Size capillary tubes for refrigeration systems. Calculate required length and diameter from cooling capacity, refrigerant type, and operating temperatures.
Capillary tubes are the simplest expansion device in refrigeration, used in domestic refrigerators, window air conditioners, and small split systems. The narrow bore (0.5-1.6 mm ID) creates the pressure drop needed between the high-pressure condenser and low-pressure evaporator, controlling refrigerant flow without any moving parts.
This calculator sizes capillary tubes by computing the required mass flow rate from cooling capacity and latent heat, then determining the tube length and diameter needed for the pressure drop. It handles R-134a, R-410A, R-22, R-290 (propane), and R-600a (isobutane) with their respective liquid properties.
Five presets cover common applications: small R-134a systems, R-410A split AC, window units, propane mini-systems, and isobutane refrigerators. The tool computes Reynolds number, flow velocity, and a tube match indicator showing whether the selected diameter/length combination passes the correct mass flow rate.
A standard tube table lists common OD/ID combinations with their typical applications and length ranges for quick reference.
Use this calculator when you need a quick capillary tube estimate for a small refrigeration system, replacement part, or prototype design. It helps compare refrigerant choices, tube dimensions, and operating temperatures without hunting through tables.
The result is most useful as a starting point for matching a real system’s mass flow and pressure drop before field testing.
Mass flow: ṁ = Q / hfg. Single-phase: Q_vol = πD⁴ΔP / (128µL) (Hagen-Poiseuille). Two-phase correction: ṁ_eff ≈ ṁ_single × 0.55. Required length: L = πD⁴ΔP × CF × ρ / (128µṁ).
Result: ṁ = 20.3 kg/h, required length ≈ 2.1 m, Re ≈ 8500
ṁ = 1000/177 = 5.65 g/s = 20.3 kg/h. ΔP ≈ 50 × 15 = 750 kPa. Hagen-Poiseuille with two-phase correction gives L ≈ 2.1 m for ID = 0.7 mm.
Capillary tubes are sensitive to refrigerant type, subcooling, and load. A small change in diameter or length can move the system from starved to flooded.
- Match the refrigerant to the actual system charge. - Keep the tube length close to the original design when replacing a part. - Coiled runs behave slightly differently from straight runs. - Verify the final setup with superheat and suction pressure after installation.
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Capillary tubes are cheap, reliable (no moving parts), and maintenance-free. They work well when the load is constant. TXVs are preferred for variable-load systems because they actively regulate superheat.
The system is "starved" — insufficient refrigerant reaches the evaporator. Symptoms: warm freezer, low suction pressure, high superheat, possible compressor overheating.
Too much refrigerant floods the evaporator — liquid may reach the compressor (liquid slugging). Symptoms: iced evaporator, high suction pressure, low superheat, compressor damage risk.
Each degree of subcooling increases mass flow by about 2-3% because more of the tube length carries single-phase liquid before flash gas forms. Subcooling delays the flash point downstream.
Yes, but R-410A operates at much higher pressures than R-22 or R-134a. The high pressure difference means shorter tubes or smaller diameters are needed. Most R-410A systems use TXVs or EEVs instead.
The point in the capillary tube where liquid refrigerant begins to flash (boil) due to the pressure dropping below the saturation pressure at the local temperature. After this point, two-phase flow begins and the pressure drop accelerates.