In the industrial world, heat is often a by-product of activities. To maintain the efficiency and lifespan of equipment, it is necessary to cool it down. An air-cooled heat exchanger is a solution that uses ambient air for this task. In this article, we will explore its operation, different technologies, how to choose the right model, its advantages and disadvantages, as well as recent innovations. You will discover how these systems work and how they can be optimised.

Key Points to Remember

• Air-cooled heat exchangers use atmospheric air to dissipate heat from industrial processes, a need that has existed since the earliest civilisations.
• Several air-cooled heat exchanger technologies exist, including finned heat exchangers, direct air cooling, and systems using water spray to improve heat transfer.
• The choice of an air-cooled heat exchanger depends on factors such as the fluid to be cooled, the thermal power to be dissipated, temperatures, available space, and noise constraints.
• Air-cooled heat exchangers offer advantages in terms of energy efficiency and water consumption, but their reliability, maintenance, and installation flexibility must be considered.
• Innovations such as hybrid systems and intelligent control aim to improve the energy efficiency and reduce the water consumption of air-cooled heat exchangers.

Fundamental Principles of Air-Cooled Heat Exchangers

History of Cooling Needs

Since the dawn of human activity, it has always been necessary to dissipate the heat generated by various actions. Whether in primitive societies or today, industrial and mechanical processes produce heat that needs to be dissipated into a colder environment. The rise of transport and industrial development over recent centuries has accentuated this need. It has become essential to exchange heat with the environment to ensure the proper functioning and longevity of installations, which have become increasingly complex. The two main media used for this cooling are water (from rivers, lakes, or seas) and atmospheric air. This document focuses specifically on devices that use atmospheric air, commonly known as air-cooled heat exchangers. These systems are employed in a wide range of applications, from oil refineries to automotive engine cooling. Here, we will address industrial applications requiring medium-power air-cooled heat exchangers, ranging from 1 to several hundred megawatts. It is important to note that the characteristics of an air-cooled heat exchanger must be defined according to the technical constraints of the installation to which it is connected. This includes the nature and flow rate of the fluid to be cooled, the thermal power to be dissipated, the fluid’s inlet and outlet temperatures, as well as available space and neighbourhood constraints, such as noise levels. To help you choose the most suitable atmospheric cooling system, we will examine the operating principle, the current state of technologies, sizing methods, and advice for the installation and operation of direct dry air-cooled heat exchangers. You will find useful information to optimise your thermal management systems.

The Role of Atmospheric Air in Heat Transfer

Atmospheric air plays a predominant role in heat transfer due to its convective properties. It acts as a natural heat transfer fluid, capable of absorbing heat from a process and dissipating it into the environment. The main mechanism is convection, where air, in contact with a warmer surface, heats up, becomes less dense, and rises, being replaced by colder air. This continuous cycle allows heat to be efficiently dissipated. To improve this transfer, air-cooled heat exchangers often use fins. These fins significantly increase the contact surface between the hot fluid and the air, thus maximising the amount of heat exchanged. The performance of an air-cooled heat exchanger will therefore depend on several air-related factors: its ambient temperature, density, speed, and humidity. For example, colder and denser air will allow for greater heat transfer. Similarly, forcing air circulation through the exchanger, via fans, increases the heat transfer coefficient. It is also possible to improve cooling by adding a fine water spray. When this water evaporates, it absorbs a significant amount of heat (latent heat of vaporisation), thus cooling the air and, consequently, the fluid being processed. This principle is similar to the feeling of coolness one experiences when spraying oneself with water in hot weather. The effectiveness of this method will depend on the ambient air humidity; it performs better in dry climates.

Industrial Applications of Air-Cooled Heat Exchangers

Air-cooled heat exchangers are useful in a multitude of industrial sectors where thermal management is a major concern. In power plants, they are used to cool steam turbine condensers, thereby maintaining an optimal vacuum and ensuring high efficiency. Petrochemical and refining industries use them to cool various process fluids, such as oils, gases, or chemicals, to control reactions and ensure operational safety. In the industrial refrigeration and air conditioning sector, air-cooled heat exchangers serve as condensers for refrigeration units, dissipating heat extracted from spaces to be cooled. The automotive industry also uses similar systems for engine cooling. Other applications include cooling compressors, cogeneration engines, electrical transformers, and process equipment in various fields such as food processing or metallurgy. In summary, wherever excess heat needs to be dissipated into the atmosphere, the air-cooled heat exchanger represents a viable and often preferred technical solution due to its relative simplicity and independence from water resources, unlike traditional cooling towers.

Operation and Technologies of Air-Cooled Heat Exchangers

The Principle of the Finned Heat Exchanger

Air-cooled heat exchangers, in their most common design, rely on the principle of the finned heat exchanger. Imagine a series of thin tubes, through which the fluid you wish to cool circulates. Around these tubes, metallic fins are attached. These fins significantly increase the contact surface between the tube and the ambient air. It is this extended surface that allows for more efficient heat transfer. Air, pushed by fans, circulates through these fins, carrying away heat from the fluid contained within the tubes. The design of the fins, their spacing, and their material are key factors in optimising this transfer. It is important to note that the efficiency of these exchangers can be affected by fouling of the fins, which highlights the importance of regular maintenance to maintain good performance.

