Autoclaves 101: What is An Autoclave and How Do They Work?

An autoclave is a heated chamber used to sterilize various types of media, by means of dry saturated steam under pressure.  In order to achieve dry saturated steam, air has to be removed from both the material inside the autoclave, and the autoclave chamber itself.  This can be accomplished in a variety of ways.

For air removal from items with high mass, but low surface area (such as bottled media), minimal air removal is required, and automatic air purging is sufficient, such as the Benchtop Autoclaves.  In this scenario, air is allowed to vent the chamber as steam enters from either an integral source (upward displacement) or from an external supply (downward displacement).  The vent only closes when all of the air is removed from the chamber.  Further advancing this method is “Freesteaming,” where the vent is allowed to stay open for a set length of time.  Turbulent steam then passes through the vent, forcing any trapped air out of the autoclave.  For more difficult loads that include a number of air pockets (such as wrapped instruments or fabrics), a more active method of air removal is required.

The most active method of air removal is via a vacuum system.  With a vacuum system, the chamber is evacuated prior to the introduction of any steam, and before Freesteaming or vacuum pulsing.  Once the air is removed from the chamber and the media, the temperature within the vessel will rise along with the pressure, until preset temperature is achieved.  Sterilization temperatures of 249.8 F are reached once steam is pressurized to at least 1.1 Bar G.  Since pressure is greater than 0.5 Bar G, the autoclave is classified as a pressure vessel, and must be designed and certified to ASME standards, such as the Laboratory Square Autoclaves.

There are a number of ways to achieve the temperature and steam within the autoclave chamber:

• Integral/External Steam Generator: Direct steam injection into the chamber via an internal source such as an integral steam generator, built into the autoclave cabinet, or an external steam generator for larger units.
• On-Site Steam Source: many facilities (research labs, hospitals, pharmaceutical companies), have their own steam source piped into the laboratory, which the autoclave can connect into.
• Heaters In Chamber: in some autoclaves, (most benchtop units), heaters are built into the base of the chamber, where water is added manually. The heaters boil the water until steam is produced.

The key advantage to either a steam generator or direct steam, is that cycle times can be considerably shorter, with immediate steam that does not require waiting for the heated water in chamber.  With basic heaters in chamber, the water has to be manually refilled, and steam is provided once water is boiling.

When the desired temperature and cycle time is achieved, the steam supply will stop, and temperature and pressure will begin to gradually drop.  If a vacuum system is included in the system, the steam can be evacuated more rapidly, to produce a dryer end product at the end of the cycle.

Once sterilized, the items (media culture, glassware, wrapped instruments, etc) can be removed and used safely and reliably in the lab; or in the case of discard waste, can leave the lab.