autoclave

autoclave

[aw-tuh-kleyv]

Vessel, usually of steel, able to withstand high temperatures and pressures. The chemical industry uses various types of autoclaves in manufacturing dyes and in other chemical reactions requiring high pressures. In bacteriology and medicine, instruments, equipment, supplies, and culture media are sterilized by superheated steam in an autoclave. In 1679 Denis Papin (1647–circa 1712) invented a prototype known as a steam digester; still used in cooking, it is now called a pressure cooker.

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An autoclave is a pressurized device designed to heat aqueous solutions above their boiling point at normal atmospheric pressure to achieve sterilization. It was invented by Charles Chamberland in 1879. The term autoclave is also used to describe an industrial machine in which elevated temperature and pressure are used in processing materials.

Introduction

Under ordinary circumstances (at standard pressure), liquid water cannot be heated above approximately 100 °C/212 °F (99.99 °C at 101.325 kPa, 99.62 °C at 100 kPa) in an open vessel except for special situations. Further heating results in boiling, but does not raise the temperature of the liquid water. However, when water is heated in a sealed vessel such as an autoclave, it is possible to heat liquid water to a much higher temperature. As the container is heated the pressure rises due to the constant volume of the container (see the ideal gas law). The boiling point of the water is raised because the amount of energy needed to form steam against the higher pressure is increased.

Air removal

When the goal of autoclaving is to achieve sterility, it is very important to ensure that all of the trapped air is removed. The reason for this is that hot air is very poor at achieving sterility. Steam at 134 °C can achieve in 3 minutes the same sterility that hot air at 160 °C takes two hours to achieve. Autoclaves may achieve air removal by various means including:

Downward displacement (or gravity type) - As steam enters the chamber, it fills the upper areas as it is less dense than air. This compresses the air to the bottom, forcing it out through a drain. Often a temperature sensing device is placed in the drain. Only when air evacuation is complete should the discharge stop. Flow is usually controlled through the use of a steam trap or a solenoid valve, but bleed holes are sometimes used, often in conjunction with a solenoid valve. As the steam and air mix it is also possible to force out the mixture from locations in the chamber other than the bottom.

Steam pulsing - Some autoclaves remove air by using a series of steam pulses, in which the chamber is alternately pressurized and then depressurized to near atmospheric pressure.

Vacuum pumps - Some autoclaves use vacuum pumps to suck air or air/steam mixtures from the chamber.

Superatmospheric - This type of cycle uses a vacuum pump. It starts with a vacuum followed by a steam pulse and then a vacuum followed by a steam pulse. The number of pulses depends on the particular autoclave and cycle chosen.

Subatmospheric - Similar to superatmospheric cycles, but chamber pressure never exceeds atmospheric until they pressurize up to the sterilizing temperature.

Uses

Autoclaves are widely used in microbiology, medicine, sterilizing instruments for body piercing, veterinary science, dentistry, podiatry and metallurgy. The large carbon-fiber composite parts for the Boeing 787, such as wing and fuselage parts, are cured in large autoclaves.

Autoclaves in medicine

A medical autoclave is a device that uses steam to sterilize equipment and other objects. This means that all bacteria, viruses, fungi, and spores are inactivated. However, prions, like those associated with Creutzfeldt-Jakob disease, may not be destroyed by autoclaving at the typical 121 degrees Celsius for 15 minutes or 134 degrees Celsius for 3 minutes, but can be destroyed with a longer sterilization cycle of 134 degrees Celsius for 18 minutes.

Autoclaves are found in many medical settings and other places that need to ensure sterility of an object. Many procedures today use single-use items rather than sterilized, reusable items. This first happened with hypodermic needles, but today many surgical instruments (such as forceps, needle holders, and scalpel handles) are commonly single-use items rather than reusable. See waste autoclave.

Because damp heat is used, heat-labile products (such as some plastics) cannot be sterilized this way or they will melt. Some paper or other products that may be damaged by the steam must also be sterilized another way. In all autoclaves, items should always be separated to allow the steam to penetrate the load evenly.

Autoclaving is often used to sterilize medical waste prior to disposal in the standard municipal solid waste stream. This application has grown as an alternative to incineration due to environmental and health concerns raised by combustion byproducts from incinerators, especially from the small units which were commonly operated at individual hospitals. Incineration or a similar thermal oxidation process is still generally mandated for pathological waste and other very toxic and/or infectious medical wastes.

Chemiclave

Unlike the humid environment produced by conventional steam, the unsaturated chemical vapor method is a low-humidity process. No time-consuming drying phase is needed, because nothing gets wet. The heat-up time is shorter than for most steam sterilizers, and the heaters stay on between cycles to minimize warm-up time and increase the instrument turnover.

Autoclave quality assurance

There are physical, chemical, and biological indicators that can be used to ensure an autoclave reaches the correct temperature for the correct amount of time.

Chemical indicators can be found on medical packaging and autoclave tape, and these change color once the correct conditions have been met. This color change indicates that the object inside the package, or under the tape, has been autoclaved sufficiently. Biological indicators include attest devices. These contain spores of a heat-resistant bacterium, Geobacillus stearothermophilus. If the autoclave does not reach the right temperature, the spores will germinate, and their metabolism will change the color of a pH-sensitive chemical. Physical indicators often consist of an alloy designed to melt only after being subjected to 121°C or 249°F for 15 minutes. If the alloy melts, the change will be visible.

In addition to these indicators, autoclaves have timers, temperature and pressure gauges that can be viewed from the outside.

There are certain plastics that can withstand repeated temperature cycling greater than the 121°C or 249°F required for the autoclaving process. PFA, polypropylene, polysulfone and Noryl® are examples.

Some computer-controlled autoclaves use an F0 (F-naught) value to control the sterilization cycle. F0 values are set as the number of minutes of equivalent sterilization at 121°C or 249°F (e.g: F0 = 15 min.). Since exact temperature control is difficult, the temperature is monitored, and the sterilization time adjusted accordingly.

See also

References

External links

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