Filter drier
Also known as: drier, filter dryer
Short definitionA filter fitted in the liquid line that traps moisture, acid and solid particles in the refrigerant. It keeps the expansion valve from blocking with ice and protects the compressor windings.
Water that gets into the circuit freezes in the narrow opening of the expansion valve and cuts off the flow. It also reacts with the oil and refrigerant to form acid, and over time the acid burns out the motor windings. The molecular sieve and activated alumina inside the filter drier absorb moisture and acid. The same core also traps swarf, brazing scale and wear debris left from installation.
It sits at the receiver outlet, ahead of the sight glass. Small units use a brazed, single-use type; large systems use a shell with a replaceable core. If the moisture indicator in the sight glass has turned yellow, the core is saturated. A temperature difference you can feel between inlet and outlet, or a sweating body, points to a blockage. Whenever the circuit is opened, for a compressor change for example, the filter is renewed too.