Handbook of air conditioning and refrigeration / Shan K

4.8 CHAPTER FOUR
calculated as
TABLE 4.3 Moisture Permeability of Clothing Ensembles
E max � h e,c(p sk, s � p a) (4.15)
where p sk, s � saturated water vapor pressure at skin surface temperature, psia (kPa)
p a � water vapor pressure of ambient air, psia (kPa )
h e,c � overall evaporative heat-transfer coefficient of clothed body, in Btu/h�ft 2 �psi
(W/m 2 �kPa)
Woodcock (1962) proposed the following relationship between h e, c and h s, the overall sensible heat
transfer coefficient, in Btu/h�ft 2 �psi (W/m 2 �kPa):
(4.16)
The moisture permeability index im denotes the moisture permeability of the clothing and is dimensionless.
Clothing ensembles worn indoors usually have an im � 0.3 to 0.5. The moisture permeability
indexes im of some clothing ensembles are presented in Table 4.3.
The Lewis relation LR in Eq. (4.16) relates the evaporative heat-transfer coefficient he and the
convective heat-transfer coefficient hc, both in Btu /h�ft2�°F�(W/m2�°C). LR � f(he /hc) has a magnitude
of 205°F/psi (16.5°C/kPa). In Eq. (4.16), the overall sensible heat-transfer coefficient hs can
be calculated as
hs �
(4.17)
1
im LR �
fcl h
�
Rt fcl hRcl � 1
he,c hs where R t � total resistance to sensible heat transfer between the skin and the indoor environment,
h�ft 2 �°F/Btu (m 2 �°C/W).
Diffusion Evaporative Heat Loss and Total Skin Wetness
Ensemble description i m
Cotton/polyester long-sleeve shirt, long
trousers, street shoes, socks, briefs 0.385
Cotton short-sleeve shirt, long trousers, work
boots, socks, briefs, cotton gloves 0.41
Cotton/nylon long-sleeve shirt, cotton/nylon
trousers, combat boots, socks, helmet liner
(army battle dress uniform) 0.36
*Measured with T rad � T a, and air velocity � 40 fpm.
Source: Adapted with permission from ASHRAE Handbook
1989, Fundamentals.
The minimum level of evaporative heat loss from the skin surface occurs when there is no regulatory
sweating and the skin wetness due to direct diffusion E df, min, Btu/h�ft 2 (W/m 2 ), is approximately
equal to 0.06E max under normal conditions, or
Edf, min � 0.06Emax (4.18)
When there is a heat loss from regulatory sweating Ersw, the diffusion evaporative heat loss
Edif, Btu/h�ft2 (W/m2 ), for the portion of skin surface that is not covered with sweat can be