Classical approach to heat and vapour resistance calculations cannot explain heat transfer in wet clothing

In this paper, we describe manikin experiments carried out on the effects of different underwear (one layer only) combined with impermeable or more or less permeable coveralls. The underwear was either dry or wet, in order to study the changes in the protective clothing insulative proper-ties as related to the materials under an impermeable coverall and under the influence of the climatic conditions, i.e. ambient temperature. The manikin was installed hanging in an upright position in the centre of the climatic chamber at CEPA and was always operated statically, i.e. without any body movement. Tests were carried at climate temperatures of 10 degrees, 20 degrees and 34 degrees C and an air velocity of 0.5 m/s. The water partial vapour pressure was maintained at 1 kPa throughout. The manikin head (4 zones), hands (2) and feet (2) were not included in the calculations. Thus the remaining covered surface still included 28 zones. All manikin surface temperatures were maintained at 34 degrees C and heat fluxes were measured continuously. In the case of wet clothing, the approach was as follows: the underwear was weighed dry and then immersed into water heated to 34 degrees C, tumbled and weighed again until it contained the required amount of water (from 125 to 500g). The underwear was immediately put into a plastic bag and then put as quickly as possible onto the manikin while equipping it also with the sensors. The manikin was afterwards turned ON. In addition to the usual results, in terms of heat fluxes and dry insulation for each condition, we obtained both the measured dry heat loss at 10 or 20 degrees C and the measured evaporative heat loss at 34 degrees C over the whole test period. We could therefore compute for the wet condition at 10 degrees and 20 degrees C the theoretical cumulative dry and the evaporative heat loss which we could then compare with the observed data. Generally, it was found that there was some energy lost from the manikin that could not be explained from the cumulative dry and humid heat losses. It must be admitted that the wet process involved some additional heat exchange. Averaging this extra heat loss from the manikin produces mean values corresponding to an instantaneous heat loss of 35 to 42 Watts. The thermal imbalance found in our condition of wet underwear under an impermeable coverall implies that some other thermal phenomenon occurred which disturbed the simple calculations usually done. In addition to a likely small experimental bias (some evaporation probably occurring during the setting up of the manikin), mechanisms of evaporation and later condensation inside the clothing layers are suspected which cannot be neglected.