Articles in PresS. J Appl Physiol (January 17, 2013). doi:10.1152/japplphysiol.01271.2012
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-Evaporative Cooling-
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Effective Latent Heat Of Evaporation In Relation To Evaporation Distance From
The Skin
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George Havenith1), Peter Bröde2), Emiel den Hartog3), Kalev Kuklane4), Ingvar Holmer4),
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Rene M. Rossi5), Mark Richards5) Brian Farnworth6) and Xiaoxin Wang7)
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1)
Environmental Ergonomics Research Centre, Loughborough Design School,
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Loughborough University, Loughborough, LE11 3TU, UK;
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Leibniz Research Centre for Working Environment and Human Factors (IfADo),
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Dortmund, D; 3) TNO Defense and Security, NL; 4) Lund University, SE; 5)EMPA
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Materials Science and Technology, CH-9014, St Gallen, CH;6) BF Scientific Inc
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Kelowna, BC,Canada,
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Oxford Brookes University, UK
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Running Head: Effective Latent Heat of Evaporation
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Contact: [email protected]
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Phone: +44 1509 223031
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Copyright © 2013 by the American Physiological Society.
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ABSTRACT
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Calculation of evaporative heat loss is essential to heat balance calculations. Despite
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recognition that the value for latent heat of evaporation, used in these calculations, may
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not always reflect the real cooling benefit to the body, only limited quantitative data on
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this is available which has found little use in recent literature. In this experiment a
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thermal manikin (MTNW, Seattle) was used to determine the effective cooling power of
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moisture evaporation. The manikin measures both heat loss and mass loss
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independently allowing a direct calculation of an effective latent heat of evaporation
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(λeff). The location of the evaporation was varied: from the skin or from the underwear or
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from the outerwear. Outerwear of different permeabilities was used and different
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numbers of layers were used. Tests took place in 20ºC, 0.5 m.s-1 at different humidities
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and were performed both dry and with a wet layer allowing the breakdown of heat loss
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in dry and evaporative components.
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For evaporation from the skin λeff is close to the theoretical value (2430J.g-1), but starts
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to drop when more clothing is worn, e.g. by 11% for underwear and permeable coverall.
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When evaporation is from the underwear, λeff reduction is 28% wearing a permeable
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outer. When evaporation is from the outermost layer only, the reduction exceeds 62%
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(no base-layer) increasing towards 80% with more layers between skin and wet
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outerwear. In semi- and impermeable outerwear the added effect of condensation in the
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clothing opposes this effect. A general formula for the calculation of λeff was developed.
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Keywords: sweat, latent heat of evaporation, protective clothing, wicking, indirect
calorimetry
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1. INTRODUCTION
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Sweat evaporation is considered to be the determining pathway for heat loss once
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environmental temperatures rise, or when heat loss is limited e.g. by protective clothing
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(14). In most research studies, the cooling power of the evaporated sweat is determined
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from the weight change of the (clothed) participant. After correction for respiratory and
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metabolic mass losses, this weight change per unit of time (g.s-1) is multiplied by the
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latent heat of sweat evaporation (J.g-1) to obtain the evaporative heat loss rate (W) (33,
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31, 29).
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Values for latent heat of evaporation (λ) of human sweat have been debated in the
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literature, considering the effects of temperature, humidity, and sweat osmolality, but
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suggested values ranging from 2,696 J.g-1 (13) to 2,595 (27, 31) and 2,398 J.g-1 (29)
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have finally converged to the latent heat of evaporation of pure water (33), only
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dependent on temperature giving a number of 2,430 J.g-1 at 30°C (12).
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On its way from the skin to the environment, the vapor may have to travel through
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clothing (Fig. 1). There it may be sorbed and subsequently desorbed by textile fibers
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(11), it may condensate in outer layers if these are colder than the skin (14, 15, 16, 17,
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18, 24) and subsequently evaporate again. It may be directly ventilated from the
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clothing microclimate through openings in the clothing or may finally diffuse through the
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outer clothing layer into the environment. Each of the phase changes mentioned will
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cause heat to be released or absorbed (24) at the location where it occurs. Once
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moisture is present as liquid, layer to layer wicking may occur (23).
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Based on studies of the moisture transport processes in clothing as discussed above,
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several authors have suggested that the commonly used calculation of evaporative heat
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loss from the clothed mass loss may not always be correct (9, 10, 17, 18, 30, 24).
