Page 218 - Industrial Ventilation Design Guidebook

5. i THERMAL COMFORT 179

the raising of a weight or other thermodynamic work (force • distance) activi
ties. Respiratory heat loss occurs from bringing ambient air into the core, rais-
ing its temperature to near core temperature, humidifying it to near saturation
at core temperature, and exhaling it. The resulting heat loss is proportional to
breathing rate and to the temperature and humidity differences. The breathing
rate or air flow through the lungs is regulated mainly by CO 2 levels in the
blood and as a result is proportional to metabolic rate.
The skin receives heat from the core by passive conduction and active skin
blood flow (Table 5.3). It transfers this heat to the surroundings by convec-
tion, radiation, and evaporative (perspiration and diffusion) mechanisms. All
of these mechanisms are unregulated or passive except evaporation from
sweating. The sweating process is actively controlled by the human's ther-
moregulatory center where the rate of sweat secretion is proportional to eleva-
tions in core and skin temperature from respective set point temperatures
(Table 5.3).
The physiologically active elements in body temperature regulation, sum-
marized in Table 5.3, function and regulate in part on deviations in body tem-
peratures from set points. In humans thermogenesis by shivering is small and
inefficient in comparison to other animals. Thus the very precise regulation of
body temperature in man is primarily due to only two active mechanisms asso-
ciated with the skin: blood flow and sweating. Under normal comfort condi-
2
tions, blood flow to skin is about 6 liters per hour per m of skin. Of this about
2
1.5 L/(h m ) is for the relatively constant minimal metabolic needs of the skin.
In hot environments and during exercise skin blood flow can be increased by
7
2
15 times to about 90 L/(h m ). When necessary to reduce heat loss in cold en-
2
vironments, the vessels can restrict blood flow to as little as 1 L/(h m ). With
continued heat exposure, the thermoregulatory system increases its sensitivity
so that blood flow increases with smaller and smaller changes in body temper-
ature as the body acclimates to the hot environment.
Sweating, the other powerful heat loss mechanism actively regulated by
the thermoregulatory center, is most developed in humans. With about 2,6
million sweat glands distributed over the skin and neurally controlled, sweat
2
secretion can vary from 0 to 1 L/(h m ). The other, lesser, passive evaporative
process of the skin is from the diffusion of water. The primary resistance to
this flow is the stratum corneum or outermost 15 jam of the skin. The diffu-
sion resistance of the skin is high in comparison to that of clothing and the
boundary layer resistance and as a result makes water loss by diffusion fairly
stable at about 500 grams/day.
When the energy flows in and out of a compartment do not balance, the
energy difference accumulates and the temperature increases or decreases. The
changes in core and skin temperature then in turn alter the physiological con-
trol signals to restore balance and thermal stability.

5. f .3 Body Control Temperatures
Body temperatures are primarily sensed by temperature sensors in the hypo-
thalamus near the center of the brain. Arterial blood flowing over and near
the hypothalamus gives it information about the average thermal condition of