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. To achieve dry saturated steam, the system must remove air from both the material inside the autoclave and the chamber itself.

You can remove air from a load in several ways:

For items with high mass but low surface area, such as bottled media, air removal needs are minimal. In these cases, an automatic air-purge system, like the one used in Benchtop Autoclaves, handles the process. As steam enters from an integral source through upward displacement or from an external supply through downward displacement, the system vents the chamber and pushes the air out. The vent closes only after the system fully purges the chamber.

A more advanced approach called freesteaming keeps the vent open for a set period. Turbulent steam flows through the chamber and sweeps out any remaining air pockets.

For more complex loads such as wrapped instruments or fabrics that tend to trap air, you will need a more active air-removal method to ensure proper sterilization.

The most active method of air removal is via a vacuum system.  With a vacuum system, evacuate the chamber before introducing steam and before starting Freesteaming or vacuum pulsing..  Once the chamber and media release all air, the temperature and pressure inside the vessel rise until they reach the preset temperature.  Once you pressurize the steam to at least 1.1 Bar G, it reaches sterilization temperatures of 249.8 °F.  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: Some autoclaves, especially benchtop units, use heaters built into the base of the chamber. You add water manually, and the heaters boil it to generate steam.

The main benefit of using a steam generator or direct steam is much shorter cycle times. Both deliver instant steam, avoiding the wait for chamber water to heat. In contrast, basic in-chamber heaters require manual refilling and only create steam once the water boils.

When the cycle reaches the target temperature and time, the system shuts off the steam supply, and both temperature and pressure begin to fall. If the autoclave includes a vacuum system, it can pull out the steam faster and deliver a drier load at the end of the cycle.

After sterilization, you can safely remove and use the items ( such as media, glassware, or wrapped instruments ) or send treated waste out of the lab.