Steam sterilization cycles


Sterilization cycle phases for a steam sterilizer

Sterilization Cycles

The sterilization cycle is composed of several phases which are the following one:

  • Phase 1: Chamber pre-heating at the startup, sending plant steam to the jacket, while the condensates are purged.
  • Phase 2: Vacuum performance, taking the air out from the chamber, and replacing it by pure steam injections. That operation could be done thanks to dilutions in order to reach a high percentage of steam VS air.
  • Phase 3: Sterilization. According the type of the materials to sterilize and especially according the bacteriological level on the load, we select the temperature and time.
  • Phase 4: Vacuum drying. Evacuation of steam using vacuum pump, and replacing it by sterile air. Dilution operations could by performed as well.
  • Phase 5: Return to atmospheric pressure for the door opening.

Utility qualities

To ensure the sterilization effectiveness, we should use the following utilities:

  • Pure steam,
  • Plant steam (no special requirement),
  • Water,
  • Air,

Pure steam quality

The result of the sterilization process is very much influenced by the quality of the steam produced by the generator. In other words, the steam quality depends on the water quality.

For a high steam quality, it is necessary to reduce as much as possible the following things:

  • Solid particle: welding, rust residues for example,
  • Liquid: except water,
  • Gas: air, hydrazine (N2H4), ammoniac (nh3),
  • Others chemical residues like mineral salts,

Another important aspect to take into account is the analysis of steam condensates. The EN 285:1996 norm indicates us the concentrations of residues that we don't have to exceed.

Once the cycle has been selected, the temperature is fixed according the type of material to be sterilized. For an efficient sterilization, it is necessary to have a saturated steam (to set the value of the pressure to be respected as a temperature function). We use Regnault table that give us the values of pressure correction as a function of temperature deviations and then to obtain a dry saturated steam.

Water quality

The steam quality depends on the water quality before its vaporization.

  • Osmosis water

The water quality for the sterilization should be high to ensure:

  1. Elimination of microorganisms
  2. Reduction of corrosive of water

This is why the water is treated with a softener and then a reverse osmosis. The EN285 normative gives us the recommended values.

  •  Soft water

The soft water is used on one hand for the osmosis water preparation, but on the other hand it is required as well for the supply of the liquid ring vacuum pump in order to cool it down.


Air quality

In order to break the vacuum inside the chamber at the end of the sterilization cycle, sterile air should go in. It uses to be air from the technical area at atmospheric pressure, going through an absolute filter (0.22 μm), to avoid the product contamination. That air is sometimes heated to make faster the drying process.


Technologies necessary for a correct operation of a steam sterilizer

Horizontal steam sterilizerThe jacket: A steam sterilizer is composed of a pressure vessel and a jacket, welded to the chamber, and having as function to avoid steam condensation on the walls of the chamber internal stainless steel sheet.

The door gaskets: It makes possible to maintain the vacuum inside the sterilizer with the doors are close. There are kind of gasket:

  • Movable gasket, using compressed air and vacuum in order to move the gasket from the chamber frame groove to the door or vice versa.
  • The inflatable gasket, injecting compressed air into the gasket placed in the chamber frame groove.

Periodic control indicators: Sensors measuring inside the autoclave, give the indications of the sterilizer functioning as well as all the cycle sterilization parameters.

  • Pressure sensors,
  • Temperature sensors,
  • Pressure gauges,
  • ...

The quality and maintenance of an autoclave are essential for a proper operation. That is why it is necessary to use those kind of features:

  • Bowie & Dick test,
  • Flow indicators,
  • Biologic indicators

RSD Engineering Solutions SL is specialized on sterilization processes (steam, oxide ethylene, superheated water) and equipment design and qualification.

Our expert team will guide you to choose the best solutions that are related to sterilization.

For more informations, contact us.