Air Handling Units

Air Handling Units

When it comes to heating, ventilation, and air conditioning (HVAC) systems, an Air Handling Unit (AHU) is a critical component that plays a pivotal role. They are essential for maintaining indoor air quality, regulating temperature, and ensuring comfort in various environments, from residential homes to commercial buildings and industrial facilities. But what exactly is an Air Handling Unit, and how does it work?

What is an Air Handling Unit (AHU)?

An Air Handling Unit (AHU) is a mechanical device designed to treat air and deliver it through duct systems. Its primary function is to take in outside air, filter it, heat or cool it, and then distribute it to various parts of a building. AHUs are in numerous applications, including residential homes, office buildings, hospitals, manufacturing facilities, and more. Their importance lies in their ability to create a comfortable and healthy indoor environment in an energy-efficient manner.

The Functions and Components of an Air Handling Unit

An AHU consists of several key components, each with a specific role in the conditioning and circulation of air. They are outlined below.

• Fan: A fan is designed to increase the pressure for air movement to overcome all pressure drops, which occur when air streams through components and the duct system, leading to flow resistance. Today, direct-driven, free-running backward-curved fans are applied in most AHUs. Belt-driven fans are less widespread due to their limited energy efficiency. The measure of the effectiveness of a fan is the fan efficiency.

• Filter Section: Air filters ensure healthy indoor air by removing harmful fine dust, pollen, bacteria, yeast, mould, and other organic and inorganic materials. Air filters also clean the air handling unit to ensure hygienic and efficient operation. For some applications, additional filters can be useful, such as filters to remove odours, grease, or corrosion molecules. The clean/initial pressure drop and the pressure increase due to dust load in the filter both have a significant influence on the overall efficiency of the filters.

• Heating and Cooling Elements: In AHUs, heating and cooling coils are used to achieve the desired temperature. Heating elements may include electric coils or hot water pipes, while cooling elements typically consist of refrigeration coils. These components are responsible for adjusting the air temperature as needed.

• Humidification and Dehumidification Systems: Humidifiers and dehumidifiers serve to maintain the desired humidity levels. Two main ways to humidify the air are with steam or liquid water.

• Mixing Chamber: The mixing chamber allows air to change from the extract unit to the supply unit side when an AHU unit contains recirculated air. Mixing helps balance air streams that adhere to the outdoor air demand while minimising thermal treatment requirements by closely reaching the requested supply air conditions.

• Energy recovery components: Thermal treatment is necessary when outdoor air temperature deviates from the required supply air conditions. An Energy Recovery System (ERS, also commonly known as a Heat Recovery System or HRS) is applied to minimize thermal air treatment's energy consumption. An ERS transfers thermal energy from the extracted air to the outdoor air. There are three, common types of heat recovery systems in air handling units. First, there are regenerative systems, which are typically rotary heat exchangers, where a wheel rotates through both air streams and, with this, transports the capacity. The second system is a recuperative system, which guides the outdoor and extract air to very small ducts and lets both airstreams exchange the capacity with the help of the surfaces that the airstreams are passing. The third system, run-around coil systems, keeps both air streams completely separated from each other so that cross-contamination is impossible.

• Silencers: Noise is generated by duct or unit components, with the fan being the main noise emitter. This makes installing sound-reducing components close to the source advantageous. Since the fan is the main source of noise in the AHU, silencers are often placed directly up or downstream of the fan. Considering energy aspects, the pressure drop of silencers should be as low as possible.

• Controls and Sensors: Modern AHUs have advanced control systems and sensors that monitor temperature, humidity, air quality, and energy consumption. These systems allow for precise regulation and optimisation of HVAC operations.

Understanding Air Handling Unit Function

Air handling units serve several essential functions within an HVAC system:

• Air Filtration: AHUs filter out impurities from the air, improving indoor air quality by removing dust, pollen, and other allergens.

• Temperature Control: AHUs help maintain a comfortable indoor environment year-round by heating or cooling the air as necessary.

• Humidity Control: In some applications, AHUs are equipped with humidification or dehumidification systems to control moisture levels, which are vital for occupant comfort and equipment protection.

• Ventilation: AHUs supply fresh air from the outdoors while exhausting stale air, ensuring a healthy and balanced air exchange.

• Energy Efficiency: Modern AHUs are designed with energy efficiency in mind. They incorporate variable speed drives, heat recovery systems, and demand-controlled ventilation to reduce energy consumption and operating costs.

How to categorise an Air Handling Unit

Various methods exist to differentiate between different AHUs. An informative article from the Eurovent Association outlines the most commonly employed ones, outlined below.

