Water-based Indoor Climate Systems, 2007
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CLIMATE CONCEPTS
CLIMATE CONCEPTS
CLIMATE CONCEPTS
TemperatureThe air temperature is the parameter that is the easiest to understand and where most people have their own experiences of an individuals’ preferences.
Operative temperature
The operative temperature is an approximate mean value of the surrounding room surfaces’ temperature and room air temperature. Thus, the perception of temperature is affected just as much by the surrounding surfaces’ temperature as by the room air. As different room surfaces such as windows, outside walls, inner walls, floor and ceiling, etc. have varying temperatures and directions within the room, the operative temperature can also vary in the different directions.
Directed operative temperature
You can perceive radiation that arises by a cold window at the same time as you experience a slightly higher temperature towards the room’s inner areas. The reason for this phenomenon is because the radiant heat transfer to the cold window is significantly greater than in towards the room.
If this results in the difference in directed operative temperature becoming large, we will get local cooling on body surfaces that face towards the window. This local lack of heat is felt as draught. The reason for the draught sensations do not necessarily need to be due to large air movements, but can also be caused by local parts of our body being exposed to a large radiant heat transfer to a cold room surface.
Temperature radiation asymmetry
We all know how uncomfortable it can be to spend a longer period of time near a cold surface such as a large window of poor quality, or a warm surface such as a warm radiator. This state has been closely examined at the “Laboratory for Heat and Climate Technology” at Denmark’s University of Technology. Here they have subjected volunteers in climate chambers to different unevenness in the thermal radiant field.
They have also discovered the limits where 5% of volunteers experienced thermal discomfort, however, this is when they are in thermal balance with the surroundings, i.e. when the comfort equation has been met. These limits are shown in Figures 7-10.
Radiation asymmetry is defined as the difference between the level radiant temperature tpd on both opposite sides of a cross section of a person’s physical centre. The figure shows these Dtpr values, which in typical situations will result in 5% experiencing thermal discomfort due to the asymmetric radiant field. The air temperature is calculated to be equal to the surface temperature on the room’s other surfaces.
It emerges that individuals can accept large unevenness in the form of cold radiation from above and warm radiation from the side. One is less tolerant to cold radiation from the side and extremely less tolerant with regard to warm radiation from the ceiling.
Figure 7.Cold surface to the side.
Dtpr < 10 K.
Figure 8.Warm surface to the side.
Dtpr < 23 K.
Figure 9.Cold surface above the head.
Dtpr < 14 K.
Figure 10.Warm surface above the head.
Dtpr < 5 K.
Air humidity
The air’s humidity is the parameter that usually has the least influence on the comfort experience. Sometimes it can even happen that other parameters such as air contamination are associated with humidity. The air’s humidity can be a troublesome factor during the summer, if the cooling installations’ capacity to dehumidify the supply air is insufficient.
Clothing
Clothing is easy to understand, yet no less complicated to handle. A normal situation on a summer’s day is that women are more lightly dressed than men, for example, skirt and a thin blouse compared with the man’s trousers and shirt and this is a sufficiently strong parameter in itself to motive different temperature levels in the building. Consequently, the summer situation can be a direct source of conflict in an open plan office where many work together.
Activity level
Activity level, or in simple terms, the work you do is also a very important parameter. More vigorous work demands a lower temperature than sedentary work and to combine several activity levels in one and the same premises can be completely inappropriate.
Air velocity
It is well known that air velocity has an effect on the experienced indoor climate, but not how it acts in the interaction between the air temperature and the air velocity. A simple comparison can be made with stretching your arm out of a side window and letting it be cooled by the breeze on a hot summer day. The experience is not as pleasant if you try the same experiment during the winter when it is -10°C in the air. You can now see that it is not just the air velocity that is of importance, but also the temperature. ISO 7730 shows this by something called the draught index (DR index), a coefficient factor that is made up of:- Air velocity
- Turbulent intensity
- Air temperature
The DR index is calculated for a given position in a room by measuring the air temperature and air velocity over a period of time (and calculate the turbulent intensity, as a measurement of how much the air velocity varies during the measurement sequence, how “restless” the air is). The result is, the colder it is in a room the lower the air velocity that can be tolerated and vice versa a high air velocity can be tolerated if the temperature is higher.
The above is usually the reason why demands on the maximum air velocity are different during the summer and winter. It is preferable during the winter that the air velocity where personnel are situated should not exceed 0.15 m/s. The corresponding velocity during the summer should not exceed 0.25 m/s.
Figure 11.Clothing plays an important part in how you experience the climate.
Figure 12.A persons perception of the indoor climate is greatly affected by the activity performed. The greater the activity the lower the preferred temperature.
Figure 13.The air velocity influences how the indoor climate is perceived.
Air quality
Uncertainty is greater when it comes to person’s perception of the air quality than for the thermal climate. Danish studies show that the number of unsatisfied users is approximately 14% if everyone is exposed to a carbon dioxide content of 800 ppm. A carbon dioxide content of 1000 ppm has also become a generally used demand level for air contamination, but it is important to know that this only applies to human generated air contamination. The carbon dioxide content is therefore not suitable as a trace parameter for other types of contamination, for example, those given off from building material.
Sometimes it can occur that you feel the air quality is poor when the problem is actually due to a too high temperature.
Lighting
As we get older our need of light increases. At sixty, for example, a person needs double the illuminance compared to a twenty year old. The demand levels for background lighting from, e.g. a ceiling luminaire are normally set to 500 lux within the reading field at the same time as the local lighting is adapted to the user’s individual requirements.
Figure 14.The air quality is affected by many factors, such as emissions from building materials, secretion from individuals, etc. Smoking is an example of contamination that has a negative effect on the air quality.