
- Fan: Moves the air through the tower. In most towers an axial fan at the top draws the air through; in some types a fan at the side pushes it in.
- Drift eliminator: Catches the water particles carried away by the air leaving the tower; it reduces both water loss and the droplets released into the surroundings.
- Cold water basin: The cooled water collects in the basin at the bottom and is pumped back to where it is used. Water lost to evaporation is topped up from the mains, and part of the water is continuously drained (blowdown) so that minerals do not concentrate.
Open-circuit and closed-circuit cooling towers
Open-circuit tower
The water to be cooled passes through the tower itself and touches the air directly. For a given duty this is the smallest and most economical type, and the one that brings the water closest to the wet-bulb temperature. The price is that the water picks up dust, pollen and oxygen from the air, and evaporation concentrates the salts in it. When an open tower serves machines that are sensitive to dirt, a plate heat exchanger is therefore usually placed between the tower and the machine, so that tower water and machine water stay in separate circuits.
Closed-circuit tower
The process water, or water with glycol, flows inside a coil in the tower and never touches the air. A separate spray water circuit wets the outside of the coil; as air passes over this wet surface, evaporation again does the cooling. Because the water on the machine side stays clean and sealed, this type is preferred for applications that are sensitive to water quality, such as induction furnaces, air compressors, hydraulic units and mould circuits. For the same capacity it is larger and more expensive than an open tower; in return, in cool weather the spray water can be switched off and the coil run dry.
Counterflow and crossflow towers
Towers are also divided into two groups by the direction in which the air moves relative to the falling water.
- Counterflow: Air rises from the bottom while water falls from the top. The footprint is small and the tower rejects more heat for the same volume. Since the water is distributed through pressurised nozzles, the pump head is somewhat higher.
- Crossflow: Air enters horizontally through the side faces of the tower and cuts across the falling water. The water is distributed by gravity from open basins at the top, so the pump head is low. For the same capacity it takes up a larger area.
How the air is moved is another way of classifying towers. The hyperbolic concrete towers of large thermal power stations use no fans: the warm, humid air inside rises by itself through the chimney effect, and these are called natural draught towers. Almost all of the factory-built packaged towers used in industry and buildings, on the other hand, have fans. A tower whose fan sits on top and pulls the air through is called induced draught; one whose fan sits at the side and pushes the air in is called forced draught. The induced draught type is more common, because it throws the humid exhaust air upwards and makes it harder for the same air to be drawn back into the tower.
Dry coolers and hybrid solutions
Where water is scarce or expensive, a dry cooler can be used instead of a tower. A dry cooler is an air-to-water heat exchanger with fans; the water stays inside a closed coil and nothing evaporates. It uses no water, but it cannot benefit from evaporation either: it can only cool water to a few degrees above the dry-bulb temperature of the outdoor air. On a 35 °C summer day this usually means water above 40 °C. Hybrid (adiabatic) coolers sit between the two: they run dry for most of the year and boost their capacity only on hot days, by humidifying the incoming air or spraying water onto the coil.
Where are cooling towers used?
- In thermal power stations, for the cooling water of the condenser that condenses the steam leaving the turbine
- In chemical plants, petrochemical plants and refineries, for reactor jackets, column condensers and plant heat exchangers
- In iron and steel works and foundries, for furnace shells, rolling mills and induction furnaces; closed-circuit towers are usually chosen here
- In the plastics and rubber industry, for the hydraulic oil coolers of injection moulding machines and for moulds that run hot; we covered which circuit is fed with tower water and which with chiller water in our article on cooling injection moulding machines
- In food and beverage plants, for hot water returning from compressors and from pasteurisation and sterilisation lines
- In large buildings such as shopping centres, hospitals, campuses and data centres, to reject the heat of the central cooling system
The difference between a cooling tower and a chiller
Both cool water, but their limits differ. Tower water stays a few degrees above the wet-bulb temperature of the air; on a humid summer day, water leaving a tower is usually in the 28–32 °C range. A tower uses no energy apart from its fans and pumps, but it cannot bring the water below the value set by outdoor conditions. A chiller, by contrast, uses a refrigeration cycle to bring water to the temperature you need regardless of the outdoor air, for example 7 °C, or below zero with glycol; in return, its compressor uses electricity.
The water temperature the machine needs tells you which one you require. For a hydraulic oil cooler, an air compressor or a power plant condenser, tower water at around 30 °C is enough. If a mould, a reactor, a laboratory instrument or a building's air conditioning needs water at 5–15 °C, you need a chiller. In large plants the two often work together; the role of the tower in the condenser circuit of a water-cooled chiller is explained in the water-cooled chiller article of our sister company Aktek Elektromekanik. Our industrial chillers are available with a water-cooled condenser connected to a tower as well as with an air-cooled condenser; for the other classes, see the chillers page.
What to look at first when choosing a tower
The right tower follows from the heat load to be rejected, the water flow rate, the water temperatures entering and leaving the tower and the summer design wet-bulb temperature of the site. After that, open or closed circuit is chosen by water quality, the fan arrangement by the space and the noise limit, and the casing and fill materials by the temperature and chemistry of the water. We went through each of these steps with worked examples in our article on choosing a cooling tower.
For your project
Tell us the water temperature your plant needs, the heat load to be rejected and your existing tower or water source; we will work out together whether tower water, a chiller or the two together is the right answer. For sizing the chiller side, see our chiller selection guide, and send your project details through our contact page.
Service, maintenance and repair of chillers and cooling systems are carried out by our group company Aktek Elektromekanik. Aktek Elektromekanik service
