Capillary tube metering devices are found mainly in domestic and small commercial applications that experience somewhat constant heat loads on their evaporators. These systems also have small refrigerant flow rates and usually employ fully hermetic compressors. Manufacturers use capillary tubes due to their simplicity and low cost. In addition, most systems employing capillary tubes as metering devices do not require high-side receivers, which add to another cost savings.
A capillary tube is nothing but a long, fixed-length tube with a very small diameter that is installed between the condenser and the evaporator. The capillary tube actually meters the refrigerant from the condenser to the evaporator. Because of its long length and small diameter, there is associated fluid friction and pressure drop as refrigerant flows through it. In fact, as subcooled liquid travels from the condenser’s bottom through the capillary tube, a portion of the liquid may flash as it experiences these pressure drops. These pressure drops bring the liquid lower than the saturation pressure for its temperature at several points along the capillary tube. This flashing is caused from the expansion of the liquid as it experiences pressure drop.