Superheated steam and saturated steam

There are many discussions on this forum regarding superheated steam, which is also known as dry steam. It is obtained by superheating saturated steam. Saturated steam is steam produced when water vaporizes as a result of an increase in temperature; it generally has a low quality, tends to contain water, and has a liquid phase present

There is a corresponding relationship between the steam pressure and temperature of saturated steam; different pressures correspond to specific saturated temperatures. In other words, under certain pressure, water vaporizes completely and continues to absorb heat without its temperature rising; it becomes saturated steam at the moment when the temperature starts to increase. Superheated steam is steam that, at a certain pressure, reaches its saturation temperature; upon continuing to absorb heat, its temperature rises above that saturation temperature.

Saturated steam refers to steam in a saturated state, which is caused by the thermal motion of gas molecules. A liquid in a saturated state is called a saturated liquid, and its corresponding vapor is saturated vapor; initially, it is wet saturated vapor, and it becomes dry saturated vapor only after all the water in the vapor has evaporated. During the process in which steam moves from unsaturated to wet saturated to dry saturated, its temperature does not increase; however, when heating continues after reaching dry saturation, the temperature rises, and the steam becomes superheated. Saturated steam contains moisture, while superheated steam contains virtually no moisture.

　When a liquid evaporates in a confined space, the liquid molecules move through the surface of the liquid into the space above, turning into vapor molecules. Due to the chaotic thermal motion of steam molecules, they collide with each other as well as with the container walls and the liquid surface. When colliding with the liquid surface, some molecules are attracted by the liquid molecules and return to the liquid to become part of it. At the beginning of evaporation, more molecules enter the space than those that return to the liquid. As evaporation continues, the density of vapor molecules in that space increases, which in turn leads to an increase in the number of molecules that return to the liquid. When the number of molecules entering the space per unit time is equal to the number of molecules returning to the liquid, evaporation and condensation are in dynamic equilibrium. At this point, although evaporation and condensation continue to occur, the density of vapor molecules in the space no longer increases; this state is known as a saturated state. A liquid in a saturated state is called a saturated liquid, and its corresponding vapor is saturated vapor; initially, it is wet saturated vapor, and it becomes dry saturated vapor only after all the water in the vapor has evaporated. During the process in which steam moves from unsaturated to wet saturated to dry saturated, its temperature does not increase; however, when heating continues after reaching dry saturation, the temperature rises, and the steam becomes superheated.

It is recommended that the poster heat the water under certain pressure; the temperature of the water rises as it continues to be heated. When the water temperature reaches a certain level, the water begins to boil, and this temperature is known as the boiling point. If heating continues with the water temperature remaining constant, the water begins to vaporize and gradually turns into steam. The boiling temperature of water at a certain pressure is also known as the saturation temperature. This temperature is related to the pressure it is under; the greater the pressure, the higher the boiling temperature ； Conversely, lower pressure results in a lower boiling temperature. Superheated steam: Steam is a rather special medium; what is generally referred to as steam is actually superheated steam. Superheated steam is a common power source, often used to drive turbines, which in turn drive generators or centrifugal compressors. Superheated steam is obtained by heating saturated steam. It contains absolutely no liquid droplets or mist, and is a real gas. The temperature and pressure parameters of superheated steam are two independent parameters, and its density should be determined by these two parameters. After being transported over long distances, superheated steam, as operating conditions such as temperature and pressure change – especially when the degree of superheating is not high – loses heat and its temperature drops, causing it to transition from a superheated state to a saturated or supersaturated state, thereby becoming saturated steam or supersaturated steam containing water droplets. When saturated steam is suddenly subjected to a significant pressure drop and the liquid undergoes adiabatic expansion, it can also become saturated steam or supersaturated steam containing water droplets. When saturated steam is suddenly subjected to a significant pressure drop and the liquid undergoes adiabatic expansion, it also turns into superheated steam, thus forming a two-phase flow of vapor and liquid. Steam that has not been heat-treated is called saturated steam. It is a colorless, tasteless gas that is non-flammable and non-corrosive. Saturated steam has the following characteristics: it condenses easily, and if heat is lost during transmission, liquid droplets or mist form within the steam, resulting in a decrease in temperature and pressure. Steam containing droplets or mist is called wet steam. Strictly speaking, saturated steam is a two-phase fluid that contains liquid droplets or mist to some extent; therefore, the same gas equation of state cannot be used to describe it in different states. The content of liquid droplets or mist in saturated steam reflects the quality of the steam, which is generally expressed by the parameter known as dryness. The dryness degree of steam refers to the percentage of dry steam in a unit volume of saturated steam, denoted by “x”.

This post was last edited by TH373637 on 2011-7-4 22:15. When the water reaches its saturation temperature, if heating is carried out at constant pressure, the saturated water begins to vaporize. Before the water is completely vaporized, the steam containing saturated water is called wet saturated steam, or simply wet steam. When wet saturated steam continues to be heated under constant pressure, the state in which all the water has vaporized into steam is called dry saturated steam. Dry saturated steam continues to be heated at constant pressure; when its temperature rises above the saturation temperature corresponding to that pressure, it becomes superheated steam.