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Cooling tower technology

Open evaporative cooling tower

The extensive name for cooling tower is open evaporative cooling tower. Cooling towers are able to draw off heat from a process or a cooling cicle in an efficient way. In a cooling tower, water is put in direct contact with surrounding air. A small part of the cooling water, i.e. 1-2%, evaporates. This evaporation causes an increase in temperature and humidity of the air and a decrease of the temperature of the water. By using the evaporation, it is possible to cool below the normal air temperature. The minimal realisable temperature is the wet bulb temperature.

Forced draught cooling tower

Fans cause a continuous air draught. When the fan is blowing, the apparatus is on overpressure, which is called forced draught. In contrast, when the fan is sucking, we have underpressure, which is called induced draught. For cooling towers with a forced draught, 2 types of fans are possible: axial (plate with blades) or radial fans (snail-shell). The disadvantage of a radial fan is that it produces more sound and that it has worse profits. The figure below shows the possible configurations of a counterflow cooling tower.

Forced Draft , Radial Fan	Forced Draft, Axial Fan	Induced Draft , Axial Fan

Counter flow

CMC and CMD cooling towers work on the principle of counter flow. This means that the water flows down while the air is pulled upwards by a fan. As the water flows further through the system, the air it encounters is fresher, in other words cooler and less saturated with moisture.

Counter flow cooling towers can approach the wet bulb temperature better compared with cross flow cooling towers.

Cross flow

Using the cross flow principle, warm water flowing down through a cooling unit is cooled by air drawn upwards by a fan. Evaporation and direct heat exchange cause a rapid drop in temperature.

Compared to counter flow cooling towers, induced-draught cross flow cooling towers are much quieter and smaller. The performance figures are next to perfect.

Beside counterflow cooling towers, Polacel developed 4 types of crossflow cooling towers. In these cooling towers the water contacts the air in crossflow. The exchange efficiency is lower, so that the cooling tower needs to be larger. However, crossflow cooling towers are often preferred because they produce less sound (no sound of falling water), are lower and use less capacity.

Water distrubution

Every cooling tower is build up from a system of nozzles (^^^^), that distribute the water over the cooling fill. The drift eliminator, which is located above this system, takes care that the water drops do not leave the cooling tower. In all counterflow cooling towers Polacel uses induced draught, axial fans that have a low air speed at the air inlet, leading to a good distribution of air. This equal distribution causes higher efficiency. Furthermore, Polacel offers a wide choice of types of fans that have particular sound characteristics and are all highly efficient. Also, the location of the fan on the outlet causes the hot air to flow far away. This limits the always-present recirculation.

Cooling tower sound reduction

A disadvantage of the principle outlined above is the sound of the water that falls in the basin of the cooling tower. This varies, dependable on the design, from 5 to 30 m3/h per square meter. Polacel advises to reduce the sound by installing floating sound attenuators that reduce the sound at its origin. An alternative solution may be to place the attenuators at the in- or outlet, but this causes an increase in resistance at the air side, which then results in more used capacity and a higher sound source.

Cooling tower fill

Besides the use of fans, modern cooling towers are also highly efficient due to the use of film fills, in which the contact surface between water and air is maximised. There are several types of fills with characteristics that have to be in accordance with the process and surrounding conditions.

Cooling water treatment

As mentioned in the first section, the cooling tower's high efficiency (less than 1 kW electric per 100 kW cooling) is due to evaporation. A disadvantage of the continuous evaporation process is that water needs to be supplied. This supplied water, however, contains salts and contaminations, which, when no measures are taken, accumulate in the cooling tower. Also, water needs to be drained in order to diminish the concentration of contaminations. When the water is treated, this drainstream can be reduced. Possible treatments are softening of the supplied water and chemical and physical treatment of the circulating water. The treatment always has to be in accordance with the present supplied water. the conditions of the process and the used material. A good water treatment not only protects the installation against the corrosive effect of the water, but also supports the legionella control. In every (open) water system a risk of contamination with legionella exists. The risk can be controlled when the manager of the installation drafts and carries out a control plan, in which regular sample-taking and periodic maintenance are included. On the basis of sample-taking can be determined whether the current control measures are sufficient or that cleaning is necessary.

Hard wearing materials

In order to prevent corrosion of cooling towers, Polacel consequently uses hard-wearing materials like stainless stell, polyester and synthetics for all components that are in contact with water. Because of this, a long life-span that does not depend on the applied water treatment or on the completed water cleanings is guaranteed.