A capillary tube is a fixed-length, narrow-bore tube used as a refrigerant metering device in small-capacity refrigeration and air conditioning systems. It creates a controlled pressure drop between the condenser (high-pressure side) and the evaporator (low-pressure side), allowing high-pressure liquid refrigerant to expand into a low-pressure, low-temperature state suitable for heat absorption. Unlike modulating devices such as thermostatic expansion valves (TXVs), the capillary tube has no moving parts and relies entirely on its internal diameter and length to regulate refrigerant flow.
Technical Details and Specifications
Capillary tubes are typically manufactured from copper or stainless steel and are characterized by very small internal diameters, generally ranging from 0.028 inches (0.71 mm) to 0.090 inches (2.29 mm). Tube lengths vary from a few inches to several feet, depending on the required pressure drop, system capacity, and refrigerant type. The restriction created by the combination of bore size and tube length meters the refrigerant flow at a relatively fixed rate.
Because the capillary tube is a fixed-orifice device, it does not adjust to changing load conditions the way a TXV or electronic expansion valve (EEV) does. This means the system must be carefully matched during design, with the tube size and length selected to provide optimal performance at the unit’s rated operating conditions. Capillary tubes are commonly used with refrigerants such as R-134a, R-410A, and R-600a in systems rated at fractional horsepower up to approximately 1.5 tons of cooling capacity.
One notable characteristic of capillary tube systems is pressure equalization during the off cycle. Because there is no valve to seal the high side from the low side, pressures equalize when the compressor shuts off. This allows the compressor to restart under a balanced pressure condition, reducing the starting torque requirement and enabling the use of less expensive, lower-starting-torque motors.
Common Applications
- Window and portable air conditioners
- Household refrigerators and freezers
- Dehumidifiers
- Small commercial refrigeration units such as beverage coolers and display cases
- Heat pump systems in smaller capacities
Practical Significance
The primary advantage of capillary tubes is their simplicity. With no moving parts, they are inexpensive to manufacture, easy to install, and highly reliable over the life of the equipment. However, they are sensitive to contamination. Even small particles of debris, moisture, or wax buildup can partially or fully block the narrow bore, leading to restricted refrigerant flow, reduced cooling capacity, and potential compressor damage. For this reason, capillary tube systems require clean refrigerant circuits, properly installed filter-driers, and careful brazing practices during assembly or service.
Technicians diagnosing capillary tube issues should look for symptoms such as elevated head pressure, low suction pressure, frost or ice formation at the tube inlet, and reduced system capacity. Replacement tubes must match the original specifications for internal diameter and length precisely, as even small deviations can significantly affect system performance and refrigerant charge balance.
Related Terms
- Metering device
- Fixed orifice
- Thermostatic expansion valve (TXV)
- Subcooling
- Superheat
- Filter-drier