Capillary tube

Capillary Tube and Its Sizing

1. A capillary tube is a long, narrow tube of constant diameter.

2. The word “capillary” is a misnomer since surface tension is not important in refrigeration

application of capillary tubes.

3. Typical tube diameters of refrigerant capillary tubes range from 0.5 mm to 3 mm and the

length ranges from 1.0 m to 6 m.

1. It is inexpensive.

2. It does not have any moving parts hence it does not require maintenance.

3. Capillary tube provides an open connection between condenser and the evaporator

hence during off-cycle, pressure equalization occurs between condenser and evaporator.

This reduces the starting torque requirement of the motor since the motor starts with

same pressure on the two sides of the compressor. Hence, a motor with low starting torque

(squirrel cage Induction motor) can be used.

4. Ideal for hermetic compressor based systems, which are critically charged and factory

assembled.

1. It cannot adjust itself to changing flow conditions in response to daily and seasonal

variation in ambient temperature and load. Hence, COP is usually low under off design

conditions.

2. It is susceptible to clogging because of narrow bore of the tube; hence, utmost care is

required at the time of assembly. A filter-drier should be used ahead of the capillary to

prevent entry of moisture or any solid particles.

3. During off-cycle liquid refrigerant flows to evaporator because of pressure difference

between condenser and evaporator. The evaporator may get flooded and the liquid

refrigerant may flow to compressor and damage it when it starts. Therefore critical charge

is used in capillary tube based systems. Further, it is used only with hermetically sealed

compressors where refrigerant does not leak so that critical charge can be used. Normally

an accumulator is provided after the evaporator to prevent slugging of compressor.

Capillary tube