Although steam‐in‐place (SIP) is the primary process for sterilization of bioreactors, sterilization guidelines do not exist for equipment design and little quantitative data are available concerning critical parameters such as length, diameter, and orientation of deadlegs. This study correlated microbiological sterilization data with the critical dimensionless parameters governing buoyancy driven convective flow within dead‐ended tubes during SIP sterilization. These parameters were shown to be the overall Grashof number, Grov, and aspect ratio, Ar. Typical values of Grov for bioprocess equipment can range from 103 to 105, while Ar may vary from near 0 to 20 during sterilization of vertical tubes at 121 °C. Reproducible sterilization resulting in a 12 log reduction in B. stearothermophilus spores required an Ar of less than 7 when Grov was 0.7 × 105, whereas an Ar of 12 could be used when Grov was 2.8 × 105.
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