In the field of chemistry, the concepts of saturated and dry steam play a fundamental role not only in purely theoretical issues, but also in their direct applications to the industrial sector.
First of all, to better understand the difference between these two notions, it is good to remember the precise definition of water vapor.
Water, as well as matter in the most general physical sense, can be in different states: the most ‘frequent ones, at least in practice, are the solid state (in the case of water, ice), the liquid state, and the aeriform. Water vapor corresponds precisely to this latter state.

However, this does not mean that there are no relationships between the different states of matter, or that each of these states is always in the same identical conditions. In fact, if a material is subjected to a certain change in pressure and / or temperature, the so-called phase transition can occur. In this way, numerous chemical bonds that bind the molecules of the material under consideration will be broken (for example, in the transition from solid to liquid) or created (as in the transition from gaseous to liquid). It is precisely this molecular reassembly that constitutes the crucial difference between the various types of states and sub-states.

So let’s consider an example to introduce the idea of saturated steam. At the pressure of 1 atmosphere (about 101325 Pascal), or the pressure that the air exerts with its weight at sea level and at a temperature of 0 ° C, the water boils at 100 ° C. This means that at this temperature, the water molecules that are in the liquid state begin to break their chemical bonds in order to transform into water vapor. A chemical transformation that brings about such a radical change, however, obviously does not happen all at once. The process requires an intermediate step: the temperature must remain fixed, at least for a short period of time, at 100 ° C. When the temperature is kept constant at 100 ° C, water is exactly between two states, liquid and gaseous, and the molecules that should break bonds are equal in number to those that should create new ones. This particular state of water is called saturated vapor .

The same is true in the general case, not only when the pressure is fixed at 1 atmosphere. The only precaution to keep in mind, in such a situation, is that the boiling temperature of the water changes (generally the difference is minimal, if not at quite high altitudes or, of course, in places where the pressure is monitored so different).

Now, we have observed that that state halfway between liquid and gaseous gives rise to a certain type of vapor, the saturated one, which in itself guarantees many advantages (as will be explained below, its high temperature allows the elimination of bacteria. ). However, it is possible to improve the degree of precision of our analysis and distinguish two further types of saturated steam:

  • dry saturated steam (often abbreviated to dry steam);
  • wet saturated steam (often abbreviated to wet steam).

The distinction between these two stages is due to the presence or absence of ‘droplets’: as experience teaches, for example, the steam from a pot produces small drops that coat the material, just as happens with fog. The appearance of these drops denotes the presence of the maximum quantity of liquid (it is the part of saturated vapor that contains the molecules that are preparing to create bonds and remain in the liquid state), and for this reason it is called wet vapor. The other stage, on the other hand, does not produce droplets and represents the remaining part of the molecules. This is dry steam.

There is also another phase that can be useful in the industrial sector, and that is that of superheated steam. Unlike the previous ones, this vapor is obtained in a state of non-equilibrium: when the temperature is considerably increased and the boiling point is exceeded, there is no more liquid to vaporize and one is in a sort of ‘stall’.

Isotherm is the name given to the curve that represents the thermal transformations taking place in a plane like the one in the figure, where the abscissas represent the volume V and the ordinates, as a function of V, represent the pressure P. The plane above is often called the plane of Clapeyron or simply volume-pressure diagram.

Saturated and dry steam applications for cleaning, degreasing and sanitizing

Let’s start with the advantages of dry saturated steam over normal and wet steam. The application of dry steam, thanks to its high temperature, allows to eliminate the bacteria present on a surface (in fact, the only bacteria capable of living at high temperatures are called thermophiles, and they cannot survive except in the vicinity of volcanoes or similar areas). This guarantees thorough sanitization which, combined with the humidity level of the dry steam, involves thorough cleaning. It could be said that dirt particles almost ‘float’ if they come into contact with the dry steam, and this facilitates their removal. The pressure also optimizes the process and improves performance.

On the totally opposite front, the bulky presence of liquid in the wet steam makes its application to cleaning surfaces of little use. It therefore follows that, from a hygiene point of view, the use of dry steam is fundamental (especially in recent years, since the arrival of the coronavirus).

Steam in the general sense, on the other hand, does not have a high enough temperature for the purpose, and therefore it too is ineffective. A similar conclusion continues to be valid in the context of superheated steam which, although very useful for performing thermal jumps in numerous machines, does not ensure optimal results from the point of view analyzed.

Ultimately, the discussion above shows that dry steam is significantly superior in terms of sanitation and cleaning compared to the other two states. Furthermore, it is important to underline a further advantage of dry steam: it does not pollute and consumes little. In fact, the pre-washing, washing and rinsing of a classic cycle ending with disinfection are incorporated into the dry steam cleaning phase, reducing waste. In numerical terms, the traditional hydrojet machine consumes between 1500 and 2000 liters of water per hour, while the steam generator only consumes 10. The advantages, even on this front, are evident.

SOURCES:
[1] William D. Wise (2005). ‟Succeed at steam sterilization” Chemical processing.
[2] ‟Saturated vs Superheat Steam Conditions". nationwideboiler.com
[3] Song, L.; Wu, J.; Xi, C. (2012). ‟Biofilms on environmental surfaces: Evaluation of the disinfection efficacy of a novel steam vapor system”. American Journal of Infection Control. Vol. 40, Issue 10, 926-930.