In a gas, loaded with moisture (gaseous water), while cooling down at a specific temperature liquid water will occur in the form of fine mist suspended in the gas or dew on surfaces by means of condensation. This specific temperature is called dew-point temperature or simply dew-point.
For many application processes neither liquid water at all nor even the tendency of forming liquid water within the gas is wanted or permitted. Therefore drying methods are applied in order to reduce the dew-point of the process gas (e.g. compressed air) so no condensation will take place and thus no liquid water can or will form within the process gas any more.
The dew-point, simply to understand since given as a temperature in °C or °F and thus directly related to many processes parameters, is the most favoured expression when measuring or indicating the moisture content in a gas. Compared to indications of a concentration, which describes the ratio of amounts of water and gas (e.g. stated as g/m³), the dew-point describes the "behaviour" of the moisture within the gas depending on the gas temperature. Thus, the dew-point itself is not suitable for a mass balance indication, but can be converted by means of formulas or tables (see water vapour content table). In return there is a direct relationship between the dew-point and the relative humidity (rh). Identical gas temperature and dew-point temperature means a relative humidity of 100% and thus a gas saturated with moisture.
Furthermore, it’s important to distinguish between dew-point and pressure dew-point:
• dew-point is related to non-pressurised, atmospheric air (atmospheric dew-point)
• pressure dew-point is related to a compressed gas (e.g. compressed air) at a specific operating pressure
When expanding a compressed gas to atmospheric pressure, its volume expands. The moisture, suspended in the gas, is dispensed to a bigger volume – the gas gets more dry. As a consequence, the dew-point of a gas, measured at atmospheric conditions, is much lower than the pressure dew-point of the same gas, measured in compressed condition. Therefore when indicating a dew-point always the related pressure must be stated as well in order to avoid a mix-up and as a consequence mistakes when selecting and sizing a dryer.
Dew-points of compressed air dryers are usually stated as pressure dew-points, but it's worth to double-check! In case, atmospheric dew-point and pressure dew-point can be converted into each other.
Summertime – sitting in a beer garden. The waitress serves a cold glass of beer. Fantastic – but something’s going on at the beer glass. The cold glass cools down the warm, non-saturated but moisture containing ambient air on its surface. As long as the temperature does not fall below the dew-point, the glass stays dry. Below dew-point, the moisture condenses to water on the cold glass surface – first as a thin wet film, later forming even droplets.