CPD Programme
> another surface. So the predominant (radiant)
heat transfer will be transferred directly to the
room occupants, contents and surfaces.
Thermal comfort
The standard index of thermal comfort for
moderate environments is the operative
temperature, θ
c
, (formerly known as
dry resultant temperature). Operative
temperature (at room air velocities of below
0.1 m·s
-1
) is given as θ
c
= (θ
ai
+ θ
r
)/2, where θ
r
is the mean radiant temperature in the room
(degrees C).
Using the CIBSE simple model for heat
losses, the predicted internal air temperature,
θ
,
ai
and mean radiant temperature, θ
,
r
may
be evaluated for rooms using:
θ
Φ 1-1.5R) + C
v
θ
ao
+ 6∑Aθ
c
ai
=
t
(
C
v
+ 6∑A
and θ
r
= 2θ
c
– θ
ai
Where ∑(A) is the area of the room surfaces
through which heat flow (m
2
). Hence rooms
can be examined to determine the effect of
the emitter type on all the key parameters
that make up the thermal environment
Figure 1 Typical radiant heating panel construction
when heating the space. As the radiant
fraction of an emitter increases (for example The make-up and selection of radiant small effect on the heat output of the panel.
[4]
if a convector type heater is replaced by a heating panels The convective heat transfer is rather more
radiant type), and the operative temperature Radiant heating panels are most frequently difficult to evaluate, and will be related to a
is controlled, the value of mean radiant heated by low-pressure/temperature hot water number of variables including: the surface
temperature θ
r
will rise, the air temperature (LPHW) and are manufactured from steel or convective heat transfer coefficient; the
θ
ai
will fall. As a result of the increase in aluminium panels that may normally be factory adjacent air temperature and speed; and
the radiant temperature, the air within a finished to suit the building’s décor. A typical the direction of heat flow. CIBSE Guide
space θ
ai
will not need heating to the same radiant heating panel consists of a serpentine B
[5]
provides tables of both the radiant and
temperature as would be required with a copper tube intimately fixed to the rear of a convective heat transfer from flat panels.
more convective type of heating system to metal panel (see Figure 1). Potentially surface mounted radiant panels
maintain the same comfort temperature This construction ensures effective will emit between 400 and 600 W.m
-2
at
θ
c
. This is likely to be reflected in reduced contact between the heating element and typical office temperatures.
energy use. the radiating surface, and maximises the Manufacturers’ data will typically quote
Particularly when there is sedentary thermal transfer to the face of the panel. A a mean water temperature and a ‘room
occupation (for example people sitting in foil faced insulation pad above the serpentine temperature’ to enable their equipment to be
offices) the radiant temperature asymmetry pipe reduces the convected and radiated heat sized. From the preceding discussion it is clear
(the difference in the perceived radiant from the upper surface of the panel. that it is challenging to provide a predictable
temperature from different directions) is Panels are available in a variety of shapes value of output. However, empirical data
recommended to be below 10K to ensure and sizes, typically ranging from widths of presented by reputable manufacturers should
reasonable comfort. Appropriate positioning 300mm to lengths of 9,000mm. provide a reasonably simple selection method
of the radiant panels (for example mounted The output from a radiant panel will be using values obtained from standardised
at high level around the perimeter of the predominantly radiant, but there will also laboratory tests. The relatively fast thermal
space) can significantly reduce potential be some convective heat output from the response of the panels will mean that they
problems from temperature asymmetry
[2]
. panel’s surface due to air movement in the can be quickly controlled to satisfy and
However care needs to be taken to ensure room. Radiant heat transfer is determined by match changing conditions in the space, so
that they are not mounted too low. If the panel emissivity (this is the ability of the reducing the detrimental consequences of
mounted too low, occupants may complain panel to radiate heat compared with that of a oversized panels.
of excessive temperatures above their head; radiant ‘black body’), which would typically It is important to note that, when panels
if mounted too high, occupants may not exceed 0.9, and the difference between the are installed vertically, e.g. as wall panels,
feel the full heating benefit. For example, panel mean surface temperature and the or freely suspended from the soffit, the
in a typical office or hospital environment, mean radiant temperature, and emissivity heat output is likely to increase over that for
where occupants remain reasonably static, of the surfaces in the room below. It has those installed in a suspended ceilings, since
a manufacturer
[3]
recommends a minimum been shown that altering the emissivities of air movement across the face of the panels
mounting height of 2.7m. the room surfaces practically has a relatively increases the convective output.
60 CIBSE Journal June 2009
www.cibsejournal.com
CIBSEjun09 pp59-62 cpd.indd 60 5/28/09 3:11:53 PM
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