The majority of temperature control shipping packages are passively controlled. This means that the design relies on the thermodynamic properties of the components to maintain the temperature. The refrigerants in the shipper should melt or freeze, and during this process they will control the temperature of the surrounding items at or near its freezing point.
This works in combination with all of the other items that comprise the total packaging system. This includes any insulation, bubble wrap, and corrugated boxes, as well as the product payload being protected by the package.
The Payload Factor
The payload can be a large factor in the total system performance and must be included in any thermal packaging evaluation.
The larger the mass of the payload is compared to the refrigerant in the package, the more of an effect it will have on the performance.
For example, a 55 gallon drum of liquid will have a large amount of thermal stability and may not need refrigerant to maintain its temperature, whereas a 10 mL vial within a chipboard carton will change temperature very quickly.
The density of the payload product is also an important property in relation to the thermal performance of the package. For a pharmaceutical package, not only does the number of liquid vials matter, but also how closely they are packed will influence the heat transfer.
A grouping of a hundred vials tightly packed in a tray will change temperature much slower than a hundred vials packed individually into chipboard cartons.
Design & Testing
As the package is designed and tested, it is important to make sure that its robustness matches the needs of the payload. As the density of the payload product decreases, the package will need more refrigerant and insulation, even though the product volume is still the same. This reality causes many packages to be designed around an empty payload volume since it represents the worst case situation for payload temperature stability.
In certain situations, this can result in an overdesigned package since the performance will be enhanced once the product is actually loaded.
Quick Tip! – If your product is a lower weight than a package was tested for, add a water-based gel pack conditioned at the same temperature. This can add thermal stability to your product and make the package work as intended.