All Air Systems: Operation and Sizing

All air systems are one of the most advanced and widespread solutions for managing air quality within buildings, both in residential and commercial, industrial, and healthcare settings. The need to ensure a healthy, comfortable, and energy-efficient environment has driven the evolution of ventilation systems that not only ensure a continuous exchange of air but also provide effective air treatment based on climatic conditions and the specific needs of the environments.

What are all air systems?

All air systems are complex systems designed for the treatment and distribution of air in enclosed spaces. Their main function is to ensure proper air quality and adequate comfort in the environment, intervening on various parameters such as temperature, humidity, pollutant concentration, and uniform air distribution. These systems differ from hybrid systems, which integrate additional technologies such as heating or cooling with radiant panels or hydronic systems, to simultaneously treat both air and the environment. All air systems primarily focus on the treatment of incoming and outgoing air, employing controlled mechanical ventilation (CMV) techniques to manage air optimally. Such systems are used in residential, commercial, industrial, and public structures, ensuring constant air exchange and regulation of temperature and humidity, with the goal of maintaining ideal environmental conditions in any season.

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Why are all air systems useful?

The fundamental principle behind all air systems is to ensure adequate air exchange. In enclosed spaces, air quality can deteriorate rapidly due to various factors, such as the production of carbon dioxide (CO_₂), moisture accumulation, the presence of fine dust, allergens, and chemical contaminants. Controlled mechanical ventilation (CMV) is the main method to counteract these phenomena, as it allows for the continuous and regulated introduction of fresh air from the outside while simultaneously expelling stale air. In this context, all air systems play a crucial role in ensuring the healthiness of the environment, balancing the flow of external and internal air, maintaining optimal oxygen levels, and reducing the concentration of pollutants.

In an all air system, the external air is first treated by an air treatment unit (ATU), which purifies it and adapts it to the needs of the environment. This treatment may include filtration operations to remove dust, pollen, bacteria, viruses, and other pollutants, as well as heating or cooling operations to regulate the temperature, ensuring that the air entering the environments is comfortable. In some cases, humidification or dehumidification may also be necessary to keep humidity levels under control, avoiding environments that are too dry or too humid, which could promote mold growth or cause discomfort. In very large environments or in high-energy-efficiency buildings, heat recovery technologies are employed to optimize energy efficiency. These technologies allow for the use of energy from outgoing air to pre-heat or pre-cool incoming air, thus reducing energy consumption for heating and cooling.

The heart of an all air system is therefore the air treatment system, which ensures the quality of the incoming air, as well as the airflow, which must be distributed evenly without creating stagnant zones. Mechanical ventilation must be designed to ensure homogeneous air distribution within the environments, avoiding areas that are too hot or cold, or with poor circulation. Additionally, the dynamic pressure within the distribution ducts must be well balanced to avoid problems of overpressure or underpressure that could compromise the efficiency of the system.

How is an all air system made?

An all air system consists of several elements, each serving a specific function. The main component of an all air ventilation system is the air treatment unit (ATU). This unit is responsible for treating the incoming air from the outside before it is distributed to the environments through ducts and vents. The main operations performed by the ATU include air filtration to eliminate impurities and solid particles present in the external environment, heating or cooling the air, which allows it to be adjusted to the desired temperature, and humidification or dehumidification, which helps regulate the relative humidity of the air to avoid discomfort and the proliferation of bacteria and fungi.

The treated air is then distributed to the environments through a network of ducts. These ducts are designed to minimize pressure losses, i.e., the resistance to the passage of air, and must be adequately sized to ensure a homogeneous and uniform flow. The materials used for the ducts vary, but the most common are galvanized steel and durable plastic materials. Within these ducts, air can be distributed through vents and grilles. Distribution vents can be mounted on the ceiling, walls, or floor, depending on the specific project needs. Extraction grilles, on the other hand, serve to remove stale air from the environment, creating a balance between incoming and outgoing air without compromising internal pressure.

In an all air system, the balance between supplied and extracted air is essential for proper operation. To this end, the extraction system is just as crucial as the distribution system. Excess or contaminated air is evacuated by extraction fans, which can be installed directly in the environments or in central ducts, depending on the system configuration. These systems are designed to ensure a constant flow, preventing stagnant air from compromising the quality of the environment.

Moreover, modern all air systems are often equipped with sophisticated control and automation systems that allow for real-time monitoring and adjustment of parameters such as temperature, humidity, air quality, and flow velocity. CO_₂, fine dust, and VOC (volatile organic compounds) sensors enable the optimization of energy consumption and the maintenance of environmental parameters within ideal limits. Control units, in turn, manage the entire system, autonomously regulating fans, heating, cooling, and dehumidification based on the detected environmental conditions.

