Driers should be used on both the liquid line and the suction line. The driers are designed differently and should be used for the purpose designated. The liquid line drier filters as well as dries the refrigerant as it leaves the condensing unit. The drier contains a desiccant and a strainer or sieve. The refrigerant flowing through the drier gives up any moisture it may be carrying to the desiccant. Any particles of a solid will be trapped and held in the drier. Most of these units are designed with a two-pound pressure drop across it. When a drier starts to become clogged, the pressure drop increases as a first indicator. Frost might form on the drier when it becomes clogged. The primary purpose of the liquid line driers is the protection of the metering device against dirt and moisture. No more than one liquid line drier should be installed in the liquid line at one time. If two or more are placed in series, the pressure drop could be severe enough to impede refrigerant flow. It is recommended that driers be installed on a vertical plane. The reason is that if the refrigerant charge becomes low, it would only flow over a small amount of the desiccant if the drier were mounted in the horizontal plane. In Fig. 12-4 a drier is shown. Notice the directional arrow that designates the direction it should be installed in the liquid line. If the drier is a bi-flow type, the directional mounting is not important because it will function in both directions. This type of bi-directional flow drier was designed for the use with heat pumps due to the reverse flow of the liquid when in cooling or heating mode. The suction line driers are bigger and usually have a Schraeder-type valve on the inlet side of the drier. By taking the pressure at the suction of the compressor and at the inlet of the drier, the pressure drop is easily found. The basic construction of this filter drier is the same as the liquid line. These filters are used to protect the compressor from contamination by acid, metal filings, etc.