The three phases of a steam sterilization process. Phase I: air present in the sterilizer chamber is replaced by steam and the aimed sterilization temperature is reached. To this end the sterilizer chamber is evacuated several times down to a pressure p v and subsequently filled with steam to a pressure p s . Phase II: the actual sterilization period. Phase III, coming to a safe state to open.
Schematic diagram of the sensing unit of the Electronic Test System (ETS, 3MTM, Neuss, Germany). The LEDs (Light-Emitting Diodes) on top of the unit indicate the status and the overall results of a steam penetration test and can provide an optical link to download the measured data into the ETS software. The lower compartment contains a Liquid Crystal Polymer (LCP) tube, which is open at the bottom end and closed at the top end. Twelve aluminum blocks are attached to this tube. The LCP tube with these blocks is insulated from the rest of the unit by a foam rubber cylinder. Between the aluminum blocks, rubber O-rings are mounted to ensure that mainly horizontal heat transfer will occur in these blocks. The pressure in the sterilizer chamber is measured with the sensor P and the chamber temperature with the sensor T. Sensors T3 and T5 measure the temperature in the corresponding aluminum blocks, counted from the top down. Dimensions are in mm.
Schematic representation of the model used in the computations. The inner channel of the ETS is modeled as a cylindrical tube with a constant radius r and length L tube. The 12 aluminum (Al) blocks are approximated by a second tube with length L block and an effective heat capacity C block, in which the distance between the original Al blocks is taken into account. The heat conductivities of this tube in the axial and radial direction are taken zero and infinitely large, respectively. The heat conductivities of the LCP tube in the radial and axial direction are used as model parameters. The part of the LCP tube in contact with the outer tube is assumed to be thermally shunted in the axial direction by the original Al blocks, which increases the effective axial heat conductivity of the LCP tube in that region by about a factor of 10. All dimensions are taken from an ETS identical to the one used in our experiments.
Reduction of the heat transfer from condensing steam by the presence of a certain fraction of air: f air = ρair/(ρair + ρvapor). 9 Black and open symbols denote the data for temperatures of 100 °C and 82 °C, respectively. Squares represent data for T sat − T w = 5 °C, triangles those for T sat − T w = 15 °C.
Example of data measured by an ETS during the experiments. In the upper plot, the solid curve denotes the pressure and the dotted curve denotes the temperature, both measured in the sterilizer chamber. The dashed curve denotes the so-called theoretical temperature, the temperature calculated from the pressure. 11 The start and end of the sterilization phase are denoted by vertical dotted lines. The lower plot shows the measured temperature of T3 and T5 (see Figure 2 ), denoted by the dotted and dashed curve, respectively. From these temperatures, an inverse time constant τ−1 is derived, which for each block i is equal to (dT i /dt)/(T chamber − T i ). The behavior of these τ−1-values is denoted by the dashed-dotted and solid curve for T3 and T5, respectively.
Temperature change per unit of time (dT/dt) at the positions of T3 and T5 of the ETS (see Figure 2 ) as a function of the volume of air injected just before the final inlet of steam. The derivatives dT/dt are calculated at the start of the sterilization phase. The open squares denote the experimental data, the dashed and solid curves reflect the corresponding model predictions assuming ideal and non-ideal film condensation, respectively.
Calculated behavior of the temperature variation of T3 and T5 and the inverse relaxation times for each block i for the situation corresponding to the experimental data presented in Figure 5 . The start and end of the sterilization phase are denoted by vertical dotted lines. The temperature variations of T3 and T5 are denoted by the dotted and dashed curves, respectively. The behavior of is denoted by the dashed-dotted and solid curve for T3 and T5, respectively. The time t * denotes the time with respect to the start of the sterilization phase.
Partial air pressures normalized to p ref = 100 kPa as a function of position in the inner channel of the LCP tube of the ETS. The pressures at the start of the sterilization phase are given by the solid curves, which are labeled according to the volume of additional air V air injected in the sterilizer chamber. The dashed curve denotes the partial air pressure for V air = 200 cm3 at the end of the sterilization phase. The gray areas represent the position of the Al blocks T3 and T5. Note that only the upper part of the inner channel is depicted.
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