The Right Steam Does The Right Thing

The previous post explained how the properties of saturated steam make it an ideal agent for sterilization. Steam often travels distances through piping before it enters the autoclave chamber and makes contact with the contaminated load. Many things can influence the quality of steam on its mission to reach the load and kill bacteria. I will illustrate what happens to steam when conditions along the way from the steam source to the chamber are not ideal and how these conditions can hamper the sterilization process yielding a less than desirable result.

Two most common conditions NOT good for sterilization are:

  • Wet steam
  • Superheated steam

In this post we'll learn how they're caused and a few basic measures taken to prevent it.

Wet steam – Not Enough Energy

Wet steam is steam at saturation temperature containing more than 5% water. Wet steam lowers the heat transfer efficiency of steam, which will result in an inefficient sterilization procedure. Wet steam causes several major problems:

  • Firstly during sterilization, the wetness in the steam clogs the pores of packed loads and prevents the steam from properly penetrating wrapped loads or sealed pouches. The wetness prevents the steam from penetrating the load.
  • The second problem associated with wet loads occurs at the end of the sterilization process. The instruments or products absorb too much humidity resulting in wet loads at process termination. This is a big problem, because the dampness is a great habitat for bacteria to thrive.
  • Lastly it is possible that the wetness will cause corrosion or spotting on the instrument being sterilized.

A Long Journey Home

Let’s follow the journey steam takes from the boiler until it reaches its destination, the chamber and the contaminated load. In hospitals steam has many uses ranging from heating water, central heating, laundry, hospital kitchen and sterilization. On average, less than 5% of hospital steam is used for sterilization. Steam is generated by a large central boiler and then transferred to distribution points via a piping system.

Hospital Boiler Room for Steam Production

Wet steam can already develop in the boiler. Usually some water is carried out of the boiler with the steam, that's fine, because as we mentioned, 3%-5% of condensate is acceptable. As steam travels, there are many places and reasons why steam cools down and condenses. When steam comes into contact with piping connections it can condense, also poorly isolated piping will cause steam to condense. Some condensate is natural and is taken care of by steam traps which are placed throughout the piping system. The steam traps collect the condensate which is returned back to the boiler. In the steam transport system there should be a steam trap at pre-defined distances along the piping to discharge the condensate. It's very important that the boiler, steam traps and piping be routinely maintained so that the problem of wet steam caused by the distribution system can be avoided. If they get filled with water and dirt, they will not work effectively, or not at all.

A short summary of the risks of Wet Steam:

  • Wet loads
  • Corrosion
  • Lower temperatures in the System

Superheated Steam - Too Hot to Condense

Another possible problem can occur when steam becomes too hot, compared to the steam table. This is called superheated steam. This can occur when the speed in the steam lines is too high or just after a pressure reducing valve when the steam is not in a balanced situation, yet. An autoclave fed with superheated steam will function like a dry heat sterilizer, in which the killing of micro-organisms is less efficient than the optimal saturated steam required for sterilization. The dry heat sterilization process takes much longer even at higher temperatures than steam sterilization. For dry heat sterilization at a temperature of 180°C an exposure time of 30 minutes is needed.

I hope this will help you to understand the nature of steam and the critical role it plays in the sterilization process. Generating the ideal conditions for saturated steam will positively impact the sterilization results. Wet steam or superheated steam are problematic. If the steam supply is within the specified pressure and temperature range and saturated, then the sterilizer is able to succeed in its mission.

After these series of posts it is quite clear to understand why steam is the most common method of sterilization.

Steam’s main advantages are:

  • Efficient process
  • Easy to control and safeguard
  • Good penetration of porous loads
  • Faster than any other method
  • No pollution of the load in any way
  • Does not release pollution into the environment
  • Reliable



In order to understand why autoclaves are most commonly used for sterilization we need to understand its driving force - steam. This post will discuss what happens when water turns into vapor and condenses back to liquid and why saturated steam is just right for getting rid of microorganisms.

Simple is Beautiful

The beauty of steam is in its simplicity. Physicists have yet to find a more efficient agent for energy transfer, which makes steam ideal for sterilization. We will now try to explain how this happens and why it's such a powerful agent for energy transfer.


Water at any temperature will slowly but surely evaporate and become a gas. This is a natural process that happens because of the movement of the water molecules. But when water is heated and reaches 100 degrees Celsius this process intensifies and the water evaporates at a much higher rate than before, and not only on the surface but also within the water. The vapor is visible as bubbles that rise from inside the water to the surface and escape into the air. This vapor is called steam. What actually happens is that the water expands in volume by 1,600 times when it changes into steam!!

Steam vapor is invisible;  notice that near the spout you can hardly see anything. The clouds we see are the steam condensate, the drops of water that form when steam hits the cooler air



A common mistake is to confuse the clouds of water coming out of a boiling kettle with steam. Steam is an invisible gas. What we actually see are the tiny drops of water that form when the invisible steam comes into contact with the colder air. The steam condenses and turns back into water again, which is what we see.

Condensation and Vaporization are opposites! The same heat energy that's required for evaporation is released during condensation.
When vapor hits the surface of a colder object, the heat energy is transferred from the steam to that colder object heating it up. Steam used in sterilization is hot enough that when it comes into contact with the sterilization load (colder object) and condenses, it transfers enough heat energy to kill all the bacteria on the load.  This is one reason why steam is such an effective agent for sterilization.

Saturated Steam - It's a Delicate Balance

Saturated steam is called saturated because the space cannot contain more gaseous water (steam).  As soon as the temperature drops the vapor will condense back into water. Saturated steam has between 3% to 5% of water and this moisture helps destroy microorganisms at a considerably lower temperature when compared to dry heat and much faster than when moisture is absent.

Deep Steam Penetration

Water vapor transfers heat much more efficiently than dry air. If we can understand why, we will understand why steam is a much more efficient sterilization agent when compared to dry heat. In our previous post we discussed how coagulation starts at a much lower temperature than oxidation. Dry heat sterilization is an oxidation method for killing microorganisms, which put simply, is burning them. But it doesn't end at that, moisture is also a carrier of heat and causes coagulation which kills microorganisms.  The amount of moisture in a microorganism also influences the ease of its destruction. Microorganisms that contain more water are killed more easily by heat because moisture transfers heat energy into the microorganism more effectively than dry heat. Spores for example, are considerably dry and therefore take longer to sterilize, but the moist environment that steam provides is significantly faster and more effective for sterilization than dry heat, which makes it ideal for the destruction of spores.

The penetration of steam is deep because it has an ability to condense: When vapor condenses into water, 1,600 liters of steam condense into 1 liter of water. This huge reduction in volume causes a strong suction, which makes sterilization highly effective, because it sucks even more steam towards the colder object being sterilized. This is why steam can penetrate so powerfully and deeply into various load types, textiles and porous loads.

A summary of the main advantages of saturated steam are:

  • The presence of some humidity, usually 3%-5%, in steam increases the killing effect of microorganisms, because coagulation starts at the low temperature of 52°C.
  • The moisture inside the cell of a microorganism makes it easier to kill
  • As soon as steam hits a cooler object that needs to be sterilized it will condense to water and release a great amount of heat
  • The change of volume when steam condenses causes suction that draws more steam, which dramatically improves penetration depth
  • The relation between temperature and pressure of saturated steam makes it easy to control

With all these advantages of steam, it's easy to understand why it's the most common and efficient sterilization agent. The next post will examine what happens when steam is superheated or when it becomes wet and why these two states are not ideal when sterilizing.