Did you know that your comfort level depends on how air is distributed around you? Selecting the most suitable air diffusers is not a technical detail, but the fundamental secret to ensuring well-being in any enclosed space.
Thermohygrometric well-being and thermal comfort
The human body can be viewed as a thermodynamic system that exchanges heat with its surroundings.
The chemical energy introduced through food is used in three main ways:
- as heat released to the outside;
- as mechanical energy produced;
- as internal reserves, which manifest themselves both in the accumulation of fat and in a change in average body temperature.
The body’s energy consumption (metabolic flow), necessary to maintain vital functions – from breathing to blood circulation, not to mention intellectual and physical activities – corresponds to the sum of the mechanical energy developed and the heat dispersed to the outside.
It follows that the feeling of well-being is not the same for everyone: it varies from individual to individual and there is no universally valid condition.
The main purpose of air conditioning systems is to ensure ideal conditions of temperature, humidity and air quality in closed environments, according to the parameters established during the design phase. In particular, comfort depends on two fundamental aspects:
- the thermo-hygrometric balance between people and the environment, i.e. the condition in which neither heat nor cold is perceived, thanks to the right balance of temperature, humidity and air movement;
- indoor air quality (IAQ), which concerns the composition of the air, in particular the concentration of pollutants (gaseous or particulate) and the level of humidity.
In addition to optimal distribution of thermo-hygrometric parameters, a comfortable environment must also guarantee:
- Acoustic comfort, meaning the absence of annoying noises or good insulation from urban noise pollution.
- Visual comfort, a sense of aesthetic harmony with the environment.
The feeling of discomfort caused by irritation affects our thermoregulatory system, upsetting the thermo-hygrometric balance that characterises our sense of well-being.
It should also be noted that sensitivity to comfort varies according to individual factors such as health, anxiety or skin reactivity, elements that cannot be expressed with precise numerical values.
For this reason, comfort can be defined as a mental state in which the individual feels satisfied with the microclimatic conditions surrounding them.
There are many factors that influence this condition:
- metabolism
- type of clothing
- age
- gender
- activity performed
- average ambient temperature
The perception of comfort is the result of a combination of thermo-hygrometric well-being, indoor air quality, visual comfort and the absence of acoustic discomfort, and depends largely on careful design choices regarding:
- the air diffusion system;
- the type and position of diffusion devices in relation to occupants and furnishings;
- the efficiency of filtration systems.
Well-being indices
When we talk about comfort, we refer to a particular condition, described through empirical parameters, which satisfies a large percentage of the occupants of a given environment.
The main regulatory reference in this area is UNI EN ISO 7730, which is based on the thermal comfort model developed by Fanger. This model allows real conditions to be described in relation to ideal comfort conditions through specific global indices, which reflect the average perception of individuals present in the environment analysed.
Two indices are used: PMV and PPD.
- The PMV (Predicted Mean Vote) represents the average of the ratings given by a sample of 1,300 subjects regarding the perceived thermal sensation in a specific environment.
- This sensation is translated into a numerical scale, shown in the following table.
This table summarises the results of experiments conducted by Fanger in a climate chamber, which made it possible to correlate the PMV value with the six main parameters that determine thermo-hygrometric comfort:
- level of activity;
- thermal resistance of clothing;
- air temperature;
- average radiant temperature;
- relative humidity;
- air velocity.
Among these parameters, the level of activity and the thermal resistance of clothing play a significant role, both of which can be assessed using analytical formulas and reference tables.
The unit Met is used to measure activity level.
A value of 1 Met corresponds to the metabolic rate – i.e. the energy expended by the body in relation to body surface area – of a person who is awake and at rest.
The Met is the unit of measurement used specifically to express the energy consumption per unit of surface area by an individual during a specific activity. 1 Met also expresses the consumption of
With regard to the thermal resistance of clothing, reference is made to clo.
Clo is the unit of measurement for the thermal resistance of clothing and is calculated as follows
Since it is clear that not all individuals react in the same way to the same environmental conditions, not everyone perceives the same thermal sensation at the same PMV.
