03/06/2025 | Altro
PhD Cristina Becchio, PhD Carola Lingua - Politecnico di Torino
Ing. Alberto Montibelli - Giacomini S.p.A.
HVAC (Heating, Ventilation, and Air-Conditioning) systems play a fundamental role in the management and regulation of the building indoor microclimate, acting on temperature, humidity, and air quality [1]. Therefore, the efficient management of these systems is essential not only to ensure optimal environmental comfort conditions for occupants, but also to maximize the overall energy efficiency of the building. One very common and often underestimated obstacle to the correct functioning of HVAC systems is the presence of air in the heat-transfer fluid. Indeed, air within the system can compromise both its operational efficiency and the system lifespan, giving rise to numerous problems, including:
- generating noise inside the pipes and on valve bodies, resulting in acoustic disturbance inside the building;
- reducing the flow rate of the heat-transfer fluid, limiting heat transfer to the various terminal devices and compromising thermal comfort inside the building;
- promoting corrosion phenomena of the various system components, damaging surfaces and materials. Corrosive products can generate impurities that deposit in the heat exchangers and inside the heating elements, reducing the effectiveness of heat exchange. In particular, corrosion induced by the presence of oxygen represents a significant critical issue, as it can damage metallic materials, reducing the efficiency of the system and causing leaks over time;
- promoting the phenomenon of cavitation, i.e., the formation of microbubbles of vapor in the heat-transfer fluid. This phenomenon occurs when the pressure drops to the vapor pressure, generating microbubbles, or cavities, containing vapor. These cavities only persist until they reach a quiescent zone; at this point, the vapor pressure is no longer sufficient to counteract the hydrostatic pressure, which is why the bubble implodes, causing localized damage and compromising the integrity of the surrounding components.
In this context, the deaeration process assumes a fundamental role in ensuring efficient and reliable operation over time. Deaeration consists of removing air and other gases present within the heat transfer fluid, typically water or a water-glycol mixture. It is often thought that paying attention during the system filling is sufficient to solve the problem. In reality, air within the system can have various origins that are not always controllable. Automatic air-vent valves are certainly useful, especially in points where the fluid is at rest, that is, where gases naturally collect due to physical laws, but they are not sufficient to solve the problem. Indeed, the integration of specific devices, deaerators, which operate continuously to eliminate air bubbles contained in the hydraulic circuits of the systems, is fundamental. Thanks to the characteristics of these components, it is possible to maintain and improve the energy efficiency of the system, reduce maintenance interventions, extend the useful life of the system components, and reduce the overall life-cycle costs of the system itself.
Several studies in the literature have highlighted the crucial role of air removal in hydronic circuits for the correct functioning of systems. In particular, the authors in [2] show that the use of deaerators can optimize heat transfer and contribute to the reduction of energy consumption. The results show that the reduction of dissolved air in the fluid leads to an improvement in its thermal conductivity, with positive effects on the overall efficiency of the system. Supporting this study, the authors in [3] analyzed and experimentally evaluated the effectiveness of deaeration systems, demonstrating that adequate air management makes it possible to maintain high levels of energy performance and extend the useful life of the system components. The continuous removal of microbubbles and residual air is also essential to prevent mechanical malfunctions and limit maintenance interventions. These scientific studies therefore confirm that deaeration represents a necessary technical measure to ensure the sustainability, reliability, and longevity of HVAC systems.
As introduced in the previous paragraph, adequate and effective deaeration can be achieved through the adoption and installation of specific devices. Indeed, the integration of these solutions into hydronic circuits allows for a significant reduction in air presence, improving the overall performance of the system. In the HVAC systems, the main devices used for air removal are:
Although deaeration is not subject to a dedicated technical Standard, its role is recognized within various regulations that emphasize its importance in the context of design, safety, and energy efficiency. In particular, the main Italian reference Standards are:
At the international level, the main reference is:
The presence of air within HVAC systems represents a critical element that can negatively impact both the operational efficiency and the longevity of hydronic systems. The main adverse effect is the reduction in heat exchange efficiency, which makes the system less responsive and compromises its effectiveness in heat distribution. In this case, air removal significantly improves heat transfer, promoting a uniform distribution of temperatures and reducing the time needed to reach optimal thermal conditions, consequently enhancing indoor environmental comfort.
Deaeration has a significant impact on safeguarding mechanical components. The presence of air bubbles can, indeed, cause cavitation phenomena, leading to high repair or replacement costs. Furthermore, air – containing oxygen – promotes corrosion reactions that can damage metallic surfaces within the circuit. Adequate deaeration contributes to significantly reducing the risk of corrosion, extending the useful life of the system and maintaining its performance over time.
The fluid flow also clearly benefits from the deaeration process. Indeed, in the absence of air, circulation occurs more smoothly and regularly, without interruptions or unwanted pressure variations. This helps prevent phenomena such as overpressure or sudden pressure drops, which can cause malfunctions or damage to the system.
Eliminating air from the system also means protecting the longevity of its components. Indeed, by minimizing phenomena such as corrosion and cavitation, the integrity of pumps, pipes, valves, and heat exchangers is preserved, improving the reliability and durability of the entire system.
Finally, from an economic point of view, a correctly deaerated system operates in optimal conditions, with lower operating costs and reduced maintenance needs.