Direct Cooling by Outdoor Air

A particularly interesting approach in the field of industrial cooling is direct cooling by outdoor air. In this system, atmospheric air is used directly to cool the process fluid, without passing through an intermediate fluid such as water. This eliminates steps and components, thereby reducing energy losses and maintenance needs. Electronic expansion valves play an important role here, as they allow for precise management of condensation pressure, even when outdoor temperature conditions vary. This method is often preferred when the refrigeration unit can be installed outdoors, for example on a roof. However, attention must be paid to the quality of the ambient air, as impurities could affect the system. If the refrigeration machine is located in an enclosed space such as a basement, adequate ventilation must be ensured to prevent heated air from recirculating, which would reduce cooling efficiency and could lead to compressor overheating. It is therefore essential to carefully study the ventilation options for cooling to ensure optimal operation.

Water Spray Systems

To further increase the cooling capacity of an air-cooled heat exchanger, one technique involves using water spray. The principle is simple: when water vaporises, it absorbs heat. By spraying fine water droplets onto the exchanger fins or into the airflow, this evaporation phenomenon is exploited to further lower the temperature. There are two main variants: either the sprayed water is that which circulates in the condensation circuit itself, or it is an independent water source. This method can significantly improve performance, especially in hot weather. However, it involves water consumption and may require treatments to prevent bacterial growth or limescale deposition. The choice between these different technologies will depend on the specific application conditions and environmental constraints.

Selection Criteria and Sizing

Choosing the right air-cooled heat exchanger is a bit like choosing a car: you need to consider the use, the necessary power, but also consumption and noise. You wouldn’t want a Formula 1 car to go and buy bread, nor a utility vehicle to cross the desert, would you? It’s the same here. You need to carefully analyse your needs to find the equipment that will best suit your installation.

Parameters Influencing the Choice of an Air-Cooled Heat Exchanger

Several elements will guide your decision. Firstly, the thermal load to be dissipated. This is the amount of heat your system needs to remove. Secondly, the desired water temperature at the outlet of the air-cooled heat exchanger. This temperature will depend on your industrial process or your air conditioning system. The ambient air temperature is also a key factor; the warmer the air, the less efficient the air-cooled heat exchanger will be. You also need to consider the available space for installation, as these pieces of equipment can be quite bulky. Don’t forget noise constraints, especially if your installation is near residential areas. Finally, the quality of the water used can influence the choice of materials to prevent corrosion, for example, by preferring coated steel over galvanised steel.

Comparison of Cooling Technologies

There are several ways to cool water. Finned air-cooled heat exchangers are common, where air passes through fins to cool a fluid. Direct cooling by outdoor air uses ambient air without an intermediary, which can be simple but less controllable. Water spray systems, often called cooling towers, use water evaporation to lower the temperature. Each technology has its advantages and disadvantages in terms of efficiency, water consumption, and complexity.

Acoustic and Environmental Considerations

Noise generated by fans can be a major concern. It is possible to choose quieter fans, such as those with forward-curved blades, even if they are slightly less energy-efficient. The addition of silencers can also help. Environmentally, water consumption must be considered, especially in regions where it is scarce. Water spray systems can produce plumes of water vapour, which can be a concern in certain contexts. It is important to check the structural strength of the equipment, as a 1000 kW tower can weigh up to 12 tonnes. Considering the installation of anti-vibration mounts to absorb vibrations is also a good idea. For a quality installation, it is recommended to choose high-quality components, such as stainless steel water pumps and long-life bearings, to minimise maintenance needs. You can consult information on similar projects to better understand these aspects, such as those related to the CNDP Bugey.

It is often advisable to use selection software, such as EvapSelect, which can help estimate consumption based on your specific data and local weather conditions. These tools can present scenarios based on different priorities, such as energy savings or maximum performance.

Advantages and Disadvantages of Air-Cooled Heat Exchangers

Energy Efficiency and Water Consumption

Air-cooled heat exchangers, particularly those that use air as the sole heat transfer fluid, offer a clear advantage in terms of water consumption. Unlike systems that rely on cooling towers, their operation does not require water make-up, which is a major asset in regions where water is a limited resource. However, this absence of water consumption has a trade-off: energy efficiency. Air, being a less efficient fluid than water for heat transfer, requires larger heat exchangers and more powerful fans. This results in higher electricity consumption, especially when ambient temperatures are high. Therefore, the cost of water versus electricity must be carefully weighed to make the right choice. For example, a reversible air-to-water heat pump can offer a good compromise for domestic cooling.

