The liquid on the way to the thermostatic expansion valve is relatively warm in comparison to the cold gas leaving the evaporator. Although the gas leaving the evaporator has absorbed heat from the air being circulated through the evaporator, its temperature is still in the vicinity of 40 F. This cool gas is fed through the subcooler where it picks up additional heat from the relatively warm liquid Freon 12 that is flowing from the receiver. This heat exchange causes the liquid to be subcooled to a level that ensures little or no flash gas on its way to the evaporator.

Figure 3-14.- Subcooler.

RECEIVER.-The receiver acts as a reservoir for the liquid Freon 12 refrigerant. The fluid level in the receiver varies with system demands. During peak cooling periods, there will be less liquid than when the load is light. The purpose of the receiver is to ensure that the thermostatic expansion valve is not starved for refrigerant under heavy cooling load conditions.

FILTER DRIER.-A filter drier unit (fig. 3-15) is installed in the plumbing between the sub- cooler and the sight glass. The unit is essentially a sheet metal housing with inlet and outlet connections and containing alumina desiccant, a filter screen, and a filter pad. Its purpose is to filter all contaminants and dry any moisture that may be present in the Freon 12 on its way to the expansion valve. The alumina desiccant acts as a moisture absorbent medium. The conical screen and fiber glass pad act as filtering devices, removing contaminants. Clean refrigerant at the expansion valve is necessary because of the critical clearances involved. Moisture may freeze at the expansion valve, causing it to hang up with a resulting starvation or flooding of the evaporator. The filter-drier unit is a "throwaway type" and is replaced whenever the charge is dumped from the unit or when filter-drier operation is doubtful.

SIGHT GAUGE.-To aid in determining whether servicing of the refrigerating unit is required, a sight gauge is installed in the line between the filter-drier and the thermostatic expansion valve. The gauge assembly consists of a fitting having windows on both sides, permitting a view of fluid passing through the line. During refrigeration unit operation, if a steady flow of Freon refrigerant is observed through the sight glass, this is an indication that a sufficient charge is present. If the unit requires additional refrigerant, an indication will be the presence of bubbles in the sight glass. Since Freon is a colorless gas or liquid, a red-colored dye may be added to the liquid to facilitate leak detection. This is usually accomplished upon initial charging of the system.

Figure 3-15-Filter drier.