Email this pageVarious States of Steam
What are Some States of Steam?
If water is heated, it becomes ‘steam generated from water,’ or water in a gaseous state. The properties of this steam vary greatly depending on the combination of the pressure and temperature conditions the steam is under.
In the article Types of Steam and Their Applications, we divided steam into categories from the viewpoint of the applications in which the different types of steam are used, but for the purposes of discussion in this article, we will divide steam into categories based on the state of the steam.
Saturated Steam
This is the most familiar type of ‘steam.’ Steam in the saturated state is made up of both water in the liquid phase and water in the gaseous phase. In other words, the rate of evaporation is equal to the rate of condensation. Steam generated using a boiler is fundamentally saturated steam. It has many properties that make it an excellent heat source, and is therefore extremely widely used as a 100 °C – 200 °C heat source.
Saturated steam is widely used as a heat source for the following reasons:
- Rapid, even heating is possible through latent heating
- Improved product quality and productivity
- Pressure and temperature can be precisely established
- Possible to control pressure in place of controlling temperature
- High heat transfer coefficient
- Small heat transfer area allows for reduced initial equipment outlay
- Originates from water, so…
- Safe and low-cost
Having said that, it is necessary to be mindful of the following when heating with saturated steam:
- Radiant heat loss causes some of the steam to condense, forming condensate, which must be removed by installing steam traps on steam transport lines
- Heating efficiency is impaired if steam other than extremely dry steam is used
- If the pressure falls due to a drop in pressure caused by friction in piping, etc., the temperature may also fall
Superheated Steam
Superheated steam is created by further heating saturated steam, yielding steam that has a higher steam temperature than saturated steam at the same pressure. This type of steam is mainly used in propulsion / drive applications, and is rarely used in heating applications.
The main reasons superheated steam is rarely used as a heat source are as follows:
- There are temperature fluctuations during heating because the superheated portion is sensible heat
- Has an effect on product quality
- Even if the pressure is constant, the temperature cannot be precisely established
- Cannot use pressure control
- Poor heat transfer efficiency due to low heat transfer coefficient
- Has an effect on productivity and initial equipment outlay
As we can see, there is no advantage to using superheated steam instead of saturated steam as the heat source in heat exchangers. On the other hand, when viewed as a heat source for direct heating as a ‘high temperature gas,’ it has an advantage over hot air in that it can be used as a heat source for heating under oxygen-free conditions, and research is being carried out on food processing applications such as cooking and drying.
The main reasons superheated steam is used as a turbine drive source are as follows:
- To maintain the dryness of the steam for steam-driven equipment, whose performance is impaired by the presence of condensate
- To improve thermal efficiency
It is advantageous to both supply and discharge the steam while in the superheated state, because no erosion-causing condensate will be generated inside steam-driven equipment. In addition, as the theoretical thermal efficiency of the turbine is calculated from the value of the enthalpy at the turbine inlet and outlet, increasing the degree of superheating as well as the pressure raises the enthalpy at the turbine inlet side, and is thereby effective in improving thermal efficiency.
Supercritical Water
Supercritical water is water in a state that exceeds the critical point of water; 22.06MPa, 373.95 °C. At the critical point, the latent heat of steam is zero. Those of you with a steam table on hand, please check this for yourselves. Which means that the specific volume of the part that is liquid is exactly the same as the specific volume of the part that is vapor.
In other words, water that is hotter / at higher pressure than this is in an indistinguishable state that is neither liquid nor gas. It is used to drive turbines in power plants which demand higher efficiency. Research is being done with an emphasis on using it as a fluid that has the properties of both a liquid and a gas, and in particular has the characteristics as a solvent for chemical reactions.
Distribution of Various Types of Steam
Distribution of Pressure and Temperature of Various Types of Steam
Click on the name of each type of steam to see the state of that type of steam.
Various Types of Steam States
Liquid State
This is water in the state that is most familiar to us. It is said that approximately 70% of the weight of the human body is from water. Water is stabilized as a liquid under normal temperature and normal pressure by the movement of the hydrogen bonds.
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Saturated Steam
Saturated steam condenses the instant it loses its latent heat. So when saturated steam is released by being vented from the piping to atmosphere, part of it condenses when it comes into contact with atmosphere, and clouds of white vapor (tiny drops of water) are formed. The heat provided raises the interior energy, and the movement of the molecules speeds up. The increased activity of the molecules causes the hydrogen bonds to break, and the water is vaporized, forming ‘steam.’
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Superheated Steam
As long as it retains its superheated state, superheated steam will not condense even if it comes into contact with atmosphere and the temperature drops. As a result of this, no clouds of vapor are formed. Superheated steam holds more heat than does saturated steam at the same pressure, and the movement of its molecules is more rapid so it is has lower density (= specific volume is greater).
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Supercritical Water
How Supercritical Water Looks to the Naked Eye
Though it is not possible to tell by looking, this is water in a form that is neither liquid nor gaseous. The general idea is of a molecular movement that is close to that of gaseous steam, and a density that is close to that of liquid water.
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