It is a vapor of water. It is invisible when pure and dry. Use as a working substance in operation of steam engines and turbines. It does not obey the laws of perfect gases until it is perfectly dry. It is a pure substance and it can be converted into any of the three states of matter/ substance (solid, liquid and gas).
Consider 1 Kg of water at 0 0C contains in the piston cylinder arrangement (fig. a). Piston and weight maintain a constant pressure in the cylinder. If the water in the cylinder heated, it will convert into steams. Water volume slightly increases with increase in temperature (fig. b). Due to the increase in water volume, the piston moves upward. Work obtained during upward movement of the piston. This increase in volume or work is neglecting for all types of calculations.
On further heating of water, the temperature reaches the boiling point. The boiling point is directly proportional to the pressure. The boiling point of water at normal atmospheric pressure (1.013bar) is 100 0C. After reaching to boiling point, the temperature will remain constant and water evaporates. Now at this stage piston moves upward against constant pressure. In simple words, pressure remains constant with an increase in the specific volume of steam (fig. c). This is termed as wet steam (contains water particles in suspension). This process will continue until the whole water converts into wet steam.
On further heating, water particles in suspension convert into steam. This is termed as dry or saturated steam (fig. d). Further heating cause rising of temperature, this state is termed as superheated steam (fig. e).
Types of Steam:
It can be classified in the following three types:
1. Wet Steam:
when the steams contain moisture or water particles in suspension, it is said to be wet steam. In this type, evaporation of water not completed and the whole of the latent heat has not been absorbed. It is a two-phase mixture, containing boiling water particles and dry steam in an equilibrium state. Its formation starts when water is heated beyond its boiling point, thereby causing the start of evaporation. A wet steams may exist in different proportions of water particles and dry steam. Accordingly, its qualities are also different. Its quality is expressed in terms of dryness fraction.
2. Dry Saturated Steam:
When the wet steam is further heated, and it does not contain any moisture or water, it is known as dry saturated steam. Dry Saturated Steam absorbed its full latent heat and behaves practically, in the same way as a perfect gas.
- The temperature quickly and precisely established.
- High heat transfer coefficient, Requires a smaller heat transfer surface area, allowing a lower initial equipment expenditure
- Rapid, even heating through latent heat transfer which improves production and product quality
- Clean, safe and low-cost
3. Superheated Steam:
When the Dry Saturated Steams is further heated at constant pressure, thus raising its temperature, it is said to be superheated steam. Since the pressure is constant, therefore the volume of superheated steams increases. It may be noted that the volume of one Kg of superheated steams is considerably greater than the volume of one Kg of dry saturated steams at the same pressure.
- With respect to steam pressure, BTU increased.
- No deposition of scales in the valve seats increases valve life
- Does not condense quickly, therefore heat losses through pipelines decreases
- No water hammering in pipelines and engine cylinders
- The thermal efficiency of engines and turbines increased
- Saving of fuel or minimum use of fuel
- Low oxygen contents, minimize corrosion in piping, valves and turbine blades
- steams consumption less
- Steams trap life increase