Thermal Analysis of Refrigeration Systems Used for Vaccine Storage
Michal J. Chojnacky, W Wyatt Miller, Dean C. Ripple, Gregory F. Strouse
Each year, billions of dollars of vaccines are stored in refrigerators at the facilities of a variety of medical providers. Many vaccines must be maintained in the range 2 °C to 8 °C to retain product potency. We have tested the performance of two types of household refrigerators to determine if these refrigerators are suitable to this task, and to identify proper storage and temperature monitoring methods. Nineteen calibrated Type T thermocouples, distributed through the refrigerator interior, served as reference thermometers. Attachment of thermocouples directly to vaccine vials gave accurate measurements of the vaccine temperature, which often differed from the air or interior wall temperatures during door openings or defrost cycles. A household, full-size freezerless refrigerator proved fully adequate at maintaining vial temperatures within the desired 2 °C to 8 °C range, independent of how the refrigerator was loaded. Tests of intermittent and continuous door opening and of simulated power outages demonstrated the value of adding water bottles to the door as a thermal ballast. The performance of compact, dormitory-style refrigerators suffered from drift of the refrigerator set point, sensitivity to load density, and high temperature non-uniformity. These problems make the dormitory-style refrigerator unsuitable for vaccine storage. We tested four electronic data loggers as a means of continuously logging refrigerator temperatures. Properly located, data loggers accurately monitored vial temperatures for extended periods.