Functions of Capillary Tubes in Refrigeration and Heat Pump Systems
1. Throttling and Pressure Reduction – Core Function
A capillary tube is a throttling and pressure-reducing device commonly used in residential air conditioners, air-source heat pumps, refrigerators, and small refrigeration systems.
When the high-pressure, warm liquid refrigerant leaving the condenser passes through the extremely narrow and long copper tube, the flow resistance causes a significant pressure drop.
High-pressure liquid refrigerant → Low-pressure, low-temperature liquid-vapor refrigerant mixture
This creates the low-pressure and low-temperature conditions required for efficient heat absorption in the evaporator.
2. Control Refrigerant Flow Rate
The cross-sectional area of a capillary tube is fixed. The tube diameter and length determine the amount of refrigerant that can flow through the system.
By controlling the refrigerant circulation rate, the capillary tube helps match the refrigerant flow with the compressor capacity, preventing excessive or insufficient refrigerant flow and maintaining stable heat exchange performance.
3. Refrigerant Cooling and Evaporation – Flash Gas Effect
After the high-pressure liquid refrigerant passes through the capillary tube and experiences a sudden pressure drop, flash evaporation occurs.
Part of the liquid refrigerant instantly vaporizes and absorbs heat from the refrigerant itself, causing a significant reduction in refrigerant temperature.
The refrigerant is therefore pre-cooled before entering the evaporator, where it can efficiently absorb heat and provide cooling.
4. Balance High and Low Pressure After Shutdown
After the compressor stops, the capillary tube provides a small flow path between the high-pressure and low-pressure sides of the system.
The refrigerant can gradually move between the two sides, allowing the system pressure to equalize.
When the compressor starts again, the pressure difference is reduced, resulting in a lower starting load and helping reduce starting current and protect the compressor motor.
Comparison of Capillary Tubes and Electronic Expansion Valves
Compared with electronic expansion valves, capillary tubes have a simple structure, low cost, and no moving parts, which helps reduce the risk of mechanical failure.
However, the refrigerant flow rate cannot be automatically adjusted. Therefore, capillary tubes are less suitable for systems with large load fluctuations, such as low-temperature heat pumps and high-capacity heat pump units.
The refrigerant charge must also be carefully controlled. Even a small deviation in refrigerant charge can affect the heating and cooling performance of the system.
Additional Considerations for Heat Pump Applications
In heating mode, the capillary tube performs the throttling and pressure reduction process in the corresponding refrigerant circuit, reducing the refrigerant pressure and temperature so that the outdoor heat exchanger can absorb heat from the ambient air.
For some heating and cooling systems, check valves and auxiliary capillary tubes are used to provide different refrigerant flow paths. Different capillary tube lengths can be selected for cooling and heating operation to better match the requirements of each operating condition.
