Steam is the gaseous / Vapor phase of water at pressurized condition.If water is heated beyond the boiling point, it vaporizes into steam, or water in the gaseous state.
However, not all steam is the same. The properties of steam vary greatly depending on the pressure and temperature to which it is subject.
Saturated (dry) steam results when water is heated to the boiling point (sensible heating) and then vaporized with additional heat (latent heating). If this steam is then further heated above the saturation point, it becomes superheated steam (sensible heating).
As indicated by the black line in the above graph, saturated steam occurs at temperatures and pressures where steam (gas) and water (liquid) can coexist. In other words, it occurs when the rate of water vaporization is equal to the rate of condensation.
Advantages of using saturated steam for heating
Saturated steam has many properties that make it an excellent heat source, particularly at temperatures of 100 °C (212°F) and higher. Some of these are:
This is the most common form of steam actually experienced by most plants. When steam is generated using a boiler, it usually contains wetness from non-vaporized water molecules that are carried over into the distributed steam. Even the best boilers may discharge steam containing 3% to 5% wetness. As the water approaches the saturation state and begins to vaporize, some water, usually in the form of mist or droplets, is entrained in the rising steam and distributed downstream. This is one of the key reasons why separation is used to dis-entrain condensate from distributed steam.
Dry Steam is saturated steam that has been very slightly superheated. This is not sufficient to change its energy appreciably, but is a sufficient rise in temperature to avoid condensation problems, given the average loss in temperature across the steam supply circuit. Towards the end of the 19th century, when superheating was still a less-than-certain technology, such steam-drying gave the condensation-avoiding benefits of superheating without requiring the sophisticated boiler or lubrication techniques of full superheating.
Superheated steam is created by further heating wet or saturated steam beyond the saturated steam point. This yields steam that has a higher temperature and lower density than saturated steam at the same pressure. Superheated steam is mainly used in propulsion/drive applications such as turbines, and is not typically used for heat transfer applications.