Saturated dry steam will be defined on this page by starting off from common knowledge.
We know that substances may exist in various physical forms: solid, liquid, steam. These different forms are called states of substances aggregation. The change from one state to the other is defined with the following words: fusion; vaporization, solidification. We also know that by heating a body, this changes its temperature and its physical state. This happens because the kinetic energy (movement) of the particles which move far away from each other increases, thus loosening the cohesion power. A solid body changes to a liquid state (fusion), and then it goes on until it becomes gas (vaporization). On the other hand, by reducing the temperature, the particles increase their state of aggregation: steam becomes liquid (condensation) and afterwards it solidifies (solidification). Some substances, moreover, may even change directly from a solid state to gas (sublimation) and vice versa (condensation). Fig.1
The three states of water and the change from one state to the other are represented in Fig.2. The vaporization temperature, with a pressure of 1 atmosphere, is 100° C. The phase in which heat is given but there is no water temperature increase is defined as latent heat, in fact, in this phase the given heat is only needed to change the state of water from solid to liquid or from liquid to steam. The latent heat of vaporization is thereafter the quantity of heat needed to transform completely in steam a liquid unitaray mass when this is at boiling temperature. We will now examine in detail the change of phase of 1 kg of water placed in a container with an initial temperature of 5° C and which can maintain a constant pressure at 1 atm. We start to heat the water: the temperature raises and the occupied volume slightly increases, while the pressure remains constant. a) A few instants before the transformation of part of the liquid into steam begins, the liquid is defined as a saturated liquid, a light increase of heat determines the beginning of the transformation from liquid to steam. b) When the change from liquid to steam occurs, the temperature and the pressure values are called respectively saturation or vaporization temperature and saturation or vaporization pressure. c) During the transformation of liquid into steam, the steam is called saturated steam. Here water is still present as micro drops. d) When all the liquid has been transformed into steam it is called saturated dry steam; all the heat which has been given was used in order to transform the water into steam, no micro drops are present. e) If heat is still applied, the temperature continues to rise and the steam volume continues to increase. If the pressure is maintained constant, the steam is called superheated steam, because it has a higher temperature than the saturation temperature of 100° C with the specific pressure of 1 atm, which was the pressure the process started off with.
The vaporization temperature, as we have seen, depends on pressure. The diagram Fig.3 indicates the vaporization temperatures according to the given pressure.