Understanding Surface Sweating

The sweat control provided by cold pipe/sheet insulation depends on the surface temperature, the dew point of air and the thermal and surface characteris tics of the insulation. The objective of the present research is to observe and better understand how surface sweating on closed cell foam insulation occurs at different relative humidities and air temperatures. An apparatus was built to measure the sur face temperature of foam insulation on a cold plate at various air temperatures and relative humidities.

From the analysis of the experimental data, it can be concluded that the surface of an insulation would visibly sweat only if adverse conditions of air temperature and relative humidity are sustained for rather long periods of time. The reasons are a strong dependence of surface sweating on surface temperature, an increase in the convective heat transfer coefficient with relative humidity at constant air tempera ture, and a possible condensation of a thin invisible layer of water on the surface.