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How does an Autoclave work?

How does an Autoclave work?

The terms Autoclave and Sterilizer are interchangeable and mean the same thing. ‘Autoclave’ is a term used mainly in laboratories whilst ‘Sterilizer’ is used more commonly for medical and pharmaceutical applications.

What is an autoclave?


An effective Autoclave or Sterilizer must contain dry saturated steam. In order to achieve this, air must be removed from both the load and the chamber which can be accomplished in a number of ways. Air removal from high mass low surface area items (i.e. comprising mainly solid areas that contain little or no air pockets such as bottled media) will need little air removal and this can be facilitated by automatic air purging. Here air is allowed to leave the chamber through a vent as steam enters the chamber either from an integral source (upward displacement) or an external supply (downward displacement) with the vent only closing when all of the air is removed from the chamber. This method can be further advanced by ‘Freesteaming’ when the vent is allowed to stay open for a set length of time. The turbulent steam then passes through the vent forcing any trapped air out of the autoclave. For more stubborn loads that contain a number of air pockets such as wrapped instruments or fabrics, a more effective method of air removal is essential.

By far the most effective way of air removal is a vacuum system, whereby a vacuum is achieved in the chamber before any introduction of steam, removing most of the air before freesteaming and/or vacuum pulsing. Once all of the air is removed from the load and chamber the temperature within the vessel will rise along with the pressure until the pre-selected temperature is reached. To achieve a typical sterilization temperature of 249.8F or more, requires the steam to be pressurized to at least 1.1 Bar G. Since pressures used are greater than 0.5 Bar G the Autoclave assembly is classified as a pressure system and must be designed to a strict engineering standard, such as PED2014/68EU/PD5500/ASME etc.

Creating the required temperature within the chamber is reached in a number of ways:

  • Steam can be injected into the chamber via an internal steam source such as an integral stainless steel steam generator, which can be built within the Autoclave cabinet or can be supplied as a separate external unit on larger autoclaves.
  • Some laboratories or hospitals have their own direct steam source on site, which would similarly be injected into the chamber.
  • In some Autoclaves heaters are built into the base of the chamber and the water is heated until it boils and produces steam.

The main advantage of using a steam generator (or the direct steam method) is that cycle times can be considerably faster as the steam is immediately available. In basic autoclaves the water is topped up manually by pouring water into the vessel whilst some autoclaves are connected to a direct water supply and the water level is maintained automatically. Some of Astell’s smaller units are available with an integral water tank, which once filled, can run for up to 20 cycles repeatedly before being replenished.

In CSSD applications it is a requirement that the steam is dry saturated steam with a known dryness and non-condensable gas content e.g. in the UK, Steam with values within the specification set out in HTM2010.

When the desired temperature is achieved for the required time then the steam supply will cease either by shutting off the power to the heaters or by cutting off the steam supply. Thus the temperature and the pressure will gradually drop. In units with a vacuum system fitted the vacuum pump can be used to remove the steam (i.e. where drying is required).


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