• Based on the direction of air movement: Unidirectional or bidirectional: An AHU can be unidirectional or bidirectional. As the names suggest, a unidirectional ventilation unit is designed to move air in a single direction and typically consists of a fan, filter, and heating or cooling element. In contrast, a bidirectional unit can supply and extract air from the interior of a building.

• By their structural design: Compact or modular: A compact AHU is installed within a single casing, typically containing ventilation components like fans, filters, and an energy recovery system. This type of AHU requires less installation space but may have limitations in performance, often necessitating the addition of components such as heating or cooling coils as duct accessories. On the other hand, modular air handling units offer greater flexibility in terms of structure, module arrangement, dimensions, and specialised functions, such as humidification and dehumidification. Each component is optimised to function as part of the ventilation system and can be customised using AHU selection software.

• Based on their intended application area: Residential or non-residential, general or industrial ventilation Residential AHUs are designed for use in apartments, single-family homes, or small multi-family houses, with airflow typically limited to 1,000 m3/h (cubic meters per hour). These units are generally less complex than non-residential AHUs and are often highly customised to suit specific building requirements. Both residential and non-residential AHUs must adhere to different standards established by EU Ecodesign regulations.

• Categorised by their location: Outdoor or indoor AHUs are commonly situated outdoors or in auxiliary premises. Outdoor units require robust, watertight construction and corrosion-resistant coatings to protect against contamination or degradation. Smaller AHUs are often installed above ceilings.

What makes a high-quality, efficient Air Handling Unit?

Eurovent 6/18 - 2022 Quality criteria for Air Handling Units offer a great reference for determining high-quality, efficient AHUs.

This recommendation aims to provide a comprehensive overview of features that determine highly efficient operation and correct service of air handling units, which serve as a measure of the AHU quality. The recommendation sets minimum quality criteria related to the design, materials, and construction of the unit's casing and components, as well as its documentation and delivery. The recommended requirements for energy efficiency and control systems are also comprehensively discussed. Detailed recommendations based on the expertise and many years of experience of Eurovent members indicate what distinguishes an excellent air-handling unit.

Read our article "How to choose the best AHU for your project."

Application areas for Air Handling Units

Air Handling Units (AHUs) play a pivotal role in maintaining optimal indoor air quality across a spectrum of environments, essential for ensuring comfort, safety, and the longevity of valuable equipment. Here’s a closer look at some key sectors where AHUs are indispensable:

• Residential: From single-family homes to entire apartment blocks, whether new or retrofit projects, AHUs provide fresh, clean, well-circulated air in large volumes indoors, allowing occupants to enjoy superior indoor air quality (IAQ) no matter where they live in the world.

• Commercial Buildings and Public Venues: AHUs are vital for effective ventilation in commercial spaces, office buildings, assembly halls, and entertainment venues. The primary goal is to mitigate CO2 levels emitted by occupants, ensuring a healthy indoor environment. AHUs calculate required airflow based on the number of people present, contributing significantly to overall indoor air quality.

• Airports and Exhibition Centers: Airports and expansive exhibition centres necessitate AHUs with robust airflow capabilities to manage substantial internal heat loads. These units efficiently regulate indoor temperatures, supporting optimal comfort for visitors and staff alike.

• Museums and Art Galleries: In the realm of cultural preservation, AHUs are indispensable for facilitating stringent control over temperature and humidity levels. They maintain precise room climates to protect exhibits from premature ageing and ensure artefacts remain pristine.

• Data Centres: Data centres rely on AHUs to dissipate internal heat loads efficiently. By blending a small volume of fresh air with recirculated air, these units effectively manage cooling. Innovative systems like regenerative cooling energy and advanced energy recovery mechanisms further enhance efficiency in this critical environment.

• Swimming Pools: Ventilation is vital in swimming pool facilities to regulate air humidity, optimizing comfort while safeguarding building structures against moisture-related damage.

• Marine Applications: AHUs deliver fresh air to cabins and entertainment areas on large cruise ships. Beyond comfort, these systems protect indoor equipment from external humidity and salty sea air, demonstrating resilience in harsh marine conditions.

• Healthcare Facilities: In medical settings, AHUs are pivotal for maintaining low particle concentrations and consistent temperature and humidity levels. These units adhere to stringent hygienic standards critical for patient care and equipment integrity.

• Pharmaceutical and Chemical Plants: Pharmaceutical and chemical manufacturing plants operate under strict cleanroom conditions. AHUs support these environments by controlling air quality and maintaining sterile conditions necessary for production.

• Oil and Gas: In the oil and gas sector, AHUs often require specialised certifications like ATEX to manage potentially explosive atmospheres. These units contribute to safety and operational efficiency in challenging industrial settings.

By catering to diverse commercial needs, modern AHUs demonstrate versatility, efficiency, and reliability across various applications, shaping healthier, more comfortable, and more productive indoor environments.