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

¹Environmental Ergonomics Research Group, Department of Human Sciences, Loughborough University, Loughborough, United Kingdom; ²EMPA Materials Science and Technology, St. Gallen, Switzerland; ³Institut für Arbeitsphysiologie an der Universität Dortmund, Dortmund, Germany; ⁴Centre Nationale de la Recherche Scientifique, Strassbourg, France; ⁵Netherlands Organisation for Applied Scientific Research (TNO), Soesterberg, The Netherlands; ⁶Lund University, Lund, Sweden; ⁷Tampere University of Technology, Tampere, Finland; and ⁸W. L. Gore, Newark, Delaware

Submitted 7 June 2007 ; accepted in final form 16 October 2007

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 and measuring heat losses from skin controlled at 34°C in ambient temperatures of 10, 20, and 34°C with constant vapor pressure (1 kPa), additional heat losses from wet skin compared with dry skin were analyzed. EVAP based on mass loss (E_mass) measurement and direct measurement of the extra heat loss by the manikin due to wet skin (E_app) were compared. A clear discrepancy was observed. E_mass overestimated E_app in warm environments, and both under and overestimations were observed in cool environments, depending on the clothing vapor permeability. At 34°C, apparent latent heat (_app) of pure evaporative cooling was lower than the physical value (; 2,430 J/g) and reduced with increasing vapor resistance up to 45%. At lower temperatures, _app 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 even exceeds by four times that value at 10°C. 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.

heat balance; sweat evaporation; condensation; protective clothing; evaporative cooling efficiency