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Effective Temperature
Since the human body uses radiation, convection and relative
humidity to mantain its temperature, the perception of warm or
cold is dependent of all this factors. Effective temperature
is a weighted average between air temperature and radiation
temperature and is closer related to the human perception. By
including both the air temperature and radiation, around 75% of
the elements that forms the perception of temperature are covered,
instead of around 25% if only air temperature is considered.
The space angles covered by the different room surfaces affect
the calculation, and thus, effective temperature varies with the
location in the room. The human body is also sensitive to variations
in radiation over the body surface and to cover this a comfort
factor is calculated.
BREEZE calculates a comfort measure together with each
effective temperature. The comfort measure is defined as the
total variation of the effective temperature over different
directions from the point of interest. To clarify this,
let's assume that we have a small convex solid body located
at the point of interest. The different sides of this body
will face different parts of the room surfaces, and the
radiation climate will vary over the surface of the body.
The comfort measure is the difference between the warmest
and the coolest part of the surface. A high value will
indicate poor comfort and the measure is thus in reality a
measure of discomfort.
The method of using a perceived temperature is becoming more
and more popular. In today's weather report, you can often see
a "Feels like" temperature that is a parallel to effective
temperature. The use of effective temperature is proven to
be a very important factor in design of energy efficient
constructions. This is mirrored in the 1978 Swedish Building
Code, by prescribing minimum surface temperatures.
The magnitude of the variations can be indicated by the
following examples.
We have a room with:
width 3 m, height 2.7 m, depth 4 m,
window surface 2 m2, double glazed window,
outdoor temperature -15C,
no solar radiation,
room air temperature 20C.
We calculate effective temperature at a point:
1.5 m above the floor,
located centrally between partitions, and
a) 1 m from the external wall
b) 1 m from the internal wall.
A) Heat from hot air, evenly distributed in the room,
unshaded window (U-value = 3.1)
window temperature 7.5C
| Point | effective temperature | comfort measure
(lower=better) |
| a) | 18.8 | 2.0 |
| b) | 19.3 | 0.43 |
B) Heat from a radiator below the window (1.2 m2),
window shaded with venetian blind between panes
(U-value = 2.6)
window temperature 9.7C
heater surface 29C
| Point | effective temperature | comfort measure
(lower=better) |
| a) | 19.5 | 1.4 |
| b) | 19.8 | 0.15 |
When BREEZE calculates comfort, the variations are restricted
to the six sides of a cube. A sphere would normally give a
slightly higher variation.
BREEZE also calculates a frequency distribution for the
effective temperature. This can serve as a basis for the
calculation of cumulative frequencies over typical climate
variations.
The effective temperature is calculated and printed for all
intervals included in the output listing. The frequency
distribution, however, is normally required just for the
period when the room is occupied. The desired period is
indicated by a reference in form 3 to a profile, taking non
zero values for hours that should be included. The profile
for load from people can often serve this purpose.
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