A cylinder made of copper or brass (of varying sizes depending on the capacity of the unit) filled with alumina or silica gel. Its function is to remove moisture that penetrates the sealed system by absorption. It is installed at the inlet of the capillary tube. The ends are either silver brazed or coupled by flare connections.

In the sealed system, only pure refrigerant and a small amount of clean refrigerant oil should be circulated. Any moisture penetrating the system will cause a lot of trouble. The air in the atmosphere can penetrate the sealed system through very tiny pinhole leaks in the tubing or at the joints. Air contains moisture. When it enters the system, the excessive low temperatures cause the moisture to freeze and block the circulation. Hence, the reason for the installation of filter-driers. Since the narrowest passage through which refrigerant moves is the capillary tube, this is the most likely place for the restriction. When a restriction occurs, the unit will no longer cool. When a sealed system is opened, moisture gets in and the filter-drier must be replaced prior to evacuating the system with a vacuum pump and recharging the unit with refrigerant.

The drier should be installed as close to the capillary tube as possible. Always keep the drier sealed until installed. (If left open, it will absorb moisture from the air and very soon become saturated.) In a properly operating system, the filter-drier should feel slightly warmer to the touch than the ambient temperature.

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