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Capillary Tube

A capillary tube meters refrigerant flow via small diameter and length. Pressure drop follows Darcy-Weisbach; ASHRAE provides empirical sizing.

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Small diameter (0.5โ€“2 mm) creates flow resistance; length adds to drop. ASHRAE charts give mass flow vs. pressure for each refrigerant. Flash gas correction accounts for subcooling at condenser outlet. R-410A, R-32 need different sizing than R-22, R-134a.

Key quantities
ฮ”P = f(L/D)(ฯvยฒ/2)
Pressure Drop
Key relation
Q โˆ Dยฒโˆšฮ”P
Flow Rate
Key relation
L from ASHRAE charts
Length
Key relation
Corrected for subcooling
Flash Gas
Key relation

Ready to run the numbers?

Why: Capillary tubes are simple, cheap expansion devices in refrigerators and AC. Correct sizing ensures proper flow and efficiency.

How: ASHRAE method uses pressure drop charts. Diameter 0.5โ€“2 mm typical. Length and diameter determine flow resistance.

Small diameter (0.5โ€“2 mm) creates flow resistance; length adds to drop.ASHRAE charts give mass flow vs. pressure for each refrigerant.
Sources:ASHRAE HandbookNIST - Refrigerant Properties

Run the calculator when you are ready.

Size Capillary TubeEnter refrigerant, condenser/evaporator pressures, and capacity for tube sizing

๐Ÿ  Residential Refrigerator

Standard home refrigerator with R-134a, 150W capacity

โ„๏ธ Window Air Conditioner

5000 BTU/h window AC unit with R-410A

๐Ÿ”ฅ Heat Pump System

Residential heat pump with R-410A, 3.5 kW capacity

๐ŸงŠ Deep Freezer

Commercial deep freezer with R-404A, -35ยฐC evaporating

๐Ÿบ Beverage Cooler

Display cooler with R-134a, 200W capacity

๐Ÿช Commercial Refrigerator

Large commercial unit with R-134a, 2.5 kW capacity

Input Parameters

For educational and informational purposes only. Verify with a qualified professional.

๐Ÿ”ฌ Physics Facts

โ„๏ธ

Capillary tube: fixed-length expansion; no moving parts.

โ€” ASHRAE

๐Ÿ’ง

Laminar flow in capillary: ฮ”P โˆ ฮผยทLยทQ/Dโด (Hagen-Poiseuille).

โ€” Fluid mechanics

๐Ÿ“

Typical lengths 1โ€“6 m; diameters 0.5โ€“2.0 mm.

โ€” HVAC design

๐ŸŒก๏ธ

Subcooling reduces flash gas; improves capacity.

โ€” Refrigeration

What is a Capillary Tube?

A capillary tube is a simple, fixed-length expansion device used in refrigeration and air conditioning systems. It consists of a small-diameter tube (typically 0.5-2.0 mm) that meters refrigerant flow from the high-pressure condenser to the low-pressure evaporator. The capillary tube relies on its small diameter and length to create the necessary pressure drop for refrigerant expansion.

Expansion Device

Capillary tubes serve as expansion devices, reducing pressure from condenser to evaporator.

Function:

Pressure reduction

Flow metering

ASHRAE Method

Industry-standard sizing method based on capacity, temperatures, and refrigerant properties.

Standard:

ASHRAE Handbook

Empirical correlations

Flash Gas Correction

Accounts for flash gas formation during expansion, affecting flow characteristics.

Effect:

Flow reduction

Quality change

How Does Capillary Tube Sizing Work?

Capillary tube sizing involves determining the appropriate diameter and length to achieve the desired pressure drop and flow rate. The ASHRAE method uses empirical correlations based on system capacity, operating temperatures, and refrigerant properties. The calculator considers pressure drop, flash gas formation, and flow regime to provide accurate sizing recommendations.

๐Ÿ”ฌ Calculation Methods

ASHRAE Method

  1. 1Input cooling capacity and operating temperatures
  2. 2Calculate saturation pressures from temperatures
  3. 3Determine mass flow rate from capacity
  4. 4Apply ASHRAE correlations for diameter and length
  5. 5Calculate pressure drop and verify flow regime

Pressure Drop Method

  • Input tube diameter and length
  • Calculate flow rate from capacity
  • Apply Darcy-Weisbach equation for pressure drop
  • Calculate flash gas and adjust for two-phase flow

When to Use Capillary Tube Calculator

Capillary tube calculators are essential for HVAC technicians, refrigeration engineers, and system designers. They're used when designing new systems, replacing capillary tubes, troubleshooting flow issues, or optimizing existing refrigeration systems. The calculator ensures proper refrigerant metering and system performance.

Refrigerators

Design and replace capillary tubes for residential and commercial refrigerators.

Applications:

  • Residential units
  • Commercial systems
  • Repair and replacement

Air Conditioning

Size capillary tubes for window AC units, split systems, and heat pumps.

Applications:

  • Window units
  • Split systems
  • Heat pumps

Freezers

Design capillary tubes for deep freezers and ultra-low temperature systems.

Applications:

  • Deep freezers
  • Medical freezers
  • Laboratory systems

Capillary Tube Calculation Formulas

Understanding capillary tube formulas is essential for refrigeration system design. These formulas relate tube dimensions, flow rates, pressure drops, and refrigerant properties to determine proper sizing and performance.

๐Ÿ“Š Core Capillary Tube Formulas

Pressure Drop (Darcy-Weisbach)

DeltaP=ffracLDfracrhov22\\Delta P = f \\frac{L}{D} \\frac{\\rho v^2}{2}

Pressure drop through capillary tube where f is friction factor, L is length, D is diameter, ฯ is density, and v is velocity.

Mass Flow Rate

dotm=fracQhfg\\dot{m} = \\frac{Q}{h_{fg}}

Mass flow rate equals cooling capacity divided by latent heat of vaporization.

Reynolds Number

Re=fracrhovDmuRe = \\frac{\\rho v D}{\\mu}

Dimensionless number characterizing flow regime (laminar, transitional, or turbulent).

Friction Factor (Laminar)

f=frac64Ref = \\frac{64}{Re}

Friction factor for laminar flow (Re < 2300) in circular tubes.

Flash Gas Percentage

xflash=fleft(fracPcondPevapright)x_{flash} = f\\left(\\frac{P_{cond}}{P_{evap}}\\right)

Percentage of refrigerant that flashes to vapor during expansion, affecting flow characteristics.

Fluid Velocity

v=fracdotmrhoA=frac4dotmpirhoD2v = \\frac{\\dot{m}}{\\rho A} = \\frac{4\\dot{m}}{\\pi \\rho D^2}

Velocity of refrigerant in capillary tube, calculated from mass flow rate, density, and cross-sectional area.

๐Ÿ“š Official Data Sources

ASHRAE Handbook โ†—
Refrigeration systems and refrigerants
Updated: 2026-02-07
NIST Refrigerants Database โ†—
Thermophysical properties of refrigerants
Updated: 2026-02-07
ANSI/ASHRAE Standard 34 โ†—
Designation and safety of refrigerants
Updated: 2026-02-07
Engineering Toolbox โ†—
Refrigeration and fluid properties
Updated: 2026-02-07

โš ๏ธ Disclaimer: Capillary tube sizing is approximate. Verify with manufacturer data and ASHRAE standards for critical applications.

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