What is a capillary tube used for?

A capillary tube is a narrow, thin-walled tube used to control or measure the flow of fluids — typically gases or liquids — in a precise and controlled manner. Its small internal diameter (often less than 1 mm) allows it to create a pressure drop or regulate flow rates accurately.

Here’s how it’s used across different fields:

1. Refrigeration and Air Conditioning Systems

In refrigeration systems, a capillary tube serves as an expansion device.

• It connects the condenser (high-pressure side) to the evaporator (low-pressure side).

• The small diameter and long length of the tube cause a pressure reduction, which allows the refrigerant to expand and cool.

• Purpose: To regulate refrigerant flow according to system load without moving parts.

• Advantages: Simple, reliable, inexpensive, and maintenance-free.

2. Laboratory and Medical Applications

In laboratories, capillary tubes are used for sampling, measuring, or transferring small liquid volumes.

• Example: Blood collection capillary tubes in hematology for micro-volume sampling.

• Example: Capillary action in thin tubes is used to draw liquids upward against gravity due to surface tension.

3. Industrial and Engineering Uses

• In hydraulic and instrumentation systems, they serve as fine fluid channels to control flow or dampen pulsations.

• In thermometers (like mercury or alcohol thermometers), capillary tubes hold the liquid column that expands with temperature.

4. Scientific Experiments

Capillary tubes are often used to study fluid dynamics, surface tension, and capillary rise phenomena, helping engineers understand how liquids behave in narrow passages.