Transient overheating in empty autoclave chambers

Abstract

The transient overheat in empty autoclave chambers is a phenomenon that frequently occurs during validation studies. This article by Italian sterilization practitioner Vittorio Mascherpa investigates the main causes of it and suggests some methods to prevent it or to cope with it. The article also includes a useful table of superheating effects due to adiabatic isenthalpic expansion of saturated steam (i.e. in ideal conditions).

Introduction

Final in-chamber drying of many sterilized items is an essential step for preserving the condition of sterility after their removal from the autoclave. Most of the energy required for drying comes by contact from the body of the sterilized items, a minor part from the chamber walls, mainly by radiation. These contributions may be sufficient or not, firstly depending on the amount of water condensed on the surface of, or inside the items. In-chamber preheating of the items reduces the steam condensation on the load during the air removal steps, both by steam injection (continuous progressive dilution of residual air after the initial vacuum pull) and steam-vacuum pulses (alternate dilution by steam supply and vacuum pull, the so called ‘fractionated vacuum’).

The air removal effect of fractionated vacuum depends only on high to low pressure ratio of the pulses, regardless to the saturation or superheating condition of the steam. Anymore, a superheated condition of the steam reduces the amount of it condensing on the load, that usually is still much colder than the saturation temperature.