Page 220 - Industrial Ventilation Design Guidebook

5. i THERMAL COMFORT I 8 I

Kg FIGURE 5.3 Schematic of skin and core temperatures for a neutral thermal sensation.

In most transient environmental situations, it is rapid changes in skin temper-
ature that affect our feelings of warmth; rapid changes in core temperature
only occur during rapid changes in metabolism and possibly during transient
radar or other microwave exposures. Diving into cold water after a hot sauna
is pleasant rather than cold because core temperature remains high and dT s^
reduces the hot thermal sensation.
In summary, core temperature is much more important than skin temper-
ature in determining how warm we feel. Core temperature is affected by meta-
bolic activity and heat storage. It is relatively isolated from the environment
except through whole-body heat balance and resulting heat storage. Feet and
hands have little metabolic heat generation themselves and depend on warm
blood from the core for their temperature. The feeling of cold feet then means
that the whole body heat balance has caused the core to lose temperature and
the hypothalamus is restricting heat flow to the feet in order to stabilize the
core temperature.
The consequence of the relationships of Table 5.3 and Fig. 5.2 is that for
a neutral thermal sensation, at steady state, the core temperature increases
while the skin temperature decreases with increased metabolic activity (Fig.
5.3). The increase in metabolism causes sweating which decreases skin tem-
perature,

5.1.4 Clothing
Thermal Insulation
Clothing affects heat and moisture loss. Increasing the thickness or num-
ber of layers of clothing increases its insulating capability and reduces body
heat loss. Clothing insulation is usually described with the clo unit. Originally,
1 clo was defined as the thermal resistance necessary for comfort while seden-
tary in a uniform still air environment of 21 °C. In conventional SI nomencla-
2
ture 1 clo has a thermal resistance of 0.155 K m /W. Some ensembles' clo
values and associated comfort temperatures are shown in Fig. 5.4.
The clothing insulation necessary for comfort or a neutral thermal sensa-
tion (TS = 0) in a thermally uniform 50% RH still-air environment is
9
graphed in Fig. 5.5. The slope of the graph is such that comfort temperature
is decreased about 0.6 °C for each 0.1 clo increase in clothing insulation. The