94 Modern Refrigeration and Air Conditioning Copyright Goodheart-Willcox Co., Inc. the compressor motor turns, the attached piston moves up and down within the cylinder. It is this reciprocat- ing movement of the compressor piston that provides both the suction of the refrigerant into the compressor and the compression of the refrigerant, which raises the heat content and pressure and expels the refriger- ant into the condenser, Figure 6-5. The compressor is the most expensive and crucial component of a system. It can be thought of as being the “heart” of the refrigeration system. Its capacity must be matched to the capacity of the other compo- nents to effectively push the refrigerant through the high side at the exact rate the vapor is being removed from the evaporator on the low side. 6.3.2 Oil Separator Compressors are lubricated by oil. This oil is placed inside the compressor crankcase or housing. It is circulated to various compressor parts. In a her- metic (sealed) system, this oil also lubricates the motor bearings. When the compressor operates, small amounts of oil are pumped out with the hot, compressed vapor. A small amount of oil throughout the system does no harm. However, too much oil entering the condenser, metering device, evaporator, and filters interferes with their operation. Oil can be separated from refrigerant by placing an oil separator between the compressor exhaust and the condenser. The operation of such a separator is shown in Figure 6-6. An oil separator is a tank or cylinder that con- tains a series of baffles or screens that collect oil. The oil, separated from the hot, compressed vapors, drops to the bottom of the oil separator. A float controls a needle valve to an oil return line connected to the com- pressor crankcase. When the oil level is high enough, the float rises and opens the needle valve. The pres- sure in the oil separator is considerably higher than the pressure in the compressor crankcase. This causes the oil to return quickly to the compressor crankcase. The oil serves as a liquid seal to prevent refrigerant from entering the return line. The float closes the needle valve when the oil level in the separator drops. Oil separators are quite efficient. They allow very lit- tle oil to pass into the rest of the system. Oil separators are most commonly used in large commercial installations. 6.4 Condensing A condenser is the part of a compression refrigera- tion system that releases heat from the vapor refriger- ant and allows the vapor to condense back into liquid. Only liquid refrigerant should leave the condenser. A condenser must be sized with consideration to the size of the compressor and evaporator. High-pressure, high-temperature vapor discharged by the compressor travels through the discharge line to V1 V2 T1 Piston T2 Cylinder End of Compression Stroke Volume (V2) = 1/2 in3 (8.2 cm3) Temperature (T2) = 250°F (121°C) End of Intake Stroke Volume (V1) = 8 in3 (131 cm3) Temperature (T1) = 50°F (10°C) Goodheart-Willcox Publisher Figure 6-5. The temperature of the vapor in the cylinder increases as the vapor is compressed into a smaller space by the piston. Oil and refrigerant mixture from compressor Refrigerant to condenser Oil return line to compressor Goodheart-Willcox Publisher Figure 6-6. An oil separator located in the discharge line. Note the flows of refrigerant and oil.