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Havenith et al. (18) studied the effects of condensation in clothing in relation to ambient
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temperature, and observed that the heat loss observed in clothing with low vapor
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permeability may be higher than suggested by the mass loss when the environment is
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cool. This ‘heat pipe’ effect was later mathematically described by Wissler and Havenith
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(34). On the other hand, Craig and Moffitt (9) studying evaporation in wet clothing, and
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McLellan et al. (26) and Aoyagi et al. (1) looking at the heat balance uncertainties in
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high sweat rates while testing protective clothing in warm environments, also observed
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inconsistencies in the heat balance calculation results. However they observed the
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opposite of what was found in the condensation studies, suggesting that in their case
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evaporative heat transfer rate may be lower than the value obtained from clothed mass
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loss rate. In such studies data were subsequently often corrected to get the heat
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balance numbers to match. e.g. Aoyagi et al. (1) adapted the skin-core temperature
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weighting used in the calculation of body heat storage, while assuming an unchanged
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clothing insulation, Chen et al. (7) assumed changes in dry heat loss with wet clothing,
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while Cheuvront et al. (8) showed that allowing for the clothing insulation to change
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during modeling the sweating response of clothed persons improved the predictive
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capabilities of the model. The real reason for the heat balance mismatch may however
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be found in the values used in the heat balance calculation for latent heat of
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evaporation. Though this physical property of water is quite stable (only a slight effect of
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temperature is present), not all of the cooling power may always benefit the person
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producing the sweat. E.g. in profuse sweating as described by Aoyagi et al., a lot of the
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sweat may migrate (wicking) into the (under)clothing and then evaporate from there. In
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this case, part of the heat for evaporation may be taken from the body, but another part
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from the environment, i.e. the evaporative efficiency ( η ) or the effective latent heat of
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evaporation (λeff) would be reduced (Evaporative heat loss=dWater/dt•(
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dWater/dt•λeff). In wet clothing (Craig and Moffitt (9); Burton and Edholm (6)) also some
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heat of evaporation will be from the clothing, again drawing part of the heat from the
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environment rather than the skin.
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This process, the effect of moisture evaporating at different distances from the skin on
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the cooling power provided, is the topic of this study. Some, mostly qualitative, data on
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this issue are available from studies with small numbers of humans (9, 10), though their
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calculations contain many estimations (e.g. estimated metabolic rate; estimated clothing
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insulation) which substantially increases the measurement uncertainty. The main
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problem in these human participant studies is that actual heat loss cannot be measured
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directly and the calculation using indirect calorimetry introduces many potential sources
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of error. To improve the accuracy of the measurements the present study will be
app)•λ=.
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performed on a thermal manikin (18). This, in contrast to human experimentation, allows
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simultaneous, independent measurements of both heat loss and mass loss, required for
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the calculation of the effective value of λ.
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The hypothesis for this study is that cooling efficiency of evaporation, or the effective
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value of λ, is affected by the location of the moisture evaporation in terms of its distance
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from the skin. In terms of test conditions this is operationalized into whether the
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moisture was placed on the skin surface, placed in the underwear layer, or put in and on
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the outer clothing layer, while manipulating the thickness or presence of other layers.
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Rather than allowing and waiting for the moisture to wick from the skin outwards to the
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underwear and further as would happen in real life, it was decided to study more
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defined conditions: wetting the skin OR the underwear OR the outerwear. Though some
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wicking will occur, in each of these conditions the main locus of evaporation will be that
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of the wetted layer, providing better defined test conditions.
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2. METHODS
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2.1 Manikin
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In order to discriminate between and determine all heat exchanges, measurements
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were made using a thermal manikin (‘Newton’, MTNW, Seattle) (18). This manikin has
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32 zones for which the surface temperature can be controlled independently and the
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total energy input required to achieve this accurately measured. This energy input is a
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direct measure of the heat loss from the manikin. This measurement and the calibration
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of the manikin are described extensively in ISO15831:2004 (20) and ASTM F1291-05
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(2). All ensembles were measured dry and with a wet layer. To provide an evaporative
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surface, the skin consisted of a thin stretch cotton layer (present in all tests), on top of
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the heating layer, that for the wet skin conditions was wetted before dressing and acted
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as a ‘sweating skin layer’ (3, 17). Insulation and Dry heat loss values were corrected for
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the insulation of the skin, while in the wet tests this was assumed to be negligible.
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Continued wettedness of the skin layer was monitored for all individual zones via their
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heat loss rate, which dropped sharply when a zone started to dry out. Apart from heat
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losses, also the mass change rate of the clothed, wet manikin was determined by
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continuous weighing (0.1 Hz) of the whole setup (Sartorius balance 150 kg, precision 1
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g; absolute accuracy to ± 5 g). This allowed continuous determination of the rate of
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water evaporation from the clothing system and thus of the real evaporative mass loss
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rate from the manikin-clothing system. The manikin was placed in front of three fans,
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mounted in a vertical plane, which produced the reference wind speed of 0.5 m·s-1.
