Page 229 - Industrial Ventilation Design Guidebook

I 90 CHAPTER 5 PHYSIOLOGICAL AND TOXICOLOGICAL CONSIDERATIONS

and after rearranging:

Substituting numerical values for h c and h r with d — 15 cm and v in m/s,

"urther, from the definition of operative temperature (T 0),

Substituting Eq. (5.24) into Eq. (5.26) and rearranging,

where subscripts h and g designate the human and globe. For 15 to 20 cm di-
ameter globes [h r/h c] =[h r/h c} h, which after substituting and rearranging
Eq. (5.27) simplifies to

Globes can be made of any opaque material. A globe of low mass is help-
ful to provide a short time constant for transient conditions. Globes are typi-
cally gray or black, but color is not important if they do not receive high
temperature radiation from the sun or other glowing objects. If significant
high-temperature radiation is present, then they should have a color similar to
that of the occupant. The comfort zones of Fig. 5,7b should be entered with T 0
when it is known that MRT ^ T a because T 0 is the temperature that the envi-
ronment feels like to the occupant of the space.

Worm Discomfort and Skin Moisture
In warm environments or situations with prolonged activities above about
1.2 met there is sweating. The sweat glands put water on the skin for evapora-
tive cooling. Since the latent heat of evaporation of water is so high very little
water is consumed in this cooling process. In the process the skin gets wet. If
the conditions are very good for evaporation the skin can remain nearly dry
while sweating occurs, as for example in windy desert conditions. In humid
still-air conditions a larger surface of water is necessary to evaporate the sweat
and the skin becomes wetter. The fraction of the surface of the skin that is cov-
19
ered with water for evaporation is called skin wettedness (u>). It is a measure
of the physiological strain or effort of evaporative cooling and has long been
associated with warm discomfort (Fig. 5.11). It is rare that a person feels com-
fortable with a skin wettedness above 20 to 25%.
Some of the discomfort of warm environments, the perception of skin
moisture, and the interactions of clothing fabrics with the skin may be due to
the moisture itself. The skin's outer layer of dead squamous cells of the stra-
tum corneum can readily absorb or lose water. With moisture addition, the
cells swell and soften. With drying, they shrink and become hard. In this set-
ting the skin's moisture may be better indicated or characterized by the rela-
24
tive humidity of the skin (RHLiJ rather than skin wettedness,