Filter driers are exactly what they sound like, they are filters. They filter the refrigerant inside the ac system. Filter driers need to be able to trap and contain moisture, acid, contaminants and wax from circulating throughout the air conditioner. Today's systems are very well designed, but problems can arise. HFC refrigerants by themselves will work for many years in a system, even when they are heated up above normal. However, under certain conditions HFC's can react and form acids. Moisture in the system can have a reaction with the refrigerants and cause hydrofluoric acid. These acids unchecked can cause havoc on your air conditioning system. Over time, pumping acid through the system will burn the insulation off the wires in the compressor. Once the wires have no more insulation, they will ark with the metal inside the compressor causing tripped breakers. We call this a grounded compressor and the only repair is a new one (very expensive). Another problem is the acid inside your air conditioner can eat away at every part or surface in the system, decomposing any surface. Over time, small pieces of parts can break free. These tiny pieces can circulate through the system, clogging capillary tubes, TXVs, or even the filter drier.
A filter drier needs to be replaced anytime the air conditioners refrigeration system is worked on. When you replace a compressor, a TXV or any time the system is open to the atmosphere, change the filter. One way to tell if the filter is clogged is to take the temperature an inch before and an inch after the filter. If you have, more than a 3 degree F drop it has clogged and needs replaced. When in doubt change it out. Filter driers are lifesavers in some cases such as a compressor burnout. A compressor burnout is when the compressor becomes grounded and when it arks it actually burns the refrigerant. Burnt refrigerant usually will turn a darker color than normal sometimes black and may become wax like in extreme conditions. Just changing your AC compressor will not cut it. The only way to ensure the new compressor runs its full life is to install new filter driers and use acid away in the system. Acid away is a product used to remove acid from dirty systems. Some manufacturers say that a suction line filter drier does a better job of removing moisture from the system. That is because the moisture tends to condense in the cold suction line and is easily trapped by the suction line filter drier. However, a suction line drier is only used for clean up purposes, say after a burnout. Suction driers installed should be left for two days to remove moisture, wax, acid and small contaminants. A suction drier should never under any circumstances be left in a system for more than 3 days. As it will become restricted very fast. Yes, filter driers will cost you more to install. However, the price of not having one could be you buying a completely new system.
Never allow anyone to open your air conditioners system without replacing the filter drier.