Theoretical and experimental study on the cooling time of a single device after sterilization based on a zero-dimensional heat transfer model

Abstract Objectives To establish a zero-dimensional heat-transfer model for an isolated single-sealed 316 stainless-steel cylindrical device after pressure-steam sterilization and to evaluate whether the 30-min cooling time required by national standards provides an adequate safety margin under controlled conditions. Methods Ambient conditions of the sterile-material storage area were used as boundary conditions. Biot number analysis was used to confirm applicability of the lumped-capacitance model, and the governing equation was derived from Newton’s law of cooling. Six 316 stainless-steel rods (diameter 4.5 mm, length 200 mm) were heated and cooled at 24 °C and 50 % relative humidity. Temperature was recorded every 5 s for 1,800 s, and each sample was tested three times. Cooling data were fitted with an exponential function in OriginPro. Results The unadjusted theoretical model with h=5 W/(mˆ2 K) predicted 36.6 °C at 30 min, whereas the experimental fit approached ambient temperature and reached 24.6 °C. The fitted parameters were A=74.74 (95 % CI, 74.45–75.03), B=0.00402 sˆ-1 (95 % CI, 0.00399–0.00405), and C=24.56 °C (95 % CI, 24.53–24.59), with Rˆ2=0.9987 and a root mean squared residual of about 0.52 °C. The unadjusted theoretical curve is therefore a conservative reference rather than a point-accurate predictor. Conclusions For the tested simplified single device, cooling followed an exponential pattern, reached body-temperature level within about 7–8 min, and approached room temperature within about 16–20 min. The 30-min rule provides an adequate margin for this bounded scenario, but the findings should not be generalized to complex instruments, packages, multiple loads, or variable airflow conditions without further validation.