We’re excited to head down to Nashville for the Furnaces North America Conference and Expo. If you plan on attending the show, please be sure to stop by our booth number 418 to learn more about our latest vacuum furnaces, and most importantly… we’re interested in learning about you, your business, and your needs. Our […]
Archives for September 2014
Vapor Pressure and Evaporation in Vacuum Furnaces
Knowledge of vapor pressure and rates of evaporation of various materials is valuable information for those operating vacuum furnaces, whether we are heat treating or brazing at high temperature and low vacuum levels or dealing with outgassing at very low temperatures and pressures.
When we think about a solid or liquid in a sealed vessel, we find that, even at room temperature and atmospheric pressure, there are molecules that leave the surface and go into the gaseous phase. The gas phase thus formed is called a vapor. The process of forming a vapor is known as evaporation and the rate of evaporation is determined by the temperature of the substance involved. In time, some of the evaporated molecules will, in all likelihood in the course of random movement, strike and stick to the surface of the vessel. This process is known as condensation and the rate of condensation is determined by the concentration of gas molecules (that is, the pressure of the evacuated gas). Eventually, the number of molecules leaving the surface of the substance is equal to the number returning to it (that is, the evaporation rate equals the condensation rate) and we have dynamic equilibrium. The (partial) pressure at which this occurs is known as the vapor pressure of the substance.2 Below this pressure, surface evaporation occurs faster than condensation, while above it, surface evaporation is slower.