What do Liquid Line Filter Driers do? Filter Driers serve a dual purpose in your A/C system. First they filter out contaminants that could slowly destroy your refrigeration or air conditioning system, and second they absorb moisture that can turn into damaging acids that erode your system. The type of Filter Drier you need is determined by the size and connections of your refrigerant lines, and also the materials used in their manufacture. Liberty Building Maintenance stocks the leading brands of Liquid Line Filter Driers for most major refrigerants and compressors.