Refrigerant Capillary Tube Length Calculator
Estimate the capillary tube length needed for your refrigerant metering device. Enter the tube inner diameter, the pressure drop across the tube, the refrigerant mass flow rate, and the liquid viscosity and density for your refrigerant.
mm
bar
g/s
μPa·s
kg/m³
Darcy-Weisbach single-phase estimate
- 1
Cross-section area
π × 0.8² ÷ 4 (mm²) = 0.5027 mm² - 2
Mean flow velocity
5 × 10⁻³ ÷ (1,081 × 0.0000005) = 9.202 m/sv = ṁ / (ρ · A) - 3
Reynolds number
61,213Turbulent flow (Re ≥ 4,000) — Blasius friction factor applies - 4
Darcy friction factor
0.02009 - 5
Capillary tube length
2 × 1,000,000 × 0.0008 ÷ (0.02009 × 1,081 × 9.202²) = 0.87
How does this calculator work?
Capillary tube length is estimated from the Darcy-Weisbach pressure-drop equation L = 2ΔP·d/(f·ρ·v²), where f is the friction factor from the Blasius correlation, v the mean liquid velocity, ρ the liquid density, and ΔP the condensing-to-evaporating pressure difference. This single-phase model is a first-pass estimate; real sizing also accounts for two-phase flash near the tube exit.
Formula
How this is calculated
A capillary tube is the simplest type of refrigerant metering device: a long, narrow tube that produces the pressure drop needed to let high-pressure liquid refrigerant expand to the low-pressure evaporator. Sizing it correctly balances the condensing and evaporating pressures at the design load.
This calculator applies the Darcy-Weisbach equation for single-phase pressure drop: ΔP = f · (L/d) · (ρ·v²/2), rearranged to solve for length L. The flow velocity v comes from the continuity equation using the tube cross-sectional area. The Darcy friction factor f is computed from the Reynolds number: 64/Re for laminar flow (Re < 2300), the Blasius correlation 0.316·Re^(−0.25) for turbulent flow (Re ≥ 4000), and a linear interpolation through the transition zone.
Important limitations: refrigerant flow inside a capillary tube is almost always turbulent. Near the tube exit the liquid flashes to a two-phase mixture, which causes an additional pressure drop not captured here. This result is therefore a first-pass engineering estimate — real sizing relies on manufacturer charts, refrigerant-specific correlations (such as the ASHRAE or Aschner methods), and system testing. Enter liquid-state viscosity and density at the condensing temperature for best accuracy.
Frequently asked questions
Pressure drop increases linearly with length (more friction) and very strongly with decreasing diameter — roughly d^5 power for fixed flow rate in the Darcy regime — so halving the diameter requires roughly 32× less length to produce the same ΔP.
Use the saturated-liquid properties at the condensing temperature for your refrigerant. For R134a at 40 °C the liquid viscosity is about 130 μPa·s and the density about 1081 kg/m³; for R410A at 40 °C they are approximately 107 μPa·s and 1063 kg/m³. Tables are available in ASHRAE handbooks and NIST REFPROP.
The single-phase model overestimates tube length whenever flashing (two-phase flow) begins inside the tube, which happens when subcooling is insufficient. If the inlet refrigerant is not fully liquid, use a two-phase correlation or system-level simulation instead.
Also known as
TG we-Calculate Editorial Team. (2026). Refrigerant Capillary Tube Length Calculator [Online calculator]. TG we-Calculate. https://we-calculate.com/calculator/refrigerant-capillary-tube-calculator
TG we-Calculate Editorial Team. "Refrigerant Capillary Tube Length Calculator." TG we-Calculate. 2026. https://we-calculate.com/calculator/refrigerant-capillary-tube-calculator.
TG we-Calculate Editorial Team, "Refrigerant Capillary Tube Length Calculator," TG we-Calculate, 2026. [Online]. Available: https://we-calculate.com/calculator/refrigerant-capillary-tube-calculator
@misc{wecalculate_refrigerant_capillary_tube_calculator, title = {Refrigerant Capillary Tube Length Calculator}, author = {{TG we-Calculate Editorial Team}}, howpublished = {\url{https://we-calculate.com/calculator/refrigerant-capillary-tube-calculator}}, year = {2026}, note = {TG we-Calculate} }
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