- If a liquid or a firm body becomes gaseous, one calls this procedure „ evaporation “. The generated gas is called “vapor” or „ steam “. Those gases are called vapor, when evaporating at normal temperatures & pressures.
- Under appropriate physical conditions, vapor can return again to the liquid (condensation) or firm phase (sublimation).
- Vapor exerts, like every other gas, a pressure, which is called „vapor pressure “.
- Vapor pressure is material- and temperature-dependent gas pressure and describes the ambient pressure, below which a liquid (at constant temperature) begins to turn into the gaseous condition.
- Saturated vapor pressure: In case, the temperature of a closed system is constant, an equilibrium adjusts itself between liquid and gaseous phases. The sated gaseous phase has a vapor pressure, called the saturated vapor pressure. If the pressure in a system increases the saturated vapor pressure the vapor condenses.
- As long as there is liquid in a system, while evacuating the system pressure cannot fall below the saturated vapor pressure, because constantly liquid evaporates.
- Water and its vapor pressure require special attention in vacuum engineering, since water vapour occurs as air humidity in large quantities. It is difficult to remove water from evacuation systems.
Vapor pressure of water at different temperatures
|Temperature [°C]||pressure [mbar]|
Vapor pressure of different liquids at 20°C
|Vacuum-oil||10-3 .. 10-9|