Why use capillary tubes instead of TXVs?

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.

What happens if the tube is too long?

The system is "starved" — insufficient refrigerant reaches the evaporator. Symptoms: warm freezer, low suction pressure, high superheat, possible compressor overheating.

What if the tube is too short?

Too much refrigerant floods the evaporator — liquid may reach the compressor (liquid slugging). Symptoms: iced evaporator, high suction pressure, low superheat, compressor damage risk.

How does subcooling affect capillary flow?

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.

Can capillary tubes be used with R-410A?

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.

What is the adiabatic flash point?

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.

Refrigerant Capillary Tube Calculator

Size capillary tubes for refrigeration systems. Calculate required length and diameter from cooling capacity, refrigerant type, and operating temperatures.

Refrigerant

Cooling Capacity (kW)

Condensing Temp (°C)

Evaporating Temp (°C)

Tube Length (m)

Tube Inner Diameter (mm)

Required Mass Flow

20.339 kg/h

Q / hfg

Tube Capacity

26.384 kg/h

With two-phase correction

Required Length

2.59 m

For target mass flow

Tube Velocity

12.17 m/s

Liquid phase

Reynolds Number

51,382

Turbulent

Pressure Drop

750 kPa

ΔT = 50°C

Refrigerant

R-134a (HFC)

hfg=177 kJ/kg, ρ=1206 kg/m³

Tube Sizing Match

⚠ Oversized tube (130%)

Standard Capillary Tube Sizes

OD	ID	Typical Use	Length Range
1.5 mm	0.5 mm	Small fridges	2-4 m
2.0 mm	0.7 mm	Window AC, small split	1.5-3 m
2.5 mm	1.0 mm	Medium split AC (1-2 ton)	2-4 m
3.0 mm	1.3 mm	Larger AC (2-3 ton)	2-5 m
3.5 mm	1.6 mm	Commercial small	3-6 m

Planning notes, formulas, and examples

About the Refrigerant Capillary Tube Calculator

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.

When This Page Helps

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.

How to Use the Inputs

Select the refrigerant type from the dropdown.
Enter the cooling capacity in kW.
Enter condensing and evaporating temperatures.
Enter the tube length and inner diameter.
Read the required mass flow, tube capacity, and sizing match.
Adjust length or diameter until the match indicator shows green.

Formula used

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).

Example Calculation

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.

Tips & Best Practices

Always cut capillary tubes slightly longer than calculated — you can always trim, but you can't add length.
Coiling the tube adds ~5-10% effective length compared to the same length straight.
Solder the cap tube to the suction line for a heat exchange effect that improves efficiency by 3-5%.
Test by measuring superheat at the evaporator outlet: target 5-10°F for optimal performance.
For field replacements, match the original tube ID and length as closely as possible.

Sizing Context

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.

Practical Checks

- 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.

Sources & Methodology

Last updated: January 15, 2025

Frequently Asked Questions

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.