Chapter 6 Basic Refrigeration Systems 97 Copyright Goodheart-Willcox Co., Inc. 2 6.4.3 Liquid Line A liquid line is tubing that carries liquid refriger- ant from the condenser or liquid receiver to the meter- ing device. Copper tubing is most commonly used. Liquid lines may be connected by brazing or using flared fittings. Because refrigerant is compressed at high pressure on the high side, a liquid line has a smaller diameter than a system’s suction line. In some systems, the liquid line may be in contact with all or part of the suction line. This arrangement forms a heat exchanger between each refrigerant line that warms the suction line and cools the liquid line. This helps to prevent condens- ing in the suction line and helps to prevent flash gas in the liquid line. Flash gas is the instantaneous evaporation of some of the liquid refrigerant. Some flash gas is acceptable in the evaporator because it cools the remaining liquid refrigerant to the desired evaporation temperature. Excessive flash gas reduces the efficiency of the evaporator. When a system has an expansion valve as its metering device, flash gas in the liquid line can cause dam- age to the expansion valve. 6.4.4 Liquid Line Filter-Drier The liquid line contains a filter-drier. Liquid line filter-driers are also called high-side filter-driers. All systems should have a liquid line filter-drier installed. A filter-drier collects moisture, dirt, metal, and other debris to prevent any from enter- ing the metering device. The drying element in the filter removes moisture. This moisture might other- wise freeze in the metering device. Moisture is also harmful when mixed with oil in a system because it forms sludge and acid. Moisture is especially harm- ful to hermetic units. A liquid line filter-drier is shown in Figure 6-10. 6.5 Metering Device The metering device controls the flow of refrig- erant into the evaporator. Metering device, refrigerant control, and refrigerant flow control are terms often used interchangeably. A metering device is located between the liquid line and the evaporator. Its function is to lower the pressure of the refrigerant by restricting the passageway into the evaporator, Figure 6-11. A metering device’s restriction allows only a small quantity of liquid refrigerant to pass. Since this small quantity of liquid refrigerant does not fill all the avail- able space in the evaporator, pressure is reduced. The space that the liquid refrigerant does not fill is filled with vapor refrigerant that flash boils because of the Goodheart-Willcox Publisher Figure 6-9. Two common types of liquid receivers. Note the liquid receiver service valves. They make service and maintenance tasks easier to perform. Vertical Liquid Receiver Inlet Service valve Mesh filters out contaminants Inlet Service valve Mesh filters out contaminants Horizontal Liquid Receiver Emerson Electric Co. Figure 6-10. This liquid line filter-drier has arrows printed on it to show the installer which direction refrigerant should be flowing through the device.