A capillary tube is the simplest type of refrigerant metering device, consisting of a long, thin copper tube with a very small internal diameter. It creates a pressure drop by using friction and the small size of the tube to restrict the flow of liquid refrigerant into the Evaporator Coil. Because it has no moving parts, it is highly reliable and commonly found in smaller Ductless Mini Splits and window units. However, unlike a TXV, it cannot adjust the flow based on the cooling load, making it less efficient in fluctuating temperatures. If the tube becomes clogged with debris or moisture, the system will lose its cooling capacity entirely. Many modern ACIq systems have upgraded to electronic valves, but the capillary tube remains a staple in budget-friendly cooling. Proper vacuuming during installation is essential to prevent the "ice-up" that occurs if moisture enters this tiny tube. It is a fundamental component of the refrigeration cycle in many Light Commercial Mini Splits.