Table of Contents
What Is a Capillary Tube?
A capillary tube is a thin copper tube used as an expansion device in small refrigeration and air conditioning systems. It meters the flow of liquid refrigerant from the high-pressure condenser side to the low-pressure evaporator side. The pressure drop occurs due to friction as the refrigerant flows through the narrow bore, causing it to partially flash into vapor and cool dramatically.
Capillary tubes are the simplest and most economical expansion devices, with no moving parts. They are commonly used in household refrigerators, window air conditioners, and small commercial systems up to about 10 tons of cooling capacity. Their main limitation is that they cannot adjust to varying load conditions like thermostatic expansion valves (TXVs) can.
Sizing Principles
The actual capillary tube sizing involves complex two-phase flow calculations. This calculator uses empirical correlations based on ASHRAE data for common refrigerants. The tube length must provide enough pressure drop to match the system's condensing and evaporating pressures while allowing the correct mass flow rate.
Standard Capillary Tube Sizes
| ID (inches) | ID (mm) | Typical Application |
|---|---|---|
| 0.026 | 0.66 | Small freezers |
| 0.031 | 0.79 | Domestic refrigerators |
| 0.042 | 1.07 | Window AC units |
| 0.055 | 1.40 | Mini-split systems |
| 0.070 | 1.78 | Small commercial units |
Practical Considerations
- Subcooling: The refrigerant entering the capillary tube should have 10-15 degrees of subcooling to prevent premature flashing.
- Cleanliness: Capillary tubes are very susceptible to clogging from contaminants. Always use filter driers.
- Coiling: Capillary tubes are often coiled to save space. Tight coiling increases effective length by about 5-10%.
- Heat exchange: Many systems solder the capillary tube to the suction line for a heat exchange effect that improves efficiency.
Frequently Asked Questions
Can I shorten a capillary tube to increase capacity?
Shortening the tube reduces restriction, allowing more refrigerant flow and potentially higher capacity. However, it also reduces the pressure drop, which can lead to incomplete evaporation, liquid slugging in the compressor, and poor system performance. Always follow manufacturer specifications.
Why does tube diameter matter so much?
Flow rate through a capillary tube is proportional to the fourth power of the diameter (Poiseuille's Law). Doubling the diameter increases flow by 16 times. This extreme sensitivity means precise tube sizing is critical -- even small changes in bore diameter significantly affect system performance.
Can capillary tubes be used with variable-speed compressors?
Capillary tubes work best at a single design condition. With variable-speed compressors, the system operates over a wide range of conditions. At low speeds, the capillary may allow too much flow; at high speeds, too little. For variable-speed systems, electronic expansion valves (EEVs) are strongly preferred.