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Evaporative Cooling: effective latent heat of evaporation in relation to evaporation distance from the skin

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Author

  • George Havenith
  • Peter Bröde
  • den Hartog Emiel
  • Kalev Kuklane
  • Ingvar Holmér
  • Rene M Rossi
  • Mark Richards
  • Brian Farnworth
  • Xiaoxin Wang

Summary, in English

Calculation of evaporative heat loss is essential to heat balance calculations. Despite recognition that the value for latent heat of evaporation, used in these calculations, may not always reflect the real cooling benefit to the body, only limited quantitative data on this is available which has found little use in recent literature. In this experiment a thermal manikin (MTNW, Seattle) was used to determine the effective cooling power of moisture evaporation. The manikin measures both heat loss and mass loss independently allowing a direct calculation of an effective latent heat of evaporation (λeff). The location of the evaporation was varied: from the skin or from the underwear or from the outerwear. Outerwear of different permeabilities was used and different numbers of layers were used. Tests took place in 20ºC, 0.5 m.s-1 at different humidities and were performed both dry and with a wet layer allowing the breakdown of heat loss in dry and evaporative components.

For evaporation from the skin λeff is close to the theoretical value (2430J.g-1), but starts to drop when more clothing is worn, e.g. by 11% for underwear and permeable coverall. When evaporation is from the underwear, λeff reduction is 28% wearing a permeable outer. When evaporation is from the outermost layer only, the reduction exceeds 62% (no base-layer) increasing towards 80% with more layers between skin and wet outerwear. In semi- and impermeable outerwear the added effect of condensation in the clothing opposes this effect. A general formula for the calculation of λeff was developed.

Publishing year

2013

Language

English

Pages

778-785

Publication/Series

Journal of Applied Physiology

Volume

114

Issue

6

Document type

Journal article

Publisher

American Physiological Society

Topic

  • Physiology

Keywords

  • sweat Latent heat of evaporation evaporative cooling efficiency protective clothing wicking indirect calorimetry

Status

Published

Project

  • EU project “THERMPROTECT, Assessment of Thermal Properties of Protective Clothing and Their Use”, contract G6RD-CT-2002-00846

Research group

  • Thermal Environment Laboratory

ISBN/ISSN/Other

  • ISSN: 1522-1601