All air system: operation

The operation unfolds through a series of phases that range from the intake of external air to its treatment, distribution, and finally expulsion, to maintain constant air quality within the building. Let’s take a closer look at the various phases of an all air system’s operation.

Intake of external air

The first step in an all air system is the intake of external air. This process occurs through an external opening, which may be positioned on a wall or on the roof of the building, depending on the design. The air coming from outside may contain a variety of substances, such as dust, atmospheric pollutants, pollen, and other impurities. This is why it is necessary to treat it before introducing it into the internal environments.

The external air is drawn in through intake ducts, which channel it towards the air treatment unit (ATU) where subsequent treatments will take place. The amount of air drawn in is calculated based on the number of people occupying the space, the size of the area, and the air quality requirements.

Air filtration

Once the air has been drawn in, the next step is filtration. The external air contains particles of dust, smog, pollen, and microorganisms that can compromise the quality of the internal environment. The filters inside the ATU are designed to capture these particles and purify the air before it is introduced into the environments. The filters are generally made of materials such as synthetic fiber, activated carbon (to remove odors and volatile organic compounds), or HEPA filters (high-efficiency), which also capture very fine particles.

The quality of filtration depends on the type of filter used and their efficiency, which is measured through the filtration class. For example, filters of class F7 or F9 (in the European context) are used to retain fine particles, while HEPA class filters can also remove bacteria and viruses.

Thermal treatment of air

Once filtered, the air may undergo thermal treatment, i.e., heating or cooling, depending on climatic and seasonal needs. This phase is crucial to ensure that the air entering the internal environments is always comfortable and maintains a stable temperature.

• heating: if the external air is cold, the air is heated before being introduced into the building. This is achieved through heating batteries within the ATU, which can be powered by various types of energy, such as gas, electricity, or centralized heating systems. The air is then brought to the set temperature, which depends on the building’s needs (for example, between 18°C and 22°C for a residential environment);
• cooling: if the external air is warm and the building requires cooling, the air is treated through a heat exchanger or a cooling battery, which lowers the air temperature through refrigeration. In some advanced systems, air cooling can be achieved with an evaporative cooling system, where the air is cooled by passing through a moist medium that absorbs heat, releasing cooler air.

Humidification or dehumidification

Another critical phase for the well-being of occupants is humidity control. The relative humidity within the environments must be maintained within ideal limits to avoid problems related to excess or lack of humidity.

• humidification: in environments where the air is too dry (for example, during winter or in environments with centralized heating), the air can be humidified through devices that spray water vapor into the air. In some all air systems, there are adiabatic humidifiers that increase humidity simply through an evaporation process;
• dehumidification: in environments with excess humidity (typically in summer or in enclosed spaces like kitchens and bathrooms), the air can be dehumidified. This process occurs through devices such as dehumidification batteries or heat exchangers that cool the air to condense moisture into a collection tank.

Distribution of treated air

After being filtered, thermally treated, and, if necessary, humidified or dehumidified, the air is introduced into the environments through a distribution duct system. These ducts, which can be made of materials such as galvanized steel or plastic, carry the treated air to different areas of the building.

Air distribution occurs through diffusion vents, which are devices positioned on the ceiling, walls, or floor, depending on the design. The vents are designed to ensure uniform air distribution and prevent the formation of stagnant zones or uncomfortable drafts. Depending on the type of system and specific needs, the vents may be adjustable, allowing users to direct the airflow in desired directions.

Extraction of stale air

Alongside the intake of fresh air, a system for extracting stale air is necessary, which removes exhausted air and replaces it with new air. Stale air is that which has lost some oxygen and has accumulated pollutants such as carbon dioxide (CO₂), vapors, odors, dust, and humidity. Extraction generally occurs through extraction fans, which can be placed in strategic points such as bathrooms, kitchens, or common areas. These fans draw in excess air and channel it outside the building through an exhaust duct system.

Control and automatic regulation

Modern all air systems are equipped with sophisticated control and automation systems that constantly monitor air quality, temperature, humidity, and other environmental parameters. Using sensors (e.g., CO₂, PM10, VOC, and humidity sensors), the system can automatically adjust the flow of treated air, heating, cooling, humidifying, or dehumidifying the air based on the detected conditions. Control units can be programmed to turn fans on or off, adjust airflow, or modify thermal treatment settings according to needs.

Types of all air systems

There are different types of all air systems, each with specific characteristics:

• natural ventilation systems: these systems exploit differences in atmospheric pressure and internal and external temperature to facilitate airflow. They may include controlled openings (such as windows and vents) and ducts designed to maximize airflow efficiency. However, their effectiveness can vary significantly based on external climatic conditions;
• controlled mechanical ventilation (CMV) systems: these systems are equipped with fans that can be adjusted to maintain a constant airflow. They can be divided into: simple flow CMV: Draws air from outside and introduces it into the internal environments without heat recovery. These systems are simpler to install but less energy-efficient;
• double flow CMV: includes a heat exchanger that allows for heat recovery from the expelled air, improving energy efficiency. This type of system is particularly advantageous in cold climates, as it reduces energy needs for heating.

