Page 225 - Industrial Ventilation Design Guidebook

I 86 CHAPTER 5 PHYSIOLOGICAL AND TOXICOLOGICAL CONSIDERATIONS
indicate that temperature has a much stronger effect than humidity on the hu-
man thermal response. That is, the ET* lines show that for the same thermal
sensation at a higher humidity the temperature must be lower. On average, for
an 11 °C increase in dew point the temperature would need to be 1 °C lower
to have the same thermal sensation. In terms of human response the bound-
aries are not hard and sharp as indicated in Fig. 5.7b but instead are more soft
and fuzzy in nature.
Optimum comfort would be in the center of each zone. Moving away from
the center, some people would be expected to have thermal sensations ap-
proaching - 0.5 and +0.5 at the cooler and warmer ET* borders. The zones of
Fig. 5.7b are for sedentary or slightly active (M ^ 1.2 met) people. If the activ-
ity level is higher than that, then the ET* line borders can be shifted about 1.4
K lower per met of increased activity. Similarly, if the clothing is different than
the 0.9 and 0.5 clo vales of Fig. 5,7a, the temperature boundaries can be de-
creased about 0.6 K for each 0.1 clo increase in clothing insulation. Another,
similar way to adjust the comfort zone for both different activity levels and clo
values is to shift the zone centered on the optimum temperature (T sedeiltary) at
50% RH as

Conditions that are warmer than the applicable still-air comfort zone of Fig.
5.7b can often be made comfortable by increasing the air speed. If the condi-
tions are 1 to 6 °C warmer than the still-air comfort zone of Fig. 5,7h, the
necessary air speed (v] to restore thermal balance and comfort can be esti-
mated from Fig. 5.8, where T-T comf is the temperature difference between
the environment and the still-air comfort temperature. Though the increased
air speed will bring the whole-body thermal sensation to the comfort level,
air motions above 0.8 m/s or so may cause other kinds of discomfort from

FIGURE 5.8 Air speed necessary at temperature T for the same thermal response as T CO)Trf in a still-
air environment (^0.2 m/s).