How to load an autoclave

Published: 31-Jul-2020

Loading an autoclave correctly not only ensures the sterilisation of its contents but also helps to maintain operational efficiency

Autoclaves come in many sizes; however there are typically only two chamber types, circular and square section. Circular pressure vessels are generally less costly to produce but have less internal ‘useable’ space than equivalent square chamber models.

Astell manufactures circular chamber autoclaves up to a capacity of 344 litres and a SQUARE range in sizes of 125 litres up to 2000 litres, which are suited to bigger loads and bulky items.

The primary reason for ensuring that any autoclave is loaded in a correct manner is to allow for adequate steam penetration throughout the load. Good steam penetration ensures maximum heat energy exchange at the point of contact with the load. (For a greater explanation see ‘What is Sterilization?’).

It is also imperative that each individual process cycle is set up with relevance to the type of load placed inside the autoclave, and that this process allows for all air contained within the vessel as the door is closed to be purged during the build-up to the sterilisation process. Removal of all air from the steam (along with good cycle program set-up) ensures consistency in temperature, promotes good penetration and aids a more thorough sterilisation process.

Why you should avoid overloading an autoclave chamber

If overloaded, or badly loaded, the contents may struggle to achieve the set sterilisation temperature, and eventually the cycle will be aborted.

Top tip: Don’t over-fill or ‘pack down’ the load as this prevents maximum steam penetration. Spread the load evenly across the surface of the autoclave tray / shelf and do not front-load or rear-load the vessel. If it is a mixed load with different types of products, ensure that the spread of items is even around the area of the tray / shelf.

It’s not only the shape and size of the autoclave chamber that suggests how you load it (but worth noting a favourable coincidence of circular vessels can be the space afforded by the ‘square peg in a round hole’ analogy which naturally allows for steam space around the load extremities, aiding penetration). Steriliser loads consisting of boxes, bags and containers also need careful consideration. Should air remain in the chamber as the set temperature is reached, steam will be unable to fully penetrate the load, and any remaining air will cause possible temperature differences throughout the pressure vessel – and it is only when pure steam comes directly into contact with the load that maximum heat energy is transferred, and a thorough sterilisation process occurs.

Benefits of a vacuum

Non-vacuum autoclaves that rely on ‘freesteaming’ or (‘air purging’) will have difficulty fully removing air from certain loads of a more complex nature, and therefore ‘correct’ loading is a bigger consideration with these models. Autoclaves fitted with pulsed vacuum systems will efficiently remove air from all but the most dense of ‘waste’ loads. It follows that if an autoclave is regularly going to be used to sterilise large loads of laboratory (or clinical) waste, addition of a vacuum stage in the build-up to sterilisation is highly recommended, and would by most be regarded as essential for particularly hazardous loads such as BSL3 waste. Vacuum, because of its efficiency, will also ensure that air is removed quickly (in comparison to freesteaming) and this will result in quicker cycle times.

Top tip: Bottled fluids and general waste should never be mixed. Each should have their own cycles.

Benefits of a load sensing probe

A load sensing probe placed in the centre of the load can help ensure that sterilisation timing does not commence until the correct temperature has been reached, but of course if the air is not successfully removed from the area immediately surrounding the probe, the cycle may not complete. (Also see ‘What is Load Sensed Process Timing?’).

Even if the load sensing probe is able to achieve sterilisation temperature, it is important the probe is placed as close to the centre as possible, to ensure that all parts of the load are likely to achieve the required temperature.

Top tip: To ensure the maximum benefit of the Load Sense Process Timing, validation of the particular load will ensure that the full load reaches the desired sterilisation temperature and all future cycles achieve consistent results.

Laboratory autoclave loading advice

Program selection: Ensure that the correct Program Cycle type is selected in order to ensure that the autoclave has the best chance of fully sterilising the load (don’t select a Liquid Cycle for a Drying Load or vice-versa).

Closed containers: Any container being used in an autoclave should have holes in the sides to allow steam to enter and air to escape.

Bags: The most difficult loads for air removal are those in plastic bags. Bags should not be sealed and the top must be opened up completely and ideally rolled down so that the maximum area of the load is exposed to the steam without the top of the bag getting in the way.

Top tip: It may be advisable to add a small amount of water into the bag upon loading to aid steam penetration within.

Open Containers: Any container holding a bag must have large holes on the sides to allow steam to enter.

Top tip: Plastics are poor conductors of heat, so the ‘harder’ the packaging material, the better the steam penetration into the load. Metal containers are by far the best product to use.

Capped bottles: Caps should be loosened if not removed entirely before being placed in the chamber (or a holding container) to aid steam penetration.

Top tip: Plastic containers or bottles are not generally recommended for autoclaving liquids since heat up/cool down times can be very long.

A variety of different autoclave containers are available from Astell that will contain a load and allow thorough air and steam passage, whilst maintaining integrity and facilitating pouring away liquid residue. Please Contact us for further details.

Finally, if multipoint validation has been commissioned in order to set up the autoclave, it is imperative to adhere to loading the chamber in exactly the same way, otherwise problems and errors may also occur. Of course, if the autoclave has been overloaded or badly loaded, there is a distinct possibility that it cannot be successfully validated – An Astell Validation Engineer can advise on the best way to load your autoclave to ensure successful cycles are achieved every time.

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