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As this paper intends to study the effect of clothing, all measurements and data in this
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paper are calculated for the clothed area (1.46 m2) only. Data from the nude head,
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hands and feet are excluded.
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2.2 Clothing
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Most of the clothing was the same as used by Havenith et al. (18) and Broede et al. (4).
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Four custom-made outer garments were used, identical in design and production, but of
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either impermeable (IMP), semipermeable (SEMI), permeable (PERM) material, or a
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highly permeable material (OPEN), providing four levels of vapor permeability (Table 1).
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These outer layers were tested alone or in combinations with one or more
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representative underwear types of similar design: cotton (CO), polyester (PES) and
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polypropylene (PP) (Table 1), selected to give a similar material heat and vapor
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resistance. Data obtained for different underwear types will be lumped together in the
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analysis. In addition, other combinations of layers were used, manipulating the distance
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of the evaporation locus to the skin or the permeability of outer layers, i.e. the type of
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covering on the outside and inside of the evaporation locus. For the wet outerwear
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condition, results for IMP, SEMI and PERM outer layer were merged. All clothing layers
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fully covered the same surface area of 1.46 m2 for which subsequent calculations were
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made. The main test conditions are defined in Table 2.
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2.3 Climate
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The main testing was performed at 20 (±0.5) ºC. Considering earlier findings (18) this
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implies that apart from dry and evaporative heat loss, some condensation may occur in
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the ensembles with lower permeability. Chamber humidity was adjusted to the expected
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evaporation rate to ensure that the manikin skin remained fully wetted during the whole
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test period (range used between 1 and 1.8 kPa). All results for evaporative heat and
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mass loss were later converted to the same vapor pressure gradient between skin and
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environment, i.e. assuming a 1 kPa vapor pressure in the environment, matching data
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of earlier work (17) using the following equation:
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Evaporative Heat Loss (at 1kPa)=
PH 2O , skin − 1
PH 2O ,skin − PH 2O ,test environment
⋅ Measured Evaporative Heat Loss
(1)
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This allows direct comparison of heat losses between clothing configurations but does
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not change the ratio between the heat loss and the mass loss method values for
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evaporation.
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2.4 Calculations and definition of terms
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The calculations follow those set out by Havenith et al. (18):
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Real Dry Heat Loss:
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DRYreal (W ⋅ m -2 ) = heat loss measured on dry manikin, dry clothing
(2)
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Apparent evaporative heat loss ( Eapp ): Increase in heat loss compared to dry when
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evaporation is present (e.g. when the manikin’s skin or any other layer is wet (i.e. heat
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loss of wet manikin – heat loss of dry manikin; at same temperature). This is referred to
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as ‘apparent’ as apart from evaporation (pathway E) it also includes heat loss due to
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wet conduction and evaporation-condensation (pathways C and D in Fig. 1). That is, it
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includes all changes in heat loss due to the wet layer.
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Eapp (W ⋅ m-2 ) = Total Manikin Heat Loss when wet – DRYreal
(3)
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As in this type of studies the temperature of the outer wet skin surface decreases
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slightly below the setpoint value of the manikin surface itself due to the evaporative
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cooling, the DRY heat loss used in equation (3) was corrected for the lower thermal
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gradient between skin and environment in the wet tests using the equation developed in
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our laboratory similar to those developed by Wang et al. (32):
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Tskin, wet = 34.13-0.012. manikin evaporative heat loss
(4)
Using the equation:
Dry Heat Loss Wet Test=
Tskin, wet − Tambient
34 − Tambient
⋅ Measured Dry Heat Loss
(5)
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though this is only a marginal correction here.
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The common way to determine evaporative heat loss in human experiments is to
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calculate it from the latent heat of evaporation of all mass that is lost from the clothed
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person (corrected for metabolic and respiratory mass changes). In the present testing
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this same value is determined by the mass loss rate of the clothed manikin as:
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Latent heat of Mass lost ( Emass ): the calculated latent heat content of the moisture that is
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evaporating from the ensemble (the “human-clothing-system”) as measured by the
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mass loss rate on the Sartorius scale in a steady state condition:
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Emass (W ⋅ m-2 ) =
dMass
( g ⋅ m−2 ⋅ s −1 ) ⋅ λ ( J ⋅ g −1 )
dt
(6)
where:
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λ = enthalpy of evaporation ( J ⋅ g −1 ) = 0.001× 2.792 ⋅106 − 160 ⋅ T − 3.43 ⋅ T 2{with T in K}
≈ 2430 at 30ºC (ref. 12)
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With these data available, the apparent (Eapp) and observed latent evaporative heat
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losses (Emass) can be compared and the evaporative cooling efficiency or the effective
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latent heat of evaporation calculated:
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Evaporative cooling efficiency ( ηapp ): The apparent evaporative heat loss of the wet
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manikin (or manikin with the clothing layers) divided by the evaporative cooling potential
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(the latent heat of the moisture evaporated) under the same temperature condition.
