Dew point
Air-water system in building structures: brief overview
The study of such a system essentially consists in determining the temperature and relative humidity of the air, as well as their possible variations and transformations (in particular the condensation and evaporation of water).
As far as temperature is concerned, it should be noted how it can vary in relation to humidity, even in the absence of heat exchange with the walls of a given room. Indeed, it can happen that part of the vapour contained in an air mass spontaneously condenses on particles suspended in the air itself (dust, etc.), which act as condensation nuclei, and the reverse process can also occur. Since the transition of water from the aeriform to the liquid state (or vice versa) is accompanied by heat transfer (or absorption), these processes (called phase transitions) alter the temperature of the air.
Example:
the temperature rise of 1 kg of air caused by the condensation of 1 gram of water vapour is approximately 2.5°C. As far as humidity is concerned, some basic concepts and definitions are given below.
Absolute humidity (AH):
is the mass of water vapour contained in the unit volume of air. It is expressed in grams of vapour per cubic metre of air.
Saturated vapour pressure (p0):
is the vapour pressure at which, for a given temperature, liquid and vapour are in equilibrium.
A vapour is said to be saturated when it is in the presence of its liquid phase and is in equilibrium with it. Under saturated conditions, the mass of vapour present in a given volume is the thermodynamically possible maximum; its value depends on the temperature and increases as it rises.
For temperatures between –5°C and 35°C, the mass of saturated vapour can vary from 4 g to 40 g per cubic metre, respectively.
The vapour pressure at a given temperature is said to be partial (p) if it is lower than the saturation pressure.
Relative humidity (RH):
percentage ratio between the value of the partial pressure of water vapour measured in the atmosphere and the value of the saturated vapour pressure of water at the same temperature. Relative humidity is therefore a function of temperature.
Example:
the partial pressure of water vapour in the atmosphere is 20.0 torr (mmHg) at a temperature of 30°C; since the saturated vapour pressure of water at this temperature is 31.8 torr, the relative humidity is:
Dew point:
the temperature (at a given pressure) at which a vapour becomes saturated and condenses on a surface.
The table shows the values of air temperature, relative air humidity and dew point of the condensation surface.