What is steam used for?

Water steam

Steam is used in a large number of industries. The most common applications are, for example, steam heating processes in factories and plants, and steam-driven turbines in power plants, but the use of steam in industry extends beyond those mentioned above.

Some of the typical applications for industries are:

  • Sterilization/Heating
  • Impulse/Movement
  • Motor
  • Atomization
  • Cleaning
  • Hydration
  • Humidification

Steam is generally produced and distributed at a positive pressure in a boiler. In most cases, this means that it is supplied to equipment at pressures greater than atmospheric pressure and at temperatures greater than 100°C.

Heating

Heating applications for pressurized steam can be found in food processing plants, chemical plants, and refineries just to name a few. Saturated steam is used as the heating source for process fluid in heat exchangers, reactors, reboilers, combustion air preheaters, and other types of heat transfer equipment.

Propulsion

In addition, steam is also regularly used for propulsion (as well as motive power) in applications such as steam turbines. The steam turbine is essential equipment for electricity generation in thermal power plants. In an effort to improve efficiency, progress has been made in using steam at even higher pressures and temperatures. There are some thermoelectric plants that use steam superheated to 250 bar and 610°C.

Generally, superheated steam is used in steam turbines to prevent damage to equipment caused by condensate ingress. However, in certain types of nuclear power plants, the use of high temperature steam should be avoided, as it could cause damage to the material used in the turbines. High pressure saturated steam is used instead. Where saturated steam is used, separators are usually installed in the steam supply line to remove condensate from the steam flow.

In addition to power generation, other typical drive/motion applications are turbine driven compressors or pumps, e.g. gas compressor, pumps for cooling towers, etc.

Motive fluid

Steam can also be used as a “motive” force to move liquid or gas streams in a pipeline. Steam ejectors are used to create vacuum in process equipment such as distillation towers that are used to purify and separate process streams. Ejectors can also be used for continuous removal of air from surface condensers to maintain a desired vacuum pressure in condensing (vacuum) turbines.

Atomization

Steam atomization is a process where steam is used to mechanically separate a fluid. For example, in some burners, steam is injected into the fuel to maximize combustion efficiency and minimize hydrocarbon (soot) production. Boilers and steam generators that use petroleum fuel will use this method to break viscous oil into small droplets to allow for more efficient combustion. Also (high) burners will commonly use steam atomization to reduce contaminants at the outlet.

Cleaning

Steam is used to clean a wide range of surfaces. An example from industry is the use of steam in soot blowers. Boilers that use coal or oil as a fuel source must be equipped with soot blowers for cyclic cleaning of the furnace walls and removal of combustion deposits from the firing surfaces to maintain boiler efficiency, capacity and reliability.

Hydration

Sometimes steam is used to hydrate the process while heat is supplied at the same time. For example, steam is used for hydration in paper production, so that paper moving on rolls at high speed does not suffer microscopic breaks. Another example is the pellet mills. Continuously, pelletizers that produce animal feed pellets use direct-steam injection to both heat and provide additional water content to that which is supplied in the conditioning section of the mill.

Humidification

Many large industrial and commercial facilities, especially in colder climates, use low-pressure saturated steam as the predominant heat source for seasonal indoor heating. HVAC coils, usually combined with steam humidifiers, are the equipment used for air conditioning, indoor comfort, records and books preservation, and infection control. When cold air is heated by the steam coils, the relative humidity of the air drips, and must then be adjusted to normal levels by adding a controlled injection of dry saturated steam in the bottom line of the air flow.

Types of water vapor

As already indicated in the previous section, water under certain conditions of pressure and temperature can occur in gaseous form, i.e., in the form of vapor.

It is called
saturated vapor
 when the pressure and temperature conditions correspond to a point of change of state, in this case, from liquid to gaseous. Thus, at that temperature, particles in both states can coexist.

In practice, dry saturated steam is when all the particles are in a gaseous state and wet saturated steam is when there are some particles in a liquid state.

On the other hand, superheated
superheated steam
 is that which has a temperature above the saturation temperature. Superheated steam is created by superheating the saturated or wet steam to reach a point higher than saturation. This means that it is a steam that contains higher temperature and lower density than saturated steam at the same pressure. Superheated steam is mainly used for motion-pulse applications such as turbines, and is not normally used for heat transfer applications.

Phase diagramThe image shows the different states of water depending on the pressure and temperature at which it is found.

The blue line shows the change from liquid to gas phase, i.e. the saturated vapor curve.

It can be observed that the higher the pressure, the phase change occurs at a higher temperature.

Advantages of using saturated steam for heating

Saturated steam has several properties that make it a great source of heat, particularly at temperatures above 100ºC. Some of these are:

  • Balanced heating through latent heat transfer with greatly improved productivity and product quality
  • The pressure can control the temperature and can be set quickly and accurately.
  • High heat transfer coefficient. The required heat transfer area is smaller, allowing the reduction
    of the initial cost of the equipment.

Having said this, it is necessary to keep the following in mind when heating with saturated steam:

Heating efficiency can be reduced if steam other than dry steam is used for heating processes.
Contrary to common perception, not all steam generated in a boiler is dry steam, but rather somewhat moist steam, which contains some unvaporized water molecules.

The loss of heat by radiation causes some of the steam to condense. Therefore, the wet steam generated becomes even wetter, and more condensate is also formed, which must be removed by installing steam traps at the appropriate points.

When steam is generated using a boiler, it generally contains moisture from unvaporized water particles which are carried into the steam distribution lines. Even the best boilers can discharge steam containing up to 3% moisture. As the water approaches saturation and begins to evaporate, a small portion of the water, usually in the form of droplets, is entrained in the steam flow and carried to the distribution points.