A filter drier is a small canister installed in the liquid line that removes moisture and debris from the refrigerant. It contains a desiccant material that "soaks up" any tiny amounts of water that might have entered during installation. This is a critical component for central condensers air conditioner condenser units to keep the expansion valve from clogging. We recommend installing a new filter drier every time the system is opened for repairs. You can find replacement parts and driers for various brands on our site. It is a simple way to protect the high-tech evaporator coils from internal contamination.