and Cooling technology Page 4
temperature differences between room air and outside air are thereby avoided,
and thus also the danger of heat shock. In addition, this measure results in
a savings of cooling energy.
Fig. 2 Sliding room temperature in summer
English translation of
technical terms and sentences in Fig. 2:
t1 Room air temperature
t2 Outside air temperature
1.1.2 Radiation temperature
Because the temperatures of the surfaces surrounding a room (walls, doors, windows,
ceiling, floor) are generally lower than the surface temperature of the human
body (25 ... 27°C), it constantly radiates heat to these relatively cooler surfaces.
This has the effect that spaces with normal air but low wall temperatures are
found to be uncomfortable. The average temperature of the surrounding surfaces,
the radiation temperature, thus has a considerable influence on comfort levels.
In order to ensure comfort, the temperature of the surfaces surrounding the room
should lie within a range in which the heat dissipation of the body is neither
restricted nor increased. A room temperature of 20 ... 22°C is generally only
suggested as the most favourable where the average wall temperature is either
identical or at least quite similar to the air temperature (Fig. 3). Where the
wall temperature is significantly lower than the air temperature, as is the case,
for example, when warming a room in winter, a room temperature of 20°C will be
felt to be too cold.
Fig.3 Radiation influence and level of comfort at a room air temperature of 21°C
The following factors should
be considered with respect to the average temperature of surrounding surfaces:
- The location of the
radiators and of the occupants of the room is of great importance. If the
radiators bring the room heat to the window-sill, the effects of the cold
exterior wall and the window surface will be compensated for by the heat radiation
of the heating unit. In addition, draughts caused by descending cold air from
the window will be avoided.
- Single-glazed windows
are not appropriate for central European conditions. This is because the radiation
loss of human beings is especially great as a result of the lower temperatures
of window surfaces. The larger the windows, the greater the heat loss.
- In summer, large windows
often result in a high air temperature due to the radiation of sunlight into
the room. This, together with the high temperature of window surfaces, detracts
from the level of comfort.
- If the average temperature
of the wall sinks by 1 K, this effects persons in a resting condition in the
same way as a drop in air temperature of 1 K. Air and wall temperature thus
have approximately the same influence upon comfort levels.