An Innovative Approach to Indoor Climate Control

By Mert Sesen | Deputy General Manager & Marketing Manager for FabricAir?Turkey?A.?

You might have felt a slight chill from a breeze while sitting in a warm room on a winter day. This sensation illustrates that thermal comfort isn't solely about temperature; it also highlights the delicate balance required for effective indoor air distribution.

Thermal comfort involves various factors beyond air temperature, such as air velocity, radiant temperature, humidity, clothing, and metabolism. Ideal thermal comfort ensures that people feel "just right" indoors, promoting well-being and health. When this balance is disrupted, it can lead to long-term health issues.

Key Factors Affecting Thermal Comfort

According to ASHRAE standards, thermal comfort is defined as the individual’s satisfaction with the surrounding conditions. This satisfaction is achieved through six main factors: air temperature, radiant temperature, humidity, air velocity, clothing insulation, and metabolic rate.

• Air Temperature: This is one of the fundamental components of thermal comfort. For individuals working while seated, the ideal temperature is 23°C ±1°C/73.4°F ±1.8°F, whereas for those in motion, it should range between 21°C/69.8°F and 24°C/75.2°F. Deviations from this range challenge the body’s thermal regulation mechanisms.
• Radiant Temperature: The heat emitted from surfaces, particularly large glass areas or exterior walls with high heat loss, affects comfort. A difference of more than 3°C/5.4°F between radiant and air temperatures is discouraged, as it can disrupt the temperature balance and alter perceived warmth. For example, in a glass-walled office facing outside, even if the room is set to 22°C/71.6°F, a window surface at 17°C/62.6°F may cause a sensation of chill.
• Humidity Level: High humidity makes it harder for the body to cool itself through perspiration, while low humidity can lead to skin dryness. The optimal humidity range for thermal comfort is between 30% and 60%. Humidity levels over 60% promote microorganism growth, while those below 30% may cause dryness in the respiratory tract, resulting in discomfort.
• Air Velocity: The speed of the air in a room significantly impacts perceived temperature. Even if the room temperature remains constant, increased air velocity can make it feel cooler. According to ASHRAE, the comfortable air velocity range is 0.1–0.15 m/s or 0.33–0.49 ft/s. Exceeding this range leads to the wind chill effect, where, for instance, at 24°C/75.2°F and an air speed of 0.3 m/s or 0.98 ft/s, people may feel as if it’s 22°C/71.6°F.
• Clothing Level (Clo Value): Clothing provides an insulation layer that regulates heat loss from the body to the surroundings. For a seated worker, the ideal insulation level is between 0.7 and 1.0 clo. For example, a T-shirt has a clo value of 0.36, and with an appropriate jacket and shirt, this value reaches around 1.0 clo.
• Metabolic Rate (Met Value): Physical activity level affects thermal comfort. A met value of 1 represents a resting metabolic rate, equivalent to about 58 W/m2. During exercise, this value can rise to 4–5 met, increasing the need for a cooler environment.

The Relationship Between Air Velocity and Temperature

The interplay between air velocity and temperature significantly impacts thermal comfort. Even if the room temperature remains stable, an increase in air velocity can make the environment feel cooler due to the body losing more heat through air movement—an effect known as the wind chill. This effect, commonly experienced outdoors, can also occur indoors due to high-speed airflows or turbulence. As air velocity rises, the body loses more heat, making the environment feel cooler.

The relationship between air velocity and temperature can be summarized as follows: a temperature drop of 1°C/1.8°F has the same effect as a 0.135 m/s or 0.44 ft/s increase in air velocity. For instance, if the temperature decreases by 1°C/1.8°F, reducing the air velocity by that amount may counteract the chilling effect.

Heat Transfer Mechanisms and Thermal Comfort

The influence of heat transfer mechanisms on thermal comfort is underscored in ASHRAE and ISO 7730 standards, which state that even indoor temperature distribution is essential for comfort. Factors like stagnant air zones and surface temperatures can result in different perceived temperatures on various parts of the body, negatively affecting thermal comfort.

