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 PERFORATED ABSORBENT PLATE . MA:ER’AL SCREEN FROM a TO REFRIOERANT SUB-COOLER SiGilT GLASS Figure 3-E-Filter drier. 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. THERMOSTATIC EXPANSION VALVE.- The thermostatic expansion valve (fig. 3-16) is mounted close to the evaporator and meters the flow of refrigerant into the evaporator, depending upon  system demand.  Efficient evaporator operation is dependent upon the precise metering of liquid refrigerant into the heat exchanger for  evaporation. As  was previuosly stated, if heat loads on the evaporator were constant, a fixed opening size could be calculated and used to regulate  the refrigerant supply. However, since the system encounters varying heat loads, a variable opening device is needed to  prevent starvation or  flooding of  the evaporator,  which would seriously  affect the evaporator and system efficiency. This variable opening effect is accomplished by the thermostatic expansion valve, which senses evaporator condi- tions and meters refrigerant to satisfy them. By sensing the temperature and the pressure of the gas leaving the evaporator, the expansion valve prevents the evaporator from being flooded, and thereby returning liquid refrigerant to the compressor. The valve consists of a housing containing an inlet port, an equalizer port, and 25 outlet ports. The flow  of refrigerant to the outlet ports is controlled by positioning  a metering valve pin. Valve pin positioning is controlled by the pressure created by the remote sensing bulb in the power section, the superheat spring setting, and the evaporator discharge pressure as supplied by the external equalizer port. The remote sensing bulb is a closed system and is filled with refrigerant. The bulb itself is placed in a well, attached to the evaporator. The pressure within the bulb corresponds to the pressure of the refrigerant leaving the evaporator. This force is felt on top of the diaphragm in the power head section of the valve, and any increase in pressure will cause the valve to move towards an open position. The bottom side of the diaphragm has the forces of the superheat spring and the external equalizer port pressure acting in a direction to close the valve pin. The valve position at any instant is determined by the resultant of these three forces. If the temperature  of the gas leaving the evaporator increases above the desired superheat value, it will be sensed by the remote bulb. The 3-22