Huanyu classification of steam in the sterilizer include: saturated steam, superheated (superheated) steam, wet steam. As different forms of steam, their states are not very different in fact, but their influence on the sterilization process can not be ignored, sometimes even directly lead to the failure of sterilization or the emergence of wet package. In this issue, we will analyze these states.
The standard state of steam: saturation. But before we dive into that, we need to do a little bit of physics, which is the three phases of water and how they transform into each other. As is known to all, the boiling point of water in the environment we live in (1 atmosphere = 1 ATM) is 100 degrees Celsius and the freezing point is 0 degrees Celsius. The liquid water cannot be heated beyond 100°C, and if you add more heat, the water will vaporize into a gaseous form. Liquid water also cannot be cooled below 0°C, and it will freeze into solid ice if the heat is removed further.
The steam that has just taken in heat and turned into a gas is still 100 degrees Celsius, and the ice that has just lost heat and turned into a solid is still 0 degrees Celsius. So liquid water and gas water vapor can coexist at 100 degrees Celsius, and liquid water and solid ice can coexist at 0 degrees Celsius. The amount of heat absorbed or released by water at the same temperature when its phase changes is what we often hear about: latent heat.
Definition of latent heat:
Latent heat, in thermochemistry, is the energy absorbed or released by a substance during a change in state (phase transition) without a change in temperature.
Definition of saturated steam: saturated steam is the temperature of the current ambient pressure corresponding to the boiling point of steam. At 1 atmosphere, 100 degree celsius liquid water has just absorbed the latent heat and then 100 degree celsius vapor is saturated vapor. At 2 atmospheres, 121 degrees celsius is saturated vapor. At 3 atmospheres, 134 degrees celsius is saturated vapor. Saturated steam, having just been vaporized from liquid water, is dry and moisture free. So you might be wondering, in the sterilization process during the sterilization period the inside of the sterilizer is supposed to be full of dry saturated steam, but as we mentioned before it can’t be sterilized in the absence of water. So how is the inside of the sterilizer filled?
The steam touches the cold apparatus as soon as it enters the sterilizer and transfers heat to the apparatus to condense as water on the surface of the apparatus. Steam keeps coming in and condensing into water. Eventually, when everything along with condensed water reaches 134°C, there will be no transfer of heat and no steam consumption. The device is covered with water at 134°C, constantly sterilizing the surface. Saturated steam, on the other hand, contains no water inside and is still dry.
Superheated steam is actually saturated steam heated again, so that the temperature of the steam is higher than the boiling point corresponding to the current ambient pressure, it becomes superheated steam. There is no upper limit on the temperature of superheated steam, such as that used by coal-fired power plants to generate electricity, which is around 800°C.
In the process of steam sterilization, the area shrouded by superhot steam is almost no sterilization effect. The reason is that the steam is so hot that no liquid water can exist in this superhot mass. Ultra-high temperature steam without any liquid water is as effective at sterilization as air at the same temperature, or as dry heat sterilization with very low efficiency. Therefore, we must try our best to avoid the generation of superheated steam in the sterilization process. Superhot steam is common in the following possibilities:
a.Incoming steam is not sufficiently cooled
b.Cotton bag is too dry, hygroscopic reaction extra heat release
c.Jacket temperature design is too high
When we boil water at ordinary times, if we carefully observe the boiling water, we can see that the 1~2 cm space on the surface of the water is colorless and transparent, and then we can see the white “water vapor”. The reason is that water vapor is colorless, so it can’t be seen in space closer to the surface of the water. As it travels upward, it comes into contact with cold air and condenses slightly to produce moisture, which is then reflected in the saturated vapor and can be seen by the naked eye. And this “white water vapor” is wet steam. Saturated steam that has been partially condensed and mixed with water. This mixture is wet steam.
Wet steam is generally found in steam pipelines. Condensation occurs when the saturated steam inevitably loses heat as it moves along the pipe. After condensation, some of the liquid water molecules will travel along with the saturated steam, and may eventually enter the sterilization chamber. These liquid water molecules are more likely to lead directly to wetness.
So that’s the analysis of the three different steam states. In the ideal state, saturated steam is the best, but in practice, it is inevitable to have super hot or wet steam. Steam traps, heat exchangers and chamber temperature control will all be added in the design of sterilizer.