Fridge capillary role in cooling process

The capillary tube acts as the expansion and metering device in domestic refrigerators, positioned between the condenser (high-pressure side) and the evaporator (low-pressure side).

Capillary tube in the refrigeration cycle. Source: ttsz / Getty Images

Core Functions in the Cooling Cycle

1. Pressure Reduction (Expansion)As high-pressure, warm liquid refrigerant leaves the filter drier, it enters the extremely narrow bore of the capillary tube (typically 0.5 mm to 1.5 mm internal diameter). The high friction resistance rapidly drops the refrigerant’s pressure. This sudden pressure drop causes a dramatic fall in boiling point, causing a portion of the liquid to flash into vapor and chilling the refrigerant mixture down to sub-zero temperatures before it hits the evaporator.
2. Refrigerant MeteringThe precise inner diameter and length of the copper tube act as a fixed restrictor. This ensures the exact mass flow rate of liquid refrigerant enters the evaporator coils—fast enough to cool efficiently, but slow enough to allow full heat absorption and vapor transition before returning to the compressor.
3. Off-Cycle Pressure EqualizationUnlike mechanical thermostatic expansion valves (TXVs), a capillary tube has no moving parts or closed valves. When the thermostat satisfies and the compressor shuts off, high-side and low-side system pressures naturally equalize through the capillary bore. This prevents high backpressure against the compressor head, allowing single-phase compressor motors to start up under low torque without stalling.

Common Failure Points

• Capillary Clogging: Debris, oxidized oil sludge, or desiccant particles from a degraded filter drier can block the tiny bore, resulting in zero cooling, low suction pressure, and high compressor head pressure.
• Moisture Freeze-Up: If moisture exists within the sealed system, it freezes into an ice plug right at the capillary exit (where the temperature drops sharply), halting refrigerant flow until the ice thaws during an off-cycle.