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ηapp (n.d .) =
Apparent evaporative heat loss of wet manikin Eapp
=
Latent heat of Mass lost
Emass
(7)
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And finally, these results for evaporative cooling efficiency can be interpreted in terms of
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the observed latent heat of evaporation that benefits the body when clothing is worn. If
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evaporative cooling efficiency is 1.0, the latent heat observed by the manikin is equal to
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the theoretical value (2430 J.g-1), while it may be lower if not all latent heat for the
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observed mass loss is taken from the body or higher if more heat is lost than
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theoretically expected based on the mass loss:
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Effective latent heat of evaporation ( λeff ): the measured energy released from the
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manikin surface divided by the observed evaporation rate from the clothed body
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λeff (J ⋅ g−1) =
Eapp
Apparent evaporative heat loss of wet manikin (W ⋅ m-2 )
=
=ηapp ⋅ λ (8)
−2
−1
mass loss rate (g ⋅ m ⋅ s )
dMass dt
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2.5 Tests preparation and protocol
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Once the internal and surface temperatures of the manikin had stabilized, the data
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acquisition started. For the wet skin experiments the ‘skin’ was wetted at this point by
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spraying it with distilled water until fully wet, while no dripping was observed. For the
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wet underwear tests 600g of moisture had been introduced by spraying the underwear
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layer several hours before the testing. The underwear was then packed in impermeable
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bags and left in the test climate for the moisture to be fully and evenly absorbed. The
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underwear was re-weighed at the start of the test to ensure the correct amount of water
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was absorbed. For the wet outer layer tests, moisture was introduced at the outer
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surface of the PERM and IMP outer garment after dressing by spraying these until wet
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and then waiting for any drippage to stop.
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For both dry and wet tests, the manikin was dressed with underwear and outerwear
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according to Table 2. The average heat flux from the manikin was seen to stabilize
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within 20 minutes after wetting or dressing. After 60 minutes the test was terminated
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and all clothing weighed again. In the wet tests, the mass loss (pathway E in Fig. 1) was
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registered (0.1 Hz) by the Sartorius Scale. Mass loss rate was calculated from the slope
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of this curve for the time period when mass loss rate and heat loss rate were both found
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to be stable as judged from the weight reduction slope and the heat loss curve.
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The parameters listed above were determined in a steady state condition of the
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boundary conditions (skin wettedness), typically over a 15 minute period.
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2.6 Statistics
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Statistical tests were performed using SYSTAT (SYSTAT INC. Version 11). P<0.05 was
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taken as significant. Main testing for ηapp was to show significant deviation from unity
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(λeff deviation from 2430 J.g-1)(one sample t-test) and differences between conditions in
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the same clothing ensembles (repeated measures ANOVA with moisture location as
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factor).
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3. RESULTS
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The heat and mass loss based results for the nude manikin and the various clothing
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configurations, as measured at 20ºC, are presented in Fig. 2. A clear reduction in heat
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and mass losses is observed with increasing clothing thickness and reducing vapor
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permeability. For the measurements with a wet skin layer and permeable garments,
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direct heat loss measurement and mass loss based calculations of evaporative heat
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loss are very close, while for wet underwear and outer wear layers, the two parameters
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clearly differ.
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Considering the ratio of measured heat loss to heat loss calculated from mass loss, i.e.
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the evaporative cooling efficiency (Eq. 5) (18) ηapp for the permeable garments (Fig. 3),
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as expected the value for evaporation efficiency of the nude wet manikin is not
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significantly different from 1 (i.e. λeff =2430 J.g-1) , implying that here virtually all of the
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heat of evaporation is actually taken from the body. The same appears to be true for the
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wet manikin with a highly permeable clothing layer (OPEN) or with underwear (UW)
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only. Adding more or less permeable clothing starts to show a reducing effect on ηapp
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and λeff. Though for UW+OPEN, this is only 3% (n.s.), for UW+PERM, ηapp and λeff are
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reduced by 11% (p<0.05).
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When the moisture is placed directly in the UW layer the effect on cooling evaporative
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efficiency becomes much bigger. Wet underwear (over the dry cotton skin) under PERM
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reduces ηapp to 0.72 (λeff =1750 J.g-1; p<0.05), i.e. a reduction in evaporative cooling
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power of 28%.
