The Principles of Sauna Physics

• Published in: Energy procedia 2015-11, Vol.78, p.1907-1912
• Main Authors: Nore, K., Kraniotis, D., Brückner, C.
• Format: Article
• Language: English
• Subjects: hygrothermal mass; hygrothermal transport; latent heat exchnage; moisture buffer capacity
• ISSN: 1876-6102; 1876-6102
• DOI: 10.1016/j.egypro.2015.11.361

The interaction of the wood surface and indoor climate has been of increasing interest the last decade. The fluctuating air humidity reacts with the wood surface to seek equilibrium moisture content. This reaction is crucial when experiencing a sauna. The instant heat contribution of the latent heat from the released damp when pouring water on the oven gives the intense perceptible sauna experience. This paper shows a set of measurements and calculations conducted in a Norwegian sauna. One sauna event includes in three parts. The heating of the sauna, increasing the temperature and drying out the wood. Then when visiting the sauna is, with the changes between warm and cold, like taking a cold bath in between being heated in the sauna. Usually also this heating includes pouring water on the oven. The physics around this moistening of the sauna is of special interest in this paper. Finally, the visitors leave the sauna and it cools down. The focus in this paper is on the increase of temperature when pouring water on the oven in the sauna. Different moisture protocols are ran with varying amounts of moistening. The protocols follow real sauna experiences. Surface temperature is carefully measured and recalculated. The key role of the wood surface sorption, providing this latent heat phenomenon, is presented in the results. The sensible heat increase is not due to the change of humidity, as often stated. The heat conductivity is actually higher in dry air compared to moist air. The damp transport energy from the sauna oven through high enthalpy in the air. The damp is absorbed in the dried wood surface and consequently emits latent heat energy back to the room. That is, the surfaces becomes substantial heating panels. The heat release due to wood surface sorption is an extreme employment of the potential of the latent heat. The authors believe that this effect may be significant also in other cases, with smaller water potentials. This is elaborated in the paper.