Autoclave

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Large autoclave for heavy transport
Autoclave

An autoclave ( Greek / Latin: self -closing) is a gas-tight, closable pressure vessel that is used for the thermal treatment of substances in the overpressure range. A pressure cooker is also an autoclave.

Autoclaves are used for sterilizing , hardening building materials, vulcanizing tires and belts and pressing fiber composite materials and are therefore used in medical technology, food technology, biology, the glass industry, the aerospace industry, in stone factories and vulcanization companies.

description

The functional principle of the autoclave is based on the work of the French physicist Denis Papin , who further developed a pressure vessel developed by Robert Hooke in 1679 into a Papin pot (steam digester).

There are autoclaves for small use volumes as well as large units for large production quantities.

The thermal treatment of the materials takes place in batches (batch operation) because of the typical seal against the surrounding atmosphere. For this purpose, the pressure vessels are usually equipped with quick-release fasteners which, compared to flanged pressure vessel openings, enable the pressure vessel to be opened and closed much faster.

use

Sterilization in medical technology and biology

A distinction is made between the following two ways of initiating sterilization:

Vacuum process (B class)
Removal of the air through repeated evacuation (pumping out) alternating with steam inflows; so-called fractionated fore-vacuum
Flow or gravitation methods (S-Class)
the air is displaced by saturated steam ( pressure cooker principle ).
Sterilization facility of the Mügelner rural outpatient clinic (1978)
Autoclave in the biochemical industry of the GDR (1976)

The objects, waste or substances to be sterilized are usually placed in the autoclave in standardized special containers, the volume of which is specified in the sterilization unit (StU). One StU corresponds to 60 × 30 × 30 cm, i.e. 54 liters.

Autoclaves are mainly used for steam pressure sterilization of culture media , medical instruments, surgical clothes, swabs and the like. Such autoclaves are therefore sometimes also referred to as steam pressure apparatus. For sterilization purposes in medicine and biology, there are autoclaves of different sizes, with an internal volume of up to a few hundred liters and more.

Sterilizers in medical technology for surgical instruments have a rectangular cross-section and vertical quick-release fasteners on both sides that allow trays to be inserted. These sterilizers are fed with pure steam that is generated outside the sterilizer. The sterilization phases and the required temperatures are recorded.

Sterilization autoclaves in biology usually have a cylindrical jacket for reasons of strength. On one side there is usually a quick release fastener, which is designed as a screw or bayonet lock. As display instruments, they have at least a thermometer and manometer . The pressure is applied either by external steam or the steam is generated in the autoclave by means of electrical heating.

Even bacterial spores (especially of Clostridium botulinum ), the resistant permanent forms of some bacteria, can be killed by sterilization . However, the aim of sterilization is not to guarantee the killing of all germs, but rather to reduce the probability of contamination of the sterile goods to 10 −6 . This means that out of a million sterile goods after sterilization, only one is contaminated with microorganisms or germs. The killing time depends on the germ load, i.e. the more germs there are, the longer the sterilization time required.

The killing rate of the germs is logarithmic, that means only 10% of the germs survive within a time interval ; varies depending on the germ (e.g. minutes for Bacillus stearothermophilus at 121 ° C hot saturated steam), but constant. The required reduction in the number of germs to 10 −6 is regular after six intervals of length , i.e. H. after a total of .

Autoclaving as a sterilization method is carried out under moist heat . The moisture mainly causes the spores of the bacteria to swell, making them less resistant than in dry heat. The procedure is divided into four sections. The first section is the rise time, during this time the interior of the autoclave is vented, i.e. the air is removed and replaced with saturated steam. The air is generally removed using the gravitational method, i.e. hot steam rises and displaces the cold air. This process is checked by a thermometer in the interior of the autoclave. After 100 ° C has been exceeded at the coldest point in the usable space, venting is completed and a valve closes the usable space pressure-tight. In this way, the preset killing temperature (often 121 ° C) can be reached. Then the equalization time begins, after this time the items to be sterilized also reach the required temperature at each point, then the actual sterilization phase (sterilization time) begins. The duration selected depends on the sterilization temperature, the type of autoclave and the microorganisms to be destroyed. For a successful sterilization, the entire room air (atmosphere) must be replaced by steam.

Pathogenic prions , such as the pathogens of the new variant of Creutzfeldt-Jakob disease (vCJD), can only be destroyed with a sterilization at 134 ° C and a duration of 60 minutes.

Objects and materials that can be treated in the autoclave are referred to as autoclavable .

Function control of medical autoclaves

For autoclaves, which are mainly used in central sterile supply departments, regular and ongoing functional checks are prescribed by the Medical Devices Act . This serves to protect both the operating staff and the patients who may have been treated with sterilized items. The electrical and mechanical device safety must be confirmed annually by a test engineer (e.g. TÜV, DEKRA, GTÜ). Different verification methods are used to verify the sterilization performance. For the ongoing control chemical indicator fields are usually used on the packaging or on paper adhesive strips, which show a color change under the defined sterilization conditions. Earth spore samples (e.g. Bacillus subtilis or Bacillus stearothermophilus, depending on the autoclave ) must be sterilized every six months and then analyzed in a certified laboratory. For sterilizers with a fractionated prevacuum, the Bowie-Dick test should be used once a day , which should measure the vacuum function with the help of a gas-permeable container.

