Book - School of Science and Technology

Criteria which affect human comfort ± definitions 9Table 1.6 Moisture content of saturated air at various temperatures (kg/kg of dry air)Temperature( C)Moisture(kg/kg)Temperature( C)Moisture(kg/kg)Temperature( C)Moisture(kg/kg)0 0.0038 18 0.0129 36 0.03892 0.0044 20 0.0148 38 0.04374 0.0050 22 0.0167 40 0.04916 0.0058 24 0.0190 42 0.05518 0.0067 26 0.0214 44 0.061710 0.0077 28 0.0242 46 0.069212 0.0088 30 0.0273 48 0.077514 0.0100 32 0.0308 50 0.086816 0.0114 34 0.0346 52 0.0972NoteAt 20 C, the moisture contents at saturation for altitudes of0.0131 kg/kg and 0.0189 kg/kg respectively.1000 m (113.9 kPa) and ‡2000 m (79.5 kPa) areHumidityThe humidity of air is a measure of the water vapour which it contains and absolutehumidity is expressed in terms of the mass of water (or water vapour) per unit mass of dryair (kg/kg) and not per unit mass of the mixture. The greatest mass of moisture which, atatmospheric pressure, can exist in a given quantity of air is dependent upon temperatureas may be seen from Table 1.6, the effect of pressure variation, for two extreme values,being given in a footnote. This condition is known as a state of saturation and if air,virtually saturated at an elevated temperature, be cooled, then a temperature is soonreached where the excess moisture becomes visible in the form of a mist, or as dew or rain.In most practical situations, both externally and within a building, the air will not besaturated and a water vapour content which may be measured relative to that situationwill exist. Two terms are used to quantify the moisture content held, the familiar relativehumidity and the less well-known percentage saturation. In fact, the precise definition ofthe former is far removed from popular usage and the latter, which is a simple ratiobetween the moisture content at a given condition and that at saturation, is now moregenerally accepted for use in calculations.Humidity levels affect the rate of evaporation from the skin and the mucosal surface. Lowhumidities may cause eye irritation and dry nose and throat. High humidities reduce the rateof evaporation from the skin and support growth of organisms, fungi and house mites.Levels between 40% and 70% relative humidity are considered to be the limits of acceptabilityfor general applications with control between 45% and 60% for optimum comfort.IonizationIonisation of gases in the air creates groups of atoms or molecules which have lost orgained electrons and have acquired a positive or negative charge in consequence. In clean,unpolluted air, ions exist in the proportion of 1200 positive to 1000 negative per cm 3 of airbut, in a city centre, the quantities and proportion change to 500 positive to 300 negativeper cm 3 . In a building ventilated through sheet metal ductwork, and furnished withsynthetic carpets and plastics furniture, the quantities and proportion will reduce stillfurther to perhaps 150 positive to 50 negative ions per cm 3 .