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Vapor and Steam

An introduction to vapor and steam.

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Vapor is a gas - there is no significant physical or chemical difference between a vapor and a gas.

  • a vapor is a substance in gaseous state - at a condition where it is ordinarily a liquid or a solid

The most common example of a vapor is steam - water vaporized during boiling or vaporation. The water vapor in the atmosphere is invisible and is often called moist. Knowledge about moist in air is important for the design of air-condition applications - like HVAC systems and industrial dryers. Moist air technology is often called Air Psychrometrics.

Evaporation from a fluid takes place when liquid molecules at the liquid surface have enough momentum to overcome the intermolecular cohesive forces and escape to the atmosphere. When heat is added to a liquid the molecular momentum and the evaporation of the liquid is increased. A reduction of the pressure above a liquid reduces the momentum needed for molecules to escape and evaporation is increased.

  • increased pressure above a liquid - reduces evaporation

This can be observed as lower water boiling temperature at higher altitudes.

Common terms in connection with vapor and steam:


  • Boiling is the formation of vapor bubbles within a fluid. Boiling is initiated when the absolute pressure in a fluid reaches vapor pressure.

Saturated Vapor

Wet Saturated Vapor

  • A wet saturated vapor carries liquid globules in suspension. A wet saturated vapor is a substance in the gaseous state which does not follow the general gas law.

Dry Saturated Vapor

  • A dry saturated vapor is free from liquid particles. All particles are vaporized - any decrease in vapor temperature or increase in vapor pressure, condensates liquid particles in the vapor. A dry saturated vapor is a substance in the gaseous state which does not follow the general gas law.

Super-heated Vapor

  • In super-heated vapor the temperature is higher than the boiling point temperature corresponding to the pressure. The superheated vapor can not exist in contact with the fluid, nor contain fluid particles. An increase in the pressure or decrease in the temperature will not - within limits - condensate out liquid particles in the vapor. Highly superheated vapors are gases that approximately follow the general gas law.

High Pressure Steam

  • Steam where the pressure greatly exceeds the atmosphere pressure.

Low Pressure Steam

  • Steam of which the pressure is less than, equal to, or not greatly above, atmospheric pressure.
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  • Engineering ToolBox, (2003). Vapor and Steam. [online] Available at: [Accessed Day Mo. Year].

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