Refrigeration & Equipment Engineering

Unit 3 - Parts of refrigeration systems

3.4.4.Capillary Tube
The capillary tub is used as an expansion devices in small capacity hermetic sealed refrigeration unit such as domestic refrigeration, water coolers, room air conditioner and freezer
It is a copper tube of small internal diameter and of varying length depending upon the application. The inside diameter varies from 0.5mm to 2.25 mm and length varies 0.5m to 5m. It is installed in the liquid line between condenser and evaporator.
In its operation the liquid refrigerant from the condenser enters the capillary tube. Due to the frictional resistance offered by a small diameter tube, the pressure drops. Since the frictional resistance directly proportional to the diameter. Therefore longer the capillary tube and smaller its inside diameter greater is than pressure drop created in the refrigerant flow In other words, greater pressure difference between the condenser and evaporator is needed for a given flow of refrigerant. The diameter and length of the capillary tube once selected for a given set of conditions and load can not operate at other conditions.

Advantages

• The cost of capillary is less than the other forms of expansion devices.
• When the compressor stops, the refrigerant continues to flow into the evaporator and equalizes the pressure between the high side and low side of the system. This considerably decreases the starting load on the compressor
• A low starting torque motor can be used to drive the compressor.

Disadvantages

• Refrigerant change in a capillary tube system is critical
• Capillary tube cannot be used in large refrigeration system because capacity control is not possible.