Page 230 - Industrial Ventilation Design Guidebook
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5.1 THERMAL COMFORT 1 9 I
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| FIGURE 5.11 Warm discomfort related to skin wettedness from various studies.
where P m is the average vapor pressure of the skin and P s sk is the saturated
vapor pressure of water at skin temperature. Typically, the water content
(water/dry skin) of the stratum corneum is about 10% but it can absorb as
much as four times its dry weight.
Skin moisture may be detected by mechanoreceptors of the skin and hair
follicles or some other neural mechanism that senses the skin's swelling and
shrinking. At high levels of skin moisture the swelling is sufficient to close or
reduce the lumen of sweat glands and reduce sweating (called hydromeiosis).
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Hydromeiosis occurs at RH sk s O.9. Conversely, under good drying condi-
tions the skin can shrink to the extent that lesions form.
As mentioned previously, the other term for characterizing skin moisture
is skin wettedness (w) or the size of the water film as a fraction of total skin
area that is necessary to account for the observed evaporative heat loss from
the skin (E si,),
where A dli is total skin area, h e is evaporative heat transfer coefficient, and P a
is the ambient vapor pressure.
Skin wettedness and skin relative humidity are related by
From Eq. (5.30) it is clear that RH sk will be greater than w except when
w = 1. It is also evident that with a constant w, RH sk increases with ambient
absolute humidity. Thus, though the ET* temperature boundaries have con-