Humidity is a key factor in drying processes. It expresses the amount of vapour in the carrier gas stream (usually air). However, there is often confusion between relative and absolute humidity. Absolute humidity is a measure of the mass of moisture in the air (g/kg, kg/ kg or g/m3 ); relative humidity (RH) is a percentage of the saturation humidity at the given temperature. Either may be important in different situations. For example, absolute humidity dictates the heat and mass balance, while relative humidity influences the equilibrium moisture content. However, a change in temperature has a major effect on RH, whereas absolute humidity is unchanged if it is quoted in mass units (e.g. g/kg). Ideally, both instrument displays and plant control rooms should show both relative and absolute humidity. Even for absolute humidity, there is confusion over terminology. Often, it is defined as a volumetric concentration in g/m3 or kg/m3 . This is acceptable in fields such as meteorology and air-conditioning which deal with air-water systems close to atmospheric pressure and ambient temperature. However, it is very inconvenient for dryer calculations because the volume varies too much over the wide range of temperatures encountered. Hence, in drying and other chemical engineering processes, “absolute humidity” is usually defined on a mass basis, and expressed in units of kg (moisture) per kg (dry gas). In other disciplines this is often called the “mixing ratio”. Users should always look at the stated units when interpreting humidity data!
Likewise, confusion often arises because volumetric flowrates differ substantially depending on whether they are stated at operating temperature (maybe 100-200° C, or even higher) or ambient temperature. Even for so-called “standard” flows (e.g. SCFM, standard cubic feet per minute) several different values are used for the “standard” temperature (ranging from 00 C to 250 C), and calculation errors can result. Hence, in drying calculations it is generally best to use mass flowrate in kg/s (or kg/h) and mass velocity in kg/m2 s (mass flux) in preference to volumetric flowrate (m3 /s) and velocity (m/s) respectively. Air flowmeters should preferably display both volumetric and mass flowrates Dewpoint is directly related to the absolute humidity (and total pressure); for example, an air-water mixture with 7.5 g water vapour per kg of air has a dewpoint of approximately 10° C (at typical atmospheric pressures).
An important step forward in reducing confusion and standardising best practice is the recent updating of British Standard BS 1339 on Humidity. Part 1 (2002) contains rigorous definitions and formulae, both for the air-water system and other solvent-gas systems, including interconversions between all the parameters given above. Part 2 (due for publication 2007) covers calculation methods, based on a spreadsheet with inbuilt functions for all the key humidity transformations. This allows users to generate calculations or tables for any desired set of conditions, and replaces several reams of lookup tables. Finally, Part 3 (2004) is a practical descriptive guide to the measurement of humidity and dewpoint.