All air systems and primary air

Air systems are distinguished into:

• all air systems: the air circuit heats or cools the environments;
• primary air systems: the air circuit serves the purpose of ventilation and pre-heating or pre-cooling.

While the all air system represents a complete system for managing ventilation and internal comfort, primary air is a specific element that provides fresh air from outside to the system. Both are essential for ensuring a healthy and comfortable environment, but they operate at different levels in the context of mechanical ventilation.

A system with primary air is a type of ventilation system that uses outside air as the base fluid for environmental treatment, but with an approach that combines air with other systems to ensure thermal comfort. In the configuration of the system with primary air, the air is introduced into the environment and is used to partially meet thermal needs, but it is not the only means of heating or cooling. Typically, in this type of system, primary air (external air) is primarily used for ventilation and ensuring air exchange, but the actual thermal treatment (heating and cooling) is carried out through other systems such as radiant panels, convectors, or heating and cooling batteries.

A typical example of primary air is the incoming air treated by a ventilation system, which can be heated or cooled through an air conditioning system, but does not constitute the only source of thermal comfort. In this case, the ventilation system works in synergy with other systems that contribute to regulating the ambient temperature, such as a radiant panel cooling system or a heat exchange battery.

Main differences

The main differences between the two systems can be identified in several aspects, including:

Operating principle

• all air system: air is the only fluid treated and distributed to ensure thermal comfort and air quality, with heating, cooling, and dehumidification occurring directly through the treatment of the air itself;
• system with primary air: external air is primarily used for air renewal and ventilation, while thermal treatment occurs through other systems such as cooling and heating batteries, radiant panels, or convectors.

Thermal treatment

• all air system: heating or cooling occurs directly in the air treatment unit (ATU), without the need for another air conditioning system;
• system with primary air: primary air is treated for air renewal, but is not solely responsible for thermal treatment, which occurs through a separate system.

Auxiliary systems

• all air system: does not require auxiliary systems for air conditioning, as air is treated and distributed directly;
• system with primary air: uses other devices (such as heaters, radiators, radiant panels) for thermal treatment, while primary air is limited to ensuring air renewal.

Applications

• all air system: suitable for buildings where constant control of air quality and temperature is necessary, such as residences, offices, schools, and hospitals;
• system with primary air: often used in commercial or industrial buildings where primary air primarily serves for ventilation, while other thermal systems handle heating or cooling.

Sizing of all air systems

The sizing of an all air system is a complex technical process that requires careful evaluation of various factors to ensure a proper balance between ventilation needs, air treatment (heating, cooling, humidification, and dehumidification), and energy efficiency. It includes the design of all the main components of the system: the air treatment unit (ATU), the air distribution network, fans, and extraction systems, as well as energy management.

Calculation of air requirements

The first fundamental step in sizing an all air system is the calculation of air requirements for each environment. This value depends on various variables, such as the number of people present, the activities performed, the size of the environments, and the required air quality.

Calculation of airflow for the Air Treatment Unit (ATU)

The airflow calculated in the previous phase determines the minimum capacities of the ATU. It is necessary to calculate the volume of air to be treated for the entire building or for individual environments. This value depends on the airflow per person and the number of people or the size of the environments. If an office of 100 m² has a required airflow of 10 m³/h per person and hosts 20 people, the total airflow needed for the environment will be:

Total airflow = 10 m³/h per person × 20 people = 200 m³/h

This value must be considered to correctly size the ATU.

Sizing of ducts and air distribution network

The design of the duct network and diffusion vents is an important step to ensure uniform air distribution in all environments. To correctly size the network, it is necessary to consider various factors: air velocity, duct diameter, air distribution.

Selection and sizing of fans

Fans are essential to ensure the necessary airflow through the system. The selection of fans depends on the required volumetric flow rate, pressure losses in the system, and static pressure. The air treatment unit, in particular, must be equipped with fans capable of maintaining the desired flow rate without compromising energy efficiency. The pressure loss in the ducts and filters must be estimated and included in the calculations to determine the power needed for the fans. The static pressure must be sufficient to overcome the pressure losses of the system (ducts, filters, grilles, etc.).

Calculation of thermal power for heating and cooling

It is necessary to calculate the thermal power required to heat or cool the incoming air, which depends on the temperature difference between the external and internal air. This calculation is essential to correctly size the heating or cooling batteries in the ATU. The required thermal power can be calculated using the formula:

P = V × C × ΔT

where:

• P is the power (in kW);
• V is the volumetric flow rate (in m³/s);
• C is the thermal capacity of air (approximately 0.33 kJ/m³ °C);
• ΔT is the temperature difference (in °C).