Filter driers are devices used in a HVAC system that are a combination of filter and dryer (or drier). A filter is used to remove any particle such as dirt, metal or chips from entering the refrigerant flow control. The refrigerant flow control device could be thermostatic expansion valve or simply a capillary tube.
The filter is sometimes also referred to as a strainer. It is critical that these particles are filtered out and prevented from going into the metering device. It can cause blockage to the passage flow of the refrigerant in the expansion valve and cause improper operation to the system.
A drier as its name imply is used to remove the moisture from the refrigerant. Sometimes it is also referred to as dehydrator or dryer. It is necessary to remove moisture from the system otherwise it may freeze inside the tube causing the flow of refrigerant to be restricted. The moisture will also cause the formation of acids and sludge when in contact with oil in the system.
These devices are usually used in bigger system where the refrigerant circuit is designed for field service. Residential units that are charged with refrigerant at factory level and hermetically sealed do not use these devices as the possibility of moisture and particles entering the system is minimal.
Types of Filter
There are basically two types in used today.
Liquid Line Type is placed after the condenser coil and before the expansion valve. The liquid refrigerant that flows from the condenser to the expansion valve is filtered from particles and moisture before entering it. Good filter will filter out particles that are 20 micron or above from the system.
Some higher end device has sight glass that enables the technician to look into it to see the level of refrigerant. Some have chemical in it that indicate the amount of moisture in the system based on the color of it.
A liquid line type filter drier
Suction Line Type is placed after the evaporator and before the compressor. This device is used to protect the compressor especially after any failure that contaminates the system.
Motor failure in the compressor usually cause acid and other contaminants to be introduced into the refrigerant system hence it is important to have the suction line filter dryer installed.
A Danfoss suction line filter drier
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