This metering device is a very popular type. It is used in both refrigeration equipment, commercial and residential, and air-conditioning equipment, both residential and commercial. The small, copper tubing is sized by its I.D. (inside diameter). The liquid line from the condensing unit might be Vi-inch copper tubing. When it enters the evaporator section, the liquid line restricts down to a capillary tube with an I.D. of perhaps .065 of an inch. By restricting the refrigerant, pressure is raised on the high side of the system. The length of the tube is also relevant to the pressures. You can see that this system has a fixed metering device. If a large heat load is placed upon this type of unit, it takes a long time to remove the heat load. You can see that the thermostatic expansion valve is far superior in responding to changes in heat load. The capillary tube system works well in home units, both air conditioning and refrigeration. Figure 15-14 shows the typical capillary tube system. Care must be taken to protect the inside of the tube from becoming restricted. This is the reason for the fine mesh strainer at the entrance of the tube. All foreign particles should be trapped here and contained.
