The receiver drier has three basic functions, to remove moisture from the refrigerant and oil, to filter out foreign materials and to act as a liquid/vapour separator ensuring a liquid supply to the TX valve at all times. As the filter in your air conditioning system, the receiver drier should be replaced every time your system is repaired or serviced, or at intervals no greater than every 3 years. Failure to replace the receiver drier can regularly result in major component failure.
The receiver-drier operates on the high side of systems that use a thermal expansion valve. This type of metering valve requires liquid refrigerant. To ensure that the valve recieves liquid refrigerant, a receiver is used. As mentioned above, the primary function of the receiver-drier is to separate gas and liquid and the secondary function is to remove moisture and filter out dirt. The receiver-drier usually has a sight glass in the top. This sight glass is often used to charge the system. Under normal operating conditions, vapor bubbles should not be visible in the sight glass. The use of the sight glass to charge the system is not recommended in R-134a systems as cloudiness and oil that has separated from the refrigerant can be mistaken for bubbles. This type of mistake can lead to a dangerous overcharged condition. There are variations of receiver-driers and several different desiccant materials are in use. Some of the moisture removing desiccants found within are not compatible with R-134a.
The desiccant type is usually identified on a sticker that is affixed to the receiver-drier. Newer receiver-driers use desiccant type XH-7 and are compatible with both R-12 and R-134a refrigerants.