Capillary Tubes in HVAC: Function & Refrigerant Pressure Control

Capillary tubes are fixed-restriction metering devices used in small refrigeration and air conditioning systems (residential refrigerators, window ACs, small chillers).

Operating Principle

• Long, small-ID (0.5–2 mm) copper tube connecting condenser outlet to evaporator inlet.
• Creates pressure drop via frictional resistance and acceleration of refrigerant (liquid + flash gas).
• No moving parts; passive, self-regulating within design limits.

Key Functions

1. Flow regulation: Matches mass flow to evaporator capacity at design conditions.
2. Pressure separation: Maintains high-side (condenser) and low-side (evaporator) pressure differential.
3. Refrigerant distribution: In multi-circuit evaporators, individual capillaries balance flow (critical for performance).
4. System balancing: Tube length/ID selected to match compressor, condenser, evaporator capacities at design point.

Advantages

• Low cost, high reliability, no maintenance, no adjustment needed.
• Allows pressure equalization during off-cycle → enables low-start-torque compressors (RSIR, CSIR).

Limitations

• Fixed geometry → performance degrades at off-design conditions (high/low ambient, varying load).
• Sensitive to refrigerant charge (critical charge systems).
• Not suitable for large/variable capacity systems (TXV/EEV preferred).
• No superheat control → risk of floodback at low load.

Sizing & Selection

Empirical charts / manufacturer software based on: refrigerant, capacity, evaporating/condensing temps, subcooling, suction line length. Field replacement must match OEM specification exactly.