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Why saturated steam: wet steam and superheated steam, isn’t it possible?

ByJules Hsu

Author: Jules Hsu 26 Feb. 2025 Catalog: Quality Control

Saturated steam: high temperature, high pressure, clean

In the medical field, steam used for sterilization must be high temperature, high pressure, saturated, and clean.

In the field of sterilization technology, industry standards define high pressure as pressure that requires human influence to achieve or is higher than a standard atmospheric pressure. Correspondingly, temperatures above 100°C are called high temperatures.

Since the thermal death of Bacillus stearothermophilus spores begins at 105°C, steam at room temperature and pressure is not enough to kill heat-resistant spores; only by increasing the pressure can the temperature of the generated steam be increased to achieve the sterilization ability to kill all microorganisms including spores.

Steam classification in thermal engineering

Engineering divides steam into the following three categories:

  • Saturated steam is steam with a temperature equal to the saturation temperature corresponding to the pressure.
  • Wet steam is a mixture of steam and liquid water at the same temperature.
  • The temperature of the superheated steam chamber is higher than the saturation temperature corresponding to the pressure.

Why other forms of steam don’t work

Since the volume of saturated steam is extremely small and exists in a free form, it can penetrate all surfaces of any non-sealed medical device, so it has a strong sterilization ability.

Wet steam

There is a large amount of liquid water in wet steam. In physics, the smallest droplet of liquid water has 600 water molecules. The volume of the droplet is much larger than the single water molecule that exists in the free ion state in saturated steam. For the penetration ability of complex devices, wet steam is far inferior to saturated steam.

Wet steam contains a relatively large amount of liquid water, and the local part of the load covered by a large amount of liquid water will lag behind the surface contacted by pure steam in terms of heating and cooling, resulting in a time lag for the local position to reach the sterilization temperature. Then the sterilization exposure time of the part may be insufficient, resulting in local sterilization failure.

Wet steam contains a large amount of condensed water. Because its latent heat has been lost, it cannot achieve secondary evaporation during the drying process, so it will remain on the surface of the medical device in the form of liquid water, resulting in wet packing.

Of course, wet steam can also be sterilized, and is generally used to sterilize items that do not require high penetration.

Superheated steam

The water molecules in superheated steam move a lot, resulting in very little water in superheated steam. Therefore, the physical characteristics of superheated steam are more like hot air, and the sterilization process using superheated steam is similar to dry heat sterilization. Then, when dry heat sterilization is used, the sterilization time will be extended by more than 4 times. If the action time is half an hour, then its sterilization will not meet the standard requirements.

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Jules Hsu

Jules Hsu is OSR’s Europe and Gulf Manager, originally from Frankfurt, Germany. Jules holds a postgraduate degree in mechanical engineering. He has been engaged in sanitary sterile equipment and filtration, serving customers mainly in Europe, the UAE and Saudi Arabia, Turkey and North Africa. “I like to share my theoretical knowledge and practical experience, and work with like-minded people to promote the advancement of human medical and health technology.”