Refrigerant Capillary Tube Calculator
Select Tube Resizer to find the equivalent tube length when you change the inside diameter, using the standard ASHRAE empirical formula. Select Flow Analysis to compute mass flow rate, Reynolds number, friction factor, exit quality, and flow regime for a given tube geometry and operating pressures. Supports R-134a, R-410A, R-22, R-600a, R-290, and R-32.
Formula
Worked example
A domestic refrigerator uses R-134a with a 1.63 mm x 2.0 m capillary tube. If you switch to a 1.83 mm bore, the new length is: 2.0 x (1.83/1.63)^4.6 = 2.0 x 1.123^4.6 = 2.0 x 1.698 = 3.396 m. For flow analysis at 1200 kPa inlet, 280 kPa outlet with the 1.63 mm tube: Reynolds number reaches roughly 18,000 (turbulent), friction factor 0.027, and mass flow about 5.5 kg/h.
What is a capillary tube in a refrigeration system?
A capillary tube is the simplest refrigerant metering device used in small refrigeration and air-conditioning systems. It is a long, narrow-bore copper tube, typically 0.5 to 3 mm inside diameter and 0.5 to 5 m long, that creates a pressure drop by friction as the high-pressure liquid refrigerant flows through it. There are no moving parts, making it extremely reliable and inexpensive. Capillary tubes are used in domestic refrigerators, window air conditioners, mini-splits, dehumidifiers, water coolers, and vending machines. They work by balancing the condensing and evaporating pressures through their inherent flow resistance, which depends on the bore diameter, length, refrigerant type, operating pressures, and degree of subcooling at the inlet.
Two calculation methods: tube resizing and flow analysis
This calculator offers two complementary modes. The Tube Resizer uses the ASHRAE empirical formula NL = OL x (D_new / D_orig)^4.6, which states that the mass flow through a capillary tube is proportional to the 4.6th power of the inside diameter at constant pressure difference and length. This formula lets you find the equivalent length when swapping to a different bore tube. The Flow Analysis mode applies the Darcy-Weisbach equation together with the Colebrook-White iterative friction factor to compute mass flow rate, flow velocity, Reynolds number, and exit quality from first principles. It accounts for refrigerant density, dynamic viscosity, tube roughness, inlet subcooling, and the pressure difference between the condensing and evaporating sides. Both methods complement each other: use Resizer for a quick replacement, then verify with Flow Analysis.
Subcooling, flash gas, and exit quality
The degree of subcooling at the capillary inlet strongly affects performance. Subcooled liquid (below the saturation temperature at condensing pressure) gives a higher density and prevents flash gas from forming in the tube, which would reduce mass flow. Increasing subcooling by 5 K typically raises mass flow by about 2-4% depending on the refrigerant. Exit quality is the vapor mass fraction at the tube outlet - a value of 0 means pure liquid, 1 means pure vapor. Typical capillary tubes produce exit qualities between 0.10 and 0.30, meaning 10-30% of the mass is already flashed to vapor as it enters the evaporator. Higher inlet pressure or lower outlet pressure increases exit quality; longer or narrower tubes reduce it by increasing residence time. The calculator estimates exit quality from an isenthalpic flash balance across the total pressure drop.
Selecting capillary tube size for a system
Capillary tube selection starts with the system cooling capacity and the design mass flow rate of the refrigerant. For a given refrigerant and operating pressures, the tube must pass exactly the required mass flow at the design condition. Too large a bore or too short a tube passes too much refrigerant, flooding the evaporator. Too small or too long starves the evaporator of refrigerant, raising superheat and reducing capacity. The reference table above lists typical bore and length combinations for common applications. For exact sizing, iterate the tube diameter in Flow Analysis mode until the computed mass flow matches the system design point. When replacing a capillary tube in the field, measure or look up the original dimensions and use the Tube Resizer mode to find the equivalent length in whatever bore size is available.
