Steam plays a pivotal role in the process and manufacturing sector for heating purposes. It is an ideal medium for transferring heat due to its high energy holding capacity, safety, and reliability while utilizing it. Most steam boilers use saturated steam for heating and processing in operations. However, in some cases, superheated steam is preferred over saturated steam. A superheated boiler pushes through the saturated steam limit to produce steam that does not include moisture (or wetness). The dryness in superheated steam boilers is an ideal alternative for turbines, along with cleaning, drying, and curing applications.
What is Superheated Steam?
If we analyze the steam closely, we notice that steam varies based on temperature and pressure. To understand steam better, we need to understand the two basic types of steam, i.e., saturated steam and superheated steam. As the saturated steam generated in the steam boiler is exposed to a higher temperature surface, its temperature increases above the evaporating temperature.
Due to its high amount of temperature through its heating above the saturation temperature, it is termed superheated steam.
Interestingly, superheat cannot be passed to the steam in the presence of water, as excess heat would evaporate more water. The saturated steam is passed through an additional heat exchanger, considered a second heat exchange stage in the boiler or a separate superheater unit. The primary heating medium is either the hot flue gas from the steam boiler or a separate fire.
The application of superheated is found in steam turbines that involve steam directed by nozzles onto a rotor that causes the rotor to turn. The energy required to accomplish it is obtained only through steam. Therefore, the steam has less energy after passing through the turbine rotor. If the steam is at saturation temperature, the energy loss causes the steam to condense. Turbines have multiple stages, including the exhaust steam from the first rotor directed to a second rotor on the same shaft. It results in saturated steam getting wetter as it progresses through the successive stages. Such occurrences lead to water hammer and the water particles causing severe erosion within the turbine. Therefore, superheated steam is provided to the turbine at the inlet and the energy is used for the rotor till its temperature/pressure reaches saturation.
Difference between Saturated Steam and Superheated Steam:
Steam is used in almost every process and manufacturing industry due to its heat transfer characteristics. Steam is one of the most efficient, effective, and reliable methods of transferring energy. Let us now understand the difference between both types of steam in detail.
The majority of steam boilers nowadays use saturated steam, generated by the transfer of heat to water that causes it to boil. It is similar to a pressure cooker in its operation. The steam is not released freely as the pressure is controlled for processing and heating purposes. Such steam is a by-product of the saturated steam curve with a relationship between temperature and pressure. Saturated steam is used for efficient and effective heat transfer purposes.
On the other hand, superheated steam is generated as the saturated steam passes out of the boiler drum and into a secondary heating area, termed superheater. It results in the saturated steam reaching beyond the saturated steam curve and into the superheated steam area, leading to the absence of moisture (or wetness) in the steam. Due to this, superheated steam is generated outside the main boiler drum. The lack of moisture in superheated steam is best-suited for turbines, drying, cleaning, and curing purposes.
Applications of Superheated Steam:
Superheated steam is not an ideal medium for transferring heat; it is sometimes utilized for heating in various steam plants, particularly in the HPIs (Hydrocarbon Processing Industries) that deliver oils and petrochemicals. As the superheated steam is available on site for power generation as the energy source for turbines. HPI usually desuperheat steam to about ten degrees of superheat that is removed easily in the first stage of the heating surface.
Superheated steam is not as suitable for process heating as saturated steam due to the following reasons:
- Superheated steam needs to decrease its temperature to saturation before it condenses to release its latent heat. The heat given up by the superheated steam to reach a saturation temperature is lower than its enthalpy of evaporation.
- Steam with a few degrees of superheat is given up promptly before condensation. With steam having a larger amount of superheat takes a longer time for cooling, and the steam releases very little energy.
- The temperature of superheated steam is not uniform, and it needs to cool to give heat. It means that the rising temperature over the heat transfer surface occurs with superheated steam.
- Using superheated steam in a heat exchanger can result in the formation of a drywall boiling zone. It quickly becomes scaled or fouled, and the resulting high temperature of the tube wall causes tube failure.
Advantages of Superheated Steam:
- As steam heats beyond the saturated steam temperature, the lack of condensate is ideal for applications like rotating equipment such as turbines as the blades are damaged from water droplets and condensate.
- The dryness of superheated steam is ideal in the steam engines of locomotives.
- Superheated steam has a higher thermal conductivity for drying and curing applications than air and reduced oxygen content, allowing efficient drying while preventing oxidation.
- Superheated steam is effective for chemical treatment in agricultural soils as the high heat energy helps in the deterioration of organic material as the steam is introduced into the soil.
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