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In the next step, transferring the wet layer to the outerwear ηapp (and λeff) drop even
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further. It was 0.38 (923 J.g-1) when no UW is worn, 0.25 (607 J.g-1) for one layer of UW
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and 0.22 (535 J.g-1) where a layer of UW and a thick mid layer are worn under the wet
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layer, showing a progressive decline in ηapp and λeff with increasing distance between
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the evaporation locus and the skin.
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For semipermeable and impermeable clothing the effect at this temperature is different
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(Fig. 4). While for wet outerwear no significant differences were observed in relation to
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the permeability of the outer clothing layer, differences are present for wet skin and wet
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UW. Going from PERM to SEMI, a small increase in ηapp and λeff takes place (n.s. for
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wet UW; p<0.05 for wet skin). Going from PERM or SEMI to the impermeable clothing
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material, a huge increase in ηapp and λeff takes place, that brings ηapp to a value
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significantly above 1 (λeff >2430 J.g-1; p<0.05) for both wet skin and wet UW.
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4. DISCUSSION
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The present study confirmed the observation made over five decades ago (6, 9, 10) that
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when clothing gets wet and/or sweat migrates from the skin into the clothing layers, the
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evaporative cooling efficiency drops, i.e. less cooling is provided to the body per gram of
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evaporated sweat/moisture (i.e. λeff is reduced). Nielsen et al. (28) qualitatively showed
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how the heat loss from a manikin changes depending on which clothing layer was wet,
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though they did not analyze this heat loss in relation to the mass loss. Using a thermal
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manikin with both heat loss and mass loss measurements, allowed quantification of this
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effect in the present study. In the older studies due to a large number of assumptions
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and estimates required for calculating the heat balance, the results had to be seen more
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qualitative in nature, explaining why this knowledge is not much used in more recent
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literature. After showing in earlier papers (18) that the amount of energy taken from the
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body for the evaporation of a given quantity of sweat (λeff) is dependent on the clothing
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permeability and on the ambient temperature, the present experiments have now clearly
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demonstrated that this ‘effective latent heat of evaporation, λeff ’ is also dependent on
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the location where the evaporation takes place in terms of its distance to the skin.
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As shown by the results for wet underwear compared to wet skin, a dramatic fall of λeff
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takes place when moisture is wicked away from the skin before it evaporates. Though
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the wicking of sweat from the skin may also have a positive effect when the skin is not
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fully wet (different from this test) by increasing the surface area of evaporation, there
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may be situations where it will lower the total available cooling power. Where both skin
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and clothing are wetted, no risk of dehydration is present, and a large amount of
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ventilation takes place in the clothing, having the extra evaporation from the fabric
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besides that from the skin may also be beneficial. In encapsulated clothing however,
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where the microenvironment may be close to becoming saturated and only a small
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fraction of produced sweat may evaporate, it would be best if this evaporates directly
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from the skin and is not wicked further out before evaporating.
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Data from the present experiment would imply that spraying clothing from the outside
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with water, as used in some field situations (e.g. decontamination work in impermeable
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clothing), will have a cooling efficiency of about 20 to 40% for the amount of evaporated
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water (i.e. a λeff reduced by 60 to 80%). In this condition, if a surplus of water is sprayed
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on to cool the person, additional conductive cooling will take place where this water is
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cooler than the person’s clothing. When this method of spraying is applied for added
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cooling, usually no water shortage is present, making the low efficiency less of an issue.
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Whether this wet outer would reduce evaporation from within the clothing needs to be
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investigated.
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Craig and Moffitt (9) calculated ηapp , or E/E’ as they called it (cf. Eq. 5), for permeable
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clothing (UW+fatigues) wetted with different amounts of water worn by a sweating
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person. In this case evaporation would take place from both the skin and the clothing,
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and the observed ηapp should be between the values for wet skin and for wet clothing
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observed in the present experiment (underwear or outer wear). For suits wetted to 0,
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400 and 1000 grams, they observed ηapp of 0.77, 0.49 and 0.35 (λeff of 1870, 1190, and
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850 respectively). For the dry suit, most evaporation should take place at the skin
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(though during the test sweat will wick into the clothing), and with increasing moisture
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content more of the total evaporation will take place in the clothing. Based on the
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present data (Fig. 3), one would expect ηapp to range between 0.89 (assuming wet skin,
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dry underwear and fatigues) and 0.55 (assuming wet UW and wet fatigues with equal
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distribution of evaporation between the two layers) for these conditions. As Craig and
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Moffitt’s heat balance calculations were based on averages over whole tests, including
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the low sweat period at the start, and as they contained many assumptions and
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estimates (clothing insulation, metabolic rate), some discrepancy is not unexpected.
