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General information about radiance
Radiance is a process by means of
which the thermal energy is transferred from one body to
another one, having different temperature. Thermal radiation
is to be considered within the wider phenomenon of
propagation of electromagnetic radiations, emitted by bodies,
and is transmitted from a warmer body to a colder one at
light speed without heating the air. It is a common
experience, in fact, that on sunny days our body can feel a
thermal comfort even with extremely low air temperatures. On
the contrary, when moving to shade, the sensation of comfort
is immediately lost, even though, in both cases, the air
temperature is the same. It is clear that a radiant heating
system will grant the comfort conditions, keeping the air
temperature lower. The temperature generating the comfort
condition is not only the air temperature, but the so called
operating temperature, arising from the combined effect of
air temperature (ta) and average radiant temperature (tmr)
of all surfaces in the room, namely ceiling, floor, vertical
walls and obviously that of radiant panels. Analytically,
the operating temperature can be approximated as the
arithmetical average of the two temperatures: top = ta + tmr
2
The temperature of a body and the composition of its surface
determine the value of heat transmission by radiance. In
order to evaluate the thermal balance of a building, the
average radiant temperature of internal surfaces must be
calculated. This temperature depends on thermal conductivity
of surfaces themselves, on external temperature, on internal
air temperature and on the radiant thermal exchange with
other surfaces, including panels. In the end, the warm
surface of the radiant panel compensates the cold surface of
walls and carries out the air heating. When designing, the
quantity of panels will be determined, which is needed to
grant the required operating temperature. To obtain this
result, a good knowledge of the general conditions and of
all the applicable variables is necessary.
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