Microclimate and thermal comfort

The term microclimate refers to a series of environmental parameters: temperature, relative humidity and air movement speed. These factors influence the transfer of heat between a person and the environment. The microclimate has a significant effect, together with the pollution of the indoor air, on the quality of the environment in which we live and work, and therefore on our comfort.
The achievement of thermal comfort, i.e. a state of complete satisfaction with the environment, is a priority and an essential condition to achieve total comfort. Man, like all mammals, is warm blooded: the internal temperature of the various parts of the human body must be maintained within a very limited range, between 35.8°C and 37.2°C. This range guarantees the conditions required for a person’s health and comfort. The temperature of the human body can remain constant only if the quantity of heat produced or absorbed by the organism is equal to that transferred to and from the environment. If this condition is met, the thermal balance is equal to zero and the internal temperature of the body is maintained within the normal range.

Thermoregulation system

The thermoregulation system of the human body, situated in the part of the brain called the hypothalamus, regulates heat exchange between the human body and the environment and maintains the body’s internal temperature constant. When it is too hot, the thermoregulation system triggers off a series of mechanisms that enable the release of heat from the body into the environment, whereas when it is too cold it intervenes limiting the dispersion of the body’s heat.
The microclimate can influence the heat exchange between a person and the environment, and in certain situations it can hinder the thermoregulation mechanisms.
For example, high humidity values of the air in summer can increase the discomfort linked to feeling hot: the high content of water vapour in the air hinders the evaporation of the water contained in sweat, and sweating is a fundamental process by which the human body disperses its excess heat.
This explains why we find it more difficult to tolerate the discomfort of heat, and body temperature leaps above that of the effective environmental temperature (measured with a thermometer), when the weather is “muggy”, i.e. with a high relative humidity value.
Vice versa, wind can increase the discomfort of feeling cold because it increases the rate at which the body loses heat.
Therefore, what is known as the perceived temperature, which is the sensation of feeling “hot” or “cold”, is linked not only to the actual temperature but also to the other environmental conditions.

Thermal comfort

When a person’s body, with a minimum involvement of the thermoregulation mechanisms, feels neither hot nor cold, he or she feels perfectly comfortable in respect of the environment; this is known as a state of “thermal comfort”.
This optimal condition occurs only if the environmental parameters – temperature, relative humidity and air movement speed – are suitably graded.
Ventilation can influence the microclimate parameters and it plays an important role in the thermoregulation process of the human body and in guaranteeing situations of environmental comfort.
Microclimate well-being and environmental comfort refer to the environmental condition in which the internal air is perceived as optimal by the majority of the occupants from the viewpoint of both the physical properties (temperature, humidity, ventilation) and the chemical properties (“clean” or “fresh” air).
The table below indicates the optimal microclimatic conditions of an environment, for moderate physical activity (sedentary), adequate clothing and without radiation, in which the majority of the “occupants” have a sensation of thermal comfort.

Season Air temperature (T) Relative humidity (RU) Air speed (S)
Winter
19-22°C
40-50%
0,01-0,1m/s
Summer
24-26°C
50-60%
0,1-0,2m/s

Standards and thermal comfort indices

The following technical standards give official values for assessing microclimate comfort in moderate thermal environments:

  • UNI EN ISO 7726:2002 Ergonomics of the thermal environment – Instruments for measuring physical quantities
  • UNI EN ISO 7730:2006 Ergonomics of the thermal environment – Analytical determination and interpretation of thermal comfort using calculation of the PMV and PPD indices and local thermal comfort criteria
  • UNI EN ISO 8996:2005 Ergonomics of the thermal environment – Determination of metabolic rate
  • UNI EN ISO 9920:2009 Ergonomics of the thermal environment – Estimation of thermal insulation and water vapour resistance of a clothing ensemble
  • UNI EN ISO 10551:2002 Ergonomics of the thermal environment – Assessment of the influence of the thermal environment using subjective judgement scales
  • UNI EN ISO 11399:2001 Ergonomics of the thermal environment – Principles and application of relevant International standards
  • UNI EN ISO 13731:2004 Ergonomics of the thermal environment – Vocabulary and symbols
  • UNI EN ISO 14505-2:2007 Ergonomics of the thermal environment – Evaluation of thermal environments in vehicles – Part 2: Determination of equivalent temperature
  • UNI EN ISO 14505-3:2006 Ergonomics of the thermal environment – Evaluation of thermal environments in vehicles – Part 3: Evaluation of thermal comfort using human subjects.

The Comfort Rate

Clean room operators usually work in better conditions than those required by the standards for normal work environments, since temperature, relative humidity, overpressure and air quality, for intrinsic needs of controlled contamination environments, are maintained constant and with stricter parameters than those of optimal microclimate conditions.
However, the need to hyper-ventilate the environments to keep the quantity of air-transported particulate under control or to dissipate the thermal energy generated by the high-tech production processes, could generate discomfort for the operators due to excess ventilation.
The Comfort-Rate therefore introduces a tool for assessing the comfort of environments subjected to forced hyper-ventilation: Clean Rooms.
There are 6 Comfort-Rate reference classes: the classes are assigned according to the quality of the ventilation and air distributions systems of the Clean Rooms.

The functional parameters taken into consideration for determining the Comfort-Rate are:

  • the temperature of the air which enters the Clean Room;
  • the desired environmental temperature;
  • the coverage factor.

Coverage factor

The Coverage Factor is an important sizing parameter of Clean Rooms: it defines the ratio between the surface of the elements that issue air into the Clean Room and the surface of the Clean Room itself.
It is good design practice to have Coverage Factors that are as high as possible: the better the distribution and efficiency of the Clean Room air input system, the higher the Coverage Factor will be, to the advantage of the operators’ comfort and of the “Recovery Time”.
The following table gives the Coverage Factors recommended for Clean Room design, according to the Clean Class required:

Clean Class Coverage factor (%)
ISO 8
5 - 10%
ISO 7
15 - 20%
ISO 6
30 - 50%
ISO 5
> 80%
ISO 4
> 90%
ISO 3
> 90%
ISO 2
> 90%
ISO 1
> 90%

Check your Comfort Rate

Check the Comfort-Rate of your Clean Room with 6 simple steps:

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