The Differences Between Superheated Steam and Saturated Steam

Steam is the heat resource that every laundry plant needs. Then, what are the differences between superheated steam and saturated steam? The differences lie in their thermodynamic state.

Saturated Steam

Saturated steam means a state where both liquid and gas coexist in equilibrium at a certain pressure. In this condition, the liquid will keep evaporating while the gas will keep condensing until the two reach a dynamic equilibrium. At that time, the temperature and the pressure of the steam are fixed. They are called saturated temperature and saturated pressure. At the beginning of the equilibrium, the steam is the wet saturated steam. If people continue to heat, the steam will become dry saturated steam after all the water in saturated water has been evaporated. The temperature of the steam from wet saturated to dry saturated remains the same. The temperature of steam does not increase during the process from wet saturation to dry saturation. At fixed pressure, if dry saturated steam is further heated, its temperature rises and it becomes superheated steam. Superheated steam contains no liquid droplets or liquid mists and is an actual gas.

❑ Example

To put it in a simple situation:

● At a certain pressure, the water begins to vaporize and gradually becomes steam after being heated to a boil. At this time, the steam temperature is equal to the saturation temperature. This temperature is related to the pressure. The higher the pressure is, the higher the saturation temperature is. Conversely, the lower the pressure is, the lower the saturation temperature is. When the pressure is 0.10MPa, the saturation temperature is 99.09°C. When the pressure is 4.05 MPa, the saturation temperature is 249.18°C. When the pressure is 10.13 MPa, its saturation temperature is 309.53°C.

Superheated Steam

The temperature will continue to rise and exceed the saturation temperature at this pressure after the saturated steam is further heated. This steam, which exceeds the saturation temperature, is called superheated steam. In this condition, the temperature and the pressure of the steam are not fixed anymore. When the heat increases, they increase as well. The superheated steam has higher thermal energy and heat capacity.

❑ Drawback

However, in actual use, using superheated steam to heat is relatively inefficient. This is because the superheated steam must be cooled to the saturation temperature to release the enthalpy of evaporation. The heat released from the cooling of superheated steam to the saturation temperature is very small compared to the enthalpy of evaporation.

● If the superheat of the steam is very small, this small part of the heat is relatively easy to release. However, when the superheat is very high, the cooling time is much longer. During this period, only a very small amount of heat can be released.

In actual heat exchange equipment, the use of superheated steam will form a dry wall inside the heat exchange equipment. This area will quickly form scale, causing the pipe wall to overheat and leading to pipe failure. Therefore, although the temperature of superheated steam at the same pressure is higher than that of saturated steam, its heating capacity is lower than that of saturated steam.

Conclusion

In the actual production, if the laundry plants want to use the high-temperature and high-pressure superheated steam produced by the power plant, the steam must first pass through a desuperheating and pressure-reducing station system to convert the superheated steam into saturated steam before it can be used. Superheated steam can only release its most useful latent heat when cooled to a saturated state.