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However the message is the same.
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Burton (6) performed similar tests (5) though with a different philosophy. He calculated
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the evaporative efficiency of sweat evaporating from the skin, in contrast to the present
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study where the measurement of evaporative efficiency was for the moisture
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evaporated from the manikin-clothing system. He then studied the effect of this moisture
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condensing further out in the clothing. In that case, the condensation energy is released
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between skin and environment and in part may flow back to the skin, thus also lowering
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evaporative efficiency. He defines an equation for evaporative efficiency at the skin (5):
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fE =
total dry insulation of clothing between skin and wet layer
total insulation of dry and wet clothing layers and air
(9)
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This problem of moisture evaporating at the skin and then condensing in the clothing
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has similarities with our problem of moisture evaporating away from the skin to the
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environment. In both cases the heat transfers (between the skin and Burton’s location of
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condensation and in our case the skin and the locus of evaporation) are dependent on
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the insulation of the clothing layers between skin and these loci and on the total
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insulation of the clothing plus the surface air layer. As in most research mass loss is
378
determined for the clothed person (accurately measuring mass loss from the skin would
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350
13
379
require undressing of the subject at intervals), the data analysis for the present paper
380
focuses on clothed mass loss efficiency.
381
The concept of looking at the location of evaporation in relation to the total insulation
382
between skin and environment seems relevant and the data of the present study were
383
re-worked in this respect. Defining the locus of evaporation in terms of the dry clothing
384
insulation between the locus of evaporation and the skin in relation to the total insulation
385
between skin and environment as:
386
Relative evaporation locus (REL) =
387
I total − I skin to wet layer
I total
=
I wet layer to environment
I total
(10)
389
390
with
I wet layer to environment = dry insulation between wet layer and environment (incl surface air layer)
I total = total dry insulation of clothing ensemble (incl surface air layer)
391
Hence if evaporation is from the skin, REL = 1 and if evaporation would take place in
392
the environment away from the clothing REL = 0. For evaporation from the outer
393
surface, REL is defined by the surface air layer insulation in relation to the total
394
insulation of clothing plus air layer (Ia/Itot).
395
Using dry insulation measurements of the various layers used, REL values were
396
calculated and data were plotted in Fig. 5 for the permeable and semipermeable
397
ensembles. A relation is evident suggesting a rather simple physical relation of
398
evaporative efficiency to the amount of insulation between skin and evaporation locus
399
and the amount between the locus and the external environment.
400
For the permeable two layer ensembles, the relationship reads:
401
402
η app = 0.998 ⋅ REL − 0.08
(11)
403
404
and for permeable and semipermeable combined:
405
η app = 1.03 ⋅ REL − 0.09
(12)
406
407
Or, without intercept, as that should theoretically be zero (evaporation away from the
408
clothing in the environment {REL=0} would not cool the body):
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388
14
η app = 0.89 ⋅ REL
409
(13)
410
411
The offset observed from the line of identity in Fig. 5 can be explained by the presence
412
of some wicking during the test. Once a wet layer is placed in an ensemble, some
413
wicking will always occur. This means that the value for REL is overestimated in that
414
case, shifting the points away from the line of identity. As this shift seems to be present
415
across the range of values in Fig. 5, equation (11) or
416
representations than equation (13).
417
In the semipermeable measurement, the values of ηapp and λeff for wet skin (0.95; 2309
418
J.g-1) is slightly higher than in the permeable outer garment (0.89; 2163 J.g-1). Based on
419
the findings of Havenith et al. (18) this must be attributed to first signs of condensation
420
in the clothing at the temperature used (20ºC), which is also responsible for the ηapp
421
value above 1 (λeff > 2430 J.g-1) for the impermeable ensemble (18) (Fig. 4). It should be
422
noted that the values presented here, for evaporation from within the clothing, will
423
change with temperature, due to the effect condensation in clothing will have on the
424
evaporative efficiency (18). For permeable clothing this effect will be minimal, but with
425
decreasing permeability the change will be more pronounced. In the latter case, ηapp
426
and λeff will have their lowest value when ambient temperature is equal to or above skin
427
temperature (no condensation) and will increase with lowering temperature (18).