Common capillary tube sizes and typical applications
| Application | Refrigerant | ID (mm) | Length (m) | Mass flow (kg/h) |
|---|---|---|---|---|
| Domestic refrigerator | R-134a | 0.66-0.84 | 2.0-4.0 | 3-6 |
| Room air conditioner (small) | R-22 | 1.2-1.6 | 1.5-3.0 | 15-30 |
| Room air conditioner (large) | R-410A | 1.4-1.8 | 1.0-2.5 | 30-60 |
| Mini-split system | R-410A | 1.6-2.0 | 0.8-2.0 | 40-80 |
| Commercial reach-in cooler | R-404A | 1.0-1.4 | 2.0-4.5 | 20-45 |
| Bottle cooler / vending | R-134a | 0.84-1.2 | 2.5-5.0 | 5-12 |
| Water cooler / dispenser | R-600a | 0.6-1.0 | 1.5-3.0 | 2-5 |
Approximate inside diameters and lengths for common small refrigeration systems. Actual values depend on refrigerant, pressure ratio, and subcooling.
Frequently asked questions
What does the 4.6 power in the tube resizing formula mean?
The ASHRAE empirical exponent 4.6 captures how strongly the inside diameter controls flow resistance in a capillary tube. Because flow in narrow tubes is governed by both laminar friction (proportional to D^4 from Hagen-Poiseuille) and turbulent effects, the actual exponent falls between 4 and 5. ASHRAE published 4.6 as the best fit to experimental data across a wide range of refrigerants and conditions. This means a 10% increase in diameter raises mass flow by about 1.1^4.6 = 1.54, or 54%, while a 10% decrease cuts it to 0.9^4.6 = 0.64, a 36% reduction. Small diameter errors have large flow consequences.
How does refrigerant type affect capillary tube sizing?
Different refrigerants have different saturation pressures, densities, viscosities, and latent heats at the same temperature. R-410A operates at much higher pressures than R-22 or R-134a, which means for the same mass flow you need a shorter or wider tube to achieve the same pressure drop. R-600a (isobutane) and R-290 (propane) have much lower densities than HFC refrigerants, requiring larger bore tubes for the same cooling capacity. Always select the refrigerant type in Flow Analysis mode to get accurate results for your system.
What is choked flow and does this calculator handle it?
Choked flow (also called critical flow) occurs when the refrigerant velocity at some point in the capillary tube reaches the local speed of sound in the two-phase mixture, typically near the tube exit. At this point, further reducing the outlet pressure does not increase mass flow. The critical pressure ratio for most refrigerants is roughly 0.55-0.60 (outlet/inlet). The current calculator uses an incompressible Darcy-Weisbach model, which is accurate for the subcooled liquid portion but does not directly simulate two-phase choked flow. For most practical residential and light-commercial applications, operating conditions are below choked flow and the Darcy-Weisbach approximation is adequate. Full two-phase choked flow analysis requires specialized software with refrigerant equation-of-state data.
My refrigerator stopped cooling after replacing the capillary tube - what went wrong?
The most common causes are a tube that is too long or too narrow (starving the evaporator) or one that has a blockage from moisture or oil. A too-long tube produces insufficient refrigerant flow, causing high evaporator superheat, warm cabinet temperature, and the compressor running continuously. A too-short or too-wide tube floods the evaporator and can slug liquid back to the compressor. Check the replacement dimensions against the original specification, verify there is no moisture in the system (a plugged drier filter can mimic a blocked cap tube), and ensure the subcooling at the condenser outlet is within the 3-10 K design range.
Can I use this calculator for R-32 or R-290 (propane) systems?
Yes, both R-32 and R-290 are included in the refrigerant dropdown. R-32 is a high-pressure refrigerant increasingly used in mini-splits as a replacement for R-410A blends, with similar sizing but higher saturation pressures. R-290 (propane) has excellent thermodynamic properties but very low density, requiring larger bore tubes for the same capacity. All hydrocarbon refrigerants require specially approved components and strict safety protocols due to flammability. Always follow the equipment manufacturer guidelines when sizing or replacing components in flammable-refrigerant systems.
What is the Reynolds number and why does it matter for a capillary tube?
The Reynolds number (Re) is a dimensionless ratio of inertial to viscous forces in the flow: Re = density x velocity x diameter / viscosity. Below 2300 the flow is laminar (smooth, layered). Above 4000 it is fully turbulent (chaotic, higher friction). In capillary tubes with typical refrigerant operating pressures, Re values of 5,000 to 50,000 are common, firmly in the turbulent regime. This is actually desirable: turbulent flow gives a well-defined friction factor that is accurately predicted by the Colebrook-White equation used in this calculator. At very low pressure differences or very small bores, flow can be laminar and friction is calculated by the simpler Hagen-Poiseuille formula (f = 64/Re), which this calculator also applies automatically.