The Receiver/Drier contains filter media to filter solids from the refrigerant (such as wear metals from the compressor), and desiccant to remove moisture from the refrigerant. Like any filter, the Receiver/Drier has a limited ability to store the debris it captures. Replace the receiver drier under any of 3 circumstances: 1. Anytime the compressor is replaced 2. Anytime the system has been open to the atmosphere for more than 30 minutes 3. Annually.