(12) may be better
429
430
5. Conclusion
431
well as a direct measurement of the mass loss rate due to evaporation, the heat loss
432
from the body per gram of moisture evaporated (the effective latent heat of evaporation,
433
λeff) was determined in relation to the location of the evaporation (skin or underwear or
434
outerwear). For evaporation from the skin this is close to the theoretical value, but starts
435
to drop when more clothing is worn, e.g. by 11% when underwear and a permeable
436
coverall is worn. When evaporation is from the underwear, the reduction is 28% wearing
437
a permeable outer. When evaporation takes place in the outermost layer only, the
Using a thermal manikin that allowed a direct measurement of evaporative heat loss as
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428
15
438
reduction is above 62% (no under clothing) increasing towards 80% with more layers
439
between the skin and the wet outerwear.
440
In semi- and impermeable outerwear the added effect of condensation in the clothing
441
opposes this effect.
442
443
Acknowledgements
444
This
445
“THERMPROTECT, Assessment of Thermal Properties of Protective Clothing and Their
446
Use” (contract G6RD-CT-2002-00846).
work
was
sponsored
within
the
EU
GROWTH
Programme,
project
447
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16
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25. Lotens WA, Havenith G. Effects of moisture absorption in clothing on the human
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ambient vapor pressure on heat strain in protective clothing. Eur J of Appl Physiol
74(6): 518-527, 1996.
27. Mitchell D, Wyndham CH, Atkins AR, Vermeulen AJ, Hofmeyr HS, Strydom NB
and Hodgson T. Direct measurement of thermal responses of nude men resting in
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28. Nielsen R, Toftum J, Madsen TL. Impact of Drying of Wet Clothing on Human Heat
Loss. In: Lotens, W.A., Havenith, G. (eds.): Proceedings of the Fifth International
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(available at www.environmental-ergonomics.org).
29. Monteith JL. Latent heat of vaporisation in thermal physiology. Nature New Biology
236 (64): 1996-1996, 1972.
30. Rossi, RM, Gross R, May H. Water vapor transfer and condensation effects in
multilayer textile combinations. Text Res J 74 (1): 1-6, 2004.
31. Snellen JW, Mitchell D, Wyndham CH. Heat of evaporation of sweat, J. Appl.
Physiol 29: 40-44, 1970.
32. Wang F, Kuklane K, Gao C, Holmér I, Havenith G. Development and Validation of
an Empirical Equation to Predict Wet Fabric Skin Surface Temperature of Thermal
Manikins, Journal of Fiber Bioengineering and Informatics JFBI Vol. 3 No.1 2010.
doi:10.3993/jfbi06201002
33. Wenger CB. Heat of evaporation of sweat: thermodynamic considerations, J. Appl.
Physiol 32(4): 456-459, 1972.
34. Wissler EH, Havenith G. A simple theoretical model of heat and moisture transport
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493
494
495
496
497
498
499
500
501
502
503
504
505
506
507
508
509
510
511
512
513
514
515
516
517
518
519
520
521
522
523
524
525
526
527
528
529
530
531
532
533
534
535
536
18
537
538
539
540
541
542
543
544
Table 1, Underwear and outer layer materials characteristics. Fabric heat resistance, Rct, and vapor
resistance, Re,cl, are measured on a sweating hot plate [ISO 11092;(19)]. im = clothing vapor
permeability index. Air permeability (AP) is measured according to EN ISO 9237 (21). f cl is the
clothing surface area factor [ISO 9920;(22)] of the fully clothed manikin. NM=not measured
Code
Material
545
546
Rct
(m2·K·W-1)
Re,cl
(m2·Pa·W-1)
im
(n.d.)
AP
(l·m-2·s-1)
f cl
(n.d.)
Hygroscopic
Hydrophilic
0.024
0.029
4.2
3.4
0.34
0.51
NM
NM
NM
NM
Hydrophobic
0.026
3.7
0.42
NM
NM
0.107
9.2
0.7
NM
NM
Impermeable 0.007
∞
0
0.24
1.32
Semipermeable
0.023
18.6
0.07
1.98
1.29
Permeable
0.025
5.6
0.25
1.02
1.28
Very
permeable
0.008
1.1
0.44
>1600
1.25
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Underwear
CO
100% Cotton
PES
100%
Polyester
PP
100%
Polypropylene
Middle Layer
ULF
Wool
Outerwear
IMP
PA webbing
with outer
PVC coating
SEMI
hydrophilic
layer with
outer PTFE
membrane
PERM
hydrophobic
layer with
inner PTFE
membrane
OPEN
PERM without
membrane
Moisture
property
19
547
548
549
Table 2: Clothing test configurations
Name
Underwear
Middle
Outerwear
Wet Layer
Layer
-
-
-
Skin
Skin/OPEN
-
-
OPEN
Skin
Skin/Underwear
CO
-
-
Skin
Skin/UW OPEN
CO
-
OPEN
Skin
Skin/UW PERM
Merged data from -
PERM
Skin
SEMI
Skin
IMP
Skin
CO/PES/PP
Skin/UW SEMI
Merged data from CO/PES/PP
Skin/UW IMP
Merged data from CO/PES/PP
UW/UW PERM
CO
-
PERM
underwear
UW/UW SEMI
CO
-
SEMI
Underwear
UW/UW IMP
CO
-
IMP
Underwear
Outer/ no Base
-
-
Merged data from Outer layer
layer
IMP, SEMI and
PERM
Outer/ one Base
CO
-
layer
Merged data from Outer layer
IMP, SEMI and
PERM
Outer/ two Base
layers
CO
Wool
Merged data from Outer layer
(Ulfrotte)
IMP, SEMI and
PERM
550
551
Downloaded from http://jap.physiology.org/ by 10.220.33.3 on June 15, 2017
Skin/Nude
20
552
553
554
Fig. 1, Schematic representation of heat transfer pathways when skin is wetted (18).
