Have you ever wondered how the pressure cooker in your kitchen is comparable to a giant boiler in an electrical power plant? Both have similarities, because they both produce steam by heating water.
Steam is produced when water is transformed into a gas from its liquid state. In other words, steam is water in its gaseous phase. Steam is often described as the mist that is produced by boiling water. Steam also used to describe energy or momentum, as when we say that the stock market has run out of steam.
While steam is easy to identify and understand when emanating from a tea kettle, there are many important aspects to consider when using process steam systems for industrial purposes.
Steam Quality and Purity
The purity of steam takes into account the contaminants present in the steam in its vaporous, liquid or solid form. On the other hand, the quality of steam normally takes into account the moisture (water particles in the liquid form) present in the steam. Ninety-eight percent quality steam means that two percent moisture is present in it. Excess moisture in the steam could be due to faulty steam traps, carryover of boiler water, or poor insulation on the steam pipes that causes excess condensation. Steam with excess moisture does not have the heat required to sterilize equipment in sterilization applications. In turbine applications, droplet condensation can lead to erosion of the turbine blades, causing poor durability and reliability. (Related reading: Combating Cavitative Corrosion and Erosive Corrosion.)
Steam purity requirements might state the maximum permissible moisture content or might specify concentration limits of various chemical impurities. Normal building heating systems and low-pressure processes require steam to have a low moisture content, whereas for the steam turbines in power plants, the maximum limits for copper, iron, silica, potassium, sodium, and total dissolved solids (TDS) is also often stipulated.
Types of Steam
There are several different types of steam, as discussed in the following sections.
Unsaturated or Wet Steam
When we use the word “steam,” we are generally referring to wet or unsaturated steam. Boilers normally deliver steam at 95% to 97% quality, or in other words containing up to 5% moisture. In order to separate the water droplets from the steam, devices known as separators (e.g., steam traps) are provided in the system.
Saturated Steam or Dry Steam
Saturated steam (also known as dry steam) is defined as the steam that is produced when the liquid and gaseous phases of water occur simultaneously at a specified pressure and temperature. Saturated steam is the point where the production rate of steam equals the vaporization rate of boiling water.
When water is heated, the temperature of the mass increases until it reaches the boiling temperature for the existing pressure. Any further increase of heating will result in vapor formation without any rise in temperature because any heat energy supplied is now used as latent heat of vaporization. Saturated steam cannot be seen by our eyes because it is free from water droplets. Saturated steam occurs at around 100°C (212°F) at normal atmospheric pressure. Most heating applications specify saturated steam as a requirement.
Superheated steam is produced by continued heating of saturated steam or unsaturated steam until the steam attains a higher temperature than the saturated steam, typically between 200°C to 800°C (392°F to 1,472°F), at a constant pressure. This type of steam stores more heat compared to saturated steam at the same pressure. Superheated steam is mainly preferred for turbine drive applications, whereas saturated steam is preferred for heating and sterilization applications.
By using superheated steam, the risk of blade erosion damage and corrosion damage in steam turbine drives is minimized because the steam does not generate water droplets due to condensation on the blades. (Related reading: Corrosion Prevention for Water Pumps, Valves, Impellers and Fittings.) However, the equipment must be made of sturdier materials because they must withstand the higher temperatures of superheated steam.
Supercritical Fluid Steam
When a critical level of pressure is applied to steam at high temperatures, the molecules are brought together to such an extent that the steam becomes a supercritical fluid, similar to water, but retaining the main features of a gas. In other words, it is neither a gas nor a liquid. This supercritical fluid is being adopted in modern power plants because of the advantages of superior thermal efficiency and lower CO2 emissions.
Applications for Steam
There are a variety of applications for steam, both in homes and in industrial facilities. Some of the principal uses of steam include:
Electric power generators
In the United States, most (almost 85%) of the electric power produced is generated by steam driven turbo-generator sets. In fact, throughout the world most of the electricity is generated by using steam. Superheated steam and supercritical fluid steam are both used in the power sector.
Apart from driving steam turbines, the steam is also used to drive large turbo compressors, turbo pumps, and air and gas compressors in the industry.
In cogeneration plants, steam is used to drive turbine-generator sets and for heating.
Atomization of fuel
Atomization is basically the process of breaking solid and liquid fuels into very small particles. This is done to achieve higher fuel economy by enabling more efficient combustion.
In addition to improving combustion, atomization reduces air pollution as well.
Drying of products
Dry steam is used to remove moisture from some industrial products. While some plants use hot air for this purpose, steam drying is safer and more economical.
In industrial plants, labs, hospitals, homes and offices, maintaining humidity in the required range is a critical issue for HVAC service providers. A too low humidity level contributes to health risks for people and damage to materials and equipment. HVAC coils with steam humidifiers are used for humidity and temperature control. When cool air is heated during colder seasons, the humidity decreases even further, so steam is injected into the heated air to increase the humidity within the air ducts.
Hospitals use saturated steam under positive pressure to sterilize utensils and wrapped items. The heat released during steam condensation kills microorganisms within a specified time duration based upon the steam's temperature and pressure. However, only items that are not damaged by heat can be sterilized by steam application.
Steam sterilization or autoclaving has the advantage of simplicity and virtually zero toxicity. Surgical equipment, dental instruments and textiles are some of the items sterilized by saturated steam.
Steam is used as a convenient medium to clean both simple and intricate surfaces that cannot be easily cleaned by other methods. For example, in the boilers that burn coal and oil, the furnace walls continuously accumulate soot that results in suboptimal performance. Steam supplied through a soot blower nozzle is used to blow off and remove this hard-to-remove soot as well as slag deposited on the wall's surface.
Steam is produced and distributed to various equipment for heating at positive pressure at a temperature exceeding 100°C. Chemical processes, oil refineries and food producers are some of the sectors needing steam for heating. Most of them prefer saturated steam.
In some processes, the steam provides heat as well as controlled moisture. In pulp and paper mills, for example, the steam provides moisture to the paper so that it can move over the rollers without being torn.
Steam can be classified as saturated, unsaturated, superheated or supercritical fluid steam. It is used globally to generate the electricity that powers the modern economy. It is also used for heating in everything from the process industry to individual homes, as well as for other applications such as sterilization and humidification.