For this reason, the UNI EN ISO 7730 standard relates PMV to PPD (Predicted Percentage of Dissatisfied), i.e. the percentage of people who are dissatisfied with the conditions present.
The PPD therefore expresses, in percentage terms, the proportion of occupants who are most likely to experience discomfort with respect to the microclimate produced by a given system.
The curve describing this index has a symmetrical pattern, as the degree of dissatisfaction increases in a similar way for conditions perceived as too cold and too hot.
A PMV value of 0 corresponds to a PPD of 5%. This means that, even in the best case scenario, an air conditioning system can only guarantee optimal conditions for 95% of occupants, while at least 5% will continue to feel uncomfortable. It is therefore impossible to achieve 100% satisfaction. Furthermore, even small deviations from the optimum point (PMV = 0) lead to a rapid increase in the percentage of dissatisfied people.
The PMV and PPD indices provide an overall assessment of the environment, but do not always accurately describe the feelings of individuals. Some individuals may experience localised discomfort, for example due to cold draughts on their necks or floors that are too cold or too hot underfoot.
For this reason, the main function of an air diffusion system is to ensure that the desired climatic conditions are achieved within a space without generating undesirable or unexpected situations at a local level.
The UNI EN ISO 7730 standard also introduces the main parameters of local discomfort, which are used to assess and control these specific conditions.
- floor temperature;
- turbulence level;
- average radiant temperature;
- vertical temperature difference;
The temperature of the floor directly affects the sensation of discomfort perceived through the feet. This aspect requires particular attention in underfloor heating systems: if the occupants are not wearing shoes, the temperature must not exceed 29 °C, as higher values can cause blood circulation problems in the lower limbs.
The turbulence level Tu of air currents within the occupied zone is expressed as:
where:
- |SD| is the standard deviation from the average air velocity value, i.e. it is the average measurement of the oscillation with respect to the average velocity value;
- va is the average air velocity.
The ‘Draught Rate’ DR is defined as the percentage of subjects dissatisfied with draughts, which also depends on the ambient air temperature value ta.
The regulations suggest maximum air velocity values within occupied areas:
- in winter equal to 0.15 m/s;
in summer equal to 0.25 m/s.
Practical experience shows that, during cooling, it is advisable to maintain the air velocity (va) at design values not exceeding 0.2 m/s. This reduces the risk of discomfort associated with increased sweating and reduced thermal insulation due to light clothing.
The mean radiant temperature (MRT) is defined as the temperature of a hypothetical environment, perfectly uniform from a thermal point of view, in which an individual would exchange the same amount of heat by radiation as they would in the real environment. The MRT is measured using a globe thermometer.
This instrument consists of a black sphere 15 cm in diameter and 0.2 mm thick, inside which a thermocouple is placed. The value measured is called the operating temperature (To). Knowing the air temperature (Ta) and air velocity (va) in the environment where the globothermometer is located, it is possible to calculate the Tmr value.
The Tmr thus obtained is an estimate of the average radiant temperature of the surfaces in the environment.
It can certainly be said that radiant asymmetry generates a strong feeling of discomfort.
Vertical temperature gradients, i.e. vertical temperature stratification whereby the ankles and head of occupants in an air-conditioned environment are exposed to different temperatures, causing discomfort. The maximum permissible gradient for a seated person is 3 °C.
Well-being conditions
At this point, it is possible to define reference values for the main parameters that influence comfort, in order to summarise the optimal conditions for well-being during both the heating and cooling phases.
Winter conditions => RH 40-45% – 1 clo – light activity ≤1.2 Met
Summer conditions => RH 55–60% – 0.5 clo – light activity ≤1.2 Met
Relative humidity (RH) has a moderate influence on the perception of thermal comfort within the typical temperature ranges found in indoor environments. In this regard, consider the following air temperature ranges:
- in winter, 19°C – 25°C;
- in summer, 23°C – 27°C;
Relative humidity can fluctuate between 30% and 70% without significantly affecting the perception of thermal comfort.
However, values below 30% can cause irritation to the mucous membranes, while values above 70% promote the formation of mould inside rooms.
In practice, to ensure adequate comfort, relative humidity values between 55% and 60% in summer and between 40% and 45% in winter are generally adopted.