System Reliability and Maintenance

In general, dry air-cooled heat exchangers are renowned for their reliability and low maintenance requirements. The absence of water circulation eliminates the risks of freezing in winter, as well as problems related to corrosion or scaling of heat exchange surfaces. Maintenance is often limited to periodic cleaning of the fins to ensure good airflow and maintain performance. Systems that incorporate water spray to improve heat transfer add a complexity that may require more extensive maintenance, particularly for the water distribution system and its treatment to prevent bacterial growth.

Installation Flexibility and Spatial Constraints

One of the great advantages of air-cooled heat exchangers is their flexibility. They can be installed outdoors, on rooftops or at ground level, which frees up valuable space inside industrial buildings. Their modular design also allows them to adapt to different required cooling capacities. However, certain constraints must be considered. The volume and weight of the units can be significant, requiring suitable support structures. Furthermore, the noise level generated by the fans can be a concern, especially in residential areas or when units are close to offices. It is therefore often necessary to provide soundproofing devices or choose locations that minimise noise pollution.

Innovations and Advanced Solutions

Eco-Energy Hybrid Air-Cooled Heat Exchangers

Are you looking to optimise the water and energy consumption of your cooling systems? Recent advances in air-cooled heat exchanger design open up new perspectives. Take, for example, hybrid models, which intelligently combine dry operation and evaporative cooling. These units are designed to switch to evaporative mode only when conditions warrant it, i.e., at higher dry bulb temperatures. This allows for significant water savings while maintaining adequate cooling performance. The integration of technologies such as the ARID Fin-Pak™ coil in the upper section, which operates dry, followed by an Ellipti-fin® coil capable of working in dry or evaporative mode, clearly illustrates this approach. These systems are often controlled by programmable logic controllers, such as the Sage2 system, which allow operating priorities to be defined to maximise water or energy savings according to your site’s specific needs. This is a concrete way to make your installations more environmentally friendly and more economical in the long term. To learn more about cooling technologies, you could consult resources on passive cooling systems.

Plume Abatement Technologies

Visible vapour plumes above air-cooled heat exchangers can sometimes pose problems, particularly in areas where icing is a concern or for aesthetic reasons. Technological solutions have been developed to minimise, or even eliminate, these plumes. One approach involves using heat recovery systems or devices that slightly reheat the air before it encounters water vapour, in order to maintain the temperature above the dew point. Another method involves optimising the design of drift eliminators to reduce the amount of water carried into the airflow, which naturally decreases plume formation. These innovations aim to improve the acceptance of air-cooled heat exchangers in various environments, especially in urban or sensitive contexts.

Intelligent Control Systems for Optimisation

The operational efficiency of air-cooled heat exchangers can be greatly improved through the implementation of intelligent control systems. These systems use advanced algorithms and sensors to monitor operating conditions in real-time, such as ambient temperature, humidity, and the thermal load of the process to be cooled. They automatically adjust the parameters of the air-cooled heat exchanger, such as fan speed or water flow rate, to maintain optimal performance while minimising energy and water consumption. The integration of these control systems allows for proactive management, anticipating needs and reacting to variations to ensure maximum energy efficiency. This results in reduced operating costs and a lower environmental footprint for your industrial installations.

In Brief: The Air-Cooled Heat Exchanger, a Pillar of Industrial Cooling

At the end of this exploration, it is clear that the air-cooled heat exchanger, in its various forms, plays an indispensable role in the modern industrial landscape. Whether for the thermal management of production processes or for the proper functioning of equipment, its ability to transfer heat to the ambient air is a proven solution. You have seen the principles governing its operation, the different technologies available, and the criteria to consider for an informed choice. By taking into account the specificities of each installation, available space, and environmental constraints, you will be able to select and implement the most suitable solution. The optimisation of these systems contributes not only to operational efficiency but also to better resource management.

Frequently Asked Questions

What is an air-cooled heat exchanger and what is it used for?

An air-cooled heat exchanger is a device that uses outdoor air to cool things, such as water or other liquids. It is often used in factories and large installations to dissipate heat.

How does an air-cooled heat exchanger work?

Outdoor air is pushed through special fins or tubes. These fins or tubes contain the hot liquid that needs to be cooled. The air takes the heat from the liquid and moves away, leaving the liquid cooler.

What are the different types of air-cooled heat exchangers?

There are several types. Some just use air for cooling (these are called ‘dry’), others add a fine mist of water to aid cooling. There are also those that use water to cool another liquid, and then this water is itself cooled by the air.

How do you choose the right air-cooled heat exchanger?

The choice depends on several things: the amount of heat to be dissipated, the outdoor air temperature, available space, and the noise the unit makes. You also need to consider water and energy consumption.

What are the advantages of air-cooled heat exchangers that do not use water (dry)?

‘Dry’ air-cooled heat exchangers do not use water, which is good for saving this resource and avoiding problems with freezing or legionellosis. However, they can be less efficient in very hot weather than those that use a little water.

Are there innovations to improve air-cooled heat exchangers?

Yes, research is being conducted to make them even more efficient and less energy-intensive. Work is underway on hybrid systems that combine several cooling methods and on intelligent controls to optimise their operation.