A foundational component of most cold chain packaging design is the gel pack, which provides a constant temperature to the product while it melts and freezes, allowing passive packaging to have temperature stability. Gel packs come in many shapes and sizes, and it can be difficult to navigate the various specifications to determine which gel pack to use in a particular application.
The first thing to consider is that the gel pack needs to provide enough energy to get the product to its destination. As long as a water-based gel is melting, it will maintain 0°C. A heavier gel will take longer to melt and maintain the product temperature for a longer period of time. A fairly straightforward concept, more water melting means longer time to melt. This also applies to having a group of gels together, as the weights are additive. It will take longer for three gel packs in a stack to melt than three gels separated (but not touching each other). This additive property is often used in designs to extend the duration of the shipment while using gel packs that are smaller and easier to place. Interestingly, a single gel pack that is the same weight as those three smaller gels will take even longer to melt. This is because the heat transfer through the large continuous mass takes longer than the three smaller gels because the overall surface area is less.
It is important to consider both of these issues when choosing a gel pack. The larger gel will take longer to melt than several smaller ones of the same weight, but might not be practical in the application. When a gel packs gets to five or six pounds, the bag starts becoming difficult to fill and seal. The larger single gel with its smaller overall surface area might not cover the product sufficiently as well. The trick is to choose a large a gel as possible, while still maintaining an ergonomical packing procedure.