Moisture in air is essential for all forms of life. The water surfaces of brooks, rivers, lakes and seas evaporate and mix with air in the gaseous state. Evaporation can also cause moisture to float in air as finely divided droplets of varying size. These droplets then form a stronger or weaker fog or mist. Such floating droplets arise, on the one hand, when vapour of a liquid is being condensed, on the other, if they are picked up by the air current from an existing accumulation of liquid or are detached from a flowing liquid. If liquids occur in the form of floating droplets, they are referred to as liquid aerosols. Depending on droplet size, one can subdivide: - Spray fog in droplets from 100 µm and more - Fine spray fog with droplets from 10 µm and more - Fog or mist with droplets from 1- 10 µm - Aerosols with droplets from 0.1 - 1 µm Droplets of less than 1 µm size count among floating substances. They can be either of a harmless or dangerous nature such as acid mist or petrol exudation or may simply cause a nuisance such as tobacco smoke for some non smokers, this consisting of microscopically fine droplets. Atmospheric air always contains a larger or smaller quantity of moisture in invisible, unsaturated vapour form and this exerts a certain vapour pressure. The quantity of vapour, which can be contained by 1 m3 of air, is limited and depends solely on the temperature of the air. The water vapour mixes with the air as a gaseous component. At high air temperatures, a relatively large quantity of vapour, up to saturation point, can be carried. Every kilogramme of air contains a certain quantity of water in grammes. This value x in g/kg is the absolute air humidity or also degree of humidity and is a result of the ratio of the quantity of water picked up to the mass of dry air. To establish what quantity of gaseous water vapour is contained in 1 kg of air, and which part thereof is transformed into condensed water upon cooling, one uses formula 1.8.1. For air and water the following is valid: RL= 29.27 mkg/kg °K
RD= 47.1 mkg/kg °K
Formula 1.8.1
Here pD is the saturation pressure obtained at a specific temperature referred to the barometric pressure pD + pL. Saturation Pressure / Atmospheric Pressure
Diagram 1.8.1
Air with a maximum concentration of water vapour is saturated. If the air contains less vapour, it is unsaturated and can pick up further water vapour right up to the saturation limit. If the air contains more water vapour than corresponds to the degree of saturation, the surplus vapour precipitates in the form of water mist. The temperature at which a quantity of air is saturated by water vapour is described as saturation temperature or dewpoint temperature. Thus the saturation quantity is the maximum quantity of water which can be carried by the air at a given temperature. Absolute Humidity / Atmospheric Air
Diagram 1.8.2
The relative humidity is formed by the relationship between the water vapour pressure and the saturation pressure and is expressed as a percentage. If both pressures have the same value, the air is saturated with moisture. If the partial pressure of the vapour is higher, the latter liquifies, forming small droplets which precipitate as condensate. If, however, the partial pressure of the water vapour becomes smaller, the air is less than saturated with humidity. The maximum quantity of water vapour in grammes which can be absorbed by the air, depends solely on temperature and volume but not on the pressure. With rising temperature, the volume expands, the space between the molecules is enlarged and the air can pick up more water vapour.
Table 1.8.1 - Absolute humidity of saturated air in g/m3| °C | g/m3 | °C | g/m3 |
|---|
| 1 | 5,15 | 26 | 24,38 |
| 2 | 5,52 | 27 | 25,78 |
| 3 | 5,92 | 28 | 27,22 |
| 4 | 6,35 | 29 | 28,77 |
| 5 | 6,80 | 30 | 30,36 |
| 6 | 7,28 | 31 | 32,02 |
| 7 | 7,77 | 32 | 33,78 |
| 8 | 8,29 | 33 | 35,64 |
| 9 | 8,84 | 34 | 37,57 |
| 10 | 9,40 | 35 | 39,60 |
| 11 | 9,99 | 36 | 41,72 |
| 12 | 10,65 | 37 | 43,91 |
| 13 | 11,35 | 38 | 46,20 |
| 14 | 12,07 | 39 | 48,62 |
| 15 | 12,82 | 40 | 51,14 |
| 16 | 13,63 | 41 | 53,76 |
| 17 | 14,48 | 42 | 56,49 |
| 18 | 15,37 | 43 | 59,35 |
| 19 | 16,32 | 44 | 62,34 |
| 20 | 17,29 | 45 | 65,44 |
| 21 | 18,31 | 46 | 68,63 |
| 22 | 19,38 | 47 | 71,92 |
| 23 | 20,53 | 48 | 75,40 |
| 24 | 21,74 | 49 | 79,08 |
| 25 | 23,04 | 50 | 82,98 |
Table 1.8.1 shows the maximum quantity of moisture in the saturated state at 100% relative humidity. The quantity of moisture is always referred to 1m3 of air. To facilitate calculations with moist air and to demonstrate the changes of state in a clear manner, the following simplified ix-diagram by Mollier should serve. The diagram is an oblique angled co-ordinate system containing the x values on the abscissa sloping towards bottom right containing the enthalpy (1+x) kg. i;x Diagram for moist air according to Mollier
Diagram 1.8.3
To facilitate the reading of the x values there is, in addition, a horizontal auxiliary axis. The saturation curve for the absolute pressure of 760 Torr (1 atmosphere) has been drawn in, as this separates the zone of unsaturated air from that of over- saturated air. Furthermore, the lines of equal relative humidity and equal density have been entered.
