After boiling, the water temperature stops rising and remains constant until complete evaporation. Vaporization is the transition from liquid to vapor, which has the same temperature parameter as the boiling liquid. This evaporation is called saturated steam. When all the water is vaporized, any subsequent addition of heat raises the temperature. Steam heated beyond the saturated level is called superheated. The industry usually uses saturated steam for heating, cooking, drying or other procedures. Overheated is used solely for the turbines. Various types of steam have different energy exchange capacity and this justifies its use in a completely different order.
Steam as one of three physical states
To better understand the properties of steam may help to understand the General molecular and atomic structure of matter, and applying this knowledge about ice, water and steam. A molecule is the smallest unit of any element or compound. She, in turn, consists of even smaller particles called atoms, which define the basic elements such as hydrogen and oxygen. Specific combinations of these atomic elements provide compound substances. One of these compounds represented by the chemical formula N2 O, whose molecules consist of 2 hydrogen atoms and 1 oxygen atom. Carbon is also abundant, it is a key component of all organic substances. Most mineral substances can exist in three physical states (solid, liquid and vapor), which are called phases.
The process of steam formation
When the water temperature approaches the boiling point, some molecules get enough kinetic energy to reach speeds that allow them to momentarily separate from the liquid in the space above the surface before returning. Further heating causes more excitation and the number of molecules that wish to leave the liquid increases. At atmospheric pressure, the saturation temperature is 100 ° C. Steam with a boiling point at this pressure is called dry saturated steam. As a phase transition from ice to water, the evaporation process is also reversible (condensation). The critical point is the highest temperature at which water can be in the liquid state. Above this point, the vapor can be regarded as a gas. The gaseous state is a similitude of the diffuse state, in which the molecules have an almost unlimited ability to move.
Interrelation of variables
At a given temperature, there is a certain vapor pressure that exists in equilibrium with liquid water. If this indicator increases, the steam overheats and is called dry. There is a relationship between pressure and temperature: knowing one value, you can define another. The state of steam is determined by three variables: pressure, temperature and volume. Dry saturated steam is a state where steam and water can be present simultaneously. In other words, this occurs when the vaporization rate is equal to the condensation rate.
Saturated steam and its properties
During the discussion of the properties of saturated steam it is often compared with a perfect gas. Do they have something in common or is it just a misconception? First, at a constant temperature level density is based on volume. Visually it is possible to imagine the following: you need to visually reduce the volume of the container with the steam without changing the temperature. The number of condensed molecules is greater than the number of evaporating, and the steam will come back into balance. As a result, the density is a constant parameter. Secondly, characteristics such as pressure and volume are independent from each other. Third, given the constancy of the volumetric characteristics, the density of the molecules increases when the temperature increases, and becomes smaller when it decreases. In fact, when heated, the water begins to evaporate faster. The balance in this case will be broken and will not be restored until the until the vapour density will not return to their former positions. In the condensation, on the contrary, the density of the vapor will decrease. Unlike an ideal gas, saturated steam can not be called a closed system because it is constantly in contact with water.
Advantages in the field of heating
Saturated is called pure steam in direct contact with liquid water. It has many characteristics that make it an excellent source of thermal energy, especially for high temperatures (above 100 ° C). Some of them:
- Rapid even heating with latent heat transfer improves product quality and productivity.
- Pressure and temperature can be controlled and set at the required level.
- High heat transfer coefficient.
- It originates from water, safe, clean and inexpensive.
Different types of steam
Steam is the gaseous phase of water. It uses heat during its formation and releases a lot of heat after that. Consequently, he
can be used as a working substance for heat engines. Known the following condition: wet saturated, dry saturated and superheated. Saturated steam is preferably superheated steam as the heating medium in heat exchangers. When it is discharged into the atmosphere from the chimneys, part of its condenseries, formed clouds of white damp vapours, containing tiny droplets of water. Superheated steam will not be subject to condensation, even when joining in direct contact with the atmosphere. In the overheated state, he will have greater heat transfer due to the acceleration of the movement of molecules and lower viscosity. The presence of moisture causes a deposition, corrosion and reducing the service duration of boilers or other heat exchange equipment. Thus, dry steam is preferred because it produces more energy and does not cause corrosion.
Dry and saturated: what is the contradiction
Many are confused with the terms "dry" and "saturated". How can there be something both at the same time? The answer lies in the terminology that we use. The term "dry" is associated with a lack of moisture, that is, "not wet." "Saturated" means "soaked", "soaked", "flooded", "piled up" and so on. All this, it would seem, confirms the contradiction. However, in steam engineering, the term "saturated" has a different meaning and in this context means the state at which boiling occurs. Thus, the temperature at which boiling occurs is known technically as saturation temperatures. Dry steam in this context does not have moisture in itself. If you watch a boiling kettle, you can see the white evaporation emerging from the tip of the kettle. In fact, it is a mixture of dry colorless steam and wet steam, which contains droplets of water that reflect light and are painted white. Therefore, the term "dry saturated steam" means that the steam is dehydrated and not overheated. A liquid free of particles, this substance is in the gaseous state, which does not follow the general gas laws.