If the refrigerant is contaminated or has absorbed moisture, the air conditioning compressor, as well as other components, will suffer. Filter-driers, which are also called accumulators in vehicles with throttle valves, prevent this from happening.

The filter-drier is a filter unit on the high-pressure side of the refrigerant circuit and is located between the air conditioning condenser and expansion valve. The refrigerant has been cooled and liquefied by the air conditioning condenser. It now enters the filter-drier.

Purpose and function


The filter-drier has the task of removing contaminants from the refrigerant and extracting moisture. This ensures the effectiveness of the system and protects the components from damage due to contamination, corrosion, or malfunctions.

The filter-drier is a filter unit and consists of a usually cylindrical drier bottle containing a filter pad filled with hygroscopic material (silica gel).

The refrigerant liquefied in the air conditioning condenser enters the filter-drier, flows through the filter pad, and exits in liquid form. A rise allows the refrigerant to move up to the outlet of the air conditioning drier and reach the expansion valve. The upper part of a filter-drier also serves as a compensation chamber, while the lower part stores refrigerant to compensate for pressure fluctuations in the system.

Regular replacement required

Important: As a rule, the filter-drier should be replaced every two years. However, if the system is damaged after an accident or you have to open it for repair, replacement is essential. Moreover, after replacing other system components such as the air conditioning compressor or air conditioning condenser, you must also replace the filter-drier, as it cannot be flushed.

Replacing the filter-drier is important to avoid poor cooling performance and costly damage to the air conditioning system. These can occur when the hygroscopic material (silica gel) is saturated. The silica gel is able to absorb 6 to 12 grams of moisture. If this potential is exhausted, moisture remains in the system and leads to corrosion, as well as ice formation in the expansion valve and evaporator. In addition, the excess moisture causes the silica gel to dissolve and be carried away by the refrigerant. The particles distribute in the system and—like ice formation—cause a reduction in cooling capacity or lead to damage to the air conditioning compressor and other components.

Different designs

The accumulator is a similar filtering device, but it is used only in vehicles with a throttle valve (orifice tube). Unlike the filter-drier, it is located on the low-pressure side between the evaporator and the air conditioning compressor. It also has a filter and storage function. In addition, it ensures that only gaseous refrigerant enters the air conditioning compressor. If liquid refrigerant is drawn into the air conditioning compressor, severe damage will occur. Liquids cannot be compressed.


To facilitate logistics and assembly, MAHLE’s range not only includes numerous air conditioning condensers but also finished modules. These are ready-made units comprising a flat-tube condenser and a collection/expansion tank with integrated filter-drier. The special replacement filter inserts for these units can be changed without having to replace the air conditioning condenser at the same time.

Causes of failure

Used filter-drier

The most common causes of failure are manifested by poor cooling performance and/or complete failure of the air conditioning system:

  • Saturated filter/excessive aging
  • Dirt particles in the system
  • Corrosion
  • Defective filter pad inside
  • Leaks at the connections or due to damage

Damage prevention

To avoid damage, you should keep a few important points in mind when working on the air conditioning system:

  • Regular maintenance
    We recommend performing an annual air conditioning check and an extended air conditioning service every two years including replacement of the filter-drier
  • Always replace the filter-drier/accumulator after opening the refrigerant circuit
  • During installation, do not remove the blanking plugs until immediately before connection
  • Before refilling the system, check the leaktightness of the system and correct fit of the connections
  • When filling the air conditioning system, observe the vacuum time (at least 20 minutes!)

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