Clothing Evaporative Resistance: Its Measurements and Application in Prediction of Heat Strain

Abstract

Clothing evaporative resistance is one of the most important inputs for both the modelling and for standards dealing with thermal comfort and heat stress. It might be determined on guarded hotplates, on sweating manikins or even on human subjects. Previous studies have demonstrated that the thermal manikin is the most ideal instrument for testing clothing evaporative resistance. However, the repeatability and reproducibility of manikin wet experiments are not very high for a number of reasons such as the use of different test protocols, manikins with different configurations, and different methods applied for calculation. The overall goals of the research presented were: (1) to examine experimental parameters that cause errors in evaporative resistance and to set up a well-defined test protocol to obtain repeatable data; and (2) to apply the reliable clothing evaporative resistance data obtained from manikin measurements and physiological data acquired from human trials to validate the Predicted Heat Strain (PHS) model (ISO 7933).

Most of the calculations on clothing evaporative resistance up until now have been based on manikin temperature rather than fabric skin temperature because the fabric skin temperature was unknown. However, the calculated evaporative resistance has been overestimated because the fabric skin temperature is usually lower than the manikin temperature. This is mainly due to that water evaporation cooling down the fabric skin. In Paper I, the error of using manikin temperature instead of fabric skin temperature for evaporative resistance calculation was examined. In Paper II, a universal empirical equation was developed to predict wet skin temperature based on the total heat loss obtained from the manikin and the controlled manikin temperature. Paper III investigated discrepancy between the two options for the calculation of clothing evaporative resistance and how to select one of them for measurements conducted in a so called isothermal condition. Paper IV studied localised clothing evaporative resistance through an inter-laboratory study. The localised dynamic evaporative resistance caused by air and body movement was examined as well. In addition, reduction factor equations for localised evaporative resistance at each local segment were established.

The thermophysiological responses of eight human subjects who wore five different vocational garments in various warm and hot environments were investigated (Paper V and Paper VI). The PHS model was validated by those human trials. Some suggestions on how to revise this model in order to achieve wider applicability were discussed and proposed.

The results showed that the prevailing method for the calculation of evaporative resistance can generate an error of up to 35.9% on the boundary air layer’s evaporative resistance Rea. In contrast, it introduced an error of up to 23.7% to the clothing total evaporative resistance Ret. The error was dependent on the value of the clothing intrinsic evaporative resistance Recl. The isothermal condition is the most preferred test condition for measurements of clothing evaporative resistance; the isothermal mass loss method is always the correct option to calculate evaporative resistance. The reduction equations developed for localised clothing evaporative resistance have demonstrated that a total evaporative resistance value provided very limited information for local clothing properties and thus, localised values should be reported. The skin temperatures predicted by the PHS model were greatly overestimated in light clothing and high humidity environments (RH>80%). Similarly, the predicted core temperatures in protective clothing FIRE in warm and hot environments were also largely overestimated. The predicted evaporation rate was always much lower than the observed data. Therefore, a further revision of this model is required. This can be achieved by performing more human subject tests and applying more sensitive mathematical equations.

Abstract (Swedish)

Popular Abstract in English

Clothing plays an important role in our lives. It serves four main functions: adornment, status, modesty and protection. Wearing popular clothing with one’s favourite decorations, contributes to a person reaching his or her mental comfort. Clothing is also a symbol of status, and was particularly so in ancient times. Moreover, it protects the human body from injury from abrasions, scratches, fire, radiations, and insect bites and helps the body maintaining core temperature.

From a heat transfer point of view, clothing acts as a thermal and moisture barrier. In cold weather, it is always good to have such a thermal barrier to prevent body heat loss. But in hot environments, clothing can greatly hinder sweat evaporation and heat dissipation. Construction workers and fire-fighters, for example, should wear protective clothing whatever the environment. They usually have a very high metabolic rate. If the heat produced cannot be balanced by sweat evaporation and/or dry heat losses, their body core temperature will rise. As body heat storage and core temperature increase, work performance will be greatly impaired, and the high body core temperature may eventually threaten their lives.

Evaporative resistance is one of the most important factors in quantifying and characterising the role of clothing as a moisture barrier. The research reported in this thesis examined several potential factors that may cause manikin measurement errors in clothing evaporative resistance. The findings can help designers to optimise functional protective clothing. They can also be a help in standardising test protocols and in enhancing measurement accuracy. An example of using clothing evaporative resistance in a heat strain model is given. The results of human trials presented in this thesis provide a picture of how humans physiologically respond to various thermal environments and protective clothing systems. Such studies contribute to the body of knowledge on how human respond to various environments.

• author: Wang, Faming
• supervisor: Ingvar Holmér; Kalev Kuklane; Chuansi Gao
• opponent: Prof. Daanen, Hein, VU University
• organization: Ergonomics and Aerosol Technology; Thermal Environment Laboratory-lup-obsolete (research group)
• publishing date: 2011
• type: Thesis
• publication status: published
• subject: Production Engineering, Human Work Science and Ergonomics
• keywords: thermal manikin, clothing ensemble, evaporative resistance, localised evaporative resistance, thermophysiological response, heat stress, heat strain, the Predicted Heat Strain (PHS) model
• in: Publication No.45
• pages: 190 pages
• defense location: Sal A:C, A-huset, Sölvegatan 24, Lund
• defense date: 2011-12-09 13:15:00
• ISSN: 1650-9773
• ISBN: 978-91-7473-158-3
• language: English
• LU publication?: yes
• id: 3bb99101-6c2f-4597-8aab-5b98917e412a (old id 2065074)
• date added to LUP: 2016-04-01 14:04:50
• date last changed: 2019-05-24 08:34:13