Air is a gas mixture of 78.1% nitrogen, 20.9% oxygen, 0.9% argon, 0.03% carbon dioxide and 0.01% water vapour as well as trace amounts of other gases such as helium, neon, krypton or xenon. Air, which consists solely of the above ingredients is dry air. Air combined with water vapour is moist air. Air is always moist, from “dry” desert air to a London fog, the air will always contain a mixture of air and water vapour.
Psychrometry is the science associated with moist air (Psychros (Greek): Cold, fresh). This is a part of thermodynamics and deals with the thermodynamic properties of air.
Absolute humidity is the mass of water vapour x contained in 1 kg of dry air; the result is therefore always (1+x)kg air-/water vapour mixture.
Relative air humidity is the moisture contained in the air, relative to the amount of moisture that the air could contain at the same temperature and 100% saturation with water vapour. It is normally stated as a percentage.
The density of air changes with its moisture content.
h,x-diagram (according to Mollier) for moist air is a psychrometric chart. The diagram shows steam/air mixtures, which gives an indication of various air states, dependent upon air temperature, air density, absolute humidity and specific enthalpy.
Humidification performance is the quantity of steam or aerosols, that a humidifier or air humidification system can normally produce in an hour. It can be determined, amongst other methods, using the h,x diagram.
Adiabatic humidification arises during air humidification using atomisers or atomising systems. These units produce aerosols, which require energy to evaporate. They take this energy from the surrounding ambient air in the form of heat. Therefore, adiabatic humidification has an additional cooling effect.
Aerosols are gases in which solid (smoke) or liquid (mist) substances are suspended in the form of fog-like distributed particles. In air humidification, aerosols are considered to be the smallest water droplets with a size of approximately 1-20 µ. They are produced by atomisers and transported into the room air. Humidification with aerosols is always adiabatic.