Saturated steam and its properties

For sure, many had to observe the picture of how standing an open container with water after a while is empty. If you cover it with a lid, the water does not go away. The reason is well known - the water evaporates. The explanation for this phenomenon is simple: some water molecules have a sufficiently high speed in order to leave the liquid. This process of the transition of a liquid to a gaseous state is called evaporation.

Another process, namely the conversion of steam into a liquid, is called condensation. These two processes, evaporation and condensation, go on constantly: some of the water evaporates, some condense. If the volume above the water surface is unlimited, then the evaporation process prevails. Evaporated water is removed, as, for example, occurs over the surface of open water, and the liquid gradually passes into a gaseous state - vapor.

But if the amount of free space above the liquid is limited, then a slightly different situation arises. Evaporated water can not leave this volume, and saturated steam forms above the surface of the water. This is the name of steam in a state of equilibrium, when the amount of evaporated water and the condensed vapor are equal. Water does not decrease and does not come, there comes a state of equilibrium between evaporation and condensation.

Now we know what saturated steam is, and its properties, perhaps, will be quite curious for us. From the very beginning, we determined that the volume of free space above the surface of the liquid is limited. Over it formed saturated steam. And if now this free volume is reduced? What will happen? In this case, the steady equilibrium between condensation and evaporation is violated. The condensation process will begin to predominate, the moisture content will increase, and the steam will decrease.

The vapor pressure at which it is in equilibrium with the liquid is called the saturated vapor pressure. If we reduce the amount of free space above the water, then the vapor pressure increases. The consequence of this is the transition of steam into water. With increased pressure, the liquid takes up less space than saturated steam. From this follows another conclusion: if the temperature is constant, then the saturated vapor pressure for any volume is the same.

There is another variant of the behavior of steam - the volume above the water surface is reduced, and the transition of vapor into the liquid does not occur. Hence, there is unsaturated vapor above the surface. Later, when the volume decreases at a constant temperature, the steam begins to turn into water, which means that saturated steam has formed. But it was not for nothing that the condition was stipulated that everything happens at a constant temperature. There is a certain value for it, in which the vapor can turn into a liquid.

This value is called the critical temperature. The substance remains a gas at a temperature above the critical temperature, but if it is below the critical temperature, the gas becomes a liquid. Each substance has its own critical temperature. It is worth noting two more features of the steam: it can be both wet and dry saturated steam. In the wet there are drops of water, and dry steam does not contain moisture.

There is also so-called superheated steam - it is dry steam with a temperature above the critical one. In this case, it is considered that there is no liquid in the closed volume, and only steam is present. Superheated steam is mainly used in engineering and power engineering. The high temperature of the superheated steam allows it to be transported by means of steam pipes and used in steam turbines. Due to the lack of water in the superheated steam, the life of the turbine increases.

The article considers what constitutes saturated steam, its types and properties, as well as the process of its formation and transformation into a liquid.