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

Abstract

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 degrees C in ambient temperatures of 10, 20, and 34 degrees 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 degrees C, apparent latent heat (lambda(app)) of pure evaporative cooling was lower than the physical value (lambda; 2,430 J/g) and reduced with increasing vapor resistance up to 45%. At lower temperatures, lambda(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, lambda(app) even exceeds lambda by four times that value at 10 degrees 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.

A simple theoretical model of heat and moisture transport in multi-layer garments in cool ambient air

“…Computed results agree well with experimental results reported by Richards, et al (2002) and Havenith, et al (2008). We discuss application of the method to human subjects for whom the rate of sweat secretion, instead of the partial pressure of water on the skin, is specified.…”

“…ISO 7933). Havenith, et al (2008) established that the effect of condensation observed in the simple, cylindrical, two-layer garments studied by Richards, et al (2002) was also observed when similar garments were evaluated on a human manikin.…”

“…Several recent studies have measured heat and vapour transport when condensation occurs within the garment (Richards, et al, 2002, andHavenith, et al, 2008). In addition to measuring cooling rates for ensembles when the skin was either wet or dry, both studies employed a flat-plate apparatus to measure resistances of individual layers.…”

“…Heat transfer through clothing owing to evaporation from skin with condensation in cool outer regions of the garment is discussed in a recent paper by Havenith, et al (2008). Sensible heat transfer by conduction and radiation down the temperature gradient from skin to ambient air and diffusion of water vapour down the water partial pressure gradient provide parallel pathways for heat removal from the skin.…”

“…Under uncomplicated, steady-state conditions, thermal and mass fluxes are uniform within the garment, and the rate of transport is simply computed as the overall temperature or water concentration difference divided by the appropriate resistance. However, that simple computation is not valid under cool ambient conditions when the vapour permeability of the garment is low, and condensation occurs within the garment.Several recent studies have measured heat and vapour transport when condensation occurs within the garment (Richards, et al, 2002, andHavenith, et al, 2008). In addition to measuring cooling rates for ensembles when the skin was either wet or dry, both studies employed a flat-plate apparatus to measure resistances of individual layers.…”