A filter drier is a sealed component in the refrigerant circuit that removes moisture, acid, and contaminants from the refrigerant and compressor oil. It is typically installed in the liquid line between the condenser and metering device.
Moisture in a refrigerant system is extremely damaging. It can freeze at the expansion device, causing restriction and system malfunction. It also reacts with refrigerant to form hydrofluoric or hydrochloric acid, which corrodes copper tubing, damages the compressor, and degrades lubricating oil.
Filter driers should be replaced whenever the refrigerant circuit is opened for service, after a compressor burnout (which releases acids), or during system installation. Suction line driers are used temporarily after burnouts to clean the system. Signs of a saturated drier include excessive superheat and restriction symptoms similar to a low refrigerant charge.