However, neither liquid water nor the tendency of forming liquid water within the gas are wanted or permitted for many application processes. Thus, drying methods are applied in order to reduce the dew-point of the process gas (e.g. compressed air) in a way that no condensation takes place and no liquid water forms within the process gas.
The dew-point is a temperature measured in °C. Therefore, it is easy to understand and directly related to many process parameters, which is why the dew-point is a favored expression when measuring or indicating the moisture content in a gas.
Compared to indications of a concentration, which describe the ratio of amounts of water and gas (e.g. stated as g/m³), the dew-point describes the "behavior" of moisture within a 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 vapor content table).
However, there is a direct relationship between the dew-point and the relative humidity (rh). Identical gas and dew-point temperatures mean a relative humidity of 100% and thus a gas saturated with moisture.
Furthermore, it is important to distinguish between dew-point and pressure dew-point:
is related to non-pressurized, atmospheric air (atmospheric 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, solute in the gas, is dispensed to a bigger volume – the gas gets drier. 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 conditions. Therefore, when indicating a dew-point, the related pressure must always be stated as well, in order to avoid a mix-up and thus mistakes when selecting and sizing a dryer.
Dew-points of compressed air dryers are usually stated as pressure dew-points, but it is always worth to double-check! In case it should be necessary, atmospheric dew-points and pressure dew-points can be converted into each other.