The Science Behind Cold Packs - Part 1: What the heck is the Latent Heat of Fusion? Apr 06, 2021 Posted by: Benjamin Shore Cold Chain, Cold Packs, Enthalpy, Gel Packs, Latent Heat of Fusion, Phase Change Materials, Refrigerants Even though we have continued to add energy to the substance, temperature is not increasing. Interesting! We add heat, but no temperature increase! What’s going on here?