Adiabatic cooling – principle of operation and applications

Adiabatic cooling is the process of reducing air temperature through the evaporation of water without adding or removing heat from the surrounding environment. This phenomenon occurs naturally in nature. Humans experience the effect of adiabatic cooling when sweat evaporates from the skin. The same principle is applied in industrial air cooling systems.

The lowest temperature to which air can be cooled during an adiabatic process is limited by the so-called wet-bulb temperature. Wet-bulb temperature is the lowest temperature that can be achieved by cooling air exclusively through water evaporation, without any additional mechanical cooling.

It is measured using a thermometer wrapped in a wet cloth. As the water evaporates from the cloth, it absorbs heat from the thermometer, causing its temperature to drop. This process continues only as long as the air is capable of absorbing additional moisture.

From a thermodynamic perspective, the process takes place at an approximately constant enthalpy of moist air. This means that during adiabatic cooling, the total energy contained in the moist air remains virtually unchanged, even though its temperature and humidity change. The energy required for water evaporation is taken from the air itself, causing its temperature to decrease while simultaneously increasing its water vapour content.

Enthalpy is a thermodynamic property that describes the total energy of a system. In the case of moist air, it mainly consists of:

• energy associated with air temperature (sensible heat),
• energy associated with the water vapour contained in the air (latent heat).

During adiabatic cooling, a portion of the water evaporates. Evaporation requires energy, which is not supplied from an external source but is instead extracted directly from the air. As a result:

• the amount of energy associated with air temperature decreases (the air cools down),
• the amount of energy associated with water vapour increases (the air becomes more humid).

This can be compared to exchanging money from one currency into another. Imagine you have assets worth €100. You exchange part of that amount into another currency. The amount of euros decreases, while the amount of the second currency increases, but the total value of your assets remains unchanged.

In this cooling technology, the loss of one form of energy is almost entirely compensated by the increase in the other, which is why the total enthalpy remains approximately constant. Consequently, the total energy contained in the air remains virtually unchanged, even though its properties are altered.

The operating principle is based on the contact between air and water in the form of a thin water film, fine mist, or wetted medium:

• warm, dry air passes through a wetted medium (such as an evaporative pad) or through a zone of atomised water mist;
• heat and mass transfer occur, with part of the water evaporating and absorbing energy from the air, resulting in a lower air temperature and higher humidity;
• the outlet air temperature decreases while its humidity increases.

For example, air at a temperature of 35°C with a relative humidity of 30% can be cooled to approximately 24–25°C. The final temperature depends on the efficiency of the installed system as well as the prevailing ambient conditions.

Technology limitations

The effectiveness of adiabatic cooling depends on the temperature and humidity of the outdoor air. In direct systems, the increase in air humidity must be taken into account, meaning the technology is not suitable for every process, particularly those sensitive to moisture. Water quality and consumption are also important considerations, as are water treatment, protection against scale formation, corrosion and microbial growth, and regular maintenance of the installation.

Direct adiabatic cooling

In a direct adiabatic cooling system, the air intended for the process or ventilation comes into direct contact with water. As a result, the air temperature decreases while its humidity increases. This solution is commonly used in production halls, warehouses, and logistics facilities, where higher humidity does not adversely affect the technological process.

Indirect adiabatic cooling

In indirect adiabatic cooling, the process air does not come into contact with water. Instead, it is cooled via a heat exchanger by a secondary air stream that undergoes the adiabatic cooling process. This makes it possible to reduce the air temperature without increasing the humidity of the supply air. Such systems are widely used in office buildings, data centres, and industrial facilities with demanding process requirements.

The range of applications for this technology continues to expand. The most common uses include:

• cooling production halls – reducing temperatures in industrial facilities while lowering energy consumption compared to conventional air conditioning systems;
• supporting refrigeration units – pre-cooling air before it enters condensers, reducing the condensing temperature and lowering compressor power consumption;
• data centres – serving as part of free cooling systems, reducing the operating time of conventional mechanical cooling equipment;
• food industry – cooling processing areas and warehouses where elevated humidity does not adversely affect products or processes;
• heavy industry (steelworks and foundries) – improving working conditions in high-temperature environments where alternative cooling methods are not cost-effective.

The greatest advantage of adiabatic cooling is its very low electricity consumption. Energy is primarily used by fans, pumps, and control systems. The adiabatic module itself does not require the energy-intensive compression of a refrigerant. When integrated with a conventional refrigeration system, it reduces compressor operating time or improves compressor operating conditions. It should be emphasised, however, that adiabatic cooling is typically implemented as a complementary technology rather than a standalone replacement for existing refrigeration systems. In many industrial applications, it serves as a support solution that reduces the workload of refrigeration equipment and improves its overall efficiency.

The temperature that can be achieved through adiabatic cooling is inherently limited. Once the air reaches saturation (100% relative humidity), evaporation effectively ceases, meaning that no further cooling is possible. Consequently, the performance of an adiabatic cooling system depends primarily on the temperature and humidity of the outdoor air. The drier the air, the greater the cooling potential.

Adiabatic cooling is a technology that utilises the natural process of water evaporation to reduce air temperature. Owing to its simple operating principle and low electricity demand, it is one of the most energy-efficient cooling solutions used in industrial applications. Although it cannot always fully replace conventional refrigeration systems, it can significantly enhance their performance and reduce operating costs in many applications. For this reason, adiabatic cooling is increasingly becoming an integral part of energy efficiency improvement strategies for industrial cooling systems.