Applications in chemistry

In general, all processes in which gases have to be reacted under pressure are carried out in pressure vessels or autoclaves (see also: Solvothermal synthesis ) . Examples of this are hydrogenation with hydrogen , the hydrolytic breakdown of fats in soap production and vulcanization . In terms of quantity, however, the most important reaction is likely to be the production of plastics such as (high-pressure) polyethylene and polypropylene from ethylene and propylene .

Experimental autoclave

For laboratories, pressure vessels are used if the safety-related reaction parameters (such as maximum pressure) are known with sufficient accuracy. This is not the case with test autoclaves and a failure of the container or triggering of safety devices (such as bursting discs ) is taken into account, which is tolerated by installation in special rooms or behind armored glass. In contrast to steam autoclaves, these autoclaves have to withstand significantly higher pressures - common laboratory autoclaves withstand approx. 150 bar. They are therefore particularly thick-walled and often consist of stainless austenitic steels (e.g. 1.4301 or 1.4571) in order to prevent corrosion and the load not to contaminate. They are also available with inner coatings made of polytetrafluoroethylene (PTFE) for carrying out experiments with very aggressive chemicals . Special designs allow pressures of up to 7000 bar and temperatures of more than 650 ° C to be achieved. Autoclaves with a volume of a few milliliters to a few liters are common in the laboratory. They usually have a manometer and a gas valve through which the desired reaction gas can be applied. A thermometer can also be part of the equipment. In the case of very small autoclaves (<20 ml), however, all of this is usually dispensed with, so they are merely steel containers that can be screwed tight. If dangerous reactions and operating conditions above the design values ​​cannot be ruled out, the autoclaves may only be operated in shielded bunkers.

Building materials industry

In the building materials industry, stone hardening boilers are used for steam hardening, which are used to harden sand-lime brick and aerated concrete . These autoclaves are horizontally arranged cylindrical pressure vessels with a quick-release fastener each on the loading and unloading side, as well as guide rails for trolleys. The sand-lime bricks formed in a press are placed on the carts and pulled into the stone hardening kettle. After the quick release fasteners have been closed, the stone hardening kettle is pressurized with steam of up to 16 bar; the resulting condensate is drawn off at the sole area. The cylinders have a diameter of 2 to 2.5 m and lengths of approx. 15 to 30 m. Bayonet locks from common manufacturers are used. The cover is placed on the counterpart of the closure welded to the jacket and rotated with a rack so that the closure elements overlap. The closure elements have an angle so that the inserted seal is pressed against the sealing surface. Only when the cover is completely closed can a safety ball valve be closed. The lid can be turned by hand or by auxiliary force (pneumatic, hydraulic).

Fiber composite production

Another area of ​​application is the production of fiber-plastic composite materials . In these autoclaves, pressures of up to 10 bar and temperatures of up to 400 ° C. are usually generated. The pressurization takes place by means of compressors , partly with a pressure accumulator. The high pressure is used to compress the individual laminate layers . The component is usually evacuated at the same time in order to completely remove excess air from the fiber composite. Normal fiber composite components made of synthetic resin (mostly epoxy resin ) with carbon or glass fibers are cured at a temperature between 100 and 250 ° C and in times between 5 minutes and several hours, depending on the resin and hardener.

Because of their high cost, such an autoclave will be primarily in the aviation and space travel , as well as the professional racing such as Formula 1 used. In the production of commercial aircraft, particularly large autoclaves are used that can accommodate entire fuselage segments. Prepregs are mainly used as semi-finished fiber products .

To produce fiber-reinforced ceramics , significantly higher temperatures of 900 ° C to 3000 ° C are used in combination with protective gas.

Laminated safety glass

Autoclave for the production of laminated safety glass

Autoclaves are used in the production of laminated safety glass (VSG). Here, two or more panes, between each of which one or more plastic films are placed, are baked in the autoclave to form a joint composite. With this process, bullet-resistant or even explosion-resistant bulletproof glass can be produced when using several glasses and different foils .

food industry

Furthermore, autoclaves are used in the food and animal feed industry to make the corresponding products (soups, stews, meal trays, etc.) long-lasting without additional cooling. Sterilization in the autoclave at 115 to 135 ° C is necessary if the product has a pH value of over 4.5, as botulinum bacteria can still germinate in this area. Pasteurisation (<100 ° C) is sufficient for lower pH values ​​(e.g. canned fruit) . However, it should be noted that molds and other bacteria are still capable of germination below this value. To maintain sterility, the products must be packaged airtight (e.g. tin cans , glass containers).

Other areas of application

Autoclaves are also used for the following purposes:

Web links

Commons : Autoclave  - collection of pictures, videos and audio files
Wiktionary: Autoclave  - explanations of meanings, word origins, synonyms, translations

Individual evidence

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Remarks

  1. ↑ After intervals of duration and therefore after a time of , the number of germs has decreased to. Obviously, the required reduction has therefore been achieved.