autoclave definition
An autoclave can be manufactured in shapes and sizes to suit the industry and application. The smallest units fit on a table top or workbench and may be used to sterilize hand tools and small surgical instruments in a dental or veterinary practice, for example. The largest laboratory autoclaves are floor standing and often of sufficient size to require dedicated space and installation to a rooms framework and services. See CUSTOM SQUARE range for details.
The concept was invented by Frenchman Denis Papin in 1679 in the form of his ‘steam digester’ – a high pressure cooker for extracting fats from bones, although the first autoclave is credited to Charles Chamberland in 1884. The name comes from Greek auto-, ultimately meaning self, and Latin clavis meaning key, thus a self-locking device.
In simple terms, an autoclave is a machine that sterilizes items placed within a chamber (known as a pressure vessel) by using dry saturated steam at a controlled temperature and pressure to destroy all microorganisms and bacteria (see also What is sterilization ). The most common temperatures used are 250°F and 270°F. These temperatures (and other high temperatures) must be maintained for a minimal time to kill microorganisms. Often a higher temperature requires a shorter sterilization process time, although the exposure time will vary depending on the item and the autoclave in use.
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.
How an autoclave works
An effective Autoclave 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 sterilizing temperature of 249.8°F 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 BPVC etc.
Creating the required temperature within the chamber is reached in a number of ways:
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 15 cycles repeatedly before being replenished.
In sterilize medical (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 sterilization 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).