In “stagnant” zones where air movement is absent, hot or cold air can accumulate, causing discomfort. For example, someone sitting in a cold dead zone may lose more heat in certain areas, increasing the feeling of chill. This disrupts thermal comfort and can lead to varying comfort levels within the same space.

A common winter issue is cold feet while the upper body remains warm, often due to cold air settling near the floor. This phenomenon, known as “cold feet,” results from improper air distribution design, which fails to mix air effectively, causing cold air to accumulate near the floor, especially in high-ceiling spaces.

Indoor air distribution must be carefully designed; even high-velocity warm air can cause a sensation of chill in winter. While low air velocities can tolerate temperature differences, higher velocities require smaller temperature differentials.

Contributions of Fabric Air Ducts to Thermal Comfort

Fabric air ducts optimize indoor thermal comfort by distributing air evenly and precisely. The primary air distribution principle of fabric ducts involves delivering large air volumes at low velocities proportional to the system’s static pressure—generally in the range of 0.1–0.15 m/s or 0.33–0.49 ft/s within occupied zones (up to 1.8 m/6 ft above the floor).

This low-velocity, high-volume distribution avoids creating an uncomfortable draft effect in areas where people sit or work, contributing to superior indoor air comfort, especially in work environments.

Balancing Temperature and Air Velocity

One of the standout features of fabric air ducts is their ability to distribute air evenly throughout a space. Unlike metal ducts, which deliver air through specific diffusers, often creating airflow inconsistencies and temperature differences, fabric ducts ensure a maximum temperature differential (ΔT) of just 0.5°C/0.9°F. According to ASHRAE Standard 55, the maximum temperature difference within a room should range between 5°C and 6°C, while metal ducts frequently encounter 2–3°C/3.6–5.4°F differentials.

Targeted Airflows for Specific Areas

A key advantage of fabric air ducts is their customization potential to direct airflow as needed. Airflows can be targeted toward specific surfaces like glass to mitigate thermal imbalances around these areas.

For instance, directing warm air toward cold glass surfaces in winter raises surface temperatures, reducing radiant cooling effects, preventing condensation, and creating a more balanced indoor temperature perception. In summer, supplying cool air to glass surfaces lowers radiant heat, helping cool the interior.

This feature allows occupants near glass or other heat-losing surfaces to remain comfortable without feeling chilled.

Fabric Air Ducts in Industrial Applications

The flexibility and area-specific design of fabric air ducts also offer advantages in industrial settings. In production facilities or storage areas with heat-generating machinery, airflow can be directed toward these units to maintain a balanced temperature around hot surfaces.

For example, fabric ducts can deliver low-velocity cool air around heat-generating machines, preventing excessive heat buildup and ensuring a comfortable working environment for employees. This approach not only enhances worker comfort but also extends equipment lifetime.

The Perfect Balance of Comfort and Efficiency

Fabric air ducts achieve optimal thermal comfort and energy efficiency by providing low air velocity, even air distribution, and custom design for each space. Their ability to deliver balanced air distribution without creating uncomfortable drafts and to be targeted toward specific surfaces makes fabric air ducts an ideal air distribution solution for workspaces, social areas, and industrial facilities where high comfort and efficiency are needed.

Fabric air ducts offer significant energy savings and cost-effective installation, making them an invaluable option for value engineering.

Additional Information

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References:

1. ASHRAE Handbook - Fundamentals (2021)
2. ISO 7730:2005 - Ergonomics of the thermal environment
3. Fanger, P.O. (2020). Advanced Thermal Comfort Analysis
4. Journal of Building Engineering, Vol. 45 (2023)
5. International Journal of Ventilation, 19(3), 2022
6. Building and Environment, Vol. 202, 2023
7. REHVA Guidebook No. 16: HVAC in Sustainable Office Buildings