Receiver dryers store excess refrigerant, filter out any contaminants that may be in the system and remove moisture from the liquid refrigerant to protect the system from internal corrosion. Placement should be between the condenser and evaporator inlet, and should be warm to the touch when the air conditioner is running.
The receiver dryer should be replaced anytime the air conditioning system is opened. Never leave the fittings uncapped prior to installation. When performing any service on the HVAC always make the receiver dryer the last component replaced.