Apparent latent heat of evaporation from clothing: attenuation and "heat pipe" effects

Investigating claims that a clothed person’s mass loss does not always represent their evaporative heat loss (EVAP), a thermal manikin study was performed measuring heat balance components in more detail than human studies would permit. Using clothing with different levels of vapor permeability, measuring heat losses from skin controlled at 34ºC, in ambient temperatures of 10, 20 and 34ºC with constant vapor pressure (1 kPa), additional heat losses from wet skin compared to dry skin were analyzed. EVAP based on mass loss ( mass E ) measurement and based on direct measurement of the extra heat loss by the manikin due to wet skin ( app E ) were compared. A clear discrepancy was observed. mass E overestimated app E in warm environments and both under and overestimations were observed in cool environments, depending on the clothing vapor permeability. At 34ºC, apparent latent heat ( app λ ) of pure evaporative cooling was lower than the physical value (λ , 2430 J·g-1), and reduced with increasing vapor resistance up to 45%. At lower temperatures app λ increases due to additional skin heat loss via evaporation of moisture that condenses inside the clothing, analogous to a heat pipe. For impermeable clothing app λ even exceeds λ by four times that value at 10ºC. These findings demonstrate that the traditional way of calculating evaporative heat loss of a clothed person can lead to substantial errors, especially for clothing with low permeability, which can be positive or negative, depending on the climate and clothing type. The model presented explains human subject data on EVAP that previously seemed contradictive.