The filter-drier is a canister typically mounted near the condenser that receives most of the liquid from the condenser. It has two primary functions in the air conditioning system:
1) Filter contaminants. This is the primary location where larger contaminants will end up.
2) The desiccant stored inside removes any residual moisture from the system that evacuating doesn?t remove. This is the most important function, as moisture is the #1 enemy of the air conditioning system.
Combined with refrigerant and lubrication, moisture will turn acidic (hydrochloric acid) and start to eat the system up from the inside out. It is necessary to replace or restore the filter-drier whenever the system is opened up to the atmosphere.