Phase change cool vests have replaceable cooling packs. 

The cooling packs have a set cooling temperature and change from solid to liquid and back again - thus under going a phase change.

The cooling packs make the cooling vest heavier and bulkier than an Arctic Heat cooling vest. The advantages are the cooling packs can be recharged in ice water, fridge or freezer, the cooling temperature is fixed and the length of cooling may be longer. An additional set of packs may be purchase to allow for continuous cooling.