Steam is water vapor, often mixed with air and/or tiny liquid water droplets. This happens through evaporation or boiling, which occurs when heat is added until water reaches the energy needed to change into vapor. Superheated or saturated steam cannot be seen, but wet steam, which is a visible mist of water droplets, is often called "steam."

When liquid water turns into steam, its volume increases by 1,700 times at standard temperature and pressure. This change in volume can be used to create mechanical work in steam engines, such as piston-type engines and steam turbines. Piston-type steam engines were important during the Industrial Revolution, and today, steam-based electricity generation provides 80 percent of the world's power. If liquid water touches a very hot surface or rapidly loses pressure below its vapor pressure, it can cause a steam explosion.

Types of steam and conversions

Steam is usually made by heating water in a boiler using coal and other fuels, but it can also be made using solar energy. Steam that contains tiny water droplets is called "wet steam." When wet steam is heated further, the droplets turn into vapor. At a high enough temperature (which depends on the pressure), all the water becomes vapor, and the system reaches a balance between vapor and liquid. At this point, the steam is called "saturated steam."

Steam that is warmer than the temperature at which water boils under a certain pressure is called "superheated steam" or "live steam." This happens only after all the liquid water has turned into vapor or been removed from the system.

Steam tables are tools that provide information about the properties of water and saturated steam. Engineers and scientists use these tables when designing and operating equipment that uses steam in thermodynamic processes. Thermodynamic phase diagrams, such as temperature-entropy diagrams or Mollier diagrams, are also helpful for analyzing steam-related systems. Steam charts are used similarly to study thermodynamic cycles.

Uses

In agriculture, steam is used to kill harmful germs in soil, which helps protect plants and improve soil quality without using dangerous chemicals.

Steam is also used in homes for cooking vegetables, cleaning fabrics, carpets, and floors, and heating buildings. In each case, water is heated in a boiler, and the steam carries the heat to the object being treated. Steam is used in irons to add moisture with heat, which helps remove wrinkles and shape clothing.

As of 2000, about 90% of all electricity was made using steam, mostly through steam turbines. In electricity generation, steam is usually cooled back into water after use and sent back to the boiler to be reused. However, in co-generation, steam is sent through pipes to buildings to provide heat after being used to make electricity. The largest steam system in the world is in New York City, which sends steam to 100,000 buildings in Manhattan from seven co-generation plants.

In other industries, steam is used to store energy, which is added or removed through pipes. Steam can hold a lot of heat because water requires a lot of energy to turn into vapor.

Fireless steam locomotives are trains that use steam stored in a large tank on the train, similar to a boiler. This steam comes from factory machines, like paper mills. These trains use pistons and connecting rods to move, like regular steam trains. They were used in places where fire from a boiler was a risk and in factories with extra steam available.

Steam engines and turbines use the expansion of steam to move pistons or turn turbines, which creates mechanical work. It is important to return condensed steam as water to the boiler at high pressure with little effort. Condensation often happens at the low-pressure end of a turbine to improve efficiency, but too much moisture can damage turbine blades. Engineers use a model called the Rankine cycle to study how steam engines work. Steam turbines are often used to make electricity.

An autoclave, which uses pressurized steam, is used in labs to kill germs. Steam, especially very hot dry steam, can clean surfaces to the same level as sterilization. Steam is a safe and non-toxic way to kill germs.

Steam is used in pipes for utility lines and to keep pipelines and containers at even temperatures. It is also used in jacketing and tracing of pipes.

Steam is used in many industries because it can transfer heat to help chemical reactions, sterilize objects, and keep temperatures steady. In the lumber industry, steam is used to bend wood, kill insects, and make wood easier to shape. Steam helps dry concrete quickly, especially in prefabricated buildings. However, care must be taken because concrete produces heat during a chemical process, and adding more heat from steam could harm the concrete. In chemical and oil industries, steam is used in chemical reactions. For example, steam cracking breaks long hydrocarbon molecules into smaller ones for fuel or other uses. Steam reforming creates syngas or hydrogen.

Steam is used to clean fibers and materials, sometimes before painting. It is also used to melt hardened grease and oil, making it useful for cleaning kitchen floors, equipment, and parts of engines. Compared to hot water, steam can reach higher temperatures and uses less water.