Penetration of water vapour into narrow channels during steam sterilization processes

Abstract

In surgery medical devices are used that should be sterile. To obtain surface steam sterilization conditions in hollow medical devices (e.g. endoscopes), sufficient water vapour should be present in the narrow channels in these devices during sterilization. In this paper, a model to calculate the water vapour distribution in narrow channels during steam sterilization processes is presented. The narrow channels in the devices are modelled as tubes with one open and one closed end. The model is restricted to isothermal situations in which no condensation takes place. To validate the model, the time evolution of the water vapour density at the closed end of a test tube is quantified by a pilot experiment based on infrared light absorption measurements. A stainless-steel test tube was used with a length of 54 cm, a radius of 1.5 mm and a wall thickness of 0.5 mm. These dimensions are comparable to the channels in medical instruments. Both the model calculations and the experiments show that for a wide range of sterilization process parameters the vapour density near the closed end of the tube is insufficient for steam sterilization. Despite the simplicity of the model, a fair overall agreement is found between the model predictions and the experimental results. This agreement can be improved significantly by an empirical modification of the boundary conditions at the open end of the tube. Our calculations show that the tube length is the most important parameter. Some possible changes in the process parameters to increase the water vapour concentration at the closed end of the tube are addressed briefly.