How the Air Conditioning Works
Your car's air conditioning system works by circulating a refrigerant that changes phase from liquid to gas and back again as it is forced through pressure and temperature changes. Cold liquid refrigerant forced through an evaporator removes heat from air pulled in from the car, and a blower pushes the cooled and dried air back out into the car. The now warmed and vaporized refrigerant circulates to the compressor, where the vapor is greatly compressed by high pressure, then to the condenser, an area of low pressure where the heat is released through the phase change back to liquid. The cold refrigerant circulates through the evaporator to again cool air from the car's interior.
About The Receiver Drier
Receiver driers are found only in systems that include expansion valves to regulate how much cold refrigerant reaches the evaporator. The cylindrical receiver drier is inserted along the pressurized lines of the air conditioning system between the condenser and the expansion valve. It serves as a filter to remove any moisture and contaminants that get into the system. Those air conditioning systems that don't have a receiver drier have an accumulator, which serves a similar function.
How The Receiver Drier Works
The receiver drier contains a dessicant sack, usually silica gel, that removes moisture from the refrigerant, preventing the moisture from reaching the compressor, where it could do serious damage. Receiver driers also filter debris from the air conditioning system, and many receiver driers have a sight glass, allowing a view of the refrigerant level in the system.
You can check the refrigerant in an air conditioning system by wiping it off and looking at the lines running through the receiver drier. While the system is running, the clear refrigerant should flow without bubbles. Only when you turn off the system should you see a few bubbles in the lines. If you see bubbles in the lines while the air conditioning system is running, your system is low on refrigerant. Low refrigerant could indicate a leak somewhere in the air conditioning system.
Other Indications of Problems
Air bubbles are not the only indications that your air conditioner is low on refrigerant. You may see oil streaks, a milky substance, or froth in the lines of the air conditioner. All of these signs indicate low refrigerant.