Small refrigerators and air conditioners use a thin copper capillary tube as the expansion device. If the original diameter isn’t available, a different diameter can be used with an adjusted length to keep the same restriction.
How to use the Refrigerant Capillary Tube Calculator
- Original inner diameter
- Original length
- Replacement inner diameter
Results update as you type, every field has its own unit menu, and everything is calculated in your browser.
How it works
Enter the original inner diameter and length, and the inner diameter of the replacement tube.
Inputs explained
| Field | What to enter |
|---|---|
| Original inner diameter | Units: in, mm |
| Original length | Units: ft, m, in |
| Replacement inner diameter | Units: in, mm |
What each result means
- Replacement length = [Original length]×([Replacement inner diameter]÷[Original inner diameter])^4.6
- Length ratio = ([Replacement inner diameter]÷[Original inner diameter])^4.6
Example values
How the main result changes when Original inner diameter changes and everything else stays at the example values above:
| Original inner diameter (in) | Replacement length (ft) |
|---|---|
| 0.0155 | 482.4649 |
| 0.0233 | 74.7216 |
| 0.031 | 19.8943 |
| 0.0388 | 7.1276 |
| 0.0465 | 3.0811 |
| 0.062 | 0.8203 |
Formula
L₂ = L₁·(d₂/d₁)^{4.6}
Worked example
Inputs — Original inner diameter: 0.031 in; Original length: 10 ft; Replacement inner diameter: 0.036 in.
Results — Replacement length = 19.8943 ft; Length ratio = 1.9894 ×.
Key terms
- Capillary tube
- Small-bore tube used as a fixed expansion device.
Frequently asked questions
Why the exponent 4.6?
Empirically, capillary flow restriction scales with about d^4.6.
Should I use a smaller or larger tube?
Either works with the adjusted length; a larger one becomes very long.
Is this exact?
It is a rule of thumb – verify the system charge and pressures.
What else affects capillary flow?
Refrigerant type, subcooling and pressures.