Wade Industrial - Steam, Valve & Pipeline Specialists

Wade Industrial Steam Specialists

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Types of Steam

What are the Different Types of Steam?

When water is heated to boiling point and beyond, it changes into steam, or, water in a gaseous state. The different combinations of pressure and temperature cause the properties of steam to vary diversely.

Saturated Steam
Superheated Steam
Supercritical Water

Saturated Steam

Saturated steam is the most common type of steam. Steam in this state is constituted of both liquid phase and gaseous phase water. This means that the rate of evaporation is equal to the rate of condensation. Steam generated by a boiler is usually saturated steam. Saturated steam has properties which make it an excellent source of heat, and is hence widely used as a 100 °C – 200 °C heat source.

Reasons why saturated steam makes for a useful source of heat:

  • Quick, efficient and even heat is produced through dormant energy
  • Temperature and pressure can be accurately measured and controlled
  • It is possible to control the pressure in place of the temperature
  • Heat transfer coefficient is highly rated
  • Initial equipment outlay can be reduced due to the relatively small heat transfer area required
  • Steam is safe to use
  • Relative to other heating processes, steam is low-cost.

When heating with saturated steam it is necessary to remember that:

  • Heat loss through radiation will cause some of the steam to condense into water which must be removed via steam traps installed along the steam transport lines
  • Only extremely dry steam should be used for maintaining heat efficiency
  • Loss of pressure for any reason within the pipe-work may cause the temperature to also drop.

Superheated Steam

Further heating of saturated steam will cause superheated steam to form. Superheated steam has a higher temperature than saturated steam at the same pressure. Superheated steam is used mainly for physical drive or propulsion applications, and is not often used for heating purposes.

Reasons why superheated steam is rarely used as a source of heat include:

  • Temperature fluctuations are common in superheated steam, which may impair product quality
  • Even when the steam pressure is constant, the temperature cannot be precisely controlled
  • Heat transfer coefficient is low, hence poor heat transfer.
Clearly there is no benefit to using superheated steam instead of saturated steam in heat exchangers. Yet superheated steam has an advantage over using hot air as it can be used as a heat source for heating under ‘oxygen-free’ conditions. This may be useful for food processing applications such as cooking and drying.

Reasons why superheated steam is useful as a turbine drive source include:

  • Maintaining dryness for steam-driven equipment where performance is impaired by the presence of condensate
  • Superheated steam can 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. This implies that the specific volume of the part that is liquid is exactly the same as the specific volume of the part that is vapor. When water is hotter, or at higher pressure than critical point, it is in a state where water and steam are indistinguishable, a state that is neither liquid nor gas. Supercritical water is used to drive turbines in power plants which demand higher efficiency.

Distribution of Pressure and Temperature of Various Types of Steam

Distribution of steam types

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