Saturated steam and superheated steam

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1. How is superheated steam generated? If the saturated steam generated in the boiler is further heated by passing it through a heat exchange surface at a higher temperature, its temperature will rise above the saturation temperature. At this point, the steam becomes superheated steam with a certain degree of superheat. When there is still water present, superheating cannot occur, because the absorbed heat first evaporates more water. Saturated steam must pass through an additional heat exchanger to achieve a higher temperature; this requires either secondary heat exchange in the boiler or passage through a separate superheater. The main heating medium can be hot flue gases or a separate heating device. 2. Main applications of superheated steam
2.1 Superheated steam is mostly used as a driving steam. For example, in the turbines of thermal power plants, steam is utilized to rotate the blades, thereby causing the generator rotor to spin and generate electricity. Steam turbines typically have multiple stages. The steam exiting from the rotor of the first stage on the same shaft must enter the rotor of the second stage to continue doing work. If saturated steam is used, some of it will condense after losing energy. That is to say, through a continuous process of work, the steam becomes increasingly wet. This not only leads to water hammer, but these water droplets also erode the steam turbine. Therefore, the best solution is to use superheated steam, utilize it to do work, and then discharge the steam once the temperature/pressure is fairly close to saturation. To improve the thermal efficiency of a steam turbine, the following two conditions must be met: A. The temperature or energy of the steam entering the turbine should be as high as possible; therefore, the pressure and temperature of the steam should be increased to the maximum extent possible. B. The temperature or energy of the exhaust steam should be as low as possible; therefore, the steam pressure and temperature need to be reduced as much as feasible, which is usually achieved by installing a condenser at the turbine exhaust. 2.2 Superheated steam is also used to heat certain products that require very high temperatures, such as some drying equipment. 3. Comparison of heat content between superheated steam and saturated steam. The table for superheated steam below shows the properties of steam at different pressures, using the same method as that employed in the table for saturated steam. However, for superheated steam, its pressure and temperature do not correspond one-to-one; there is no direct relationship between them. Therefore, at a given pressure, superheated steam can correspond to a wide range of temperature values. Examples of superheated steam tables: As can be seen from Table 1 above, at atmospheric pressure—that is, at a pressure of 1.013 bar a (0 bar g) and a temperature of 400°C—the enthalpy of superheated steam is hg = 3278 kJ/kg (from the superheated steam table). As shown in Table 2, at atmospheric pressure—i.e., at 1.013 bar a (0 bar g) and 100°C—the enthalpy of saturated steam is hg = 2676 kJ/kg (from the saturated steam table). The enthalpy difference between the two states is 3278 – 2676 = 602 kJ/kg. On the surface, it seems that superheated steam contains more usable energy; however, in reality, for applications where steam is used for heating purposes, the situation is quite the opposite—and it proves to be very problematic. Based on the energy of the superheated portion, the specific heat capacity can be calculated using the temperature difference between saturated steam (100°C) and superheated steam (400°C). However, unlike the specific heat capacity of water (which is 4.19 kJ/kg·°C and remains constant), the specific heat capacity of superheated steam varies depending on pressure and temperature, and is not a constant. Therefore, the calculated value of the specific heat capacity, 2.0 kJ/(kg·°C), represents only the value within a specific temperature range at a pressure of 0 bar g. There is no direct correspondence between the temperature, pressure, and specific heat of superheated steam. However, when the degree of superheat is relatively small, the specific heat capacity gradually increases with increasing pressure. 4. Why is superheated steam less suitable for process heating than saturated steam? Superheated steam must first be cooled to the saturation temperature before it can release its latent heat of vaporization; the heat released during this cooling process is relatively small compared to the latent heat of vaporization. If the steam superheat is low, this small amount of heat can be released relatively easily; however, when the superheat is high, the cooling time is much longer, and only a small portion of heat can be released during that period. Unlike saturated steam, the temperature of superheated steam is not fixed; it must be cooled in order to release heat, whereas saturated steam releases heat only as a result of phase change. This means that when superheated steam releases heat, a temperature gradient can occur along the heat transfer path. In heat exchangers, the use of superheated steam can create a dry wall near the tube sheet of shell-and-tube heat exchangers. The dry wall area becomes fouled rapidly, causing the tube walls to overheat and ultimately leading to tube failure. These all clearly indicate that in heat exchange applications, the use of steam with excessive superheat is rare, because: very little heat is released before the steam cools to its saturation temperature ； The temperature gradient on the heat exchange surface is relatively large ； The heat transfer rate for superheated steam is low ； More heat transfer surfaces are needed. Thus, in heat transfer applications, superheated steam is less efficient than saturated steam. This seems to go against our usual understanding, as the heat transfer rate is generally proportional to the temperature difference across the heat exchange surface. If, at the same pressure, superheated steam has a higher temperature than saturated steam, then it should naturally be able to release more heat But the answer is no, because: Q = UAΔT. For any heat exchange device, the heat transfer area A remains constant, but the value of the heat transfer coefficient U is not fixed. The value of “U” for superheated steam varies depending on the process, but it is much smaller than that of saturated steam; furthermore, the higher the degree of superheating, the lower the value of “U”. The \"U\" value for superheated steam is generally between 50 and 100 W/(m2·℃), whereas that for saturated steam is 1200 W/(m2·℃). Therefore, although the temperature of superheated steam is always higher than that of saturated steam at the same pressure, its heat transfer capacity is much lower than that of saturated steam. Therefore, in the heat transfer process under the same pressure, the efficiency of superheated steam is much lower than that of saturated steam. Therefore, in actual production, especially these days as more and more users rely on centralized heating provided by thermal power plants, the steam generated by these plants is high-temperature and high-pressure superheated steam. It must first pass through a temperature and pressure reduction system to be converted into saturated steam before it can be supplied to various users. Only when the superheated steam is cooled to a saturated state can its most useful latent heat be released.