ITM Italia EURODRY EFT Serie Manual De Instrucciones página 18

at the state of saturation in the same conditions of pressure and temperature. This index, which is always less than
one, is called the hygrometric degree of the air and, along with our knowledge of the temperature, it allows us to
univocally define a certain mixture of air and water vapor. However in practice it is more convenient to use a
definition different from the relative humidity index which refers to the partial pressures of the dry air and the vapor.
The relative humidity of the air (RH) is defined as the ratio between the partial pressure of the vapor in a given
mixture and the partial pressure of the vapor at the state of saturation having the same pressure and temperature
as the mixture being examined. The latter definition is expressed in a percentage (RH%) in practice and is less
intuitive of the hygrometric degree but expresses the same empirical concept on a different numeric scale. In
practice the parameter volumetric humidity of the air is often expressed in g/m³. Finally, it should be pointed out
that, relative humidity being equal, the vapor content in the air (absolute humidity or vapor content), increases as
the temperature rises and for a univocal definition of the condition of moist air, at least two parameters are needed:
these are temperature and relative humidity. In approximate terms Table 1 shows the values of volumetric
humidity of the air in certain significant conditions.
Air Temperature (°C)
-5
0
5
10
15
20
25
30
35
2.2.2 Effects of moist air
Moist air can interact with the objects exposed to it in different ways. First and foremost, in the presence of
hygroscopic materials, i.e. materials that can absorb and give off moisture as a function of certain environmental
conditions, the air can affect their state of conservation. In fact certain substances undergo chemical and/or
physical decay mainly as a result (among other things) of their moisture content and the temperature they are
subjected to. Some chemicals corrode the surface of objects. The speed and extent of this corrosion depends on
ambient moisture because above a certain threshold the electrochemical processes are accelerated. All these
phenomena make it necessary to control the characteristics of the ambient air, especially its relative humidity. Moist
air can also slow down a drying process during the desiccation of certain substances. As a result dehumidifiers are
used to control the relative humidity of the air.
2.3 Operating principle
The above paragraphs have explained that one possible way for dehumidifying the air is to cool it to below dew
point and then collect the liquid condensate.
Naturally, dehumidifying by condensation, which is the function of the machine being described here, is not the only
way to keep relative humidity under control. As we saw in the previous paragraph, this process also depends on
temperature. To this end it would be possible to use a heating system with an air exchange function. However
there are certain limitations of this system, such as:
High energy consumption caused by the wasting of heat during the air exchange phase;
If the environmental conditions are unfavorable, i.e. temperature and humidity above the optimal values for
an indoor environment, it is certainly uneconomical, or even impossible, to reach the desired conditions by
air exchange and heating.
This is why there is a widespread use of dehumidifiers. These units can often solve problems that would otherwise
be impossible to solve and sometimes they allow you to save a remarkable amount of energy. The following
paragraph makes this even clearer.
Table 1: Volumetric air moisture in g/m³
Relative humidity (RH%)
30% 40% 50%
1.0 1.4 1.7
1.5 1.9 2.4
2.0 2.7 3.4
2.8 3.8 4.7
3.8 5.1 6.4
5.2 6.9 8.6
6.9 9.2 11.5
9.1 12.2 15.2
11.9 15.9 19.9
GB 5
60% 70% 80%
2.0 2.4 2.7
2.9 3.4 3.9
4.1 4.8 5.4
5.6 6.6 7.5
7.7 9.0 10.3
10.4 12.1 13.9
13.8 16.2 18.5
18.3 21.3 24.4
23.9 27.9 31.9
90% 100%
3.1 3.4
4.4 4.8
6.1 6.8
8.5 9.4
11.6 12.8
15.6 17.3
20.8 23.1
27.5 30.5
35.9 39.9