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

Abstract

Havenith G, Bröde P, den Hartog E, Kuklane K, Holmer I, Rossi RM, Richards M, Farnworth B, Wang X. Evaporative cooling: effective latent heat of evaporation in relation to evaporation distance from the skin. J Appl Physiol 114: 778 -785, 2013. First published January 17, 2013 doi:10.1152/japplphysiol.01271.2012.-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, WA) 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 (eff). 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 20°C, 0.5 m/s 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, eff is close to the theoretical value (2,430 J/g) but starts to drop when more clothing is worn, e.g., by 11% for underwear and permeable coverall. When evaporation is from the underwear, eff reduction is 28% wearing a permeable outer. When evaporation is from the outermost layer only, the reduction exceeds 62% (no base layer), increasing toward 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 eff was developed.

Tactile cues significantly modulate the perception of sweat-induced skin wetness independently of the level of physical skin wetness

“…Since Gagge's seminal work (1937), the measurement of w has received great attention, particularly in the context of predicting the body's heat balance during conditions of increased metabolic heat production (e.g., resulting from exercising muscles) and decreased gradient for heat loss to the environment (e.g., resulting from high ambient temperatures) (Candas et al 1979;Havenith 2001;Havenith et al 2013;Nadel and Stolwijk 1973). However, although much is known on the biophysical role of w in contributing to thermal homeostasis, surprisingly little has been done to elucidate how humans sense wetness on their skin and how the level of "physical" skin wetness relates to the level of "perceived" skin wetness (Montell 2008).…”

A review on advanced imaging technologies for the quantification of wicking in textiles

“…Adapted from Hu et al 44 based on the wicking speed in-plane, through-plane and total moisture absorption. The MMT is also described by the AATCC test method 195-2009 4,9 and was used, for example, to evaluate knitted fabrics of bamboo yarns. 50 Wardinigsih and Troynikov 50 investigated bamboo fabrics with the MMT and found that the cover factor (i.e.…”

“…In sports, the reduced cooling efficiency of sweat may increase the risk of heat stress. 4 Perspiration that is not taken up and wicked by the multi-layer protective garments of firefighters and thus left on the skin can lead to steam burns. 5 Wet textiles in contact with the skin have a higher friction coefficient and can cause blisters 6,7 and decubitus in immobile patients.…”

Fabric Cooling by Water Evaporation

“…The overall cooling effects of sweat evaporation for clothed persons has been carried out on human subjects as well as using thermal manikins [3][4][5][6]. However, the outcomes have been difficult to related to clothing design and fabric selection because the data are confounded by heat loss due to the latent heat of vaporization which cannot be assessed separately [7]. The cooling generated by wet clothing, independent of body metabolic heat loss, and the associated latent heat of vaporization, have not been separated.…”