555
556
557
Fig. 2, Measured evaporative heat loss and latent heat of mass loss calculated from mass loss rate at
558
the same time point (steady state) for different evaporation loci.
559
560
Fig. 3, Evaporative Cooling Efficiency ( ηapp ) and effective latent heat of cooling ( λeff ), measured in
562
undressed state or for permeable clothing, for different evaporation loci. ηapp and λeff have a fixed
563
relation, with the standard latent heat of water at 30 ºC, 2430 J.g-1, equivalent to an apparent
564
evaporative cooling efficiency of 1.0. For wetted outerwear, all clothing types are included as the
565
vapor resistance of the clothing is then not relevant as the moisture does not travel through it.
566
567
Fig. 4, Evaporative Cooling Efficiency ( ηapp ), measured in dressed state and for different outer
568
clothing types, for different evaporation loci.
569
570
571
Fig. 5, Evaporative cooling efficiency ( ηapp ) and effective latent heat of evaporation ( λeff ), in relation to
572
REL, the ratio of dry insulation on the outside of the wet/evaporating layer to the total dry insulation
573
(eq. 8). ηapp and λeff have a fixed relation, with the standard latent heat of water at 30 ºC, 2430 J.g-1,
574
equivalent to an apparent evaporative cooling efficiency of 1.0. Line represents theoretical relation
575
between ηapp and REL, assuming a linear relation between the two fixed points: REL= 0, ηapp =0
576
(evaporation in the environment) and REL=1, ηapp =1 (for evaporation from the skin).
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561
Fig. 1
Skin
Outer Garment
B
C
Conduction
Radiation
Convection
D
Evaporation
F
Conduction
Radiation
Convection
Wet
Conduction
Condensation
on Outer Layer
po
re
r
ab
nf
i
s
Environment
ic
Environment
E
Wicking
outwards
Environment
A
ga
Environment
rm
e
nt
Outer Garment
Downloaded from http://jap.physiology.org/ by 10.220.33.3 on June 15, 2017
Skin
Ventilation (Dry)
op
en
ing
s
Fig 2
400.0
Heat loss calculated from mass loss
Actual heat loss from the body
300.0
250.0
200.0
150.0
100.0
50.0
0.0
wet skin
wet
under wear
Downloaded from http://jap.physiology.org/ by 10.220.33.3 on June 15, 2017
Heat Loss ((W.m-2)
350.0
wet outerwear
1.2 [2916]
wet skin
1.0 [2430]
wet
under
wear
0.8 [1944]
0 6 [1458]
0.6
0.4 [972]
0.2 [486]
0.0 [0]
Downloaded from http://jap.physiology.org/ by 10.220.33.3 on June 15, 2017
Evap. Co
ooling Efficie
ency (n.d.) [λ
λeff (J.g-1)]
Fig 3
wet outerwear
3.5
Evapora
ative Coo
oling Effic
ciency
3.0
IMP
SEMI
2.5
25
PERM
2.0
1.5
1.0
0.5
0.0
Wet skin
under UW
+ outer
Wet UW
under
outer
Wet Outer
No UW
Wet Outer
over 1 BL
Wet Outer
over 2 BL
Location of Wet Layer in clothing system
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Fig 4
1.2 [2916]
PERM
1
0 [2430]
1.0
SEMI
NUDE/OPEN
0.8 [1944]
0.6 [1458]
0.4 [972]
0.2 [486]
0 0 [0]
0.0
0.0
0.2
0.4
0.6
0.8
1.0
REL
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Evap. Cooliing Efficienc
cy (n.d.) [λefff (J.g-1)]
Fig 5
1.2