The greatest enemy in a refrigeration system is moisture: water mixed with the refrigerant freezes in the small orifice of the expansion valve and blocks the flow, reacts with the oil and refrigerant to form acid, and the acid burns out the compressor motor windings. The filter drier absorbs moisture and acid with its cartridge of molecular sieve and activated alumina; at the same time it traps swarf, brazing scale and wear particles.
It is fitted in the liquid line, at the receiver outlet and before the sight glass. If the moisture indicator in the sight glass is green the system is dry; if it is yellow the filter is saturated and must be replaced. A noticeable temperature difference between filter inlet and outlet (cold outlet, sweating body) is a sign of blockage. The filter should be renewed after every intervention (compressor replacement, opening the line, charging).
There are brazed (single-use) and replaceable-core shell types; in large systems a temporary "burn-out" clean-up filter is also fitted in the suction line. All Sonkar condensing units are delivered with a filter drier and sight glass.