Vapor and Steam

Introduction to vapor and steam

Vapor is a gas -

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

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

Our most common example of a vapor is steam - water vaporized during boiling or evaporation. The water vapor surrounding us in the atmosphere is invisible and is often called moist. Knowledge about moist in air is important in air-condition applications like HVAC systems and dryers. Moist air technology is called air psychrometrics.

Evaporation from fluids takes place when the 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 the liquid the molecular momentum and the evaporation increases. A reduction of the pressure above a liquid will reduce the momentum needed for molecules to escape the liquid and increase the evaporation.

• increasing the pressure above the liquid reduces the evaporation

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

Common terms used in connection with vapor and steam:

Boiling

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

Saturated Vapor

• Vapor at the temperature of the boiling point which corresponds to its pressure.

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 the vapor temperature or increase in the vapor pressure, will condensate liquid particles in the vapor. A dry saturated vapor is a substance in the gaseous state which does not follow the general gas law.

Superheated Vapor

• In superheated vapor the temperature is higher than the boiling point temperature corresponding to the pressure. The vapor can not exist in contact with the fluid, nor contain fluid particles. An increase in pressure or decrease in 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, that of the atmosphere.

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