Way back in the early 1980s, when I was working on Air Force radios, we received repaired modules from the depot. They were packed with dessicant packs of a certain capacity, and these fabric bag packages had printed instructions for how to "recharge" them. Basically, they called for putting them in an oven and baking them for a while at a low temperature to drive out the absorbed water.
I don't bother with that stuff anymore. I put "crystal" cat litter in fabric bags I make from cheap remnants at fabric stores and sew them up. If they aren't still really "crunchy" sounding when they're handled, indicating they have absorbed some water, I pull the thread closing up the bag, dump the contents in the trash, and refill them. It's inexpensive and it doesn't take much (< 1 cup for a 40mm ammo can) to protect a large volume. Problem solved.